Chemical Equilibrium Introduction to the Law of Mass Action

Chemical Equilibrium The state of a chemical reaction in which its forward and reverse reactions occur at equal rates so that the concentration of the reactants and products does not change with time.

Law of Mass Action aA + bB + cC +...  pP + qQ + rR +... Equilibrium Constant [P] p [Q] q [R] r... K = [A] a [B] b [C] c...

The Ammonia Synthesis Equilibrium

Meaning of Equilibrium Constant K>>1: reaction is product-favored; equilibrium concentrations of products are greater than equilibrium concentrations of reactants. K<<1: reaction is reactant-favored; equilibrium concentrations of reactants are greater than equilibrium concentrations of products.

Chemical Equilibrium The state where the concentrations of all reactants and products remain constant with time. On the molecular level, there is frantic activity. Equilibrium is not static, but is a highly dynamic situation.

The Law of Mass Action For jA + kB  lC + mD The law of mass action is represented by the equilibrium expression:

Equilibrium Expression 4NH 3 (g) + 7O 2 (g)  4NO 2 (g) + 6H 2 O(g)

Notes on Equilibrium Expressions (EE) 4 The Equilibrium Expression for a reaction is the reciprocal of that for the reaction written in reverse. 4 When the equation for a reaction is multiplied by n, EE new = (EE original ) n 4 The units for K are usually ignored.

K c v. K p For jA + kB  lC + mD K p = K(RT)  n  n = sum of coefficients of gaseous products minus sum of coefficients of gaseous reactants.

Heterogeneous Equilibria... are equilibria that involve more than one phase. CaCO 3 (s)  CaO(s) + CO 2 (g) K = [CO 2 ] The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present.

CaCO 3 (s) CaO(s) + CO 2 (g)

Reaction Quotient... helps to determine the direction of the move toward equilibrium. The law of mass action is applied with initial concentrations.

Reaction Quotient (continued) H 2 (g) + F 2 (g)  2HF(g)

Solving Equilibrium Problems 1.Balance the equation. 2.Write the equilibrium expression. 3.List the initial concentrations. 4.Calculate Q and determine the shift to equilibrium.

Solving Equilibrium Problems (continued ) 5.Define equilibrium concentrations. 6.Substitute equilibrium concentrations into equilibrium expression and solve. 7.Check calculated concentrations by calculating K.