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In today's T-Level Chemistry lesson, we are going to study A eleven point three, Catalysts and Reaction Rate.

By the end of this lesson, you should be able to define a catalyst accurately, use evidence to decide whether a substance is acting as a catalyst, explain how a catalyst provides an alternative pathway with lower activation energy, and interpret that on a Maxwell–Boltzmann distribution.

Retrieve collision theory. Activation energy is the minimum collision energy required for a successful reaction.

Particles must collide with sufficient energy and the correct spatial orientation.

At a fixed temperature, particles have a distribution of kinetic energies, shown by a Maxwell–Boltzmann distribution.

How could a reaction be made faster without increasing temperature? One method is to use a catalyst.

Specification point A eleven point three point one. A catalyst is a substance that increases the rate of a chemical reaction without itself being permanently chemically changed.

The catalyst may participate in the reaction mechanism, but it is regenerated overall, so it is not used up permanently.

A correct definition must include both increased rate and no permanent chemical change.

A catalyst does not mean more reactant is present. It does not, by definition, increase the temperature. And it is not simply a substance that makes a reaction happen. The key effect is a faster reaction through a different reaction pathway.

A catalyst provides an alternative reaction pathway. That pathway has a lower activation energy than the uncatalysed pathway.

Watch the energy profile. The uncatalysed route has a higher energy barrier. The catalysed route has a lower barrier.

At the same temperature, the particles have the same overall energy distribution as before. Because the activation-energy threshold is lower, more particles have sufficient energy to react.

Link that to collision theory. Without a catalyst, only collisions with energy at least the higher E a can satisfy the energy requirement.

With a catalyst, the lower E a means a greater proportion of collisions have sufficient energy. Correct spatial orientation is still required. More successful collisions per unit time means an increased rate of reaction.

Use evidence. A reaction without substance X takes one hundred and twenty seconds to reach the stated endpoint. With substance X it takes forty five seconds.

Substance X is recovered after the reaction with no permanent chemical change. That evidence is consistent with X acting as a catalyst.

Rate evidence alone is not enough. You also need evidence that the substance is not permanently chemically changed.

Specification point A eleven point three point two. A catalyst does not need a new Maxwell–Boltzmann curve at the same temperature.

The particle-energy distribution is unchanged because temperature is unchanged. What changes is the activation-energy threshold. The catalysed pathway has a lower E a.

On one distribution, the catalysed E a is positioned to the left of the uncatalysed E a.

The area to the right of the catalysed E a is larger, so a greater proportion of particles have sufficient energy to react.

Do not confuse a catalyst with temperature. Increasing temperature changes the Maxwell–Boltzmann distribution.

Adding a catalyst at the same temperature does not change that distribution. A catalyst lowers the activation energy by providing an alternative pathway.

Both can increase the proportion of particles able to react, but they do so in different ways.

Guided application. A reaction has E a of eighty five kilojoules per mole without a catalyst, and fifty two kilojoules per mole with a catalyst. Temperature is unchanged.

The fifty two kilojoules per mole line lies further left. The shape of the distribution does not change. The catalysed pathway has the larger proportion of particles with energy at least E a, so successful collisions occur more frequently and rate increases.

Correct the misconceptions.

A catalyst does not give particles more energy. At the same temperature the energy distribution is unchanged.

A catalyst does not lower the energy of every particle. It lowers the activation-energy requirement.

A catalyst is not used up. It is not permanently chemically changed overall.

And a catalyst does not change rate by changing temperature. Its defining role is an alternative lower E a pathway.

Examination-style question.

Part a. Define a catalyst. Two marks.

A substance that increases the rate of a chemical reaction, without itself being permanently chemically changed.

Part b. Explain how a catalyst increases rate in terms of activation energy. Two marks.

It provides an alternative reaction pathway with lower activation energy.

Part c. A Maxwell–Boltzmann distribution is drawn at one temperature. Explain how the graph should show catalysed and uncatalysed pathways. Three marks.

Use the same distribution at the same temperature. Show a lower E a for the catalysed pathway. The area to the right of the catalysed E a is larger.

Part d. Explain why the catalysed pathway produces a greater proportion of successful collisions. Two marks.

A greater proportion of particles have energy at least the lower E a, so more collisions satisfy the energy requirement and can be successful.

Before we finish, complete the model.

A catalyst increases rate without permanent chemical change. It provides an alternative pathway with lower activation energy. At constant temperature the Maxwell–Boltzmann distribution stays the same. The lower E a increases the area of particles able to react, so successful collisions occur more frequently.

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