Factors Affecting Reaction Rates
The rate of a reaction describes how fast reactants are converted into products. This topic covers the five main factors that influence this speed: concentration, temperature, particle size, catalysts, and pressure for gases.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Increasing the concentration of reactants in a solution increases the reaction rate because there are more particles in a given volume, leading to more frequent collisions.
- Increasing the temperature increases the reaction rate. This is because particles have more kinetic energy, leading to more frequent collisions, and more importantly, a greater proportion of these collisions have energy exceeding the activation energy.
- Decreasing the particle size of a solid reactant (i.e., increasing its surface area) increases the reaction rate. This exposes more particles at the surface, making them available for collisions.
- For reactions involving gases, increasing the pressure increases the reaction rate. This forces the gas particles closer together, increasing their concentration and thus the frequency of collisions.
- Adding a catalyst increases the reaction rate by providing an alternative reaction pathway with a lower activation energy. This increases the proportion of collisions that are successful, without the catalyst being consumed.
Definitions
- Rate of Reaction
- The speed at which a chemical reaction proceeds, typically measured as the change in the amount or concentration of a reactant or product per unit of time.
- Collision Theory
- A model stating that for a reaction to occur, reactant particles must collide with both the correct orientation and with sufficient kinetic energy (the activation energy).
- Catalyst
- A substance that speeds up a chemical reaction by providing an alternative mechanism with a lower activation energy, and is not chemically changed at the end of the reaction.
Worked example
Marble chips (a form of solid calcium carbonate) react with hydrochloric acid in an open beaker, producing carbon dioxide gas. Three separate experiments are conducted. Experiment 1 is the baseline. Describe the effect on the initial rate of reaction for Experiment 2 and Experiment 3 compared to Experiment 1, and provide a brief explanation for each. Experiment 1: 5 g of large marble chips with 50 cm³ of 1.0 mol/dm³ HCl at 25°C. Experiment 2: 5 g of finely powdered marble with 50 cm³ of 1.0 mol/dm³ HCl at 25°C. Experiment 3: 5 g of large marble chips with 50 cm³ of 1.0 mol/dm³ HCl at 50°C.
- 1
Step 1 (Experiment 2 vs 1):
Identify the change.
The marble chips are replaced with powder.
This increases the total surface area of the solid reactant.
- 2
Step 2 (Explanation for Exp 2):
According to collision theory, a larger surface area exposes more calcium carbonate particles.
This increases the frequency of collisions between the acid and the reactant, leading to a faster initial reaction rate.
- 3
Step 3 (Experiment 3 vs 1):
Identify the change.
The temperature is increased from 25°C to 50°C.
- 4
Step 4 (Explanation for Exp 3):
Increasing the temperature gives the reactant particles more kinetic energy.
This results in more frequent collisions, and crucially, a significantly higher proportion of collisions having energy equal to or greater than the activation energy.
This increases the frequency of successful collisions, leading to a much faster initial reaction rate.
Answer: Experiment 2: The rate will be faster than Experiment 1 because the powdered marble has a larger surface area, increasing collision frequency. Experiment 3: The rate will be much faster than Experiment 1 because the higher temperature means collisions are more frequent and, more importantly, a greater fraction of them are successful as more particles have energy exceeding the activation energy.
Common mistakes
- ×Forgetting the constraint on pressure. Stating 'increasing pressure increases rate' is incomplete. It must be specified that this applies to reactions involving gases.
- ×Only giving half the reason for temperature's effect. Many students state that higher temperature means particles move faster, causing more collisions. The dominant reason is that a greater proportion of particles have energy exceeding the activation energy, increasing the success rate of collisions.
- ×Applying 'concentration' to solids. The rate for a solid reactant depends on its surface area (particle size), not its concentration. Concentration applies to species in solution or gases.
No-calculator tips
- ✓To compare the impact of different changes, remember the general hierarchy of effectiveness: adding a catalyst usually has the largest effect, followed by a significant temperature increase. Changes in concentration or particle size are typically less dramatic.
- ✓Always frame your explanations in terms of collision theory. Ask yourself: does this change affect the FREQUENCY of collisions, or the ENERGY (success rate) of collisions, or both?
- ✓Visualise the factors. Imagine more particles squeezed into a box (concentration/pressure), particles moving faster and more violently (temperature), or breaking a large block into powder to reveal more surface (particle size).