What controls the rate of a reaction, what is dynamic equilibrium, and how do we use moles to measure amounts?
The factors affecting the rate of reaction (concentration, temperature, surface area, catalysts) and collision theory, the mole and concentration calculations, reversible reactions and dynamic equilibrium, Le Chatelier's principle, and the Haber process.
A focused answer to the OCR Gateway GCSE Combined Science A topics C5 and C6 on monitoring and controlling reactions, covering the factors affecting rate and collision theory, the mole and concentration calculations, reversible reactions and dynamic equilibrium, and the Haber process.
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What this topic is asking
OCR wants you to explain the factors affecting reaction rate using collision theory, carry out mole and concentration calculations, describe reversible reactions and dynamic equilibrium, apply Le Chatelier's principle, and describe the Haber process.
Rate of reaction and collision theory
Four factors increase the rate, each explained by collision theory:
- Concentration (or pressure for gases): more particles in the same volume means more frequent collisions.
- Temperature: particles move faster, so they collide more often, and a greater proportion of collisions have enough energy to react.
- Surface area: breaking a solid into smaller pieces or a powder exposes more particles, so there are more frequent collisions on the surface.
- Catalyst: provides an alternative pathway with a lower activation energy, so more collisions are successful, without the catalyst being used up.
You can measure rate by following the volume of gas produced over time, or the loss of mass, or the time for a solution to turn cloudy. The rate at any moment is the gradient of the graph; the reaction is fastest at the start and slows as reactants are used up.
Moles and concentration
The mole connects the mass you can weigh out to the number of particles reacting, which lets you predict the masses of reactants and products from a balanced equation. For solutions, concentration can be given in grams per cubic decimetre or in moles per cubic decimetre, and a titration (a required practical) is used to find an unknown concentration of acid or alkali by reacting it with a solution of known concentration until neutralisation.
Equilibrium and the Haber process
A reversible reaction can go both ways, shown by the symbol that means "reacts in both directions". In a closed container it can reach dynamic equilibrium, where the forward and backward reactions happen at the same rate, so the amounts of reactants and products stay constant (though the reactions are still going on). Le Chatelier's principle says that if a condition is changed, the equilibrium shifts to oppose the change: raising the pressure shifts it towards the side with fewer gas molecules; raising the temperature shifts it in the endothermic direction. The Haber process makes ammonia from nitrogen and hydrogen () and is a key example: it uses a high pressure (around atmospheres) to favour ammonia, a moderate temperature (around degrees Celsius) as a compromise between rate and yield, and an iron catalyst to speed it up. The ammonia is used to make fertilisers, which helps feed the growing human population (a C6 global challenge).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20196 marksExplain, using collision theory, how increasing the temperature and increasing the concentration each increase the rate of a chemical reaction.Show worked answer →
A Chemistry Paper 4 six-mark extended response, marked on levels. Reward, using collision theory (reactions happen when particles collide with enough energy, the activation energy): increasing the temperature gives the particles more kinetic energy, so they move faster and collide more often, and crucially a greater proportion of collisions have enough energy to react, so the rate increases. Increasing the concentration means there are more particles in the same volume, so collisions are more frequent, and the rate increases. Top answers refer to more frequent collisions and (for temperature) more collisions exceeding the activation energy, not just "particles move faster".
OCR 20214 marksCalculate the number of moles in 20 g of sodium hydroxide, NaOH. The relative atomic masses are Na = 23, O = 16, H = 1.Show worked answer →
A C5 quantitative calculation. Method: first find the relative formula mass () of NaOH by adding the relative atomic masses: . Then use moles mol. Markers award the correct of and the correct use of the moles equation to give mol. A common error is to forget to add all three atoms when finding , or to divide the wrong way round; checking that a smaller mass than the formula mass gives less than one mole is a useful sense-check.
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