Formulas, Equations, & Mathematics CHAPTERS 4 & 5 Overview & Memory Refreshers Universal language of symbols for identifying all of the elements on earth; using 1, 2, or 3 letters 1st letter is always capitalized, all others are lower case Some letters are for the English names, some are for Latin or Greek (K – kalium, Na – natrium) Subscripts – written if more than one atom is present Diatomic molecules – oxygen, hydrogen, nitrogen, Group 17 Chemical Formulas Compounds are combinations of elements chemically joined/bonded in definite proportions Formulas use chemical symbols, subscripts, and numbers to show qualitative and quantitative information about a substance Qualitative information relates to things no counted/measured, like what elements the compound is comprised of Quantitative information deals with things that can be counted/measured, like how many atoms of each element are present Chemical Formulas (cont) Symbols supply qualitative info – CO: carbon & oxygen (what’s the difference between CO & Co?) Subscripts tell you how many of each atom are there – quantitative Two types of formulas – empirical & molecular Empirical – all types of compounds Molecular – compounds formed from sharing atoms (what type of bond?) Chemical Formulas (cont) Empirical Formula – represents the simplest integer ratio in which atoms combine to form the compound (most reduced) Ionic substances don’t form molecules, but form an array of ions Ionic formulas indicate the ratio of atoms in a compound (MgCl2) Ionic formulas are empirical formulas Chemical Formulas (cont) Covalently bonded substances form discrete units called molecules Sometimes the empirical formula represents the simplest & actual ratio (H2O) Molecular formula may often be a multiple of the empirical formula C6H12O6 is the molecular formula of glucose, but the empirical formula is CH2O Atoms, Compounds, & Ions Atoms naturally have a neutral charge Ions are positively or negatively charged Positively charged ions attract negatively charged ions in a ratio that will produce a neutral compound The charge of an ion is represented by a superscript following a symbol 1+ or 1-, the 1 is omitted (only charge is shown): Na 1+ = Na+, Cl 1- = Cl- Anything else shows size and sign of charge Al3+, O2- Atoms, Compounds, & Ions (cont) Polyatomic ion – atoms covalently bonded together that has a charge (Table E in your reference table) Parentheses are used to enclose polyatomic ions when there is more than one of the ion in a compound, with a subscript indicating how many Compounds can form several ways, one such way is by oppositely charged ions attracting each other in a ratio that produces a neutral charge (NH4)3PO4 Atoms, Compounds, & Ions (cont) Subscripts indicate quantities of atoms present, superscripts indicate charge, coefficients tell us how many units of the formula are present 2H2O – 2 molecules of water; 4 hydrogen & 2 oxygen atoms 4Mg(NO3)2 – 4 Mg, 8 N, 24 O Atoms, Compounds, & Ions (cont) When water from some ionic solutions evaporates, the solute (left over substance) forms a crystal lattice that binds water within the structure This is called a hydrate These crystals have a definite number of water molecules for each unit of the compound Barium chloride (BaCl2) traps two water molecules; BaCl2∙2H2O In a chemical reaction, the water in a hydrate does not react, but it does add mass to the compound Writing Formulas & Naming Compounds All compounds must be electrically neutral (what does that mean?) Compounds achieve neutrality by having an equal number of positive and negative charges; NaCl When ions have unequal charges, simply write the charge of one ion as the subscript for the other (but not the sign, just the number) Ex: MgCl2 polyatomic ions form compounds the same way; NaNO3, Ca(NO3)2 Writing Formulas & Naming Compounds (cont) Compounds are named according to the types of elements that form them Ionic compounds are named one way, covalent compounds containing only nonmetals are named another Binary ionic compounds – containing only 2 elements Ionic compound – positively charged particle (usually a metal) comes first Negatively charged ion ends the compound Name of negative ion changed to end in -ide NaCl: sodium chloride Writing Formulas & Naming Compounds (cont) Name the following ionic compounds: KCl, MgS, AlN Naming compounds with polyatomic ions: if positive portion is a metal, use the unchanged metal name plus the negative polyatomic ion name KNO3 – potassium nitrate Most polyatomic ions are negatively charged, but NH4+ is not If a compound contains the ammonium ion (+), and the negative is a nonmetal, the ending is –ide, but if it is combined with another polyatomic ion, they both keep their names Name these: NH4Cl; NH4NO3 Writing Formulas & Naming Compounds (cont) If a binary (2 atom) compound contains 2 nonmetals, it is molecular (covalent bonds, no ions) Lower electronegativity value written first Name of compound will end in –ide prefixes are used to tell how many atoms of each element are present CO, CO2 If only one atom of the first atom is present, mono is not used If element starts with a vowel, final a or o in prefix is not used NO, N2O4 Writing Formulas & Naming Compounds (cont) Prefixes Used Number of Atoms Prefix 1 Mono- 2 Di- 3 Tri- 4 Tetra- 5 Penta- 6 Hexa- 7 Hepta- 8 Octa- Writing Formulas & Naming Compounds (cont) Some metals have more than one common oxidation state Fe: +2, +3 Can be a problem when naming a compound because you don’t know which iron is being used Solution is to use roman numerals after the name of the metal for the oxidation state Iron(II) chloride; FeCl2 Iron(III) chloride; FeCl3 Chemical Rxns & Equations A physical change does not form a new substance, but merely the appearance of the material (melting, boiling, etc) A chemical change is when the substance produced is different from the starting material (think of something burning) A chemical reaction is a well-defined chemical change A chemical equation shows what takes place during a chemical rxn A substance that enters into an equation is called a reactant, and is on the left side of the rxn arrow A product is a substance that is produced by the rxn, and appears on the right of the arrow Chemical Rxns & Equations (cont) Plus signs separate reactants and products, while the arrow represents a chemical rxn (read as “produces” or “yields”) Chemical Rxns & Equations (cont) Chemical & physical changes involve the loss or gain of energy, usually in the form of heat There are two major groups of rxns, depending on whether the rxn gains or loses energy; what are they? Chemical Rxns & Equations (cont) Type of Rxn Surrounding Temp Potential Energy of Reactants Potential Energy of Products Energy Change Endothermic Decrease Less More Positive Exothermic Increases More Less negative Chemical Rxns & Equations (cont) Endothermic rxns require energy to occur; i.e. energy is entered into the system ice melting is endothermic Energy is absorbed from the surroundings, lowering the surrounding temperature Reactants absorb energy as they become products, so the products have more potential energy than the reactants Chemical Rxns & Equations (cont) Exothermic rxns release thermal energy when they occur; i.e. energy is removed into the system Water freezing is exothermic Energy is released and given off to the surroundings, raising the surrounding temperature Products have less potential energy than the reactants Chemical Rxns & Equations (cont) – Balancing Chemical Equations A correctly written chemical equation has the same number of each type of atom on both sides of the equation The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction An equation that conserves atoms and agrees with the law of conservation of mass is called a balanced equation An equation that does not conserve atoms between reactants and products is called an unbalanced equation, and must be balanced Can you change the subscripts of an atom/formula in order to balance an equation? Chemical Rxns & Equations (cont) – Balancing Chemical Equations The only way to balance a chemical equation with the correct formulas is to change the coefficients in the equation What is a coefficient? Think of the equation as a seesaw: both sides must be balanced in order to have an equilibrium (law of conservation of mass) How would you balance the following equation? H2(g) + O2(g) → H2O(g) Chemical Rxns & Equations (cont) – Balancing Chemical Equations H2(g) + O2(g) → H2O(g) H2(g) + O2(g) → 2H2O(g) 2H2(g) + O2(g) → 2H2O(g) Chemical Rxns & Equations (cont) – Balancing Chemical Equations Symbol Meaning + Separates reactants or products → Separates reactants from products; Reads as yields or produces (s) Identifies a substance as a solid (ℓ) Identifies a substance as a liquid (g) Identifies a substance as a gas (aq) Identifies a substance as being dissolved in aqueous (water) solution Chemical Rxns & Equations (cont) – Types of Rxns It would be difficult to categorize all types of chemical rxns, but there are 4 major types: Synthesis Decomposition (analysis) Single replacement Double replacement Chemical Rxns & Equations (cont) – Types of Rxns: Synthesis Synthesis: 2 or more reactants combine to form a single product General equation form is A + B → AB Chemical Rxns & Equations (cont) – Types of Rxns: Decomposition Decomposition (analysis): is the opposite of a synthesis rxn; a reactant is broken down (decomposed) into two or more simpler substances (products) all decomposition rxns begin with just one reactant AB → A + B Chemical Rxns & Equations (cont) – Types