Reaction A: 4HCl (g) + O2 (g) ⇄ 2Cl2 (g) + 2H2O (g)

Reaction B: N2O4 (g) ⇄ 2NO2 (g)

Reaction C: H2 (g) + I2 (g) ⇄ 2 HI

Reaction D: 2NH3 (g) ⇄ N2 (g) + 3H2 (g)

The reactions represented above are carried out in sealed, rigid containers and allow to reach equilibrium. If the volume of each container is reduced from 1.0 L to 0.5 L at constant temp, for which of the reactions will the amount of product (s) be increased?

A) Reaction A

B) Reaction B

C) Reaction C

D) Reaction D

Reaction A

Explanation:

Le Chatelier's Principle (or the equilibrium law) states that if any change (such as temperature or pressure) is made to a system in equilibrium, the system will adjust to oppose that change. Thus, if volume is reduced (at a constant temperature) causing pressure to increase, the reaction will shift to reduce pressure and return the system to equilibrium. As pressure is caused by the movement of molecules in a container and their impact against the container, the system will shift to reduce the number of molecules. The equilibrium will shift towards the reaction where there are more moles of reactants as compared to moles of products. Reaction A is the only reaction where the number of moles of reactants is greater than the number of moles of the products (5:4). For Reaction B and D, the reverse reactions will increase, reducing the products and increasing the reactants, while for Reaction C an increase in pressure will have no impact on equilibrium as there is an equal number of moles on both sides.