Catalysts (VCE SSCE Chemistry): Revision Notes

Catalysts

Introduction

Catalysts are substances that dramatically increase the rate of chemical reactions without being consumed in the process. They play crucial roles in many areas of chemistry, from industrial manufacturing to environmental protection. For example, catalysts are essential in petroleum refineries for breaking down large hydrocarbon molecules into smaller, more useful compounds, and in catalytic converters to reduce air pollution from vehicle exhaust.

Understanding how catalysts work requires knowledge of collision theory and the energy changes that occur during chemical reactions. This note explores the mechanism by which catalysts increase reaction rates and examines their practical applications.

How catalysts work

Understanding activation energy

Before we can understand catalysts, we need to revisit the concept of activation energy. The activation energy ($E_a$) is the minimum amount of energy that reactant particles must possess for a chemical reaction to occur. Think of it as an energy barrier that must be overcome for reactants to transform into products.

Some reactions occur readily at room temperature because they have very low activation energies—only a small amount of energy is needed to break the bonds in the reactants. However, many reactions have high activation energies and proceed very slowly under normal conditions. This is where catalysts become important.

The role of catalysts

Catalysts work by providing an alternative reaction pathway that has a significantly lower activation energy than the uncatalysed reaction. This is the key to understanding their effectiveness.

When a catalyst is present:

• The reaction can proceed via a different route with a lower energy barrier
• More colliding particles have sufficient energy to exceed this reduced activation energy
• A greater proportion of collisions are successful (result in product formation)
• The overall reaction rate increases substantially

Because catalysts are not consumed during the reaction, they do not appear as reactants or products in balanced chemical equations. A small amount of catalyst can facilitate the conversion of large amounts of reactants.

The mountain analogy

Types of catalysts

Catalysts can be classified into two main categories based on their physical state relative to the reactants and products:

Homogeneous catalysts

Homogeneous catalysts are in the same physical state (solid, liquid, or gas) as the reactants and products they are catalysing.

Heterogeneous catalysts

Heterogeneous catalysts are in a different physical state to the reactants and products of the reaction they catalyse.

Catalysts in industry

Why heterogeneous catalysts are preferred

The chemical industry extensively uses catalysts to improve efficiency and reduce costs. For industrial processes, heterogeneous catalysts are generally preferred over homogeneous catalysts because they offer several practical advantages:

• Easier separation: Because they are in a different physical state, heterogeneous catalysts can be more easily separated from the reaction products
• Reusability: Once separated, heterogeneous catalysts can be reused multiple times, making them more economical
• High temperature stability: Many heterogeneous catalysts can withstand the high temperatures often required in industrial processes

The importance of surface area

Heterogeneous catalysts work through a process called adsorption. Particles at the surface of certain solids can form bonds with gas molecules that collide with the surface. When reactant molecules adsorb onto the catalyst surface, their bonds become distorted or may even break completely. This makes it much easier for the reaction to proceed.

This is why industrial catalysts are often designed with porous structures or are supported on materials with high surface areas, such as honeycomb ceramics.

Case study: heterogeneous catalysts in industry

Catalytic converters in cars

Since 1986, all new cars sold in Australia must have a catalytic converter fitted between the engine and the exhaust pipe. This device uses heterogeneous catalysis to clean exhaust gases and significantly reduce air pollution.

The catalytic converter's purpose is to convert toxic gases produced during combustion into less harmful substances:

The catalyst typically consists of a mixture of platinum, palladium, and rhodium metals coated onto a honeycomb-shaped ceramic support. The honeycomb structure creates millions of tiny pores, providing an enormous surface area for the catalytic reactions.

As exhaust gases flow through the catalyst chamber, they pass over the metal surfaces where the harmful gases (carbon monoxide, nitrogen oxides, and unburnt hydrocarbons) are converted to less harmful products (carbon dioxide, nitrogen, and water). The catalyst remains unchanged and can function effectively for many years without replacement.

The Haber process

The Haber process is one of the most important industrial chemical reactions, producing ammonia ($\text{NH}_3$) which is used to manufacture fertilisers, nylon, explosives, and some pharmaceuticals. This process also employs heterogeneous catalysis.

The reaction uses powdered iron as a heterogeneous catalyst. Here's how the catalytic process works:

1. Hydrogen ($\text{H}_2$) and nitrogen ($\text{N}_2$) gas molecules adsorb onto the iron surface
2. As they attach to the surface, the strong covalent bonds within the molecules break
3. The individual hydrogen and nitrogen atoms are now much more reactive
4. These atoms readily combine on the catalyst surface to form ammonia molecules
5. The ammonia molecules then leave the iron surface, freeing up sites for more reactant molecules
6. The iron catalyst remains unaltered and continues to facilitate the reaction

Without the iron catalyst, the reaction between nitrogen and hydrogen would be extremely slow because the $\text{N} \equiv \text{N}$ triple bond in nitrogen gas is very strong and requires significant energy to break. The catalyst provides an alternative pathway that makes this bond-breaking process much easier.

Remember!