Five factors change the rate of reaction: the concentration of a solution, the pressure of a gas, the temperature, the total surface area (particle size) of a solid, and a catalyst. Increasing concentration, pressure, temperature or surface area speeds a reaction up; a positive catalyst speeds it up without being used up.
This page covers one Form 4 content standard: the factors affecting the rate of reaction. It builds on the concept of rate from the previous standard and prepares you for collision theory, which is the explanation of everything below. Here the job is to know each factor, know which way it moves the rate, and be able to design a fair experiment that tests one factor at a time.
The five factors
There are five factors you must know for the exam:
- Concentration of a reactant in solution.
- Pressure of a reactant that is a gas.
- Temperature of the reaction mixture.
- Total surface area of a solid reactant (controlled by particle size).
- Catalyst, a substance that changes the rate without being used up.
For the first four, increasing the factor increases the rate; for a positive catalyst, adding it increases the rate. Each one is a favourite Paper 2 topic, so learn both the effect and the reason.
Concentration
The higher the concentration of a dissolved reactant, the faster the reaction. There are more reactant particles in each unit of volume, so reactions such as magnesium with hydrochloric acid speed up when a more concentrated acid is used. On a graph of gas volume against time, a higher concentration gives a steeper curve at the start.
Pressure (for gases)
For reactions between gases, increasing the pressure pushes the gas particles closer together, it is effectively increasing their concentration, so the rate increases. Pressure only matters when at least one reactant is a gas; it has no effect on reactions between solids and solutions.
Temperature
Raising the temperature increases the rate strongly. The particles gain kinetic energy and move faster. Temperature is examined in the classic sodium thiosulfate + hydrochloric acid experiment: the higher the temperature, the shorter the time for the sulfur precipitate to hide a cross drawn under the flask, which means a faster rate.
Surface area (particle size)
For a solid reactant, breaking it into smaller pieces or a powder increases the total surface area exposed to the other reactant, so the rate increases. Powdered calcium carbonate reacts with acid faster than the same mass of large marble chips. Note that a larger surface area comes from a smaller particle size, students often mix up the direction.
Catalyst
A catalyst speeds up a reaction without being permanently used up. Manganese(IV) oxide, MnO₂, catalyses the decomposition of hydrogen peroxide into water and oxygen. The catalyst is recovered unchanged in mass at the end, and it does not change the amount of product formed, only how quickly it forms. Catalysts are studied in full in the next standard.
Worked example
Question. Two experiments react excess calcium carbonate with 50 cm³ of 1.0 mol dm⁻³ hydrochloric acid at the same temperature. In Experiment A the carbonate is a single large lump; in Experiment B it is a fine powder of the same total mass. (a) State which experiment has the higher initial rate. (b) State how the final volume of carbon dioxide compares. (c) Explain your answer to (b).
Step 1, Identify what changed. Only the particle size (surface area) differs; the amount of acid and its concentration are the same.
Step 2, Decide the effect on rate. The powder (Experiment B) has the larger total surface area, so its initial rate is higher, its curve is steeper at the start.
Step 3, Decide the final volume. The final volume of gas is the same in both experiments.
Step 4, Explain the final volume. The acid is the limiting reactant (the carbonate is in excess), and both experiments use the same amount of acid. The same amount of acid produces the same number of moles of carbon dioxide, so the total volume of gas is identical; only the speed of reaching it differs.
Answer. (a) Experiment B (the powder) has the higher initial rate. (b) The final volume of carbon dioxide is the same in both. (c) Because the same amount of acid reacts in each, the same number of moles of CO₂ is produced; surface area changes the rate, not the quantity of product.
Practice question
Question. A student investigates how temperature affects the rate of the reaction between sodium thiosulfate solution and dilute hydrochloric acid by timing how long a cross under the flask takes to disappear. At 30 °C the time is 60 s; at 50 °C it is 25 s. (a) State which temperature gives the faster reaction. (b) Explain why. (c) Name one variable that must be kept constant for a fair test.
Answer. (a) 50 °C gives the faster reaction, because the cross disappears in a shorter time (25 s versus 60 s). (b) At the higher temperature the particles have more kinetic energy and move faster, so the sulfur precipitate forms more quickly. (c) Any one of: the concentration and volume of the sodium thiosulfate solution, the concentration and volume of the acid, or the depth/size of the cross, kept the same so that only temperature is changed.
Exam tip
Whenever a question compares two experiments, first ask which single factor was changed and check that everything else is the same, that is the fair-test discipline the examiner is looking for. Then remember the crucial distinction: changing concentration, pressure, temperature or surface area changes the rate (how steep the graph is and how soon it finishes), but if the amount of the limiting reactant is unchanged, the final volume or mass of product is the same. Losing sight of this leads to the classic wrong answer that a faster reaction “makes more gas”.
Designing fair experiments
The whole point of this standard is controlled comparison. To test concentration, you keep temperature, volume, and the amount and form of any solid the same, and change only the concentration. To test surface area, you keep concentration, volume and temperature fixed and change only the particle size. The measured quantity, gas volume, loss of mass, or time to a fixed cloudiness, must be recorded in the same way each time. When you write these up, name the manipulated variable (the factor you change), the responding variable (rate, or a quantity from which rate is found) and at least one controlled variable, exactly as you learnt in the Introduction to Chemistry chapter.
Where this fits
This is content standard 7.2 of the Rate of Reaction chapter. It states what changes the rate; the next standard, collision theory, explains why. Reinforce it with the chapter revision notes and test yourself on the practice questions. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, a one-to-one teacher can check that your fair-test answers always separate rate from final quantity, which is the single most common slip in this standard.
Quick recap
- Five factors: concentration, pressure, temperature, surface area, catalyst.
- Increasing concentration, pressure, temperature or surface area increases the rate.
- Smaller particles → larger surface area → faster reaction.
- Changing a factor changes the rate, not the amount of product (if the limiting reactant is unchanged).
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