Kc Equilibrium Constant… We have discussed the shifting of equilibrium position in a qualitative way, but the degree of shift can be quantified Kc

[reactant] : [product] Equilibrium Constant… The value of Kc tells us how the equilibrium position is shifted: Kc = 1 [reactant] : [product] 1 : 1

[reactant] < [product] Equilibrium Constant… The value of Kc tells us how the equilibrium position is shifted: Kc > 1 [reactant] < [product] Equilibrium is to the right

[reactant] > [product] Equilibrium Constant… The value of Kc tells us how the equilibrium position is shifted: Kc < 1 [reactant] > [product] Equilibrium is to the left

Equilibrium Constant… aA + bB cC + dD Kc = [C]c[D]d [A]a[B]b

Equilibrium Constant… 2SO2 + O2 2SO3 Kc = [SO3]2 [SO2]2[O2] = 0.72 0.4 moldm-3 0.2 moldm-3 0.7 moldm-3 Kc = [SO3]2 [SO2]2[O2] = 0.72 0.42*0.2 = 15.3 What does this tell us about the eqm position?

Equilibrium Constant… 2SO2 + O2 2SO3 Kc = 15.3 0.4 moldm-3 0.2 moldm-3 0.7 moldm-3 Kc = 15.3 Kc is greater than one so the equilibrium is shifted toward the right.

Equilibrium Constant… 2SO2 + O2 2SO3 0.4 moldm-3 0.2 moldm-3 0.7 moldm-3 Now we need to look at the units for the equilibrium constant. Kc = (moldm-3)2 (moldm-3)2 (moldm-3) = 1 (moldm-3) = mol-1dm3

Equilibrium Constant… 2SO2 + O2 2SO3 Therefore Kc = 15.3 mol-1dm3

Equilibrium Constant… What ways can we change the value of Kc?

Equilibrium Constant… The only factor affecting Kc is: Temperature Concentration and pressure do not affect Kc. As catalysts do not change the equilibrium position they will not affect Kc either.