chem-201-chapter-6 - 10/6/11 Equilibrium Constants Chemical...

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10/6/11 1 Chemical Equilibrium Equilibrium Constants For a generic chemical reaction, the equilibrium constant is defined as: aA + bB cC + dD (1) The equilibrium constant , K eq , for a chemical reaction indicates whether the reactants or the products will be favored in an equilibrium process b a d c eq [B] [A] [D] [C] K = 1 Equilibrium Constants For the reverse reaction cC + dD aA + bB (2) the equilibrium constant is the inverse of the forward reaction: d c b a eq1 2 eq [D] [C] [B] [A] K 1 K = = Equilibrium Constants When reactions are added to produce a net reaction, the net equilibrium constant is the product of the K eq s for each reaction: HA H + + A - (1) H + + C HC + (2) HA + C HC + + A - (net) ] C ][ HA [ ] A ][ [HC K K K ] C ][ H [ ] [HC K [HA] ] [A ] [H K 2 1 net 2 - 1 + + + + = = = = Thermodynamics Enthalpy, Δ H, is a measure of the change in heat content between reactants and products CH 4 + 2O 2 CO 2 + 2H 2 O(g) Δ H o rxn = -802.34 kJ Methane releases heat to the surrounding when combusted NaCl(s) Na + (aq) + Cl - (aq) Δ H o rxn = 3.87 kJ Sodium chloride takes heat from the surroundings when dissolved Thermodynamics Entropy, Δ S, is a measure of the change of disorder when going from reactants to products CH 4 + 2O 2 CO 2 + 2H 2 O(g) Δ S o rxn = -5.14 J/K The products are more ordered than the reactants NaCl(s) Na + (aq) + Cl - (aq) Δ S o rxn = 43.4 J/K The products are more disordered than the reactants—more space between ions in solution
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10/6/11 2 Thermodynamics Gibb s Free Energy, Δ G, is a measure of the energy available to do work following reaction Definition: Δ G = Δ H - T Δ S CH 4 + 2O 2 CO 2 + 2H 2 O(g) Δ G o rxn = -800.78 kJ NaCl(s) Na + (aq) + Cl - (aq) Δ G o rxn = -9.00 kJ Thermodynamics Δ G is also a measure of where the equilibrium for a reaction will lie: If Δ G rxn < 0, the reaction is product favored If Δ G rxn > 0, the reaction is reactant favored If Δ G rxn = 0, the reaction is in equilibrium The equilibrium constant is related to the Gibb s Free Energy: K eq = exp{- Δ G o rxn /RT} R = gas constant = 8.314 J/mol K T = temperature (in K) Solubility Products The solubility product, K sp , for a salt is a specific type of equilibrium constant Given an excess of salt, K sp for the salt will determine how much of the salt will dissolve in water: M x A y (s) x M y+ (aq) + y A x- (aq) y x x y sp ] A [ ] [M K + = Solubility Products
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chem-201-chapter-6 - 10/6/11 Equilibrium Constants Chemical...

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