AP Chemistry: Total Notes Review Table of Contents 1. Stoichiometry 2. Electronic Structure 3. Bonding and Molecular Structure 4. Intermolecular Forces 5. Properties of Solutions 6. Kinetics 7. Equilibrium 8. Thermodynamics 9. Electrochemistry 10. Nuclear Chemistry 11. Organic Chemistry 12. Reaction Writing #1: STOICHIOMETRY o Found in Chapters 3 and 4 of the textbook o Limiting Reactants: divide # moles by whole number coefficients – the smallest is the limiter (use that to find the resulting moles of product) o 4 basic steps: 1: write the balanced chemical equation 2: convert to moles 3: use the mole: mole ratio 4: convert to asked for units But that’s for easy stuff like, “determine the number of [moles/grams/liters/etc.] in [substance] for [number and units] of [element] according to the equation below” If you want to get to the really hard-core stuff, look at” Empirical Formulas and Analyses This stuff tells you what the empirical formula of a reactant is based on its products. For example: “Ascorbic Acid contains 40.92% H and 54.50% O by mass. What is the empirical formula of ascorbic acid?” How to Solve: 1. Assume that there’s a 100g sample of the substance (that means that the percents equal their mass) 2. convert samples to moles 3. divide each by the smallest number of moles 4. make a ratio 5. use the ratio to write the equation Take a Look: Sometimes with analysis, you have to find the molecular formula. That’s like the empirical formula, but it’s not in its lowest ratio. For example: “Mesitylene, a hydrocarbon that occurs in small amounts in crude oil, has an empirical formula of C3H4. The experimentally determined molecular weight of this substance in 121 amu. What is the molecular formula of mesitylene?” How to Solve: 1. find the empirical formula weight (use the formula given in the question) 2. put numbers into the equation whole # multiple = molecular mass / empirical mass 3. solve 4. multiply subscripts by whole numbers Take a Look: It’s important to remember that chemistry doesn’t always do the problems the way examples show them. Most of the time you have to manipulate equations and whatnot using the techniques you learn from the examples. Look out for questions that combine these two concepts or otherwise flip them around. Combustion Analysis This is a lot like the two things above, but it’s different at the same time (that’s chem. For you). Okay, here’s what it is: the question will tell you the products of the combustion of usually a hydrocarbon (there are about a million different kinds, so be prepared. Oh, and look at the organic chem. Chapter if you don’t believe me). For example: “Isopropyl alcohol, a substance sold as rubbing alcohol, is composed of C, H, and O. Combustion of 0.255g of isopropyl alcohol produces 0.561g of CO2 and 0.306g of H2O. Determine the empirical formula of isopropyl alcohol.” How to Solve: 1. Find the mass of C and H in the substance: i. convert CO2 and H2O to moles ii. use mol: mol ratios to isolate C and H iii. change moles to grams 2. subtract the mass of C and H from the total mass to find the mass of O 3. convert grams to moles 4. divide by the smallest Take a Look: Limiting Reactants Might as well do one of these so you don’t screw it up on the test. These have a reactant that limits the amount of product produced because one of the reactants runs out. For example: “A 55.8g sample of pure iron is submerged in a solution containing 339.0g of silver nitrate. Fe(NO3)3 and solid silver are the products. a) write the balanced chemical equation b) determine the limiting reactant c) determine the moles of each product produced d) determine the mass of excess reactant remaining” How to Solve: 1. write the balanced chemical equation 2. convert reactants to moles 3. find the limiter (divide # of moles by the coefficient and the smallest is the limiter) 4. use the limiter in an I.C.E. table to find the moles of products and excess reactant remaining 5. convert to units Take a Look: Theoretical Yeilds The quantity of product that is calculated to form when all of the limiting reactant reacts is the theoretical yield. The amount that actually reacts is the actual yield. Percent yield = actual amount / theoretical yield x 100 It’s not too hard, so we won’t do an example. Solution Stoichiometry (AKA: titrations) Use this to determine how much (volume or concentration) of a particular ion there is in a solution. Equivalence point: pretty much the most important part of a titration, it’s when the moles of “A” equal the moles of “B.” It’s also called the “neutralizing point.” Use the same steps as every other stoich. problem to solve these (they just look scarier). For example: “A sample of iron ore is dissolved in acid, and the iron is converted to Fe+2 . The sample is titrated with 47.20 mL of 0.02240 M MnO4- solution. The oxidation-reduction reaction that occurs during the titration is as follows: a) b) c) d) how many moles of MnO4- were added to the solution? How many moles of Fe+2 were in the solution? How many grams of iron were in the sample? If the sample had a mass of 0.8890g, what is the percentage of iron in the sample? How to Solve: Use the basics: 1. use the balanced equation 2. convert to moles (MnO4-) 3. use mol to mol ratio (Fe+2) 4. convert to units Take a Look: #2: ELECTRONIC STRUCTURE o chapters 6: Electronic Structure of Atoms Terms to Know: o Electromagnetic Radiation (radiant energy): moves through a vacuum at the speed of light (c = 3 x 108) Includes: Gamma rays, X rays, Ultraviolet, visible light, infrared, microwaves, and radio frequency o Electronic structure: describes the energies and arrangements of electrons around an atom Electrons behave as both particles and waves (wavicles) o Wave characteristics described with wavelength (λ) and frequency (ν) λxν=c o Photon: wavicle o h = Planck’s constant (6.62 E -34 J/s) o quantum theory: energy is quantized; it can only have certain values o Photoelectric Effect: emitting electrons from a metal’s surface by light (proposing that light behaves as if it consists of quantized packets – photons) o minimum amount of energy that an object can gain or lose is related to the frequency of radiation E=hxν For example: “A yellow Na vapor lamp (street light) has a beam with λ = 589 nm. Calculate a) the frequency of the light b) the energy in a mole of photons of the light c) how many photons are in a 10 mJ burst of this light.” How to Solve: 1: use λ x ν = c to solve for frequency 2: use E = h x ν to solve for J/mol 3: convert 10 mJ to J and use ratios to find # of photons Take a Look: More terms: o Spectrum: the result of dispersion of radiation into its component wavelengths o Continuous spectrum: a spectrum containing all of the wavelengths o Line spectrum: a spectrum that only contains certain wavelengths o Bohr proposed a model explaining H’s line spectrum: ~ energy of H depends on n (the quantum number) ~ n must be a positive integer, each value corresponding to a different specific energy ~ energy of an atom increases as n increases ~ ground state: n = 1 ~ if an electron drops from a lower energy state to a higher one, it emits light; light must be absorbed to do the opposite ~ frequency of light emitted must be such that hv equals the difference of energy between the two allowed states of the atom Equations that represent the movement of an electron from one “allowed energy state” to another E = -2.18E-18 n2 1 = (RH)( 1 - 1 ) λ n2i n2f where n is a whole # (principle quantum #) Rydgerg eq.: calc. wavelength of the spectral line of H) RH = 1.096776 E 7 m-1 (Rydberg constant) Some questions that might come up: “Calculate the energy of an electron in the H atom when n = 3” How to Solve: Plug and chug in Bohr’s equation. Take a Look: “Find the wavelength of radiation emitted when an electron moves from n = 3 to n = 1 in the H atom. What region of the electromagnetic spectrum is the emitted radiation in?” How to Solve: Plug and chug in Rydberg equation Take a Look: More and More Terms: o De Broglie: matter (like electrons) should have wave-like properties o The characteristic wavelength of an object depends on momentum: λ = h / m x v (m = mass in kg; v = velocity) (more mass, less visible wavelength) o Uncertainty principle: limit to accuracy with which the position and momentum of a particle can be measured at the same time For example: “What is the wavelength of an electron moving with a speed of 5.97 E 6 m/s? (electron mass = 9.11 E -28g) How to Solve: Plug and chug into equation. Take a Look: Orbitals and all their Wonder o Wave Functions: ψ; behavior of the electron in the quantum mechanical model of the H atom ~ each has a precisely known energy ~ location can’t be exactly determined o Probability density: ψ2; the probability that the electron will be at a particular point in space ~ Electron density: a map of the probability of finding the electron at all points in space o Orbitals: the allowed wave functions of an H atom ~ combo. of an integer and a letter ~ n: principle quantum number (positive integer) ~ l: azmuthal quantum number (s: 0, sphere; p: 1, dumbbell; d: 2, flower; f: 3, rare), defines the shape of the orbital ~m1: magnetic quantum number; (-1 to 1); describes orientation of the orbital ~m2: spin magnetic quantum number; (+1/2 or -1/2), two directions of an electron spinning about an orbital axis o Electron shell: the set of all orbitals with the same value n ex) 3s, 3p, 3d o Subshell: set of one or more orbitals with the same n and l values ex) 