What affects how fast a reaction goes, and how do we measure it?
The factors that affect the rate of reaction (concentration, temperature, surface area and catalysts), how rate is measured by gas volume or mass loss, and interpreting rate graphs.
A CCEA GCSE Chemistry answer on rates of reaction, covering how concentration, temperature, surface area and catalysts change the rate, how rate is measured by following gas volume or mass loss, and how to read and interpret rate graphs.
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What this dot point is asking
CCEA wants you to state the four factors that change the rate of a reaction, describe how rate is measured by following gas volume or mass loss over time, and interpret the shape of a rate graph.
The factors that affect rate
Each works by changing how often particles collide successfully, an idea explained fully by collision theory.
Measuring rate
There are two common practical methods:
- Volume of gas produced: collect the gas in a gas syringe and record the volume at set times. Used when a gas is made, such as hydrogen or carbon dioxide.
- Loss of mass: stand the flask on a balance and record the falling mass as gas escapes. Used when the gas is allowed to leave.
A third method follows a colour or cloudiness change, for example timing how long a cross takes to disappear when sulfur is produced.
Reading a rate graph
Comparing two reactions, the one that is steeper at the start and reaches the flat part sooner is the faster reaction. The final height (total gas) depends on the amount of reactant, not the rate.
Worked example
Examples in context
Example 1. Storing food in a fridge. Lowering the temperature slows the reactions that spoil food, because cold particles collide less often and with less energy. The everyday practice of refrigeration is a direct use of the temperature factor on reaction rate.
Example 2. Dust explosions in flour mills. Finely powdered flour or sawdust has a huge surface area, so it can burn extremely fast, even explosively, when sparked. The surface-area factor that speeds up lab reactions also explains a serious industrial hazard.
Try this
Q1. State two factors that increase the rate of a reaction. [2 marks]
- Cue. Any two of: higher concentration, higher temperature, larger surface area, a catalyst.
Q2. State what a flat (horizontal) line on a gas-volume against time graph shows. [1 mark]
- Cue. The reaction has finished (no more gas is being produced).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20194 marksDescribe two ways you could increase the rate of reaction between marble chips (calcium carbonate) and dilute hydrochloric acid, and explain each.Show worked answer →
Markers want two valid factors, each with an explanation.
Increase the temperature. Heating the acid gives the particles more energy, so they move faster and collide more often, and more collisions have enough energy to react. So the rate increases.
Increase the surface area (use smaller marble chips or powder). Breaking the marble into smaller pieces exposes more surface, so more acid particles can collide with the solid at once, increasing the rate.
(Increasing the acid concentration is also accepted: more acid particles per volume means more frequent collisions.)
Markers reward two distinct factors, each linked to more frequent (or more energetic) successful collisions.
CCEA 20213 marksIn a reaction that produces a gas, describe how you would measure the rate, and explain what a graph of gas volume against time shows about how the rate changes.Show worked answer →
The marks are for the method and the shape of the graph.
Method: collect the gas in a gas syringe (or measuring cylinder over water) and record the volume at regular time intervals.
Graph: plot gas volume against time. The line is steepest at the start, where the rate is fastest because the reactants are most concentrated. It gradually becomes less steep as reactants are used up, and finally goes flat (horizontal) when the reaction has finished and no more gas is produced.
Markers reward measuring gas volume over time, a steep start showing the fastest rate, and a flat line showing the reaction is complete.
Related dot points
- Collision theory, activation energy, how concentration, temperature and surface area change the frequency and energy of collisions, and how catalysts work by lowering activation energy.
A CCEA GCSE Chemistry answer on collision theory, covering how successful collisions need enough energy (the activation energy), how each rate factor changes the frequency or energy of collisions, and how catalysts speed reactions by providing a lower-activation-energy pathway.
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A CCEA GCSE Chemistry answer on reversible reactions and equilibrium, covering dynamic equilibrium in a closed system, how changing temperature, pressure and concentration shift the equilibrium, and the conditions used in the Haber process to make ammonia as a compromise.
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A CCEA GCSE Chemistry answer on energy changes, covering exothermic and endothermic reactions, energy level diagrams and activation energy, and how calorimetry is used to measure and compare the energy released when fuels burn.
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A CCEA GCSE Chemistry answer on nanoparticles, covering their size, the large surface area to volume ratio that makes them so reactive and useful, their applications in sunscreens, catalysts and medicine, and the benefits and possible risks of nanotechnology.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)