Advanced Chem Lecture #8 Sections 4.1-4.5 General Properties of Aqueous Solutions Molarity Precipitation Reactions Acid-Base Reactions Oxidation-Reduction Reactions General Properties of Aqueous Solutions A solution is a homogenous mixture of two or more substances Solute is substance present in smaller amounts Solvent is substance present in larger amounts Solutions can be liquid (seawater), solid (alloy), or gaseous (air) Here, we will only discuss liquid solutions where the solvent is water, and the solute is initially a liquid or solid All solutes that dissolve in water fit into one of two categories: Electrolyte: when dissolved in H2O, can conduct electricity Non-electrolyte: does not conduct electricity when dissolved in H2O Pure H2O is in fact a very poor conductor of electricity However, add a small amount of NaCl, and current will flow in water NaCl fully disassociates into Na+ and ClStrong electrolyte (full disassociation into cation and anion) Also H2SO4, HCl, NaOH, KI In other words, all H+ separates from all ClAcetic acid and ammonia do not fully disassociate in water Weak electrolytes (incomplete disassociation) HF, H2O That is, not all H+ separates from OHWhat term do we use for the degree of separation? Non-electrolytes do not disassociate at all in water Sugars, alcohols Battery connected to electrodes in (draw on board): Distilled water Salt water (NaOH, HCl) Sugar water Results: Ions in salt solution move to electrodes to conduct electricity Solutions without ions do not conduct In reality, pure water will not conduct electricity Still, probably not a good idea to swim in lightning storms Why is this? When ionic compounds disassociate, hydration occurs Process by which ions are surrounded by water molecules +/- regions of H2O are attracted to opposite charge in solution Prevents reformation of ionic compound from cation and anion Ionization of acetic acid represented as: CH3COOH CH3COO- + H+ COOH is the group with the ionizing proton Where CH3COO- is the acetate ion Double arrows show reversible reaction Some CH3COO- will react with H+ to form CH3COOH Eventually a state is reached where ionization and Reionization occur at the same rate: chemical equilibrium By contrast, HCl has no tendency to re-ionize Precipitation Reactions Reaction that results in an insoluble product, or precipitate Pb(NO3)2 + 2KI PbI2 + 2KNO3 How can we predict whether a precipitate will form or not? Depends on the solubility of the solute Maximum amount of solute that will dissolve in a given quantity of solute at a given temperature Molecular Equations, Ionic Equations, Net Ionic Equations Molecular equations: formulas of compounds are written as though all species existed as molecules or whole units Pb(NO3)2 + 2KI PbI2 + 2KNO3 Useful to recreate the reaction in the laboratory However, does not accurate describe how ions are behaving Ionic equations: shows dissolved species as free ions Pb2+ + 2NO3- + 2K+ + 2I- PbI2 + 2K+ + 2NO3Precipitate product and spectator ions K and NO3 simply stay disassociated in solution Net ionic equation: shows only reacting species Pb2+ + 2I- PbI2 More accurate describes reacting species However, incomplete for the laboratory chemist to carry out rxn Acid-Base Reactions Arrhenius definition of an acid: ionizes in water to produce H+ ions free protons – charged particles HCl H+ + ClArrhenius definition of a Base: ionizes in water to form OH-, or hydroxide ions NaOH Na+ + OHHowever, it is important to remember that these charged particles don’t really exist alone as shown… something we’ll address later Also, this definition only holds for reactions in water! Bronsted-Lowry definition of an acid: any substance that donates a proton HCl + H2O H3O+ + ClStrong evidence that H+ never exists alone Nature strongly dislikes free charges Basis for many biological processes - respiration H+ binds with H2O to give H3O, hydronium ion Bronsted-Lowry definition of a base: any substance that accepts a proton NaOH + H2O Na+ + H2O + OHNH3 + H2O NH4+ + OHWater donates a proton to ammonia to form ammonium Lewis acid: electron pair acceptor BF3 + NH3 H3N:BF3 Lewis base: electron pair donor NH3 + H2O NH4 + OHHere, hydrogen ion is acid, and donates e- to ammonia Note that ammonia is a base under all three definitions Conjugate Acids and Bases Compounds formed after reaction of acid and base Conjugate base formed when acid surrenders H+ H3O H20 + H+ Conjugate acid formed when base gains H+ H2O + H+ H3O More practically: HCl(a) + H2O(b) H3O(ca) + Cl-(cb) H3O(a) + Cl-(b) HCl(ca) + H2O (cb) A little something on acid-base reactions Acids and bases tend to react to neutralize one another Thus, can add more of one to get acidic of basic solution Acids create more H+ ions lower pH ph 4 = 0.01M, pH 2 = 0.0001M Bases remove H+ ions (by forming OH ions) and raise pH Acid + Base = Water + Salt HCl + NaOH H2O + NaCl Pure water self-ionizes at 10^-7M, forms trace amounts of H and OH Thus, neutral is said to be pH 7