3s, 3p, and 3d are each subshells of the n = 3 shell ~ there is one orbital in an s subshell ~ 3 orbitals in a p subshell ~ 5 orbitals in a d subshell ~ 7 orbitals in an f subshell o Radial probability function: the probability that an electron will be a certain distance away from the nucleus o Node: an area where there is 0 probability that the electron will be found Electron Configuration and the Periodic Table o Different subshells of the same shell have different energies ~ For a given value of n, the energy of the subshells increases as the value of l increases (ns < np < nd < nf) ~ degenerate: orbitals within the same subshell, hence the same energy o Pauli exclusion principle: no two electrons in an atom can have the same values for n, l, m1, and ms ~limits the amount of electrons that can occupy an orbital to 2 o Electron configuration: describes how the electrons are distributed among the orbitals fo an atom o Hund’s Rule: the lowest energy is attained by maximizing the number of electrons with the same electron spin (so don’t pair up until an electron of the same spin occupies each orbital) o Elements in the same group on the Periodic Table have the same type of electron arrangement in their outermost shells ex) F, [He]2s22p5; and Cl[Ne]3s23p5 ~ Outer-shell electrons: those that lie outside the orbitals occupied in the nextlowest noble gas element ex)[He]2s22p5 ~ Valence electrons: outer-shell electrons involved in chemical bonding (elements with atomic number 30 or less, all outer-shell electrons are valence electrons) ~ Core elements: electrons that aren’t valence electrons o Representative (main group) elements: elements in which the outermost subshell is an s or a p subshell (alkali metals and noble gases) o Transition elements: elements in which the d subshell is being filled o Lanthanide (rare earth) elements: elements in which the 4f subshell is being filled o Actinide elements: elements in which the 5f subshell is being filled ~ Lanthanide and Actinide elements: f-block metals You can use these facts to utilize the Periodic Table to write electron configurations of elements. Example: “Write the electron configuration of Al.” How to Solve: 1: write the n value that’s closest to the nucleus 2: write the l value 3: Go to the next n value and write the l value 4: repeat steps until you reach Al on the Periodic Table Take a Look: This tells you that there are: 2 electrons in the 1s sublevel, 2 electrons in the 2s sublevel, 6 electrons in the 2p sublevel, 3 electrons in the 3s sublevel, and 1 electron in the 3p sublevel. Effective Nuclear Charge o Valence Orbitals: elements in the same group have the same number of electrons in their valence orbitals ~ there are differences between elements of different groups because their valence orbitals are in different shells o Effective Nuclear Charge: a representation of the averaged electrical field experienced by the electron; the average environment created by the nucleus and other electrons in the atom; expressed as Zeff ~ As you get closer to the nucleus, Zeff increases; as you get farther away, there are more electrons “shielding” the one on the outside, so it experiences a smaller Zeff ~ P.T. trend: left to right -> decreased Zeff (more electrons shielding) So, F has a lower effective nuclear charge than C Sizes of Atoms and Ions o Bonding atomic radius: used to determine the size of an atom ~ P.T. trend: radii increase as you go down a column (increase n value) and decrease as you go right across a period (fewer protons in elements on the left, so smaller Zeff) ~ the charge of the nucleus on an atom or monatomic ion is the same as the atomic number o Cations: smaller than parent atoms (lose an electron) o Anions: bigger than parent atoms (gain an electron) o Ions with the same charge: size increases as you go down a column o Isoelectronic Series: a series of ions with the same number of electrons ~ size decreases with increasing nuclear charge (electrons are more strongly attracted to the nucleus) ~ Larger ions have lower atomic numbers Ex) Na+1, Mg+2, Al+3, F-, and O-2 For example: “Arrange these atoms and ions in order of decreasing size: Mg+2, Ca+2, and Ca” How to Solve: 1: know that cations are smaller than parent atoms, so Ca+2 is smaller than Ca 2: Ca is under Mg on the periodic table, so Mg+2 is bigger than Ca+2 3: write it out Ca > Mg > Ca+2 Ionization Energy o Ionization energy: the minimum amount of energy needed to remove an electron from an atom in its gaseous phase (forms a cation) ~ first ionization energy: taking the outermost electron (or first electron) ~ second ionization energy: removing a second electron ~ always positive (need to put energy into the system to take the electron out) ~ sharp increase in ionization energy after removing all of the valence electrons (higher effective nuclear charge) ~ P.T. Trend: increase as you go left to right and decrease as you go down (because smaller atoms’ electrons are closer to the nucleus) (opposite of atomic radii) o Writing electron configurations for ions: write the configuration of the neutral atom (see page 11), then remove or add the right number of electrons ~ add electrons to orbitals with the lowest value n ~ remove electrons from the highest value n For example: “Arrange the following in order of decreasing first ionization energy: Ne, Na, P, Ar, and K” How to Solve: 1: locate each element on the periodic table 2: Na, P, and Ar are in the same period, so they must vary in the order Na < P < Ar 3: Ne is above Ar, so Ar < Ne 4: K is below Na, so K< Na 5: write it out Ne > Ar > P > Na > K “Write the electron configuration of Ca+2” How to Solve: 1: Write out parent atom’s electron configuration: [Ar]4s2 2: Remove 2 electrons from orbitals with the highest value n [Ar] or 1s22s2p63s2p6 Electron Affinities Electron affinity: the energy change upon adding an electron to an atom in the gas phase (opposite of ionization energy) ~ more negative electron affinity means the anion’s more stable ~ P.T. Trend: more negative as you go left to right (until you get to noble gases) and more negative as you go up ~ Noble gases are always positive because they would have to gain a new subshell to gain an electron ~ Halogens have the most negative electron affinities (Cl is the most negative) ~ Enthalpy measurement: ΔH = kJ/mol #3: BONDING AND MOLECULAR STRUCTURE o Chapters 8 and 9 Chemical Bonds 3 types: 1: Ionic 2: Covalent 3: Metallic Ionic Bonding o Electrostatic attraction between ions (one takes the other’s electron) o Metal + metal, metal + non-metal, metal + polyatomic ion, or 2 polyatomic ions o Lattice Energy: the energy required to completely separate a mole of a solid ionic compound into its gaseous ions Coulomb’s Law: Eel = K Q1 x Q2 d where Q is charge and d is distance between ions ~ increases with the charge of the ions and decreasing size of ions (charge plays bigger role, though) For example: “Arrange the compounds from least to greatest lattice energy: NaF, CsI, CaO” How to Solve: 1: look at charges (Na+, F-; Cs+, I-; Ca+2, O-2)(CaO has more than all, because stronger charges) 2: Compare distances (Cs is bigger than Na, and I is bigger than F, so NaF > CsI) CsI < NaF < CaO Covalent Bonding o Atoms share electrons (usually 2 non-metals) o Electronegativity: how much an atom “wants” an electron ~ P.T. Trend: increases from bottom left to top right ~ differences in electronegativity between bonded things causes polarity o Polarity: unequal sharing of electrons ~ greater the electronegativity difference, the more polar Lewis Electron Dot Structures o Show molecules with all electrons, bonded and non-bonded o Steps to Making one: 1: Sum the valence electrons (use Periodic Table) (anion – add electron; cation – subtract electron) 2: make the central atom the least electronegative one 3: Fill the octets 4: use double and triple bonds as necessary o Formal charges: subtract the amount of electrons on the periodic table (for that element) from the electrons you drew in ~ 0 means right on ~ the negative charge should be on the most electronegative atom o Resonance: when one Lewis structure can’t accurately describe a molecule (due to something like a double bond that “resonates” between atoms as in ozone or benzene); the electrons in the shifting bond are delocalized o Exceptions to the octet rule: 1: Odd number of electrons: rare, but you’ll just have to cope with it 2: Fewer than 8 electrons: if filling the octet of the central atom results in a negative charge on the central atom and a positive charge on the more electronegative outer atom, don’t fill the octet of the central atom (as in BF3) 3: More than 8 electrons: atoms in the 3rd row or below on the Periodic Table can have expanded octets due to empty d orbitals; always check formal charges to make sure it all works Bond Enthalpy o Strength measured by how much energy is required to break the bond o Called: Bond Enthalpy o Always positive because bond breaking is endothermic Enthalpies of Reaction: Estimate ΔH: ΔH = Σ(bond enthalpies of reactants (or broken)) – Σ(bond enthalpies of products (or formed)) Molecular Shapes o Shape plays a key role in reactivity o Can predict the shape of a molecule by noting the number of bonded and nonbonded pairs of electrons ~ electron domains: pairs of electrons o VSEPR (Valence Shell Electron Pair Repulsion Theory): the best arrangement of a given number of electron domains is the one that minimizes the repulsions among them Electron Domains Non-bonded domains Bonded Domains Bond Angle 2 3 3 4 4 4 2 3 2 4 3 2 0 0 1 0 1 2 5 5 5 5 6 6 6 5 4 3 2 6 5 4 0 1 2 3 0 1 2 180 120 <120 109 107 104.5 90, 120 <120 equit. 