Chemistry Course Help with Catalyst

4.8 Catalyst

A catalyst is a substance, which increases the rate of a 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 negative catalyst. Obviously, the catalyst accelerating the rate will be positive catalyst. However, the term positive is seldom used and catalyst itself implies positive catalyst.

4.8.1 General characteristics of catalyst

1. A catalyst does not initiate the reaction. It simply fastens it.
2. Only a small amount of catalyst can catalyse the reaction.
3. A catalyst does not alter the position of equilibrium i.e. magnitude of equilibrium constant DG°. 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.
4. A catalyst drives the reaction through a different route for which energy barrier is of shortest height and hence Ea is of lower magnitude. That is, the function of the catalyst is to lower down the activation.

Ea = Energy of activation in absence of catalyst.

E¢a = Energy of activation in presence of catalyst.

Ea – E¢a = lowering of activation energy by catalyst.

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