Collision theory and activation energy — AQA GCSE Combined Science
Test yourself on Collision theory and activation energy with AQA GCSE practice questions.
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Collision theory and activation energy explained
Collision theory states that particles must collide for a reaction to happen, but most collisions do not cause a reaction.
Read the full explanation
A successful collision needs enough energy, called the activation energy, to break bonds in the reactants. Factors such as concentration, pressure, surface area, temperature and catalysts change the rate by altering either the frequency of collisions or the proportion of particles with energy greater than or equal to the activation energy. For example, increasing temperature makes particles move faster, so they collide more often and with more energy; adding a catalyst provides an alternative pathway with lower activation energy.
Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.
Reaction rate depends on how often reactant particles collide with at least the activation energy. In solution, raising concentration packs more reactant particles into the same volume, so collisions become more frequent. For gases, raising pressure squeezes the same particles into a smaller volume, again increasing collision frequency. For solids, breaking a lump into powder exposes more surface, so more particles can collide at once. In each case the particles themselves are not made more energetic; only the number of collisions per second rises, so the rate increases. For example, marble chips react faster with 2 mol/dm³ hydrochloric acid than with 1 mol/dm³ acid, and powdered calcium carbonate reacts faster than the same mass of large chips.
Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction.
Temperature affects reaction rate in two ways. First, warming a reactant gives its particles more kinetic energy, so they move faster and collide more often. Second, each collision carries more energy, so a greater proportion of collisions has at least the activation energy needed to react. Both effects raise the rate. For example, sodium thiosulfate solution reacts with dilute hydrochloric acid faster at 40 °C than at 20 °C: the sulfur cloud appears sooner because more collisions are successful each second. A common practical method is to time how long the cross under the flask disappears at different temperatures, then compare the times.
predict and explain using collision theory the effects of changing conditions of concentration, pressure and temperature on the rate of a reaction
This statement requires students to use collision theory to predict and explain how concentration, pressure and temperature affect reaction rate. For concentration, increasing the amount of dissolved reactant in a given volume means more particles in the same space, so collisions become more frequent and the rate increases. For pressure, increasing the pressure of reacting gases compresses the same number of particles into a smaller volume, again increasing collision frequency. For temperature, particles move faster, so collisions are more frequent and a greater proportion of collisions have energy greater than or equal to the activation energy. A student might predict that raising the temperature of sodium thiosulfate solution with dilute hydrochloric acid makes the cross disappear sooner, then explain this using collision theory.
predict and explain the effects of changes in the size of pieces of a reacting solid in terms of surface area to volume ratio
For a solid reactant, only particles at the exposed surface can collide with particles of the other reactant, so the rate depends on how much surface is available. Breaking a solid into smaller pieces increases its total surface area without changing its mass or volume, so the surface area to volume ratio rises. A greater ratio means more solid particles are exposed to collisions per unit of volume, so collisions occur more frequently and the rate of reaction increases. For example, 10 g of marble chips in excess acid reacts slowly, but the same 10 g powdered exposes far more surface and fizzes faster, although the same total volume of gas is eventually produced. The ratio is calculated by dividing surface area by volume, so halving particle size roughly doubles the ratio for similarly shaped pieces.
use simple ideas about proportionality when using collision theory to explain the effect of a factor on the rate of a reaction.
Collision theory says that particles must collide with at least the activation energy for a reaction to occur. The rate depends on the frequency of these successful collisions. Simple proportionality means that if a factor increases the frequency of successful collisions by a given factor, the rate increases by roughly the same factor, provided other conditions stay constant. For example, doubling the concentration of a reactant in solution roughly doubles the number of particles in a given volume, so successful collisions per second and the rate roughly double. Similarly, increasing the surface area to volume ratio of a solid by breaking it into smaller pieces increases the exposed area and so increases the rate in proportion, until other factors limit it. Proportionality is approximate because factors interact and measurements have uncertainty.
Your focus
- Describe collision theory and the conditions needed for a successful collision.
- Define activation energy and use it to explain why some collisions do not lead to reaction.
- Apply collision theory to explain how concentration, temperature, surface area, pressure and catalysts affect reaction rate.
