What controls how fast a reaction goes, and how do we measure rate?
The factors that affect the rate of reaction, collision theory, the effect of a catalyst, and methods of measuring and calculating rate from experimental data.
A focused answer to Edexcel GCSE Combined Science Topic 7 (CC7), covering the factors that affect the rate of reaction, collision theory, catalysts, and how rate is measured and calculated from experimental data.
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What this dot point is asking
Edexcel wants you to state the factors that affect the rate of a reaction, explain them using collision theory, describe the effect of a catalyst, and measure and calculate the rate of reaction from experimental data.
Factors that affect rate
Collision theory
Each factor speeds up the reaction through collisions:
- Temperature: particles move faster, so they collide more often and more collisions have the activation energy.
- Concentration / pressure: more particles in the same volume, so collisions are more frequent.
- Surface area: breaking a solid into smaller pieces (or a powder) exposes more particles, so there are more frequent collisions.
Catalysts
A catalyst speeds up a reaction by providing a different reaction pathway with a lower activation energy, so more collisions are successful. A catalyst is not used up in the reaction, so a small amount can be used repeatedly, which makes catalysts valuable in industry because they cut energy costs. Different reactions need different catalysts. Enzymes are biological catalysts that control the reactions in living cells.
Measuring rate
You can follow a reaction by measuring how quickly a product forms or a reactant is used up:
- Volume of gas produced (gas syringe), against time.
- Mass loss (balance) if a gas escapes, against time.
- Time for a solution to turn cloudy (for example sodium thiosulfate with acid).
The rate is the change in amount divided by the time. The slope (gradient) of a graph of product against time gives the rate; a steeper slope means a faster reaction.
A typical rate graph (for example volume of gas against time) is steepest at the start, when the reactant concentration is highest, and gradually becomes less steep as the reactants are used up. It finally goes flat when the reaction is complete, because one of the reactants has run out. The point at which the graph levels off shows how much product was made in total, while the steepness at any moment shows how fast the reaction was going then. Comparing two such curves (for example with and without a catalyst) is a very common exam task: the faster reaction has the steeper initial slope but reaches the same final amount if the same quantities of reactants are used.
Try this
Q1. State three factors that increase the rate of a reaction. [3 marks]
- Cue. Higher temperature, higher concentration (or pressure), larger surface area (a catalyst is also acceptable).
Q2. Explain how a catalyst speeds up a reaction. [2 marks]
- Cue. It provides a pathway with a lower activation energy, so more collisions are successful; it is not used up.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksUse collision theory to explain why increasing the temperature increases the rate of a chemical reaction.Show worked answer →
A 4-mark explain question on collision theory.
Increasing the temperature gives the reacting particles more kinetic energy, so they move faster (1 mark). This means they collide more frequently (1 mark). More importantly, more of the collisions have enough energy (the activation energy) to be successful (1 mark). Because there are more frequent and more energetic successful collisions, the rate of reaction increases (1 mark).
Markers reward more frequent collisions and, crucially, more collisions having the activation energy. The energy point is what distinguishes top answers.
Edexcel 20215 marksA student measures the volume of gas produced when magnesium reacts with hydrochloric acid, recording of gas in the first 30 seconds. Calculate the mean rate of reaction over this time, and describe how the rate changes as the reaction proceeds.Show worked answer →
A 5-mark calculation and description question.
Mean rate = amount of product / time = (2 marks for the value and unit). The rate is fastest at the start, because the concentration of the acid is highest, so there are the most frequent successful collisions (1 mark). As the reaction proceeds, the acid is used up and its concentration falls, so the rate decreases (1 mark). The reaction stops when one of the reactants is used up, shown by the graph levelling off (1 mark).
Markers reward the rate calculation with units, and linking the falling rate to the falling reactant concentration over time.
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Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)