Consider the following reaction that has reached equilibrium:

NH3 (aq) + C2H4O2 (aq) ! NH4+ (aq) + C2H3O2- (aq) ∆H = -102.1 kJ



a. What will happen to the concentration of NH4+ (aq) if the temperature is raised?



b. What will happen to the concentration of NH4+ (aq) if the concentration of NH3 is lowered?

Two questions on Le Chatelier's Principle. You can read about the Principle in texts but boiled down to every day language here it what it says. Pay attention.

For a system at equilibrium, it will try to undo what we do to it.
Therefore, let me re-write the equation as
NH3 (aq) + C2H4O2 (aq) ! NH4+ (aq) + C2H3O2- (aq) ∆H = -102.1 kJ to the following.

NH3 (aq) + C2H4O2 (aq) ! NH4+ (aq) + C2H3O2- (aq) + heat
Now, if we add heat, the reaction will shift so as to undo it; therefore, the reaction will move to use up the heat. That means it moves to the left (there are always only three answers to these problems. 1. shift to the right 2. shift to the left 3. no change.)

2. If we remove NH3 from the reaction, it will shift to produce more NH3 and the only way it can do that is to shift to the left. Shifting to the left means NH4^+ is used so (NH4^+) will be lowered.

To answer these questions, we need to understand the concept of Le Chatelier's principle, which states that when a system at equilibrium is subjected to a change, the system will shift in a way that partially opposes the change until a new equilibrium is established.

In the given reaction:

NH3 (aq) + C2H4O2 (aq) ⇌ NH4+ (aq) + C2H3O2- (aq)

a. What will happen to the concentration of NH4+ (aq) if the temperature is raised?

If the temperature is raised, the reaction is in an exothermic direction as indicated by the negative enthalpy change (∆H = -102.1 kJ). According to Le Chatelier's principle, an increase in temperature will cause the reaction to shift in the endothermic direction to absorb the excess heat. Hence, the concentration of NH4+ (aq) will decrease.

b. What will happen to the concentration of NH4+ (aq) if the concentration of NH3 is lowered?

If the concentration of NH3 is lowered, the reaction is no longer in equilibrium. According to Le Chatelier's principle, the reaction will shift to re-establish equilibrium. In this case, since NH3 is a reactant, lowering its concentration will cause the reaction to shift towards the reactant side. As a result, more NH4+ (aq) will be formed to compensate for the decrease in NH3 concentration, leading to an increase in the concentration of NH4+ (aq).