90 90 180 90 <90 90 Name linear trigonal planar bent tetrahedral trigonal pyramidal bent trigonal bipyramidal seesaw t-shape linear octahedral square pyramidal square planar o Polarity: just because a molecule has polar bonds, doesn’t mean the whole molecule is polar; you can tell whether a molecule is polar or not by its symmetry Hybridization o Overlap: increased overlap brings electrons closer together while simultaneously decreasing electron-electron repulsion ~ BUT: if atoms get too close, the internuclear repulsion greatly raises the energy o Once you know the electron-domain geometry, you know the hybridization state of the atom (count on your fingers to find out the hybridization: sp, sp2, sp3, sp3d, sp3d2) o Two ways orbitals can overlap: Sigma bonds (σ): single bonds; head-to-head overlap; stronger Pi bonds (π): form with double or triple bonds; side-to-side overlap #4: INTERMOLECULAR FORCES o Chapter 11 o Intermolecular Forces: forces between molecules in non-metals ~ often confused with ionic bonds, but those are intramolecular forces How things “Stick Together” LDFs (London Dispersion Forces): momentary polarization due to electrons’ revolution about the nucleus (at one instant, electrons can all be on one side creating an instantaneous dipole so that the molecule can attract to other molecules undergoing the same thing); everything has LDFs; they’re not super strong; bigger molecule leads to greater LDFs because there are more electrons; shape of molecules affect LDFs’ strength (surface area) Dipole-Dipole Forces: negative and positive ends of two polar molecules attract to each other; some LDFs involved; only between polar molecules; not instantaneous; has greater effect than LDFs Hydrogen Bonding: a kind of dipole-dipole, but stronger; only with “FON” (Fluorine, Oxygen, and Nitrogen); electronegativity of F, O, and N causes a super strong dipoledipole moment Ion-Dipole: ion attracted to molecule with a dipole moment How strongly things stick together o Depends on LDFs and D-D Forces For example: “List the substances BaCl2, H2, CO, HF, and Ne in order of increasing boiling points.” How to Solve: 1: identify what types of forces hold the molecules together BaCl2: ionic bonds; H2: LDFs (non-polar); CO: D-D (polar); HF: H-bonds (FON); Ne: LDFs 2: recognize differences between atoms, ions, and molecules as well as the strength of the forces LDFs are the weakest, then D-D, then H-bonds H2 < Ne < Co < Hf < BaCl2 Phase Changes o Phase changes strain intermolecular forces (the strength of intermolecular forces determines when the substance will change phases) o Condensed states: solids and liquids (that’s why it’s harder to go from liquid to gas than solid to liquid) o Every phase change involves a change in enthalpy Solving for enthalpy of phase changes: ΔH = q= mcΔT phase change: ΔH = q = ΔHvaporization x #mole = ΔHfusion x #mole For example: “Calculate the enthalph upon converting 1.00 mol of ice at -25 C to water vapor at 125 C under a constant pressure of 1 atm. The specific heat of ice, water, and steam are 2.09 J/g-K, 4.18 J/g-K, and 1.84 J/g-K, respectively. For H2O, ΔHfus = 6.01 J/g-K and ΔHvap = 40.67 kJ/mol.” How to Solve: 1: find the enthalpy change for going from 25 C to 0 C 2: find the enthalpy of the phase change (use ΔHfus) 3: find enthalpy change for going from 0 C to 100 C 4: find enthalpy of the phase change (use ΔHvap) 5: find enthalpy change for going from 100 C to 125 C 6: add all of those enthalpies up to find the total enthalpy Take a Look: Some Key Words: o Solid: most condensed phase; doesn’t flow; closely packed o Liquid: another condensed phase; less closely packed; flows into a container o Gas: not condensed; free moving molecules (see chapter o Melt: going from solid to liquid (use heat of fusion) o Freeze: going from liquid to solid (also use heat of fusion) o Vaporize: liquid to gas (use heat of vaporization) o Condense: gas to liquid (also use heat of vaporization) o Sublime: solid to gas (use heat of sublimation) o Depose: gas to solid (also use heat of sublimation) o Evaporate: molecules escaping from the surface of a liquid into the gas phase o Volatile: evaporates quickly/readily o Normal boiling point: the boiling point of a liquid at 1 atm pressure (occurs when vapor pressure equals the pressure of the atmosphere around it) o Vapor pressure: the pressure exerted by the vapor of a liquid when the liquid and vapor states are in dynamic equilibrium o Equilibrium: when evaporation and condensation occur at equal rates Phase Diagrams: o AB line: vapor interface (each point is the boiling point at that pressure) o A: triple point (all three states in equilibrium; liquids can’t exist below it) o B: critical point (above this, liquid and vapor are indistinguishable from eachother) o AD line: liquid/solid interface (each point is the melting point at that pressure) o AC line: solid/gas interface (each point is the sublimation point at that pressure) Structures of Solids: o Crystalline (structured) or amorphous (no order) Types of Questions You Can Expect: o Questions about comparing intermolecular forces ~ which is stronger? ~ what type of force does the substance have? o Calculating Enthalpies of Phase Changes o Phase diagrams ~ what phase is the substance in? ~ what do certain points mean? ~ interpretation o Questions about the different properties of Crystalline Solids #5: PROPERTIES OF SOLUTIONS o o o o Chapter 13 Solution: homogenous mixture of two or more pure substances A solute is dispersed throughout a solvent Formation of Solutions: solvent pulls solute particles apart and surrounds (solvates) them Types of Solutions State of Solution Gas Liquid Liquid Liquid Solid Solid Solid State of Solvent Gas Liquid Liquid Liquid Solid Solid Solid State of Solute Gas Gas Liquid Solid Gas Liquid Solid Example Air oxygen in water alcohol in water salt in water hydrogen in palladium mercury in silver silver in gold Energy Changes in Solution 3 Processes Affect Energetics of Solutions 1: separation of solute particles 2: separation of solvent particles 3: new interactions between solute and solvent ΔHsoln = ΔH1 + ΔH2 + ΔH3 Entropy: the amount of randomness in a solution; the more random a solution, the lower the energy So, although enthalpy increases, if the system becomes more disordered (entropy increases), the overall energy can decrease. To What Extent is the Solution a Solution? o Saturated: solvent holds as much solute as possible at that temperature o Unsaturated: less than the maximum amount of solute for that temperature is dissolved in the solvent o Supersaturated: solvent holds more solute than is possible for the given temperature (usually unstable) Factors Affecting Solubility o Temperature: solubility of solids in liquids usually increases with increasing temperature; solubility of gases usually increases with decreasing temperature Remember! “Like dissolves like” ~ polar substances dissolve in polar solutions ~ non-polar substances dissolve in non-polar solutions (think oil and water) Why? More similar intermolecular forces Gases o The solubility of a gas increases in direct proportion to its partial pressure above the solution So, pretty much, the more pressure a gas exerts over a solution, the more soluble it is (I guess that’s why you keep soda in highly pressurized cans or bottles, eh?) Henry’s Law: Sg = kPg Sg : solubility of the gas of the gas k: Henry’s constant for that gas solute Pg: partial pressure The Wonderful World of Concentrations Ways to express it: o Mass percentage Mass of A in solution x 100 Total mass of solution o Mole Fraction: Moles of A Total moles in solution o Molarity (M): Mol solute L of solution o Molality (m): Mol of solute Kg of solution Note: you can change from M to m if you know the density (g/L) of the solution Colligative Properties Some properties change when you make solutions. (The changes depend on the number of solutes, not their identity) Such as: o Vapor Pressure Lowering: higher concentration on non-volatile solutes makes it harder for the solvent to escape into the vapor phase Raoult’s Law: PA = XAP°A (find the new vapor pressure) o Boiling Point Elevation: solutions want to stay in the liquid phase longer; the elevation is proportional to the molality of the solution ΔTb = Kb x m (Kb is the boiling point elevation constant) o Freezing Point Depression: solutions want to stay in the liquid phase longer; the depression is proportional to the molality of the solution ΔTf = Kf x m o Osmotic Pressure: the net movement of solvent is always toward the solution with the higher solute concentration; obeys a law similar to the ideal-gas law πV = nRT (π = osmotic pressure) so, π = (n/V)RT = MRT ~ isotonic: two solutions of identical π ~ hypotonic: a more concentrated solution ~ hypertonic: a more dilute solution The van’t Hoff Factor Reassociation is more likely with high concentrations, so you can’t really expect something like NaCl, for example, to totally split up into 2 moles of ions. That’s why we have this equation: ΔTf = Kf x m x i (i = how much the substance dissociates) NOTE: you can determine the molar mass of a substance using colligative properties by manipulating the equations Colloids Suspensions of particles larger than individual ions or molecules, but to small to be settled out by gravity o Tyndall Effect: the