AS Chemistry 9701: Homogeneous vs Heterogeneous Catalysts

(a) State what is meant by a homogeneous catalyst and by a heterogeneous catalyst, in terms of the physical state (phase) of the catalyst relative to the reactants. [2]

(b) A gas-phase reaction between two colourless gases, $\text{P(g)}$ and $\text{Q(g)}$ , occurs extremely slowly at $400\text{ K}$ . When a small amount of a transition metal is added as a solid, gas particles adsorb onto its surface and the rate increases dramatically; the mass of the metal is unchanged at the end of the reaction. State whether this catalyst is acting homogeneously or heterogeneously, and explain, in terms of activation energy and reaction pathway, why the rate increases. [4]

(c) The same solid catalyst is used in two further experiments, each using the same total mass of metal: in one, the metal is a single, large block; in the other, the same mass is used as a very fine powder. State and explain which of these two experiments gives the greater initial rate of reaction. [2]

Part (a): Homogeneous vs heterogeneous catalysts

The key distinction is the physical state (phase) of the catalyst compared with the reactants:

A homogeneous catalyst is in the same phase as the reactants, for example, a catalyst dissolved in the same aqueous solution, or present within the same gas mixture. Reaction occurs throughout the bulk of that single phase.
A heterogeneous catalyst is in a different phase from the reactants, most commonly a solid catalyst used with gaseous or liquid/aqueous reactants. Reaction occurs at the surface (interface) between the catalyst and the reactants, rather than throughout the bulk.

Part (b): Identifying and explaining the catalyst’s action

The reactants, $\text{P(g)}$ and $\text{Q(g)}$ , are gases, while the catalyst is a solid transition metal, a different phase from the reactants. This catalyst is therefore acting heterogeneously.

Gas molecules of P and Q adsorb onto active sites on the metal’s surface. This adsorption:

weakens the bonds within the adsorbed molecules, and
holds the reactant particles close together, in a favourable orientation for reaction.

Together, these effects provide an alternative reaction pathway with a lower activation energy, $E_A$ , than the uncatalysed gas-phase reaction. At the same temperature ( $400\text{ K}$ ), a much greater fraction of collisions on this lower-energy surface pathway now have enough energy to react, so the rate increases substantially. Once formed, the products desorb from the surface, which regenerates the free active sites. This is why the mass of the metal catalyst is unchanged at the end of the reaction.

Part (c): Effect of surface area (powder vs. block)

Both experiments use the same total mass (and so the same number of atoms) of catalyst, but this mass is distributed very differently:

A single large block has a relatively small surface area compared with its mass/volume. Most of the metal atoms are “locked” inside the block and cannot come into contact with gas molecules.
The same mass as a fine powder consists of many tiny particles, which together have a much greater total surface area exposed to the gas.

A greater exposed surface area means many more active sites are available for P and Q molecules to adsorb onto per unit time, so more successful adsorption/reaction events occur per second. The powdered catalyst therefore gives the greater initial rate of reaction, even though the total mass (and hence the total catalytic material available, in the long run) is the same in both experiments.

Final answers

(a) Homogeneous catalyst: same phase as reactants. Heterogeneous catalyst: different phase from reactants (reaction at the surface).
(b) Heterogeneous (solid catalyst, gaseous reactants), adsorption onto active sites provides a lower- $E_A$ pathway, increasing the fraction of successful collisions; desorption of products regenerates the catalyst.
(c) The powder gives the greater initial rate, since it exposes a much greater surface area (more active sites) for the same mass of metal.