Final answer:
Increasing the concentration of a reactant causes the forward reaction to speed up immediately, as there are more reactant molecules to form products, aligning with Le Châtelier's principle. Over time, the reverse reaction also increases due to the higher product concentration, until equilibrium is re-established.
Step-by-step explanation:
When the concentration of a reactant is increased in a chemical equilibrium, the position of the equilibrium shifts towards the formation of more products. This change affects the rates of the forward and reverse reactions differently. Initially, the forward reaction speeds up because there are more reactant molecules available to react, thus increasing the rate at which the products are formed. This process is in accordance with Le Châtelier's principle, which states that a system in equilibrium will adjust to minimize the change imposed on it.
Over time, as the forward reaction proceeds, more products are formed, and the rate of the reverse reaction will increase as well, eventually reaching a point where the forward and reverse reaction rates are equal once again, and a new equilibrium is established. The reverse reaction does not speed up immediately upon adding the reactant, but rather as a response to the increased product concentration resulting from the faster forward reaction.
Therefore, the correct answer to the question 'What effect does adding a reactant have on the rates of the forward and reverse reactions?' is (e) the forward reaction speeds up.