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Reversible Reactions and Equilibrium: Question 10

Syllabus 6.3

Multiple choice Extended 1 mark

The following reaction reaches dynamic equilibrium in a sealed container at constant temperature and constant volume:

2L(g)+M(g)⇌2N(g)2\text{L}(g) + \text{M}(g) \rightleftharpoons 2\text{N}(g)

The forward reaction is exothermic.

A small additional amount of gas L\text{L} is then injected into the container, while the temperature and the volume of the container are kept constant.

Which statement correctly describes the effect on the position of equilibrium?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the rule for a concentration change

By Le Chatelier’s principle, if the concentration of a substance in an equilibrium mixture is increased, the position of equilibrium shifts in the direction that uses up some of that extra substance, opposing the change.

Step 2: Apply this to the injected gas L

L\text{L} is a reactant. Injecting more L(g)\text{L}(g) increases its concentration, so the equilibrium shifts in the direction that consumes L\text{L}. That is, towards the right, forming more N(g)\text{N}(g):

2L(g)+M(g)⇌2N(g)2\text{L}(g) + \text{M}(g) \rightleftharpoons 2\text{N}(g)

Step 3: Check each option

  • A: Incorrect direction. Increasing a reactant’s concentration shifts equilibrium towards the products, not away from them.
  • B: Correct. The equilibrium shifts right, so more N(g)\text{N}(g) is formed and its concentration increases.
  • C: Incorrect (concentration changes, not just temperature changes, can shift the position of equilibrium. (Note: this rule only applies to gases and dissolved species) for a solid or a pure liquid, changing the “amount present” does not change its concentration.)
  • D: The direction (right) is correct, but the reasoning about L\text{L} is wrong: as the equilibrium shifts right to oppose the increase, some of the injected L\text{L} is used up in forming N\text{N}. Its concentration does not keep rising. It settles at a new value that is higher than the original equilibrium concentration, but lower than it was immediately after injection.

Step 4: Select the correct statement

Only option B correctly describes both the direction of the shift and its effect on the concentration of N\text{N}.

Final answer

  • Injecting more L(g)\text{L}(g) shifts the equilibrium to the right, increasing the concentration of N\text{N}, option B\boxed{\text{option B}}.