Show all 18 objectives
- Describe how concentration, pressure and surface area affect the rate of a reaction.
- Explain the effect of each factor in terms of collision frequency.
- Apply collision theory to a named practical example involving a solution, a gas or a solid.
- Describe how temperature affects the rate of a reaction.
- Explain the effect of temperature using collision frequency and collision energy.
- Interpret rate data from a temperature-based practical, such as the sodium thiosulfate cross method.
- Predict the direction of rate change when concentration, pressure or temperature is altered.
- Explain each predicted change using collision frequency and activation energy.
- Apply collision theory consistently to unfamiliar reactions and practical contexts.
- Describe how breaking a solid into smaller pieces changes its surface area to volume ratio.
- Explain, using collision theory, why a greater surface area to volume ratio increases the rate of reaction.
- Predict and justify the effect of changing the size of pieces of a reacting solid on the rate and on the total product formed.
- Describe how collision theory links successful collision frequency to the rate of a reaction.
- Apply simple proportionality to predict how changing concentration, pressure or surface area affects the rate.
- Explain why proportional predictions are approximate and depend on controlling other variables.
Collision theory and activation energy exam tips
Marking Points
- State that reacting particles must collide with each other for a reaction to occur.
- Explain that collisions must have at least the activation energy to be successful.
- Define activation energy as the minimum energy that particles need to react.
- Use collision theory to explain how a named factor changes the rate, for example higher concentration increases collision frequency.
- Explain how a catalyst increases rate by providing an alternative reaction pathway with lower activation energy.
- State that rate depends on the frequency of collisions between reactant particles.
- Explain that higher concentration means more reactant particles per unit volume in solution, so collisions are more frequent.
- Explain that higher gas pressure means the same particles occupy a smaller volume, increasing collision frequency.
- Explain that greater surface area of a solid exposes more particles, so more collisions occur per second.
- Link the increased collision frequency to a faster rate of reaction.
- Use a named example, such as marble chips with dilute versus concentrated hydrochloric acid, or powder versus lumps.
- State that raising temperature gives particles more kinetic energy.
- Explain that faster-moving particles collide more frequently.
- Explain that collisions are more energetic, so more collisions reach or exceed the activation energy.
- Link both effects to an increased rate of reaction.
- Use a named example, such as sodium thiosulfate with hydrochloric acid at different temperatures.
- Describe a method that measures rate, such as timing the disappearance of a cross or measuring gas volume over time.
- Predicts that increasing concentration increases the rate of reaction.
- Explains that increasing concentration means more reactant particles in the same volume, so collisions are more frequent.
- Predicts that increasing the pressure of reacting gases increases the rate of reaction.
- Explains that increasing pressure brings gas particles closer together, so collisions are more frequent.
- Predicts that increasing temperature increases the rate of reaction.
- Explains that increasing temperature increases particle speed, so collisions are more frequent and a greater proportion of particles have energy greater than or equal to the activation energy.
- Uses the term successful collisions to describe collisions that lead to reaction.
- Only particles at the surface of a solid can collide with particles of the other reactant, so reaction occurs at the surface.
- Breaking a solid into smaller pieces increases its total surface area while mass and volume stay the same, so the surface area to volume ratio increases.
- A higher surface area to volume ratio exposes more solid particles to collisions per unit volume, increasing collision frequency.
- More frequent collisions between reactant particles increase the rate of reaction, so the reaction finishes sooner.
- The total amount of product formed eventually is unchanged because the same mass of solid reacts; only the rate changes.
- Comparing equal masses of large chips and powder in the same volume of reactant solution allows a fair test of the effect.
- Particles must collide with energy greater than or equal to the activation energy for a reaction to occur.
- Rate depends on the frequency of successful collisions, so a factor that increases this frequency increases the rate.
- If a factor increases the frequency of successful collisions by a given factor, the rate increases by approximately the same factor when other variables are controlled.
- Doubling the concentration of a reactant in solution roughly doubles the number of particles per unit volume, so the rate roughly doubles.
- Increasing the surface area to volume ratio of a solid exposes more particles to collision, so the rate increases in proportion to the exposed area.
