The Equilibrium Constant:

1. 2 HI(g) H2(g) + I2(g) = [H2][I2] [HI]2 • The Equilibrium Constant: • For any equilibrium reaction, at a certain temperature, the equilibrium ratio of product concentration to reactant concentration will always be the same (constant) eg: Keq “Equilibrium expression” Why is [HI] squared?

2. = [H2][I2] [HI][HI] = [H2][I2] [HI]2 HI(g) + HI(g) H2(g) + I2(g) Keq

3. a A + b B c C + d D = [C]c [D]d [A]a [B]b General equation for any equilibrium: Keq Where [ ] represents the equilibrium concentration of each species.

4. eg: Calculate the value of Keq for N2(g) + 3 H2(g) 2 NH3(g) = [NH3]2 [H2]3 [N2] = (84.3 M)2 (1.75 M)3 (2.12) if the equilibrium concentrations are: [N2] = 2.12 M, [H2] = 1.75 M, [NH3] = 84.3 M Keq = 625 M-2 625

5. Keq gives important information about an equilibrium reaction: If Keq is close to 1 then neither reaction is favoured and [reactants] = [products] ~ Large Keq means forward reaction is favoured. [products] > [reactants] ie: Small Keq ( < 1, but not negative)means reverse reaction is favoured. [reactants] > [products] ie:

6. 2 C(s) + 3 H2(g) C2H6(g) = [C2H6] [H2]3 Note: Only species which can vary in concentration are included in the equilibrium expression: Keq Don’t include pure liquids or solids(only aqueous and gaseous substances)

7. A + B AB + Energy = [AB] [A][B] Effect of Temperature on Keq: • For any equilibrium reaction, the only factor that effects the value of Keq is temperature. eg: Keq • Increasing temperature causes equilibrium to shift backwards which causes Keq to decrease. • Decreasing temperature causes equilibrium to shift forwards which causes Keq to increase. Why doesn’t changing concentration cause Keq to change?

8. Assignment Homework: • P.556 “Interpret & Apply” #1-5 (skip 1e,f) • Worksheet #25-28