Equilibrium Constant and Reaction Quotient Example Problem

This example problem demonstrates how to use the reaction quotient to predict the direction of a chemical reaction towards equilibrium.

Problem:

Hydrogen and Iodine gas react together to form hydrogen iodide gas. The equation for this reaction is

H₂(g) + I₂(g) ↔ 2HI(g)

The equilibrium constant for this reaction is 7.1 x 10² at 25 °C. If the current concentration of gases are

[H₂]₀ = 0.81 M
[I₂]₀ = 0.44 M
[HI]₀ = 0.58 M

what direction will the reaction shift to reach equilibrium?

Solution

To predict the direction of equilibrium of a reaction, the reaction quotient is used. The reaction quotient, Q, is calculated in the same way as the equilibrium constant, K. Q uses the current or initial concentrations instead of the equilibrium concentrations used to calculate K.

Once found, the reaction quotient is compared to the equilibrium constant.

If Q < K, then there are more reactants present than equilibrium and the reaction will need to produce more products to reach equilibrium and will shift to the right.

If Q > K, then there are more products present than equilibrium and the reaction will need to produce more reactants shifting the reaction to the left.

If Q = K, then the reaction is already at equilibrium and there will be no shift.

Step 1 - Find Q

Q = [HI]₀²/[H₂]₀·[I₂]₀
Q = (0.58 M)²/(0.81 M)(0.44 M)
Q = 0.34/.35
Q = 0.94

Step 2 - Compare Q to K

K = 7.1 x 10²
Q = 0.94

Q < K, therefore the reaction will shift to the right.

Answer:

The reaction will shift to the right to produce more hydrogen iodide gas to reach equilibrium.