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 MnO2 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 CO2, N2 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 45oC– 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 40oC

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, EA

• A catalyst increases the rate of reaction by reducing the EA

Activity

• Page 216• Exercise 7.4 on worksheet• Read about photochemical reactions and do

questions on page 218