Predicting the Direction of Reaction

How could we predict the direction in which a reaction at non-equilibrium conditions will shift to re-establish equilibrium?

The reaction quotient, Qc, is an expression that has the same form as the equilibrium- constant expression but whose concentrations are not necessarily at equilibrium. Substitute the current concentrations into the Qc expression and compare it to Kc.

Reaction Quotient Qc

For the general reaction:

aA + bB ↔ cC + dD

The Qc expression would be:

Q_C = [C]_i [D]_i  /  [A]_i [B]_i

Where [C]_i is the concentration of C at a particular instant in time (not necessarily at equilibrium)

Predicting the Direction of Reaction

• If Qc > Kc, the reaction will shift left; toward reactants
• If Qc < Kc, the reaction will shift right; toward products
• If Qc = Kc, then the reaction is at equilibrium as follows: Reactants ↔ Products

Consider the following equilibrium:

N₂ (g) + 3H₂ (g) — 2NH₃ (g)

A 50.0 dm³ vessel contains 1.00 mol of N₂, 3.00 mol of H₂ and 0.500 mol of NH₃. In which direction (toward reactants or toward products) will the system shift to establish equilibrium at 400° C? Kc for the reaction at 400°C is equal to 0.500.

First, calculate concentrations from moles of substances as follows:

[N₂] = 1.00 / 50.0 = 0.020 M,

[H₂] = 3.00 / 50.0 = 0.060 M, and

[NH₃] = 5.00 / 50.0 = 0.010 M.