Chemical Equilibrium l The Nature of Chemical Equilibrium l Shifting Equilibrium l Equilibria of Acids, Bases, and Salts l Solubility Equilibrium

Chemical Equilibrium l The Nature of Chemical Equilibrium – Reversible Reactions: a reaction in which the products can re-form the reactants – Equilibrium: a Dynamic State – The Equilibrium Expression: nA + mB ↔ xC + yD

Chemical Equilibrium l The Nature of Chemical Equilibrium – Reversible Reactions: Chemical Equilibrium: when the forward reaction rate is equal to the reverse reaction rate –At equilibrium the concentrations of reactants and products remain constant

Chemical Equilibrium l Equilibrium is a Dynamic State – Favorable Reactions: the equilibrium concentrations of products are greater than reactants – Unfavorable Reactions: the equilibrium concentrations of products are less than reactants

Chemical Equilibrium l The Equilibrium Expression: aA + bB ↔ cC + dD – Equilibrium Constant: is the product of the product concentrations divided by the product of the reactant concentration, where each concentration is raised to the power of its coefficient – Chemical Equilibrium Expression: K = [C] c [D] d / [A] a [B] b

Chemical Equilibrium l The Equilibrium Example: – The H 2, I 2, HI Equilibrium System The Equilibrium Expression: H 2 (g) + I 2 (g) ↔ 2 HI (g) Chemical Equilibrium Expression: K = [HI] 2 / [H 2 ] [I 2 ]

Chemical Equilibrium l Shifting Equilibrium – Predicting the Direction of Shift – Reactions that Go to Completion – Common-Ion Effect: if adding an ion for two solutes causes precipitation or reduced ionization

Shifting Equilibrium l Predicting the Direction of Shift – Le Chatelier’s Principle: equilibrium shift is in the direction that relieves the applied stress – Changes in Pressure – Changes in Concentration – Changes in Temperature

Le Chatelier’s Principle l Changes in Pressure – Increased pressure favors fewer moles of gas – Pressure change on confined gasses has no effect on the equilibrium constant

Le Chatelier’s Principle l Changes in Concentration – A pure substance in a condensed phase solids (s), liquids (l) can be eliminated from the expression – Concentration change has no effect on the equilibrium constant

Le Chatelier’s Principle l Changes in Temperature – Increasing Energy (Heat) favors the endothermic reaction – Decreasing Energy (Heat) favors the exothermic reaction

Shifting Equilibrium l Reactions that Go to Completion – Formation of a Gas – Formation of a Precipitate – Formation of a Slightly Ionized Product l Common-Ion Effect: if adding an ion for two solutes causes precipitation or reduced ionization

Chemical Equilibrium l Equilibria of Acids, Bases, and Salts – Ionization Constant of a Weak Acid: aA + bH 2 O ↔ cC + dH 3 O + – Buffers: a substance that can resist changes in pH – Ionization Constant of Water: – Hydrolysis of Salts

Chemical Equilibrium l Equilibria of Acids, Bases, and Salts – Ionization Constant of a Weak Acid: aA + bH 2 O ↔ cC + dH 3 O + Acid-Ionization Constant (K a ) is the product of the product concentrations divided by the product of the reactant concentration, where each concentration is raised to the power of its coefficient – Acid-Ionization Expression: K a = [C] c [H 3 O + ] d / [A] a [H 2 O] b

Chemical Equilibrium l Equilibria of Acids, Bases, and Salts – Buffers: a substance that can resist changes in pH – Ionization Constant of Water: K w = [H 3 O + ][OH - ] / [H 2 O][H 2 O] K w = [1.0 x M][1.0 x M] / [1][1] K w = 1.0 x M 2

Chemical Equilibrium l Equilibria of Acids, Bases, and Salts – Hydrolysis of Salts Hydrolysis: a reaction between water molecules and ions of dissolved salt Anion Hydrolysis: –HA (aq) + H 2 O (aq) ↔ H 3 O + (aq) + A - (aq) –K a = [H 3 O + ][A - ] / [HA] Cation Hydrolysis: –B (aq) + H 2 O (aq) ↔ BH + (aq) + OH - (aq) –K b = [BH + ] [OH - ] / [B] Hydrolysis in Acid-Base Reactions

Chemical Equilibrium l Solubility Equilibrium – Solubility Product – Calculating Solubility – Precipitation Calculations – Limitations on the use of K sp

Solubility Equilibrium l Solubility Product – Quantitative Classifications Soluble: Solubility greater than 1g per 100g water. Slightly Soluble: Solubility between 1g to 0.1g per 100g water. Insoluble: Solubility less than 0.1g per 100g water.

Solubility Equilibrium l Solubility Product – Solubility Product Constant (K sp ): the product of the molar concentrations of ions in a saturated solution, each raised to the power that is the coefficient in the chemical equation – Example: AB 2 (s) ↔ A +2 (aq) + 2B -1 (aq) K sp = [A +2 ][B -1 ] 2

Chemical Equilibrium l Solubility Equilibrium – Calculating Solubility – Precipitation Calculations – Limitations on the use of K sp