effect of colloidal suspensions scattering rays of light o Hydrophyllic: water-loving colloids o Hydrophobic: water-hating colloids (need to be stabilized in water) Types of Questions you can Expect o Different concentration calculations o Colligative Property calculations ~ vapor pressure, boiling point, freezing point, and osmotic pressure ~ molar mass o Predicting whether or not a substance will dissolve o Calculate molality and Molarity from each other #6: KINETICS Chapter 14 4 Factors that Affect the Rate of a Reaction 1: nature of the reactants 2: Concentration/pressure of the reactants 3: Temperature 4: presence of a catalyst Reaction Rates Rate = Δ concentration Δ time (Δ = final – initial) Use mole ratios to determine the different rates Rate and Concentration Rate = k[A]x[B]y (x and y = exponents of rate-determining step; k = rate constant) o Orders: Exponent to which the specie is raised o Overall order: Add the exponents (orders) of each specie Now let’s do a problem…. Find the Rate of a Reaction “The initial rate of a reaction A + B C was measured for several different concentrations of A and B. The results are below. Use the data to determine: a) the rate law for the reaction b) the magnitude of the rate constant c) the rate of the reaction when [A] = 0.050M and [B] = 0.100M” Experiment Number [A] (M) 1 2 3 [B] (M) 0.1 0.1 0.2 0.1 0.2 0.1 Initial rate (M/s) 4.0 x 10^-5 4.0 x 10^-5 16.0 x 10^-5 How to Solve 1: write out the basic formula for a rate law (rate = k[A]x[B]y) 2: use given values to determine “x” 3: use given values to determine “y” Now you know the rate law Be careful, because sometimes they like to make it hard for you. Keep your mind open to different ways to manipulate the equations (sorry, I can’t find any super awesome examples so you’ll have to cope with that great piece of advice). 4: Use an “initial rate” value from the table and the corresponding [A] and [B] values to solve for “k” Now you know the magnitude and units of k Make sure to be careful with the units of k. Check the table below to see what they usually are. 5: plug and chug all of the values you just found to get the value when [A] and [B] are what they ask for Now you’re done! Take a Look Change of Concentration Over Time (integrated rate laws) Rate Law Equation Integrated Rate Law Zero Order First Order Second Order Rate = k[A]0 A products [A] = -k x t + ln[A]0 Rate = k[A]1 A products ln[A] = -k x t + ln[A]0 Rate = k[A]2 A products [A]-1 = k x t + [A]0-1 t1/2 = [A]0 x k 2 y = mx +b t1/2 = ln(1/2) = 0.693 k k y = mx + b t1/2 = 1 k[A]0 k = slope [A] vs. time k = slope ln[A] vs. time k = slope 1/[A] vs. time M/s 1/s 1/Ms y = mx + b Half-Life Equation Graphically, what makes it linear k and its units The first and second order integrated rate laws are on the bottom left-hand corner of the crappy periodic table. Most of the problems involving these will be simple plug-and-chuggers, so we won’t do any samples. Just memorize the equations. Energy Profile Diagrams o vertical axis: Energy o horizontal axis: reaction progress o Activation energy: the minimum amount of energy needed to start a reaction o lower activation energy (Ea) means faster reaction o Catalysts: provide alternate pathways for a reaction, thus lower its activation energy The Arrhenius Equation The increase in rate with increasing temperature is nonlinear. The Arrhenius Equation relates 3 factors that affect reaction rates: 1) fraction of molecules possessing Ea 2) collisions that occur per second 3) fraction of collisions that have appropriate orientation factor k = A x e ^ -Ea/RT k = rate constant; A = free factor; -Ea = activation energy; R = gas constant; T = temperature in Kelvins OR lnk = -Ea x 1/RT + lnA (This one’s on the crappy on the very bottom left) Note: the slope = activation energy Reaction Mechanisms o coefficients need to match between element steps and exponents of the rate law o the slow step is the rate determiner o intermediates may be used as long as they’re used up by the end o rate laws are only determined experimentally o elementary reactions: single steps o molecularity: indicates the number of molecules that act as reactants in a reaction o unimolecular: A o bimolecular: A + B o termolecular: A + B + C o multi-step mechanisms: always have to add up to give the equation of the overall process (like Hess Law in Thermodynamics) For example, “The conversion of ozone into O2 proceeds by a two-step mechanism. O3 O2 + O O3 + O 2O2 a) describe the molecularity of each elementary reaction in this mechanism b) write the equation for the overall reaction c) identify the intermediates.” Take a Look # 7 EQULIBRIUM Chapters 15, 16, and 17 What is equilibrium? o All reactions are reversible, but we want to know to what extent they are reversible. o Equilibrium occurs when a reaction and its reverse proceed at the same rate o Depicted by a double arrow The oh-so-magnificent Rate Constant o Symbolized by K (the ratio of the rate constants at that temperature) Finding K: Rate of forward reaction: rate= kf[reactants] Rate of reverse reaction: rate = kr[reactants] So, at equilibrium: Ratef = Rater => kf/kr = products over reactants = K Generally, aA + bB cC +dD Kc = [C]c[D]d [A]a[B]b o Only gases are used in the expression o The reverse and forward equilibrium constant expressions are reciprocals o If you multiply a reaction, the K is raised to that power (always do one operation above) o Pressure and concentration are proportional, so K can also be written according to pressures (Kp) Kp = Kc(RT)Δn (equation to change from concentration to pressure or vice versa) The meaning of K: K tells you how much something dissociates. Basically, it tells you whether the reaction is product-favored (k >> 1) or reactant-favored (k << 1). Different kinds of K: K: equilibrium constant kf: forward rate kr: reverse rate Kc: equilibrium with concentrations Kp: equilibrium with pressures Kw: equil. of water Ka: dissociation of an acid Kb: dissociation of a base Ksp: solubility product constant We’ll discuss all of these in this chapter. For example: “Oxalic acid, H2C2O4, is a diprotic acid with K1 = 5.36 x 10-2 and K2 = 5.3 x 10-5. For the reaction below, what is the equilibrium constant? H2C2O4 + 2 H2O 2H3O+ + C2O4-2” How to Solve: 1: write out the two equations involved in the dissociation of oxalic acid 2: manipulate those equations until they look like the one in the problem (like Hess’s Law) 3: if you multiplied or divided any equations, you have to either raise K to that power or take the root of that power; if you reversed them, take the inverse of K (K-1) 4: once it’s all situated, multiply the K values together Take a Look: OR: “A closed system containing 1.000E-3 M H2 and 2.000E-3 M I2 at 448 C is allowed to reach equilibrium. Analysis of the mixture shows that the concentration of HI is 1.87E-3 M. Calculate Kc at 448 C for the reaction H2 + I2 2 HI” How to Solve: 1: make an I.C.E table with the initial concentrations 2: add the change in concentration (1.87E-3) to the product 3: use stoichiometry to subtract that amount from the reactants 4: now you know the concentrations, so put them in to the equilibrium constant expression Take a Look I.C.E. Tables o Initial Change End Tables demonstrate the concentrations of substances undergoing equilibrium o They can be used for about some really important things: ~ calculating K ~ using with a known K value to find concentrations o Remember: in titrations, the values always have to be in moles (we’ll get to that later, though) (we do about a million of these, so I don’t think I’ll do an example here) The Reaction Quotient o Q is the reaction quotient. It gives the same ratio as K, but the system isn’t at equilibrium. o You can compare K to Q to figure out how far along the reaction is Q > K: there’s too much product, so the reaction shifts to the left (toward the reactants) Q < K: there’s too much reactant, so the reaction shifts to the right (toward the products) Q = K: it’s at equilibrium If a question asks about the concentrations of a system when it’s not at equilibrium, plug it into a Q expression. Usually they have you compare it to K and ask what this indicates about the reaction. Le Chatlier’s Principle o This is used so much. I’m not even lying. It comes up everywhere. o Here’s what Le Chat had to say: “If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance.” Let’s break it down: Change in Concentration: ~ increase: shift to opposite side ~ decrease: shift to that side ~ I kind of think of it like a Star Wars spaceship going into hyperspace. It charges up the concentration, then splurts to the other side. Well, that’s a bad analogy, but think of it as splurting to one side or the other based on how much stuff there is. Change in Pressure: ~ increase: shift to the side with the fewest moles of gas ~ decrease: shift to the side with the most moles of gas Change in Temperature: ~ if it’s exothermic Hotter shift to reactants ~ if it’s endothermic Hotter shift to products (write heat as either a product or reactant, depending on ΔH, and think about it like concentration) Note: this is the only one that affects K Catalysts: ~ DO NOT AFFECT EQUILIBRIUM! ~ they only provide an alternate pathway, which lowers the activation energy, and speeds up the reaction Acids and Bases o Arrhenius acids and bases: Acid: substance that increases the concentration of H+ in solution when dissolved Base: substance that increases the concentration of OH- in solution when dissolved