Collision theory, activation energy, the effect of concentration, temperature, surface area and catalysts on rate, and the Maxwell-Boltzmann distribution.

What this dot point is asking

WJEC wants you to use collision theory and activation energy to explain how concentration, temperature, surface area and catalysts affect reaction rate, and to interpret the Maxwell-Boltzmann distribution.

The answer

Collision theory and activation energy

The factors that change rate

The Maxwell-Boltzmann distribution

The distribution plots the number of molecules against energy. The area beyond $E_a$ represents molecules able to react. Raising the temperature shifts the curve to higher energies and flattens the peak, so a much larger fraction exceeds $E_a$, which is why a small temperature rise produces a large rate increase. A catalyst lowers $E_a$, moving the threshold leftward so more molecules can react at the same temperature.

How a catalyst changes the picture

A catalyst provides an alternative reaction pathway with a lower activation energy. On the Maxwell-Boltzmann distribution, lowering $E_a$ moves the threshold to the left, so a greater proportion of molecules already have enough energy to react at the same temperature, and the rate rises. Crucially, a catalyst is not used up and does not change the products, the enthalpy change or, for a reversible reaction, the position of equilibrium; it only helps the reaction reach that state faster. Heterogeneous catalysts (a different phase, like solid iron in the Haber process) work by adsorbing reactants on their surface.

Measuring a rate in the lab

Rate is found by measuring how a property linked to concentration changes with time. Common methods include collecting and measuring the volume of gas produced, recording the mass loss as a gas escapes, or timing how long a precipitate takes to obscure a mark (the classic sodium thiosulfate and acid experiment). Plotting concentration against time gives a curve whose gradient is the rate, steepest at the start when concentrations are highest and levelling off as reactants are used up.

Examples in context

Catalytic converters. Platinum and rhodium catalysts in car exhausts lower the activation energy for converting toxic $\text{CO}$ and $\text{NO}_x$ into $\text{CO}_2$ and $\text{N}_2$, a vital rate application. Enzymes in the body. Biological catalysts lower activation energies so metabolic reactions proceed fast at body temperature, the same Maxwell-Boltzmann logic applied to living systems.

Try this

Q1. State the two conditions a collision must meet to be successful. [1 mark]

• Cue. Energy at least equal to the activation energy and correct orientation.

Q2. Explain how a catalyst increases the rate of reaction. [2 marks]

• Cue. It provides an alternative pathway with a lower activation energy, so a greater proportion of collisions are successful.

Q3. State the effect of increasing the surface area of a solid reactant on rate. [1 mark]

• Cue. It increases the rate by exposing more particles to collision.

Q4. State the effect of a catalyst on the activation energy and on the enthalpy change of a reaction. [1 mark]

• Cue. It lowers the activation energy but leaves the enthalpy change unchanged.

Q5. Name one method of following the rate of a reaction that produces a gas. [1 mark]

• Cue. Measure the volume of gas collected over time (or the mass lost over time).