Rates of Reactions – Part 2

Factors affecting the rate of reaction The surface area The concentration of the reactant The temperature at which the reaction is carried out The use of a catalyst The influence of light

4. Rate of Reaction – Effect of Catalyst Hydrogen peroxide water + oxygen Powdered manganese IV oxide MnO 2 is added as a catalyst. The more finely ground up the manganese oxide is the faster the reaction goes. The mass of the manganese oxide remains the same. It is a catalyst.

Effect of Catalyst The more catalyst you use the faster the reaction. The type of catalyst you use will effect the reaction.

Definition of a Catalyst A catalyst is a substance that increases the rate of a chemical reaction. The catalyst remains chemically unchanged.

How does a catayst work? The provide a surface on which other molecules or atoms can react.

Catalysts in Industry Catalysts are very useful as you only need a small amount to dramatically change the rate of a reaction. The catalyst is not used up so can be used again and again. Used in the production of polythene, painkillers, fertilizers and fabrics. The processes to make these compounds, without catalysts, would need much more energy and be much more costly.

Catalytic Convertors Used on cars to reduce pollution. Car exhausts produce CO, NO and unburnt hydrocarbons. The catalytic convertor converts these to less harmful products CO 2, N 2 and water. The catalytic convertor removes polluting oxides and completes the oxidation of unburnt hydrocarbons.

Catalytic Converters Rhodium and platinum – precious metals used

Biological Catalysts Living cells also produce catalysts, these are protein molecules called enyzmes. Characteristics; – Enzymes are proteins – Very specific – control only one reaction – Temperature sensitive – stop working above 45 o C – pH sensitive – work best at pH 7

Enzymes in Industry Biological washing powders use enzymes to remove biological stains (sweat, blood, food). The enzymes break down protiens and fats. Because the enzymes are temperature dependent they need a wash of no more than 40 o C

Surface Catalysts and Collision Theory One main type of catalyst works by absorbing the reactant molecules onto a solid surface. This process of adsorption brings the reacting molecules closer together.

Nickel Catalyzed Hydrogenation Finely divided nickel is used as a catalyst to add hydrogen to unsaturated hydrocarbons.

Collision Theory If a reaction only occurs when particles collide with the surface of a solid…. – Then it follows that the more surface area you have, the more collisions you have, the faster the reaction is. This also applies if; – solutions are more concentrated – Gases are more compressed Increasing the temperature increases the speed the particles are moving again increasing the probability of collision.

Activation Energy Not every collision produces a reaction. A certain amount of energy needs to be present to break the bonds, E A A catalyst increases the rate of reaction by reducing the E A

Activity Page 216 Exercise 7.4 on worksheet Read about photochemical reactions and do questions on page 218