o Bronsted-Lowry acids and bases: Acid: proton donor Base: proton acceptor o Lewis acids and bases: Acid: electron acceptor Base: electron donor o Amphoteric: can be either an acid or a base (HCO3-, HSO4-, H2O) Note: in any acid-base reaction, the equilibrium will favor the reaction that moves the proton to the stronger base. pH and pOH o o o o o “p” just means to take the negative logarithm of something pH = -log[H+] and pOH = -log[OH-] pH + pOH = 14 [H+][OH-] = 1.0 x 10-14 All of these equations are on the crappy and they’re used a lot Dissociation The equilibrium expression for the general dissociation of an acid or base is Ka or Kb. The higher the value, the stronger the acid or base (because it dissociates more). Ka x Kb = 1 x 10-14 pKa + pKb = 14 You can use these values to calculate pH and percent ionization or vice versa. But keep your eyes peeled, because chemistry likes to throw new stuff at you (it’s either to make sure you know what you’re doing or make you feel like an idiot, I’m not sure which yet). Percent Ionization [H3O+]eq x 100 [HA]initial [OH-]eq x 100 [B]initial Use these equations to find out how much of a substance is ionized. If the percent ionization > 5%, you can’t ignore x when solving for concentrations with K. Let’s do Some Problems Finding Ka “A solution of 0.10M formic acid (HCHO2) is found to have a pH = 2.38. Calculate the Ka and the percent ionization in 0.10M solution.” How to Solve: 1: write the balanced chemical equation 2: make an I.C.E. table 3: fill in the values (you know the pH, so you know the [H+], so you can figure out the rest of the values from that) 4: use the end values from the I.C.E. table and plug them into the Ka expression 5: find the percent ionization by plugging and chugging the values you found into the equation Take a Look Use Ka to calculate pH “The Ka of Niacin is 1.5E-5. What is the pH of a 0.01M solution of niacin?” How to Solve: 1: write out the chemical equation (you don’t need to know all of the chemical formulas) 2: write out the Ka expression and make an I.C.E. table 3: use “x”s to express the concentrations gained or lost 4: plug the end values (xs) into the equilibrium constant expression and solve for x 5: x = [H+] (be careful with stoichiometry here, though, because it’s not always 1:1) so take the –log of that to get pH Take a Look Use Ka to calculate percent ionization “Calculate the percent ionization of HF molecules in a 0.010 M solution of HF.” How to Solve 1: write out the equation and set up an I.C.E. table to find the [H+] at equilibrium 2: solve the I.C.E. table using xs and the Ka value as in the previous example 3: plug in the value of [H+] that you found as well as the initial concentration of the solution to the equation and solve Take a Look You can do all of this with bases, too. Just make sure you use Kb and solve for [OH-] instead of Ka and [H+]. Also, if you need to find pH or Ka, you can use the equations above to go back and forth between them. Polyprotic Acids o These acids have more than one proton on them (H2SO4, H3PO4, etc.) o These acids dissociate more than once, thanks to these extra protons, so they have multiple dissociation constants o It’s always easier to remove the first proton from a polyprotic acid than to remove the second (so you’ll have a lower concentration of the final conjugate base, and almost all of the [H+] comes from the first dissociation) Calculating the pH of a polyprotic acid solution “The solubility of CO2 in pure water at 25 C and 1 atm is 0.0037M. The common practice is to assume that all of the dissolved CO2 is in the form of carbonic acid (H2CO3), which is produced by a reaction between the CO2 and H2O: CO2 + H2O H2CO3 What is the pH of a 0.0037M solution of H2CO3? And what is the [CO3-2] in that solution?” How to Solve 1: write out the first dissociation and make an I.C.E. table 2: put in values and use x-s to indicate the change in concentration (as before) 3: use the Ka1 value to find the [H+], and thus the pH (because the first dissociation creates more H+) 4: write out the second dissociation and make another I.C.E. table. This time, put in the [H+] you found in the first dissociation for its initial value instead of leaving it as 0. 5: solve through for x again using the ka2 constant to find the [CO3-2] Take a Look Acid-Base Properties of Salt Solutions Acid + Base salt + water o Salts can act as acids or bases when dissolved in water. Here’s a little summary of how that all works 1) if an anion is the conjugate base of a strong acid, it won’t affect the pH 2) if an anion is the conjugate base of a weak acid, it will cause an increase in the pH (more basic) 3) if a cation is the conjugate acid of a strong base, it won’t affect the pH 4) if a cation is the conjugate acid of a weak base, it will cause a decrease in pH (more acidic) 5) other metal ions (that don’t form strong bases) will cause a decrease in pH 6) when a solution has both the conjugate base of a weak acid and the conjugate acid of a weak base, the ion with the larger Ka or Kb will have the greater influence on pH o pretty much, when you see a salt, ask “what are its parents?” and if its parent is strong, it won’t affect the pH. If it’s weak, it will either make the solution more acidic (if it’s a weak base) or more basic (if it’s a weak acid). For Example: “List the following inorder if increasing pH: 0.1M Ba(C2H3O2), 0.1M NH4Cl, 0.1M NH3Br, 0.1M KNO3” How to Solve 1: split up each salt into its ions 2: ask what acid or base each ion came from, and use that information to surmise whether or not the salt acts basic or acidic Take a Look Factors that Affect Acid Strength o Polarity (difference in electronegativity) o Bond Strength (strong, but not too strong) o Stability of conjugate base Four kinds of Acids 1) Binary: H-X ~HBr, HCl, HI ~ bond strength affects whether or not these acids are strong 2) Oxyacids: non-metal – O - H ~ HClO, HClO2, HClO3, HClO4, etc, ~ the strength of these acids is affected by polarity (adding more O makes the bond stronger because it’s more polar and can donate an H+ better) 2) Carboxilic acids: hydrocarbon – carboxyl group (COOH) ~ HC2H3O2 (acetic acid) ~ these are affected by the strength of their conjugate bases and the number of electronegative ions in the acid 3) Amino Acids: “base acids” (can react with themselves) Common Ion Effect This is related to Le Chatlier’s Principle. If you add a strong electrolyte to a weak electrolyte and they have an ion in common, the extent of ionization of the weak electrolyte is decreased. This is because you increase the concentration of an ion on one side of the equation, which will move the equilibrium to the left, away from the products. NaF in an HF solution. NaF Na+ + F- and HF H+ + FF- is a common ion, so there will already be some in the HF dissociation from the NaF. That will shift the reaction to the left. For example, “Calculate the pH of a solution containing 0.085M nitrous acid (HNO2; Ka = 4.5x14) and 0.10M potassium nitrite (KNO2).” How to Solve 1: identify the major species in the solution and decide whether they’re strong or weak 2: identify the equilibrium of interest that affects [H+] 3: use an I.C.E. table to find the concentrations of the ions involved 4: use K to find [H+] and pH Take a Look Buffers A buffer is a solution that resists a change in pH. It contains either a weak acid and its conjugate base or a weak base and its conjugate acid. Some equations: Dissociation constant Ka = [H3O+][X-] [HX] Henderson-Hasselbalch Equation pH = pKa – log ([HX] / [X-]) or pOH = pKb + log ([X-] / [HX]) generally, pH = pKa + log ([base] / [acid]) Use this to calculate the pH of a buffer. For example: “How many moles of NH4Cl must be added to 2.0L of 0.10M NH3 to form a buffer whose pH is 9.00? (assume no volume change)” How to Solve 1: set up the Hendersen-Hasselbalch equation equaling 9 (that’s the pH you want) 2: you want to know the [acid], so set it up using the values you know and solve for x (or [A]) Take a Look: “A buffer is made by adding 0.300mol HC2H3O2 and 0.300mol NaC2H3O2 to enough water to make 1.0L of solution. The pH of the buffer is 4.74. Calculate the pH of this solution after: a) 0.020 mol NaOH is added b) 0.020mol HBr is added.” How to Solve: a) 1: write out the balanced chemical equation and I.C.E. table it 2: NaOH is the limiter, so use 0.020M as the change value 3: notice how there’s a weak acid and its conjugate base left. Use the HendersenHasselbalch equation to find the pH Careful! NaOH is a strong base, so it will be added to HC2H3O2. Be sure you plug in the [A] and [B] into the appropriate spots in the equation. b) 1: write out the balanced chemical equation and I.C.E. table it 2: HBr is the limiter, so use 0.020M as the change value 3: this is a buffer too, so use the H-H equation to find the pH Careful! HBr is a strong acid, so it will be added to C2H3O2-. Be sure you plug in the [A] and [B] values into the right spots! Take a Look Titrations A + B +ion + -ion + HOH Some trends to remember: o If both are strong: ions don’t affect pH and the pH is 7 o If it’s a weak acid + strong base: slightly basic (due to the weak acid’s conjugate base) and the pH > 7 o If it’s a strong acid + weak base: slightly acid (due to the weak base’s conjugate acid) and the pH < 7 o If it’s a weak acid + weak base: depends on the strength of the weak base vs. the weak acid (write out an I.C.E. table and equilibrium of interest) Equivalence point: moles acid = moles base Half-way to the