What controls how fast a reaction goes, and how do we explain the effect of each factor?
Collision theory, the effect of concentration, pressure, surface area, temperature and catalysts on rate, the Maxwell-Boltzmann distribution, activation energy and how catalysts work.
An Eduqas A-Level Chemistry C2.3 answer on collision theory, the factors affecting reaction rate, the Maxwell-Boltzmann distribution, activation energy and the action of catalysts.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
Eduqas topic C2.3 covers collision theory, the qualitative effect of concentration, pressure, surface area, temperature and catalysts on reaction rate, the Maxwell-Boltzmann distribution of molecular energies, the meaning of activation energy, and how catalysts work. The quantitative rate equations come later in the Physical and Inorganic section; here the treatment explains rate using collisions and energy distributions.
Collision theory
Factors that change the rate
Concentration and pressure. Higher concentration (in solution) or higher pressure (for gases) packs the particles closer, so they collide more often and the rate rises. Surface area. Breaking a solid into smaller pieces exposes more particles to collision, so a powder reacts faster than a lump. Temperature and catalysts are best explained with the energy distribution, below.
The Maxwell-Boltzmann distribution
The Maxwell-Boltzmann distribution plots the number of molecules against their kinetic energy. It starts at the origin, rises to a peak (the most probable energy) and tails off at high energy; the area under the curve is the total number of molecules. Only the molecules to the right of can react.
How catalysts work
On the Maxwell-Boltzmann distribution, the lower line sits further left, so a greater proportion of molecules exceed it and more collisions succeed. A catalyst changes the rate, not the position of equilibrium or the enthalpy change.
Examples in context
Example 1. Catalytic converters. Platinum, palladium and rhodium catalysts in a car exhaust lower the activation energy for converting toxic carbon monoxide and nitrogen oxides into and , letting the reactions proceed fast enough at exhaust temperatures.
Example 2. Flour-mill explosions. Finely divided flour dust has an enormous surface area, so combustion is extremely fast; a spark can ignite a dust cloud explosively, a dramatic illustration of the surface-area effect on rate.
Try this
Q1. State the two conditions that must be met for a collision between particles to be successful. [2 marks]
- Cue. The particles must collide with energy greater than or equal to the activation energy, and with the correct orientation.
Q2. Explain why increasing the surface area of a solid reactant increases the rate of reaction. [2 marks]
- Cue. More particles are exposed at the surface, so the frequency of collisions between reactant particles increases, giving more successful collisions per second.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksUsing the Maxwell-Boltzmann distribution, explain why increasing the temperature increases the rate of a reaction.Show worked answer →
At a higher temperature the molecules have a greater average kinetic energy, so the whole distribution shifts to higher energies and flattens (1).
A greater proportion of molecules now have energy greater than or equal to the activation energy (1), so a greater proportion of collisions are successful (1). The molecules also collide more frequently, but the dominant effect is the larger fraction exceeding the activation energy, so the rate increases sharply (1).
Eduqas 20213 marksExplain, in terms of activation energy and the Maxwell-Boltzmann distribution, how a catalyst increases the rate of a reaction.Show worked answer →
A catalyst provides an alternative reaction pathway with a lower activation energy (1).
On the Maxwell-Boltzmann distribution, lowering the activation energy means a greater proportion of molecules now have energy greater than or equal to (1), so a greater proportion of collisions are successful and the rate increases. The catalyst is not used up (1).
Related dot points
- Reversible reactions and dynamic equilibrium, Le Chatelier's principle, the Bronsted-Lowry theory of acids and bases, strong and weak acids, and the reactions of acids.
An Eduqas A-Level Chemistry C2.1 answer on dynamic equilibrium, Le Chatelier's principle, the Bronsted-Lowry theory, strong and weak acids, and the typical reactions of acids.
- Enthalpy changes, exothermic and endothermic reactions, standard enthalpy changes of reaction, formation and combustion, calorimetry, Hess's law and mean bond enthalpies.
An Eduqas A-Level Chemistry C2.2 answer on enthalpy changes, standard enthalpies of reaction, formation and combustion, calorimetry, Hess's law and mean bond enthalpies.
- Rate equations and orders of reaction, the rate constant, determining orders from concentration-time and rate-concentration data, the rate-determining step, and the Arrhenius relationship with activation energy.
An Eduqas A-Level Chemistry PI3 answer on rate equations and orders of reaction, the rate constant, determining orders from data, the rate-determining step and the Arrhenius equation.
- Green chemistry and sustainability, atom economy and percentage yield as measures of efficiency, the use of catalysts and renewable feedstocks, and reducing the environmental impact of chemical processes.
An Eduqas A-Level Chemistry C2.4 answer on green chemistry and sustainability, atom economy and yield as efficiency measures, the role of catalysts and feedstocks, and reducing environmental impact.
Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)