Question

Consider the reaction A ⇌ B which is exothermic in the forward direction. Which of the following increase(s) when the temperature increases. You may select more than one, or none, of the options. (a) Ea (reverse) (b) the forward rate (c) Ea (forward) (d) the reverse rate

Answer 1

Given that the reaction is exothermic in the forward direction.

Hence, heat is essentially a product of the forward reaction, we can write

Now, when we increase the temperature, it is essentially adding more heat to the equilibrium. So we are adding more products. Hence, the reaction will shift towards the left (towards reactants) according to Le Chatelier principle. Another way to explain it would be to think in terms of the reverse endothermic reaction . Here heat is a reactant, hence increase in temperature will drive the reaction to form more A.

Hence, the reverse rate (rate of backward reaction) will increase as more reactant A are formed.

Hence, the correct choice for the property that increases with increase in temperature is

(d) the reverse rate

Note: activation energies do not change with temperature change since the reactants and products are same and they go through the same transition state irrespective of the temperature by following the same mechanism. The only change that happens because of temperature increase is that now many more of the molecules can cross the activation barrier.

Hence, option (a) and (c) are incorrect.

option (b) the forward rate is incorrect since the reaction moves to the left.