7.1Speed of reaction

Factors affecting the rate of reaction are:

Concentration (pressure in gases): the higher the concentration of reactants is (higher pressure in gases) the faster the reaction will happen. This is because there is more chance of reactant particles colliding, so there will be more collision per unit of time.

Particle size: For the same mass of particles (e.g. 1kg) the smaller the particles are the faster the reaction will take place. This is because many small particles will have more surface area than fewer large particles. More surface area means that there is more area in which particles can collide. More collisions per unit of time means a faster reaction.

Catalysts (including enzymes: biological catalysts): catalysts are substances which speed up reactions, whilst remaining chemically unchanged. Catalysts work by lowering the activation energy and providing an alternative pathway for the reaction. Reactions can take place with less energy.

Enzymes: proteins that function as biological catalysts Factors that control how well enzymes work:

Temperature: enzymes have an optimum temperature: the temperature at which they work best giving the fastest reaction. In humans, most enzymes have an optimum temperature of 37°C, but in plants it is around 25°C. When temperature increases, the molecules move faster so collide with an enzyme in less time, having more energy makes them more likely to bind to the active site: the part of an enzyme where a specific substrate will fit perfectly. If the temperature is too high, the enzyme molecules vibrate too vigorously and the enzyme is denatured: it loses its 3D shape and will no longer bind with a substrate. When the temperature is too low there is not enough kinetic energy for the reaction so it reacts too slowly.

pH: The base or acid conditions can denature enzymes too, but the enzyme can be denatured if the pH is too low OR too high. Enzymes have an optimum pH too, for example amylase has an optimum pH of 7.5, and pepsin’s is pH 2.

Temperature: higher temperature speeds up reactions in two ways: 1) particles travel faster so they collide with other particles faster, so a reaction (which needs collisions) will take less time and 2) particles will more often overcome the activation energy.


  1. Hydrochloric acid and sodium thiosulphate solution are mixed in a flask, and a stopwatch They react forming sulphur which is insoluble in water so precipitates. The flask is on top of a cross drawn on a piece of paper. You measure the amount of time taken for the cross to not be visible because there is enough sulphur. The diameter of the cylinder should be kept constant (since that and the volume of reactants will control the depth of the mixture; deeper means that the cross will be invisible sooner). Variables that can be changed: temperature and concentration of reactants.
  2. Reactions with different sized particles (e.g. magnesium powder vs. ribbon + acid, marble chips vs. smaller chips). Time taken for a certain amount of gas to be produced is measured, or change in mass, because the gas escapes (e.g. hydrogen for Mg + acid reaction, carbon dioxide for marble chip + acid experiment.) Marble is calcium carbonate. Interpret data: less time taken for a reaction means a faster rate; the reaction in terms of data can be a certain amount of gas produced or a change in mass etc. Large surface area can mean danger: Flour dust, wood dust, custard powder, instant coffee, sugar, and dried milk have large surface areas, and are combustible. A spark from a machine, or a lit match, can cause an explosion, this also applies to gases from mines (another syllabus specified  example).
  3. Fill a gas jar with a mixture of hydrogen and oxygen, and cover it. Even if you leave it for hours, no reaction will happen. Then dip a platinum wire into the mouth of the jar. The gas mixture explodes immediately with a pop, producing water.

Light can affect the rate of reaction: photochemical reactions for example in photosynthesis. Light provides energy for the reaction, chlorophyll is a dye that absorbs light.

carbon dioxide + water → (light + chlorophyll) → glucose + oxygen 6CO2 + 6H2O → (light + chlorophyll) → C6H12O6 + 6O2

Silver salts in photographic film: Silver bromide breaks down, where light strikes the film, so silver is reduced. Silver

ions are reduced to silver.