The key idea
Rate of reaction = amount of reactant used (or product formed) ÷ time.Increasing temperature, concentration, surface area or using a catalyst all increase reaction rate by increasing the frequency of successful collisions.
rate = amount of product formed ÷ time
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Collision theory
For a reaction to occur, particles must collide with sufficient energy (at least equal to the activation energy) and with the correct orientation.Increasing the frequency of successful collisions increases the reaction rate.Four factors do this: temperature (faster particles), concentration or pressure (more particles per volume), surface area (more particles exposed) and catalysts (lower activation energy).
Effect of temperature
Raising temperature increases the kinetic energy of particles, so they move faster and collide more often.More importantly, a greater proportion of particles have energy greater than or equal to the activation energy.This is shown on a Maxwell–Boltzmann distribution, where the curve shifts to the right and the shaded area beyond the activation energy increases significantly with small temperature rises.
Catalysts
A catalyst increases the rate of reaction without being used up.It provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions are successful.On an energy profile diagram, a catalyst lowers the peak of the energy barrier.Enzymes are biological catalysts; transition metals such as iron (Haber process) and platinum (catalytic converters) are industrial catalysts.
Measuring rates experimentally
Rates can be measured by recording the volume of gas produced per unit time (gas syringe), the mass lost per unit time (mass balance), the change in absorbance of light (colorimetry), or the time for a precipitate to obscure a mark (cross method).The initial rate is the steepest gradient at the start of a rate graph, when reactant concentrations are highest.
Definitions to learn
Rate of reaction
The change in amount of reactant or product per unit time.
Collision theory
The theory that reactions occur when particles collide with sufficient energy and correct orientation.
Activation energy
The minimum energy required for a collision to result in a reaction.
Catalyst
A substance that increases the rate of reaction by providing an alternative pathway with lower activation energy, without being used up.
Maxwell–Boltzmann distribution
A graph showing the spread of kinetic energies among particles in a sample at a given temperature.
Explain why increasing the temperature increases the rate of a reaction.
Higher temperature gives particles more kinetic energy.
Particles move faster and collide more frequently.
More collisions have energy equal to or greater than the activation energy.
Therefore, the rate of successful collisions (and the rate of reaction) increases.
Higher temperature increases rate because more collisions exceed the activation energy.
For collision theory answers, always state BOTH that collisions are more frequent AND that more collisions have energy equal to or greater than the activation energy.
Do not say particles collide more — you must add that MORE collisions have energy greater than or equal to the activation energy.
How to score full marks
- 1For any rate explanation, mention BOTH increased collision frequency AND more collisions exceeding the activation energy.
- 2A catalyst lowers activation energy — always include this when explaining how a catalyst works.
- 3On a rate-of-reaction graph, the gradient (steepness) represents the rate — steepest at the start, levels off when reaction is complete.
Try these yourself
Open each answer only after you have explained the full chemical process.
1State the equation for rate of reaction.[1 mark]
- 1.Link amount of change to time.
2Give two factors that increase the rate of a reaction.[2 marks]
- 1.Recall the four main factors.
3Explain, using collision theory, why increasing concentration increases reaction rate.[4 marks]
- 1.Link number of particles to frequency of collisions.
4Explain the effect of a catalyst on reaction rate and on activation energy.[4 marks]
- 1.Describe how a catalyst provides an alternative pathway.
5Describe an experiment to investigate the effect of concentration on the rate of the reaction between sodium thiosulfate and hydrochloric acid.[5 marks]
- 1.Include method, measurement and how rate is calculated.
6Explain why powdering a solid reactant increases the rate of reaction.[4 marks]
- 1.Link surface area to collision frequency.
7On a Maxwell–Boltzmann distribution, sketch the effect of increasing temperature on the distribution of molecular energies.[4 marks]
- 1.Describe or annotate the key change.
8A student measures the volume of gas produced at regular intervals in a reaction. Sketch the shape of the volume vs time graph and explain the shape.[4 marks]
- 1.Describe why the gradient decreases.