Chemistry, Third edition

262 14 · SPEED OF CHEMICAL REACTIONS

Catalysis

How catalysts work

The idea of an overall reaction as consisting of several stages helps to explain how cat-

alysts work. Catalysts cause the reactants to make products through a differentmechan-

ism in which the sloweststep in the new mechanism is faster (i.e. possesses a lower

activation energy) than the slowest step in the mechanism which applies to the

uncatalysed reaction. This has the effect of speeding up the rate of the overall reaction.

Homogeneous catalysis

Catalysts which are in the same phase as the reactants (e.g. they are all in solution)

are termed homogeneous catalysts. Examples of homogeneous catalysis include the

hydrolysis of sugar (page 254), and the catalysis of ozone destruction by Cl•(see page

394). Another example is the reaction of persulfate (S 2 O 82 (aq)) and iodide (I(aq))

ions which obeys the overall reaction

2I(aq) S 2 O 82 (aq)I 2 (aq) 2SO 42 (aq) (14.6)

The direct reaction of Iwith S 2 O 82 possesses a high activation energy because the

reacting ions are both negatively charged and so they repel each other. This makes the

direct reaction slow, but the reaction is speeded up if Fe3+ions are added. Although

the reaction mechanism is complicated, the catalytic effect of Fe^3 ions probably

involves the following (simplified) stages. First, the Fe3+reacts with the iodide,

2I(aq) 2Fe3+(aq)I 2 (aq) 2Fe2+(aq) (14.7)

Fe2+then reacts with S 2 O 82 ,

S 2 O 82 (aq) 2Fe2+(aq)2SO 42 (aq) 2Fe3+(aq) (14.8)

regenerating the Fe3+.

The catalytic steps are faster than direct reaction of Iwith S 2 O 82 because steps

(14.7) and (14.8) involve two oppositely charged ions and so both reactions are

expected to be very rapid. (Note that addition of equations (14.7) and (14.8) pro-

duces (14.6), the equation of the overall reaction.)

Heterogeneous catalysis

Catalysts which are in a different phase as the reactants (usually a solid catalyst

speeding up reactants in a solution or gaseous mixture) are termed heterogeneous

catalysts. Industrially important heterogeneous catalysts include solid nickel,

platinium and rhodium. These catalysts work by allowing reactant molecules to stick

(adsorb) to the surface of the metal catalyst, so causing the bonds between atoms in

the reactant molecules to weaken, making molecular rearrangement easier.

As an example of heterogeneous catalysis, consider the reaction of ethene with

hydrogen using a heated nickel catalyst:

400 oC

CH 2 =CH 2 (g)H 2 (g) —C 2 H 6 (g)

ethene Ni ethane

1.The starting materials, nickel, hydrogen and ethene, are mixed (Fig. 14.12(a)).

2.The ethene and hydrogen molecules adsorb(‘stick’) on the surface of the surface

of the nickel (Fig. 14.12(b)).

14.8