How does pressure affect speed of reaction? In reactions involving gases, changing the pressure of a gas is similar to changing its concentration. At low pressures, the particles are spread far apart. At high pressures, the particles are closer together. There is a greater probability of effective collisions. Hence, the speed of reaction increases when pressure increases. Collisions between the particles become more frequent.

How does particle size affect speed of reaction? If a solid is cut into smaller pieces, its surface area gets larger. So extra surfaces are exposed for reactant particles to collide into. There is a greater probability of effective collisions. Hence, the speed of reaction increases when particle size decreases (or surface area increases). This results in more reactions taking place in a shorter time.

How does particle size and pressure affect us? Small pieces of food have a larger total surface area than larger pieces of food. Thus, they can be cooked faster and more thoroughly. In coal mines, there are many fine coal particles (small particle size) which can explode easily. Gases that can combust in air, such as methane, are present. As these gases accumulate, pressure is built up, causing explosions.

How does temperature affect speed of reaction? On heating, the particles of the reacting substances absorb energy. When the particles gain energy, they move faster. There is a greater probability of effective collisions. Hence, the speed of reaction increases when temperature increases. Collisions between the particles become more frequent.

How does temperature affect us? In deep-freeze compartments where the temperature is about –20 °C, food is kept fresh for long periods because the very low temperature slows down chemical reactions in food.

Experiment – How Temperature affects speed of reaction When dilute hydrochloric acid is added to sodium thiosulphate solution, a fine precipitate of sulphur slowly forms and the solution becomes cloudy. Na 2 S 2 O 3 (aq) + 2HCl(aq)  2NaCl(aq) + H 2 O(l) + SO 2 (g) + S(s)

Apparatus and Procedure 5 cm 3 of dilute hydrochloric acid is quickly poured into the sodium thiosulphate solution and the stopwatch is started immediately. The mixture is swirled once and the stopwatch is stopped at the moment the cross disappears from view. The time taken is recorded. The experiment is repeated but at different temperatures.

Results of Experiment The higher the temperature, the shorter the time taken for the cross to disappear from view. This means that the higher the temperature, the faster the speed of reaction. The speed of reaction increases rapidly as the temperature increases.

Results of Experiment To see how the speed of reaction varies with temperature more clearly, we can plot values of 1/Time against temperature. It is the same as plotting the speed of reaction against temperature. The speed of reaction increases rapidly as the temperature increases.

Catalysts and Enzymes A catalyst is a substance which increases the speed of a chemical reaction and remains chemically unchanged at the end of the reaction. An example of catalyst is manganese(IV) oxide which catalyses the decomposition of hydrogen peroxide. 2H 2 O 2 (aq)  2H 2 O(l) + O 2 (g)

Characteristics of Catalysts Only a small amount of a catalyst is needed to speed up the reaction. A catalyst increases the speed and not the yield of a chemical reaction. A catalyst is selective in its action. This means one catalyst cannot act on or speed up all types of reactions. Different catalysts speed up different reactions.

Characteristics of Catalysts A catalyst lowers the activation energy of a reaction. Uncatalysed reactionCatalysed reaction

Characteristics of Catalysts The physical appearance of a catalyst may change at the end of the reaction, but its chemical properties remain unchanged. A catalyst is not used up during the reaction. The same amount of catalyst is present at the beginning and at the end of the reaction.

Enzymes They are proteins. They are very specific in their actions. Amylase, for example, is the enzyme present in saliva. It only catalyses the reaction which changes starch into sugar. Enzymes are biological catalysts. They can be denatured by heating. They are sensitive to pH changes.