Practice Problem 2

Assume that the concentrations of H₂, I₂, and HI can be measured for the following reaction at any moment in time.

H₂(g) + I₂(g) 2 HI(g) K_c = 60

For each of the following sets of concentrations, determine whether the reaction is at equilibrium. If it isn't, decide in which direction it must go to reach equilibrium.

(a) (H₂) = (I₂) = (HI) = 0.010 M

(b) (HI) = 0.30 M; (H₂) = 0.01 M; (I₂) = 0.15 M

(c) (H₂) = (HI) = 0.10 M; (I₂) = 0.0010 M

Answer

a) The only way to decide whether the reaction is at equilibrium is to compare the reaction quotient with the equilibrium constant for the reaction.

The reaction quotient in this case is smaller than the equilibrium constant. The only way to get this system to equilibrium is to increase the magnitude of the reaction quotient. This can be done by converting some of the H₂ and I₂ into HI. The reaction therefore has to shift to the right to reach equilibrium.

(b) The reaction quotient for this set of concentrations is equal to the equilibrium constant for the reaction.

The reaction is therefore at equilibrium.

(c) The reaction quotient for this set of concentrations is larger than the equilibrium constant for the reaction.

In order to reach equilibrium, the concentrations of the reactants and products must be adjusted until the reaction quotient is equal to the equilibrium constant. This involves converting some of the HI back into H₂ and I₂. In other words, the reaction has to shift to the left to reach equilibrium.