(Note: Some words can be repeated) a. Combustion reactions occur at very different rates. The reaction will be increases when large lumps of coal and a limited supply of air is used. This is because the burning only takes place on the of the coal. When the coal is ground into very fine particles the reaction is because there is a large area of fuel exposed to the oxygen. In diesel engines the liquid fluid reacts within a few milliseconds producing an which is an extremely rapid reaction. b. Combustion is just one example of chemical reaction which occurs at different rates. One factor that determines the speed rate at which a reaction occurs is the energy. Usually slow reactions have values for this. Chemists alter the rate at which a reaction occurs by changing one or more of the following factors: size of the or by using a Increasing the rate at which c. For any reaction to occur the reacting particles must this occurs will the rate of the reactions. per unit volume. Increasing the d. Concentration is a measure of a number of concentration means there are particles per unit volume so the chance of collisions between particles and this, in turn, increases the reaction rate.