equivalence point is the area of most buffering Some graphs: Note: Always ask yourself, “What drives the pH?” It could be a buffer, a strong base/acid, or a weak base/acid. Each has a different way of solving for the pH. ~Strong acid/base: I.C.E. table it to find the [H+] or [OH-] and then the pH ~ Buffer: Henderson-Hasselbalch equation ~ weak conjugate base/acid: I.C.E. table it and use Kb/Ka values to find the [H+] or [OH-] For example: “Calculate the pH when the following quantities of 0.10M NaOH solution are added to 50.0mL of 0.100M HCl: a) 49.0mL b) 50.0mL c) 51.0mL.” How to Solve: 1: find the moles of A and B (NaOH and HCl) 2: write the chemical equation of A + B (NaOH + HCl HOH + NaCl) 3: I.C.E. table it for each of the different quantities of NaOH a) There’s fewer moles of NaOH, so it’s the limiter. Use this as the change value. There’s some HCl left, so find the [H+] to find the pH. b) The moles of NaOH and the moles of HCl are equal, so the pH is 7 c) HCl is the limiter, and there is some NaOH left. So, find the [OH-] to find the pOH, then the pH. Take a Look “Calculate the pH when the following quantities of 0.10M NaOH solution are added to 50.0mL of 0.100M HC2H3O2. Ka = 1.8x10-5. a) 45.0mL b) 50.0mL c) 51.0mL” How to Solve: a) 1: write out the chemical equation and make an I.C.E. table 2: find the number of moles of NaOH and acetic acid 3: there are fewer moles of NaOH, so use 0.0045 as the change value 4: a weak acid (acetic acid) and its conjugate base are left … it’s a buffer! 5: use the H-H equation to find the pH b) 1: write out the chemical equation and make an I.C.E. table 2: find the number of moles of each 3: the #moles are equal, so use 0.005 as the change value 4: you’re left with a weak conjugate base (C2H3O2-), so react it with water and I.C.E. table it to find the [OH-] and the pH (you need to find Kb here, which is just 1E-14 / 1.8E-5) c) 1: write the chemical equation and I.C.E. table 2: find the #moles of each 3: there are fewer moles of acetic acid, so it’s the limiter. Use 0.005 as the change value 4: you’re left with a strong base (NaOH) in solution. NaOH and OH- are in a 1:1 ratio, so find the [NaOH] by dividing moles by total L and that equals the [OH-]. You can find the pH using that. Take a Look “Calculate the pH at the equivalence point when: a) 40.0mL of 0.025M benzoic acid is titrated with 0.050M NaOH (Ka = 6.3 E -5)” How to Solve: 1: set up the chemical equation (you don’t need to know the formula for benzoic acid) 2: find the #moles of benzoic acid. This is equivalent to the #moles NaOH, so you can find the volume of NaOH to use later. Also, use this number as the change value. 3: look at what’s remaining in solution. The only thing that drives the pH is the conjugate base of benzoic acid 4: find the [X-] by adding up the volumes of NaOH and benzoic acid (you found V NaOH is step 2) and dividing the #moles by that quantity 5: change the Ka into Kb (because it’s a conjugate base) and solve for x ([OH-]). Use this to find the pH Take a Look: Solubility Equilibria This is pretty much the dissociation of non-soluble salts. You can tell just how much nonsoluble salts dissociate by looking at the dissociation constant as with acids and bases. … but this time it’s different. Simple K is now Ksp! Non-soluble salt + ions + - ions Ksp = [+ ions][- ions] (not divided by reactants because they’re solid) What affects solubility? 3 factors: 1) the presence of common ions 2) the pH of the solution 3) the presence of complexing agents 4) amphoterism (haha, and you thought there were only 3) Common Ions Look on page 41 for more info. about this effect. Pretty much, when you have slightly soluble salt and you add another solute that brings along a common ion when it dissociates, the solubility of the slightly soluble salt will decrease. (Think like Le Chatlier) pH of the Solution The solubility of slightly soluble salts containing basic anions increases as [H+] increases (as pH lowers). The more basic the anion, the more the solubility is influenced by the pH of the solution. Complex ion formation A complex ion is a metal ion and Lewis bases bonded to it. The solubility of metal salts increases in the presence of suitable Lewis bases (NH3-, CN-, or OH-). The stability of a complex ion in solution is determined by its equilibrium constant of formation (Kf). Amphoterism Substances that can act as either an acid or base are amphoteric. These substances dissolve better in strong acid and strong base solutions than in water (yeah, they dissolve well in both acidic and basic solutions). It’s because of the formation of complex anions with a bunch of OH- ions bound to a metal. For example: “What is the solubility of LaF3 in moles per liter if Ksp = 2 E -19?” How to Solve: 1: write out the equation and make an I.C.E. table 2: ignore the reactant (it’s solid) and use “x”s to show the change and end values 3: put the “x” values into the Ksp expression and solve for x to get the [LaF3] Take a Look: “Calculate the molar solubility of CaF2 in a solution that is: a) 0.0100M in Ca(NO3)2 b) 0.010M in NaF (Ksp = 3.9 E -11” How to Solve: a) 1: write equation and I.C.E. table 2: use “x”s and Ksp expression to find [CaF2] b) 1: write equation and I.C.E. table (Notice the common ion, F-, from the NaF. Make sure that’s accounted for as an initial value) 2: same thing as before Take a Look Note: the [CaF2]in part b was less than part a due to Le Chatlier’s Principle. Will a Precipitate Form? These are the last types of questions you need to know about for equilibria. Woot! These ask you, “Will a precipitate form when…” or “What concentration is necessary to form a precipitate when…” For example: “ Will a precipitate form when 0.10L of 8.0 E -3 M Pb(NO3)2 is added to 0.40L of 5.0 E 3 M Na2SO4? (Ksp = 6.3 E -7)” How to Solve: 1: write out the chemical equation to determine whether an insoluble salt forms or not 2: write out the equilibrium of interest 3: find Q (page 33) Be careful with additive volumes! The concentrations of ions don’t stay the same because it’s a new volume! 4: compare Q with Ksp Take a Look: “A solution contains 1.0 x 10-2M Ag+ and 2.0x10-2M Pb+2. When Cl- is added to the solution, both AgCl and PbCl2 precipitate form. What concentration of Cl- is necessary to begin the precipitation? Which salt will precipitate first?” How to Solve: 1: find the Ksp values of the two insoluble salts 2: use the Ksp values to find the [Cl-] necessary 3: the smaller [Cl-] means that it precipitates first Take a Look: Wow, that’s a lot of stuff on equilibrium, so here’s a little summary of how to solve basic pH problems. Another helpful trick is knowing how to estimate logarithms. Logs to Remember log 1 = 0 log 3 = 0.5 log 5 = 0.7 log 7 = 0.85 log 10 = 1 Inverse Logs to Remember 10^-0.1 = 0.8 10^-0.3 = 0.5 10^-0.5 = 0.3 10^-0.7 = 0.2 10^-0.9 = 0.12 When you have to do a logarithm or inverse of a logarithm without a calculator, break up the logarithm, find each log by estimating, then add them together. For example: Find pH: [H+] = 3.2 x 10-5 Do that all of the time and you’ll be set. #8: THERMODYNAMICS Chapters 5 and 19 Are you ready to switch gears completely? Too bad, because we are. Thermochemistry Basics o 1st Law of Thermodynamics: energy is neither gained or lost, it is conserved o Energy: the ability to do work or transfer heat o Joule: the unit used to express energy (calories are used too, but let’s face it, we have enough of those already) J = 1 kg x m2/ s2 o Work: energy used to move something (w = F x d) o Potential energy: energy an object possesses by virtue of its chemical composition o Kinetic energy: energy an object possesses by virtue of its motion (KE = 0.5mv2) o State Function: something that only depends on the present state of something—not the path it took to get to that state o System: the thing we study o Electrostatic energy: energy that holds atoms together (k x Q1Q2 / d) (page 15) o ΔH: enthalpy; heat at constant pressure o Exothermic: a system that releases energy to surroundings o Endothermic: a system that takes in energy from surroundings o Thermochemical Equation: shows the chemical equation of a reaction and gives the ΔH for it in J or kJ (it shows how much energy is given off or taken in by a mol of each of the reactants or products) AB2 A + 2B ΔH = +100kJ kJ per mol. So, when AB2 decomposes, it takes in 100 5 Ways to Find the Enthalpy of a Process 1) Information from the Equation 2) Calorimetry 3) Hess Law 4) Enthalpy of Formation 5) Bond Enthalpies Information from an Equation “How much heat is released when 4.50g of methane gas is burned in a constant pressure system? CH4 + 2O2 CO2 + 2H2O ΔH = -890 kJ” How to Solve: 1: convert to moles 2: solve for kJ This isn’t unlike stoichiometry. Take a Look: Note: it’s not always a 1:1 ratio. Also, they might ask questions about the reverse of the reaction, in which case you just switch the sign of ΔH. “What is the ΔH when 5.0g of H2O2 decomposes? 