Picture (1)represents the equilibrium mixture for the reaction A + B ⇌ 2 AB at 298 K.
-If this reaction is exothermic,which picture (2)-(4)represents the equilibrium mixture at 400 K?

Question

Picture (1)represents the equilibrium mixture for the reaction A + B ⇌ 2 AB at 298 K. 
-If this reaction is exothermic,which picture (2)-(4)represents the equilibrium mixture at 400 K?

Quizplus · Accepted Answer

Since the reaction is exothermic, increasing the temperature would shift the equilibrium to the left (towards the reactants) in order to absorb the excess heat. This means that the concentration of A and B would increase, while the concentration of AB would decrease.

Looking at the three options, only picture (4) shows an increase in the concentration of A and B, and a decrease in the concentration of AB. However, picture (4) also shows that the reaction has gone to completion (meaning all the A and B has reacted to form AB), which is unlikely to happen at equilibrium.

Picture (2) shows an increase in the concentration of AB, which is the opposite of what we expect at a higher temperature. Picture (3) shows an increase in the concentration of A and AB, and a decrease in the concentration of B which is also opposite of what we expect at a higher temperature.

Therefore, the best choice is (C) picture (4), even though it shows the unlikely scenario of complete reaction.

Picture (1)represents the Equilibrium Mixture for the Reaction a