Equilibrium Chemistry— Introduction

Chemical Equilibrium State for a chemical reaction where the reaction stops BEFORE all of the limiting reactant has been used up. Point in a chemical reaction where NO change is seen between [reactants] and [products] Rate of forward reaction = Rate of reverse reaction Happens in a closed system Example: 2NO 2 (g)   N 2 O 4 (g)

Equilibrium Constant Expression ONLY at equilibrium Equation representing the relationship between [products] and [reactants] at equilibrium Ratio of [products] and [reactants] Equilibrium constant (K C )— Numerical ratio for the equilibrium expression Indicates equilibrium when the ratio of the product and reactant concentrations = K C

Equilibrium Constant Expression (cont.) Concentration must be in molarity (M = mol/L) K C = [products] [reactants]

Gas Equilibrium Constant (K p ) Equilibrium constant expression based on partial pressures of gases Partial Pressure Pressure exerted by a particular gas Total system pressure = Σ partial pressures of gases in system More detail on partial pressures later with gases.

Writing Equilibrium Constant Expressions…. Solids and liquids are NOT placed in expression. Removing or adding small amounts of water/liquid does not greatly alter water concentration. Solid concentrations do not change. Gases and aqueous substances ARE included in expression.

Example 1: Write the Equilibrium constant Expression for the following….. 2NO 2 (g)  N 2 O 4 (g)

Example 2: Write the Equilibrium Constant Expression for the following…. 1)2SO 3 (g)  2SO 2 (g) + O 2 (g) 2) N 2 (g) + H 2 (g)  2NH 3 (g)

Example 3: Write the equilibrium expressions for the following….. 1)NiO (s) + CO (g)  Ni (s) + CO 2(g) 2)4HCl (aq) + O 2(g)  2Cl 2(g) + 2H 2 O (l)

Relationship between K C and K P K p = K C (RT) Δn R = L  atm/mol  K T = temperature (Kelvin) Δn = moles of gas products – moles of gas reactants

Example 4: Calculate K C and K p for the following reaction at 475°C H 2(g) + Br 2(g)  2HBr (g) Given: [H 2 ] = 0.140M, [Br 2 ] = 0.019M, and [HBr] = 0.395M

K C and Temperature Influence Equilibrium constant (K C ) value remains the same at a specific temperature Equilibrium constants change as temperature changes. Think of expression….. K C = [products] [reactants]  K C with  temperature  [products] so reaction shifts towards PRODUCTS  K C with  temperature  [reactants] so reaciton shifts towards REACTANTS

What does K eq really mean…. K eq > 10, favors products, reach equilibrium towards right. K eq < 0.1, favors reactants, reach equilibrium towards left K eq ~ 1, equilibrium has been reached with equal reactant and product concentrations

Reaction Quotient (Q C ) Indicates which direction (towards product, towards reactant) the reaction must shift to reach equilibrium. Same method as K C but deals with ANY moment in time, not just equilibrium Q C < K C too much reactant so forward reaction to get to equilibrium Q C = K C equilibrium established for chemical reaction Q C > K C too much product, so reverse reaction to get to equilibrium

Reactant Quotient (Q C ) cont. THINK…… K C = [products] [reactants] [reactants] HUGE forward reaction goes faster [products] HUGE reverse reaction goes faster

Example 3: H 2 (g) + I 2 (g)  2HI (g) Write both the equilibrium constant expression and the reaction quotient expression.

Homework Equilibrium Problems #1-6