2H2O2 2H2O + O2 ΔH = -196 kJ” Solve this one on your own… Answer: -14.4 kJ Calorimetry Equations: q = m c ΔT q = -Ccal x ΔT Cp = ΔH / ΔT coffee-cup bomb calorimetry Heat capacity: the temperature change experienced by an object when it absorbs a certain amount of heat Specific Heat: the heat capacity of one gram of a substance (c) (water’s is 4.184 J / gK) For example: “When a student mixes 50mL of 1.0M HCl and 50mL of 1.0M NaOH in a coffee-cup calorimeter, the temperature of the resultant solution increases from 21.0 C to 27.5 C. Calculate the enthalpy change for the reaction, assuming the calorimeter loses only a negligible quantity of heat, that the total volume of the solution is 100mL, that its density is 1 g/mL, and that its specific heat si 4.184 J/gK” How to Solve: Plug and chug, my friend. Plug and chug. (q = m c ΔT) Take a Look: “A 0.5865g sample of lactic acid (HC3H5O3) is burned in a bomb calorimeter whose heat capacity is 4.182 kJ/C. The temperature increased from 23.10 C to 24.95 C. Calculate the heat of combustion of the lactic acid a) per gram b) per mole” How to Solve: 1: plug and chug values into bomb calorimetry equation 2: divide by mass to get kJ/g (a) 3: multiply by the molar mass to get kJ/mol (b) Take a Look: Hess Law If a reaction is carried out in a series of steps or reactions, the enthalpy change for the overall reaction will equal the sum of the enthalpy changes of each individual step. Basically, you have a bunch of different equations, but you only want to know the ΔH for one. That one is the sum of all of the others, but you may need to do some manipulation of those individual ones to end up with the equation you want. If you do manipulate an equation, make sure you change its ΔH the same way. For example: “Calculate the ΔH for the reaction: NO + O NO2 given the following information: NO + O3 NO2 + O2 ΔH = -198.9 kJ O3 3/2 O2 ΔH = -142.3 kJ O2 2O ΔH = 495.0 kJ” How to Solve: 1: flip the O2 2O and multiply it by ½ (do the same with ΔH) 2: flip the O3 3/2 O2 equation (do the same with the ΔH) 3: add and cancel the chemical equations to get the equation they’re asking for 4: if it all cancels out correctly, add up the ΔHs for all of them Take a Look: Note: this is a lot like the reaction mechanisms we did on page 30. Standard Enthalpies of Formation ΔHrxn = Σ ΔHf products – Σ ΔHf reactants ΔHf = enthalpy of formation (usually the problem gives this to you) “Using the given values, determine the enthalpy of reaction for the combustion propane gas to form carbon dioxide gas and liquid water. C3H8 -103.85 kJ CO2 -393.5 kJ H2O -285.8 kJ How to Solve: 1: write out the balanced chemical equation for the reaction 2: plug in the ΔHf values into the ΔHrxn (be sure to include coefficients!) Take a Look: Note: these problems don’t always simply ask for the ΔHrxn, sometimes they ask for the ΔHf values of an individual species. In that case, just use algebra. Also, the ΔHf value for elements in their standard state is 0. Bond Enthalpies ΔHrxn = bonds broken – bonds formed Note: This is reactants – products, which is kind of weird. To solve questions about this, write out the Lewis electron dot configuration of each of the reactants and products (page 16), look up the bond enthalpies of each bond that exists, sum them up, and plug them into the equation. For example: “Use the bond enthalpies given to estimate the enthalpy of the reaction for the combustion of gaseous ethane, producing carbon dioxide gas and water.” C–H 413 kJ C–C 348 kJ O=O 495 kJ C=O 799 kJ O-H 463 How to Solve: 1: write out the balanced chemical equation for the reaction 2: re-write the equation with Lewis electron dot configurations instead of chemical formulas 3: plug in the bond enthalpies of each bond in each element (careful for coefficients!) into the equation Take a Look: Now let’s get a little more complicated… Spontaneity Spontaneity tells you whether or not a reaction will occur naturally. If a reaction is nonspontaneous, it needs some energy put into it to occur. Just because a reaction is spontaneous, doesn’t mean it’s rapidly occurring There are 3 variables that describe this: ΔH: enthalpy (heat at constant pressure) ΔS: entropy (disorganization) ΔG: free energy (tells whether or not a reaction is spontaneous) We already talked a lot about ΔH, so we’re going to move on. Entropy ΔS o 2nd Law of Thermodynamics: nature wants entropy to increase (kind of like your room: it will keep getting unorganized unless you put some energy into cleaning it) o 3rd Law of Thermodynamics: the entropy of a pure crystalline substance at absolute zero is 0. S = k lnW (but you don’t really need to know that) The only thing you really need to take away from this is that W = possible # of microstates. o Microstates: the possible position and energy of the individual gas molecules. More of these means higher entropy. How to get more microstates: ~ increase temperature ~ increase volume ~ increase the number of independently moving particles The system gets MORE DISORDERED!!! Calculating Entropy ΔS = ΔSfinal – ΔSinitial ΔS increases when: ~ gases are formed from solids or liquids ~ liquids are formed from solids ~ more gas molecules are created during a chemical reaction That means that the ΔS value is positive, and the system is more disorganized. A negative ΔS value means the system is getting more organized. For example: “Calculate the ΔS for the synthesis of ammonia from N2 and H2 at 298K: N2 + 3H2 2NH3” NH3 192.5 J/mol K N2 191.5 J/mol K H2 130.6 J/mol K How to Solve: 1: plug in the values into the ΔS equation above (watch out for coefficients!) Kablamo, you’re done. Take a Look: Note: This is like the ΔH questions, they don’t always ask for the whole reaction; sometimes they ask for an individual species. Entropy change in surroundings: ΔSsurr = -qsys / T (-qsys = ΔHrxn) but that’s also not really important ΔSuniverse = ΔSsys + ΔSsurr (when this is positive, the reaction is spontaneous) Free Energy This was proposed by Josiah Gibbs to show how to use ΔH and ΔS to predict the spontaneity of a reaction. It has a really useful equation that you can find on the left side of the crappy periodic table equation sheet. ΔG = ΔH - T ΔS If ΔG is negative, the reaction is spontaneous in the forward direction; if it’s 0, the reaction is at equilibrium; if it’s positive, the reaction is spontaneous in the reverse direction. Note: in any spontaneous process at constant temperature and pressure, the free energy always decreases. Calculate the Free Energy of a Reaction ΔGrxn = ΔGproducts - ΔGreactants The ΔGf for an element at standard conditions is 0. You don’t always have to calculate ΔG, though. Most of the time, they ask you to predict the ΔG based on ΔH or ΔS values that you find. In that case, use the G = H – TS equation to estimate: ~ ΔH - and ΔS +: reaction is spontaneous (ΔG is negative) ~ ΔH + and ΔS -: reaction is non-spontaneous (ΔG is positive) ~ ΔH + and ΔS +: spontaneity depends on T (high T makes it spontaneous) ~ ΔH – and ΔS -: spontaneity depends on T (low T makes it spontaneous) What if it’s not at standard conditions? Then the equilibrium constant comes in. ΔG = ΔG° + R x T x lnQ We can manipulate this when it’s at equilibrium (ΔG = 0): 0 = ΔG° + R x T x ln K ΔG° = -R x T x ln K K = e ^ - ΔG° / RT (R = gas constant; use 8.314 J / mol K) Let’s do some problems now… “The Haber Process: N2 + 3H2 2NH3. a) Predict the direction in which ΔG° for this reaction changes with increasing temperature b) calculate the values of ΔG for the reaction at 25 C and 500 C c) calculate the equilibrium constant K” How to Solve: a) 1: determine whether the ΔS value will be positive or negative 2: because ΔS is negative, the -T ΔS part of the equation will get bigger as T increases, so ΔG also gets bigger b) 1: calculate the value of ΔH for the reaction using standard enthalpies of formation 2: calculate the value of ΔS for the reaction using standard entropies of formation 3: plug these values along with the T into the equation c) 1: establish which equation to use (the third one from the previous page) 2: plug in values to solve for K Take a Look: “Calculate the K for PbCO2 at equilibrium at 773 K.” How to Solve: 1: write out the balanced chemical equation 2: notice how it’s not at standard conditions. This means that you need to calculate the ΔG from the ΔH and ΔS values, so find them using products – reactants 3: use the G = H – TS equation to find ΔG 4: plug in the ΔG value you just found into the equation to find the K Take a Look: # 9: ELECTROCHEMISTRY Chapter 20 Redox Reactions Basics o Oxidation: loss of electrons (bigger oxidation number) o Reduction: gain of electrons (smaller oxidation number) o Redox Reaction: a reaction in which one or more elements gains an electron (is reduced or oxidizing agent) and one or more elements loses an electron (is oxidized or reducing agent) o Assigning Oxidation Numbers: Rules: 1: if an atom is in elemental form, its charge/oxidation number is 0 2: for any monatomic ion, the charge is whatever is on the ion 3: O is almost always -2 (except in H2O2, so look out for that in identifying redox reactions) 4: H is always +1 with a non-metal and -1 with a metal 5: F is always -1 6: the sum of oxidation numbers in a neutral compound is 0 7: the sum of oxidations numbers in an ion equals the charge of the ion Practice assigning Oxidation Numbers: 1) H2S 2) S8 3) SCl2 4) Na2SO3 5) SO4-2 Answers: 1) H = +1, S = -2 2) S = 0 3) S = +2, Cl = -1 4) Na = +1, S = +4, O = -2 5) S = +4, O = -2 o Reduction Potentials: use these to tell what’s reduced and oxidized in a spontaneous redox reaction; the more positive the reduction potential, the better it’s reduced (this is also known as standard electrode potential, electromotive force, emf, E°, and is measured in V) Electrochemical Cells Two half-cells, a salt bridge, and a wire Two kinds: Galvanic (Voltaic) and Electrolysis Galvanic (Voltaic) Some Terms o Produce energy through a spontaneous reaction o Anode: where oxidation occurs (“an ox”) o Cathode: where reduction occurs (“red cat”) o Electrode: strip of metal o ½ Cell: electrode in solution o Salt Bridge: used to neutralize ions in solution o Electrode potential: how much an electrode wants to gain or lose electrons (use reduction potentials) o Coulomb (C): unit of charge (how many electrons) o Faraday’s Constant: 96,500 C / mol (on crappy) o Volt: measure of cell potential ( V = J / C) o Ampere: unit of current (rate of electron movement) (A = C / sec) o Cell Potential = Ecell ox (cathode) – Ecell red (anode) o Line notation: anode anode solution cathode solution cathode (double line represents a porous barrier) Balance Redox Reactions: 1) assign oxidation numbers 2) divide into 2 half-reactions (one for oxidation and one for reduction) 3) balance all species except H and O in each reaction 4) balance all Os by adding HOH as needed 5) balance all Hs by adding H+ as needed 6) balance the charge by adding electrons as needed 7) multiply the half-reactions by integers if necessary so that the number of electrons are equal 8) add the half-reactions (simplify when possible) 9) check for mass and charge 10) if it’s basic, add OH- s to each side Watch: MnO4- + C2O4-2 Mn-2 + CO2 For example: Electrode A is made of lead and the solution is lead (II) nitrate Electrode B is made of manganese and the solution in manganese (II) nitrate 1) Which is the most easily reduced metal? 