Edexcel IGCSE Chemistry · Spec 3.9-3.11
Rates of Reaction: Factors & Collision Theory
The factors that affect rate of reaction, explained by collision theory.
Chemistry revision video
Rates of Reaction: Factors & Collision Theory
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Explained
Collision theory, and the four factors that follow from it
Some reactions finish in a flash and others take years. Collision theory explains the difference with two conditions. Particles must collide, and they must collide with at least the activation energy. A collision with too little energy achieves nothing.
So there are only two ways to speed a reaction up: make collisions happen more often, or make a greater proportion of them successful. Every factor in this topic does one or the other, and knowing which is what the marks are for.
Temperature
Raising the temperature does both. The particles gain kinetic energy and move faster, so they collide more frequently. They also arrive with more energy, so a greater proportion of collisions have at least the activation energy and are successful.
Both halves are needed for full credit, and the second is the one usually left out.
Concentration and pressure
A more concentrated solution has more particles in the same volume, so collisions happen more often. Higher pressure in a gas does the same thing by squeezing the same particles into a smaller space.
Neither changes how fast the particles move, so the proportion of successful collisions is unchanged. Only the frequency rises.
Surface area
Breaking a solid into smaller pieces or grinding it to a powder exposes more of it. More particles are available at the surface for the other reactant to reach, so collisions happen more often.
Say increased surface area, not smaller pieces, because the surface area is the thing that has changed.
Catalysts
A catalyst provides a different route with a lower activation energy, so a greater proportion of the collisions already happening are successful. It does not make particles move faster and it does not increase the number of collisions.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Chemistry sets out exactly what a four mark temperature question needed: the rate of reaction increases, the particles move faster, there are more frequent collisions, and there are more successful collisions. One response gained three marks with no mention of more successful collisions or of more particles having the activation energy. Another mentioned kinetic energy and rate but neither frequency nor success, and gained two. The report states that candidates must talk about more frequent and successful collisions.
The reports are more positive about surface area. In a later series, learners confidently linked the use of powder to increased surface area and rate of reaction, and the overall quality of responses on rate was described as very good.
On measuring rate from a graph, another report is strict. Where the question says to draw a tangent, not drawing one loses two marks straight away. It also records a response where the gradient was right but the unit was written as g/m instead of g/min, costing a mark.
Measuring a rate
Rate is the change in an amount divided by the time taken. In practice you follow something you can measure: the volume of gas collected, the loss of mass, or the time for a cross to disappear behind a cloudy precipitate.
A graph of product against time starts steep, because the reactants are at their most concentrated, and flattens as they are used up. It becomes horizontal when the reaction stops. The gradient at any point is the rate at that moment, which is why a tangent is needed to find it, and the overall gradient of a straight portion is an average rate.
Spec 3.9-3.11
What you need to know
- Explain reactions using collision theory
- List the factors that affect the rate
- Explain why each factor changes the rate
Active recall
Quick check
Answer each question before opening the answer.
Name four factors that affect the rate of a reaction.
Temperature, concentration (or pressure for gases), surface area, and use of a catalyst.
According to collision theory, what is needed for a reaction to happen?
Particles must collide with enough energy (the activation energy) and in the correct orientation.
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