- Proportionality is a simple model and may not hold exactly because of experimental uncertainty, limiting factors or changes in temperature.
Examiner Tips
- 💡Use the phrase 'collide with sufficient energy' when explaining successful collisions.
- 💡When explaining a rate change, state whether collision frequency, collision energy, or both are affected.
- 💡Link catalyst explanations to a lower activation energy and an alternative pathway.
- 💡Name the variable you are changing and state clearly that collision frequency increases.
- 💡For surface area questions, compare powder with lumps of the same mass rather than different masses.
- 💡Use the phrase 'more frequent collisions' rather than 'more collisions' alone, and avoid claiming particles gain energy unless temperature changes.
- 💡Include both the frequency effect and the energy effect when explaining temperature.
- 💡Refer to the activation energy explicitly to show why more collisions are successful.
- 💡When describing a practical, state the variable measured and how it indicates rate, such as time for a colour change.
- 💡Structure each explanation as condition changed, effect on particles, effect on collisions, effect on rate.
- 💡Use the phrase energy greater than or equal to the activation energy when explaining temperature effects.
- 💡For pressure questions, make clear that the change applies to gases and that volume decreases if pressure increases at constant temperature.
- 💡Name the variable you change (size of pieces) and the variable you measure (rate, for example gas volume per unit time or loss in mass per unit time).
- 💡Use the phrase surface area to volume ratio and explain that it increases when a solid is broken into smaller pieces.
- 💡Link the ratio to collision frequency and then to rate, and state that the total product is unchanged for the same mass of solid.
- 💡State the factor changed, the factor kept constant and the measured rate, then use the phrase roughly proportional or in proportion.
- 💡Use a numerical example, such as doubling concentration and predicting the rate roughly doubles, to show proportional reasoning.
- 💡Explain the link in order: factor changes particle proximity or exposed area, so successful collision frequency changes, so rate changes.
Common Mistakes
- Saying that any collision causes a reaction; correct this by stating that collisions must also have sufficient energy, at least the activation energy.
- Confusing activation energy with the overall energy change of a reaction; correct this by defining activation energy as the minimum energy needed to start the reaction, not the energy difference between reactants and products.
- Claiming that a catalyst increases the energy of particles; correct this by explaining that a catalyst lowers the activation energy by providing an alternative pathway.
- Saying that increasing concentration makes particles move faster; correction: it increases the number of particles per unit volume, so collisions are more frequent, not more energetic.
- Confusing pressure with concentration for gases; correction: pressure changes the volume the gas occupies, which changes how often particles meet.
- Thinking surface area changes the total mass of solid; correction: it changes how much solid is exposed, so the same mass reacts faster when powdered.
- Claiming temperature only increases collision frequency; correction: it also makes collisions more energetic, so more particles have at least the activation energy.
- Saying that all collisions cause reaction at higher temperature; correction: only collisions with at least the activation energy are successful.
- Confusing rate with yield; correction: temperature changes how fast the reaction goes, not necessarily how much product forms.
- Claiming that increasing concentration increases the activation energy; correction: concentration changes collision frequency, not the activation energy.
- Saying that increasing pressure increases the number of gas particles; correction: pressure increases because the same particles occupy a smaller volume, increasing collision frequency.
- Explaining temperature only by saying particles move faster without mentioning activation energy; correction: include both collision frequency and the proportion of collisions with sufficient energy.
- Thinking that powder has more mass or reacts to give more product: the mass is unchanged, so the same total volume of gas or amount of product forms, but faster.
- Saying smaller pieces have more surface area without linking it to volume: the key idea is the surface area to volume ratio, which increases as pieces get smaller.
- Believing smaller pieces are more reactive because their particles have more energy: particle energy and activation energy are unchanged; the increase is in collision frequency at the exposed surface.
- Treating proportionality as exact: doubling concentration may not exactly double the rate because of measurement uncertainty and other limiting factors.
- Confusing rate with total amount: a proportional change in rate does not change the total amount of product formed from a fixed amount of reactant.
- Saying a factor changes the activation energy: concentration, pressure and surface area change collision frequency, not the activation energy, while temperature changes both collision frequency and the proportion of particles with enough energy.