KEY2CHEM

Catalysis

A catalyst is a substance that speeds up a chemical reaction without itself getting used it (it may change forms, but it is regenerated by the end of the reaction). A catalyst functions by lowering the activation energy of a chemical reaction by providing an alternate pathway (reaction mechanism) by which the reaction can occur.

Example 1.

Which is a catalyst in the following two-step reaction mechanism?

$$\require{mhchem}\ce{H2O2 + I- -> H2O + OI-}$$

$$\require{mhchem}\ce{OI- + H2O2 -> H2O + O2 + I-}$$

A. $$\require{mhchem}\ce{H2O2}$$

B. $$\require{mhchem}\ce{I-}$$

C. $$\require{mhchem}\ce{OI-}$$

Solution

B. $$\require{mhchem}\ce{I-}$$

A catalyst is added to the reaction (as a reactants), and may change forms before it ultimately is regenerated at the end (as a product). It differs from an intermediate in that an intermediate is formed then used up.

Example 2.

In comparing the reaction energy diagrams of catalyzed vs uncatalyzed reactions, which statement is true?

A. A catalyst speeds up a reaction by making the reaction more exothermic.

B. A catalyst speeds up a reaction by lowering the activation energy of the forward reaction only.

C. A catalyst speeds up a reaction by changing the reaction mechanism of the reaction.

Solution

C. A catalyst speeds up a reaction by changing the reaction mechanism of the reaction.

A catalyst functions by offering an alternate pathway for the reaction to follow, thereby lowering the activation energy (of both the forward and reverse reactions).

Example 3.

A catalyst can function by stabilizing the transition state, thus lowering the ________ and increasing the rate of the reaction.

A. activation energy

B. energy of reactants

C. energy of products

Solution

A. activation energy

A catalyst lowers the activation energy (either by stabilization of the transition state or via an alternate reaction mechanism), leading to an increase in reaction rate. The energy of the reactants and products are unaffected by a catalyst.