Question

Which is(are) correct statements about the role of a catalyst in a reaction?

• A. It is reactant in a rate-determining step and then a product of some subsequent step.
• B. It provides an alternate mechanism with a lower energy of activation.
• C. It increases the rate of chemical reaction but does not itself undergo a permanent change during the course of the reaction.
• D. Enzyme catalysts are protein molecules.
• E. All of the above.

Answer 1

The correct option is E All of the above.

A catalyst is a substance which increases the rate of the reaction without itself being consumed at the end of the reaction and the phenomenon is called catalysis. There are some catalysts which decrease the rate of reaction and such catalysts are called a negative catalyst. Obviously, the catalyst accelerating the rate will be a positive catalyst. However, the term positive is seldom used and catalyst itself implies a positive catalyst. Catalysts are generally foreign substances but sometimes one of the products formed may act as a catalyst and such a catalyst is called "auto catalyst" and the phenomenon is called autocatalysis.

General Characteristic of Catalyst:

A catalyst does not initiate the reaction, it simply fastens it.
Only a small amount of catalyst can catalyse the reaction.
A catalyst does not alter the position of equilibrium i.e. magnitude of equilibrium constant and hence, G. It simply lowers the time needed to attain equilibrium. This means if a reversible reaction in absence of catalyst completes to go to the extent of $75\%$ till attainment of equilibrium and this state of equilibrium is attained in $20$ minutes then in presence of a catalyst also the reaction will go to $75\%$ of completion before the attainment of equilibrium but the time needed for this will be less than $20$ minutes.
A catalyst drives the reaction through a different route for which energy barrier is of shortest height and hence, $E_a$ is of lower magnitude. The function of the catalyst is to lower down the activation energy.