2) What is the balanced equation showing the spontaneous reaction that occurs? 3) What is the maximum emf the cell can produce if everything exists at standard conditions? 4) What is the direction of the flow of electrons in the wire? 5) What is the direction of the positive ion flow in the salt bridge? 6) Which electrode is increasing in size? 7) Which electrode is decreasing in size? 8) What is happening to the concentration is Electrode A’s solution? 9) Identify the i: anode ii: cathode iii: positive electrode iv: negative electrode 10) If Electrode A were replaced with a S.H.E., what would be the i: emf of the cell ii: direction of electron flow Some Quick Hints for Acing Those Types of Questions: o Write absolutely everything you know on the picture (reduction potentials, equations, positive/negative charges, electron flow, everything) o In the salt bridge: cations move to the cathode and anions move to the anode o Red cat and an ox o Emf = cathode – anode o The salt bridge keeps the solutions neutral (they like to ask questions about it) o S.H.E.s (Standard Hydrogen Electrode) have a reduction potential of 0 V o Pay attention to what ions they’re asking about and the charge of the ion Free Energy and Redox ΔG = -nFE (n = moles electrons, F = Faraday’s constant, E = cell potential, emf) o When E is positive, the reaction is spontaneous (G is negative) o When E = 0, ΔG = 0 (it’s at equilibrium… hold on to your socks) ΔG = -nFE = -R x T x lnK So, Ecell = R x T x lnk = 0.0592 log K nF n When it’s not at standard conditions (when there isn’t a concentration of 1M): Nernst equation: E = E° - 0.0592 / n log Q For example: “Find E°, ΔG, and K for 2 Ag + ½ O2 + 2H+ 2Ag+ + H2O” How to Solve: 1: Find E by looking up reduction potentials and doing cathode – anode 2: find ΔG by using the –nFE equation 3: find K using the ΔG = -RTlnK equation Take a Look: “Calculate the emf at 298K generated by: CrO7-2 + 14H+ + 6I- 2Cr+3 + 3I2 + 7H2O When the concentrations are 2.0M, 1.0M, 1.0M, and 1 E -5 M, respectively.” How to Solve: 1: write out the balanced half-reactions 2: look up reduction potentials and find the E° 3: this isn’t at standard conditions, so write the Q expression ([products] / [reactants]) 4: use the given concentrations to find Q 5: plug in the values into the Nernst equation Take a Look: Note: They usually ask you how changing the concentration of a species affects E or K. You can just use Le Chatlier’s to describe this, but the Nernst equation works too. Also, they might ask to find the concentration of an individual species and give you the emf, so just manipulate the equation. For example: “The voltage of a cell is 0.45V at 298K when [Zn+2] = 1.0M and PH2 = 1.0 atm, what is the [H+]?” How to Solve: 1: write out the Q expression and plug in the values you know (leave [H+] as a variable) 2: find the E° 3: write out the Nernst equation using the values you know, leaving log Q as a variable 4: find what Q (on its own) equals with those values 5: put in the expression you found in step 1 for Q and solve for [H+] Take a Look: Note: You can use this to find pH, too. Electrolysis o Electrolysis: using electricity to create a non-spontaneous charge o E° = negative and ΔG = positive o Use Faraday’s constant a lot with these problems o Use it a lot with molten salts 5 Basic Steps to Solve: 1) balance half-reactions 2) convert to charge 3) use Faraday’s constant to find moles of electrons 4) use ratios to find moles of substance 5) convert to units Sounds a lot like stoichiometry, eh? For example: “Calculate the number of grams of aluminum produced in 1.00 hours by the electrolysis of molten AlCl3 if the electrical current is 10.0A.” How to Solve: 1: Write the balanced half-reactions for Al and Cl 2: convert Amperes and hours into Coulombs 3: use Faraday’s constant to find the moles of electrons 4: use the ratio from the reaction in step 1 to find the moles Al 5: convert to grams Take a Look: # 10: NUCLEAR CHEMISTRY Chapter 21 Just the Basics: o As an atom gets bigger, it needs more neutrons to stabilize it o When an atom decays, it breaks down into other elements o 5 kinds of decay: 1: β (beta) particle emission (an electron emitted from the nucleus) 2: α (alpha) particle emission (Helium-4 particle 42He emitted from nucleus) 3: gamma radiation (represents energy lost when remaining nucleons become more stable; not shown in nuclear equations) 4: positron emission (a particle with the same mass as an electron, but the opposite charge) 5: electron capture (nucleus captures an electron from the electron cloud) o annihilation: when a positron and an electron run into each other (releases a ton of energy) o things usually decay into lead because it’s super stable o N stands for the amount of radioactive particles o Half-life: the amount of time it takes for half of the neutrons in an element to decay (use lots of algebra to figure out those questions or just use the first order half-life equation on page 28) o Geiger counter: used to detect radioactivity o Nuclear fission: large nuclei split o Nuclear fusion: small nuclei fuse together (like in the Sun) o Subcritical: when you have less radiation than you need Nuclear Problems: These don’t show up to much, but if and when they do, you just have to make sure that the sums of the mass numbers and atomic numbers (numbers on the top and bottom) equal each other on both sides. #11: ORGANIC CHEMISTRY Chapter 25 Organic Chemistry is the chemistry of all living or once-living things. All of them have Carbon! Structures o Governed by bonds on C (always 4 bonds) 2 classifications: saturated and unsaturated Alkanes: saturated (4 single bonds); -ane Alkenes: unsaturated (at least 1 double bond); -ene Alkynes: unsaturated (at least 1 triple bond); -yne Aromatic: form a ring (benzene) Alkanes CnH2n+2 Methane (CH4), Ethane (C2H6), Propane (C3H8), etc. How to Make a Structure: 1: write out Cs 2: add H Isomers: same molecular formula, but different arrangement. Naming: 1: count how many Cs there are on the longest chain (to find prefix) 2: number the longest chain 3: add the methyl (CH3), ethyl (C2H5), or propyl (C3H7) groups to the C with the smallest number For example: “2-methylpentane” That means there’s a methyl group off of the 2nd C in the pentane chain. This is an isomer of hexane (6 C) o Cycloalkanes: exist in a ring of single bonds Alkenes CnH2n -cis -trans Some common Alkenes are Ethene (ethylene) Propene (propylene) For isomers of these, you need to indicate where the double bond is by numbering the longest C chain and putting it on the smallest (like naming Alkane isomers) Alkynes CnH2n-2 Common Alkynes: Ethyne (acetylene) propyne Reactions with Alkenes/Alkynes Something like a Halogen will break a double bond and replace it (Halogenation) Aromatic Cyclic (form a ring) Contain benzene Function Groups Functional Group type of compound suffix/prefix example systematic name alkene word - ene ethene alkyne word - yne ethyne alcohol word - ol methanol ether ether dimethyl ether haloalkanes halo- chloromethane amine (act basic) word - amine ethylamine aldehide word - al ethanal ketone word - one propanone carboxilic acid word - oic acid ethanoic acid (acetic acid) ester word -oate methylethanoate amide (good base) word - amide ethanamide THE EXAM 2 Parts Part I: 50% o 75 multiple choice questions in 90 minutes o No calculators o Periodic Table provided o No equation sheet o No penalty for guessing (like the ACT) Part II: 50% o 6 Free Response questions in 95 minutes o 2 “Sub-parts” 1: 55 minutes; 20% #1: equilibrium #2: equilibrium, thermodynamics, kinetics, or electrochemistry #3: equilibrium, thermodynamics, kinetics, or electrochemistry You can use your calculator, periodic table, and equation sheet on this sub-part. 2: 40 minutes; 30% #4: write net-ionic equations and answer short questions for 3 reactions #5: atomic theory, bonding, intermolecular forces, or colligative properties #6: atomic teory, bonding, intermolecular forces, or colligative properties You can use your periodic table and equation sheet on this part, but no calculator. Of #2, #3,#5, or #6, one will have to do with a lab