Equilibrium: A State of Dynamic Balance Chapter 18.1

Equilibrium Most chemical reactions do not go to completion. (Most reactants do not turn completely to products.) Most chemical reactions do not go to completion. (Most reactants do not turn completely to products.) reversible reaction = one that can occur in both the forward and the reverse directions. reversible reaction = one that can occur in both the forward and the reverse directions. N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) ↔ means that when products build to a certain level, products start making reactants. ↔ means that when products build to a certain level, products start making reactants.

Equilibrium chemical equilibrium = a state in which the forward and reverse reactions balance each other because they take place at equal rates. chemical equilibrium = a state in which the forward and reverse reactions balance each other because they take place at equal rates. Law of chemical equilibrium = states that at a given temperature, a chemical system may reach a state in which a particular ratio of reactant and product concentrations has a constant value. Law of chemical equilibrium = states that at a given temperature, a chemical system may reach a state in which a particular ratio of reactant and product concentrations has a constant value.

Equilibrium aA + bB ↔ cC + dD aA + bB ↔ cC + dD Equilibrium constant expression: K eq = [C] c [D] d K eq = [C] c [D] d [A] a [B] b [A] a [B] b

Equilibrium Equilibrium constant, k eq = numerical value of the ratio of product concentrations to reactant concentrations, with each concentration raised to the power corresponding to its coefficient in the balanced equation. Equilibrium constant, k eq = numerical value of the ratio of product concentrations to reactant concentrations, with each concentration raised to the power corresponding to its coefficient in the balanced equation. K eq = [C] c [D] d [A] a [B] b [A] a [B] b

Example p 565:1 aA + bB ↔ cC + dD aA + bB ↔ cC + dD K eq = [C] c [D] d [A] a [B] b [A] a [B] b N 2 O 4 (g)↔ 2NO 2 (g) K eq = [ K eq = [NO 2 ] 2 [N 2 O 4 ] [N 2 O 4 ]

Homogeneous equilibrium = all the reactants and products are in the same physical state Homogeneous equilibrium = all the reactants and products are in the same physical state Heterogeneous equilibrium = reactants and products are in more than one physical state. Heterogeneous equilibrium = reactants and products are in more than one physical state.

Do not include SOLIDS and LIQUIDS in heterogeneous equilibrium expression because their concentration is constant. Do not include SOLIDS and LIQUIDS in heterogeneous equilibrium expression because their concentration is constant. 2 NaHCO 3 (s) ↔ Na 2 CO 3 (s) + CO 2 (g) + H 2 O(g) K eq = [CO 2 ] [H 2 O] Practice problems 2 p 567

Determining the Value of Eq. Constants Write the equilibrium expression from the balance equation. Write the equilibrium expression from the balance equation. Substitute given values. Substitute given values. Solve. Solve.

Problem 18-3 p 568 N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) Concentrations: nitrogen =.533 M; hydrogen =1.600 M; ammonia=.933 M Concentrations: nitrogen =.533 M; hydrogen =1.600 M; ammonia=.933 M K eq = [NH 3 ] 2 K eq = [NH 3 ] 2 [N 2 ] [H 2 ] 3 [N 2 ] [H 2 ] 3 K eq = [.933] 2 K eq = [.933] 2 [.533] [1.600] 3 [.533] [1.600] 3 K eq =.399

Equilibrium K eq >1: more products than reactants at equilibrium K eq >1: more products than reactants at equilibrium K eq <1: more reactants than products at equilibrium K eq <1: more reactants than products at equilibrium