of Rxns: Single Replacement Single replacement: one element replaces another in a compound always involves an element and a compound A + BX → B + AX How do we know if a reaction will take place? Activity Series (Table J, pg 5) [RT quiz] The activity series is arranged so that an element on the table will react with anything below it All metals above H2 will react with acids to produce hydrogen gas and produce a salt Chemical Rxns & Equations (cont) – Types of Rxns: Single Replacement The second column of Table J contains a few nonmetals that will replace a less active nonmetal: A + XB → B + XA Chemical Rxns & Equations (cont) – Types of Rxns: Double Replacement Double replacement: usually involves 2 soluble ionic compounds that react in solution to produce a precipitate, gas, or molecular compound (H2O) AB + CD → AD + CB Just like single replacement rxns, not all combinations of reactants will produce a rxn Chemical Rxns & Equations (cont) – Types of Rxns: Double Replacement 3 situations that might cause a double replacement rxn: 1. A rxn will occur if one of the products is a solid (precipitate) Solubility Guidelines (Table F, pg 2) Check the solubility of the two ionic products If one product is insoluble, the rxn will occur (precipitate is formed) 2. a rxn will occur if one of the products is a gas 3. a rxn will occur if a molecular (covalent bond) substance, such as water, is formed Chemical Rxns & Equations (cont) What is the law of conservation of mass? If you are given an unknown in a balanced equation, you should be able to use process of elimination and simple math to determine what element (and how many of it) is missing The mass of a missing substance can also be found; the total mass of reactants must be the total mass of the products The Mathematics of Formulas Remember that atomic mass is a relative value based on carbon-12 (this is the standard) How much larger is Mg than C? Formula mass is the sum of the atomic masses of all atoms present Gram formula mass is the formula mass expressed in grams instead of amu Some substances form molecules, and the gram formula masses are expressed as gram molecular masses The Mathematics of Formulas (cont) Formulas represent the composition of a substance, so using the subscripts and coefficients of the elements in a substance, you can find the percent mass of the substance The percentage composition of a substance is the composition (by %) of each element compared with the total mass of the compound What is the % of O in potassium chlorate? Flash back to hydrates; what are they? The Mathematics of Formulas (cont) Ionic substance with trapped water molecules within their crystal structure are called hydrates, substances without water are called anhydrous If you need to calculate the percentage of water in a hydrate, count the water molecule as a single unit (all units of H2O in the hydrate) What is the % by mass of water in Na2CO3 ∙ 10H2O ? The Mole Collective nouns are used to describe quantities that we are familiar with; a dozen = 12, a gross = 144, and a ream is 500 In chemistry, there is a collective noun used to describe a usable number of particles A mole is the number of atoms of carbon present in 12.000 grams of C-12 (everything comes back to carbon-12 being the standard) The number of particles in a mole of a substance is 6.022 x 1023, which is referred to as Avogadro’s number The Mole (cont) It’s not actually possible to count the number of particles in a mole, the mass of one mole can be found by determining the gram formula mass 6.02 x 1023 particles of that substance Gram formula mass of any substance is the mass of one mole of the substance The abbreviation for mole is mol The Mole (cont) To convert grams to moles and moles to grams: moles = # of grams grams = # of moles x X 1 mol gram formula mass gram formula mass 1 mol The Mole (cont) How many grams are present in 40.5 moles of H2SO4 ? How many moles are present in 4.75 grams of NaOH ? The Mole (cont) When the molecular mass & its empirical formula are known, it’s possible to determine the correct molecular formula Propene: molecular mass is 42 amu, empirical formula is is CH2, and the molecular mass of CH2 is 14 amu Divide the molecular mass of the compound by the molecular mass of the empirical formula, and you’ll get an integer (42/14 = 3) Multiply the empirical formula by the integer found to get the molecular formula C3H6 Mole Relations in Balanced Equations Coefficients in a balanced equation represent the number of mole ratios The mole ratios from a balanced equation will remain constant no matter what How many liters of carbon dioxide gas will be produced from the complete combustion of 30.0 liters of ethane as shown by the following equation? 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g)
