The rate and extent of chemical change — GCSE Chemistry Revision
Everything you need to revise the rate and extent of chemical change for GCSE Chemistry: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Rates of Reaction
The rate of a chemical reaction tells us how quickly reactants are used up or products are formed. A faster reaction means changes occur more rapidly. We can measure reaction rates by observing changes like gas volume produced, mass lost, or the formation of a precipitate. To calculate the rate, we use the formula: Rate = Change in amount of reactant or product / Time taken. Understanding reaction rates is crucial in industry to optimise processes.
Factors Affecting Reaction Rate
Several factors can influence how fast a reaction proceeds. These include temperature, concentration of reactants, surface area of solids, and the presence of a catalyst. Increasing temperature provides particles with more kinetic energy, leading to more frequent and energetic collisions. Higher concentration means more particles in a given volume, increasing collision frequency. Larger surface area exposes more reactant particles to collide.
Collision Theory
Collision theory explains how chemical reactions occur. For a reaction to happen, reactant particles must collide with each other. However, not all collisions are successful. Particles must collide with sufficient energy, known as the activation energy, and with the correct orientation. Any factor that increases the frequency of successful collisions will increase the rate of reaction. This explains the effect of temperature and concentration.
Catalysts
A catalyst is a substance that increases the rate of a chemical reaction without being used up itself. Catalysts work by providing an alternative reaction pathway with a lower activation energy. This means more reactant particles will have enough energy to react upon collision, leading to a higher frequency of successful collisions. Catalysts are highly specific and are important in industrial processes to save energy and time.
Key facts
- Reaction rate is how fast reactants are consumed or products formed.|Higher temperature increases reaction rate.|Higher concentration increases reaction rate.|Increased surface area increases reaction rate.|Catalysts speed up reactions by lowering activation energy.|Collision theory explains successful particle collisions.
Key terms
- Rate of reaction::The speed at which a chemical reaction occurs, often measured by the change in concentration of a reactant or product over time.|Collision theory::A model explaining that reactions occur when particles collide with sufficient energy (activation energy) and correct orientation.|Activation energy::The minimum amount of energy that reactant particles must possess for a collision to result in a successful reaction.|Catalyst::A substance that speeds up the rate of a chemical reaction without being chemically changed or used up itself.|Surface area::The total area of the exposed surface of a substance, which can affect reaction rate, especially for solid reactants.|Concentration::The amount of solute dissolved in a given volume of solution; higher concentration typically means more particles per unit volume.
Common mistakes
- Thinking a catalyst is used up in the reaction (it isn't, it just provides an alternative pathway).|Confusing reaction rate with reaction yield (rate is how fast, yield is how much product).|Not mentioning activation energy when explaining collision theory for successful collisions.|Assuming all collisions lead to a reaction; only successful ones do.
Exam tips
- When explaining factors affecting rate, always link back to collision frequency and/or energy of collisions.|Be precise with 'increase' or 'decrease' when describing changes in rate or factor values.|Remember to state that catalysts provide an alternative pathway with lower activation energy.|Practise interpreting and sketching rate graphs, identifying initial rate and points where reaction stops.
Quick quiz
1. Which of the following would NOT increase the rate of a reaction?
- Increasing the temperature
- Increasing the concentration of reactants
- Decreasing the surface area of a solid reactant
- Adding a suitable catalyst
Show answer
Decreasing the surface area of a solid reactant — Decreasing the surface area of a solid reactant means fewer particles are exposed for collisions, which would decrease the reaction rate, not increase it. All other options increase the rate.
2. According to collision theory, for a reaction to occur, particles must collide with:
- High frequency only
- Correct orientation only
- Sufficient kinetic energy and correct orientation
- Low kinetic energy and correct orientation
Show answer
Sufficient kinetic energy and correct orientation — Collision theory states that successful collisions require particles to have at least the activation energy (sufficient kinetic energy) and to collide in the correct alignment (orientation).
3. A catalyst increases the rate of a reaction by:
- Increasing the temperature of the reactants
- Increasing the activation energy
- Decreasing the activation energy
- Increasing the concentration of reactants
Show answer
Decreasing the activation energy — Catalysts work by providing an alternative reaction pathway that has a lower activation energy, meaning more particles have enough energy to react during collisions.
4. Which method is suitable for measuring the rate of a reaction that produces a gas?
- Monitoring the change in pH
- Measuring the temperature change
- Collecting the gas and measuring its volume over time
- Observing the colour change of a solution
Show answer
Collecting the gas and measuring its volume over time — For reactions producing a gas, collecting the gas and measuring its volume or mass loss over time are direct ways to monitor the reaction's progress and thus its rate.
5. If you double the concentration of a reactant in a solution, what generally happens to the reaction rate?
- It decreases
- It stays the same
- It increases
- It stops completely
Show answer
It increases — Doubling the concentration means there are more reactant particles in the same volume, leading to more frequent collisions and therefore an increased chance of successful collisions, speeding up the reaction.
Exam-style questions
Explain why increasing the temperature increases the rate of a chemical reaction. [3 marks]
Show mark scheme
- Particles gain more kinetic energy / move faster.
- Increased frequency of collisions.
- Increased proportion of collisions have energy greater than or equal to the activation energy / more successful collisions.
Describe how a catalyst speeds up a reaction. [2 marks]
Show mark scheme
- It provides an alternative reaction pathway.
- Which has a lower activation energy.
A student investigates the rate of reaction between marble chips (calcium carbonate) and hydrochloric acid. State two ways the student could increase the rate of this reaction, apart from changing the temperature. [2 marks]
Show mark scheme
- Increase the concentration of hydrochloric acid.
- Decrease the size of the marble chips / increase surface area of marble chips.
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