Catalysts
A A substance that changes the rate of a chemical reaction without being changed by the reaction itself. is a substance that:
- increases the rate of a reaction
- does not alter the A substance formed in a chemical reaction. of the reaction
- is unchanged chemically and in The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). at the end of the reaction
Only a very small mass of catalyst is needed to increase the rate of a reaction. However, not all reactions have suitable catalysts.
Catalysts only affect the rate of reaction - they do not affect the The mass of product made in a chemical reaction. The percentage yield is a measure of the yield obtained compared to the maximum possible yield. of the reaction. A catalysed reaction produces the same amount of product as an uncatalysed reaction but it produces the product at a faster rate.
Different substances catalyse different reactions. The table describes three common catalysts.
| Catalyst | Reaction catalysed |
| Iron | Haber process (making ammonia) |
| Vanadium (V) oxide | Contact process (a stage in making sulfuric acid) |
| Manganese dioxide | Decomposition of hydrogen peroxide (produces water and oxygen) |
| Catalyst | Iron |
|---|---|
| Reaction catalysed | Haber process (making ammonia) |
| Catalyst | Vanadium (V) oxide |
|---|---|
| Reaction catalysed | Contact process (a stage in making sulfuric acid) |
| Catalyst | Manganese dioxide |
|---|---|
| Reaction catalysed | Decomposition of hydrogen peroxide (produces water and oxygen) |
Notice that these catalysts are transition metals or compounds of transition metals.
How catalysts work
A catalyst provides an alternative The sequence of reactions needed to produce a desired product from a particular set of raw materials. that has a lower The minimum amount of energy that colliding particles must have for them to react. than the uncatalysed reaction. This does not change the frequency of collisions. However, it does increase the frequency of successful collisions because a greater proportion of collisions now exceeds this lower activation energy.
The effect of a catalyst on the activation energy is shown on a chart called a Chart showing how the energy of reactants and products changes during a reaction.. This shows how the The capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. of the A substance that reacts together with another substance to form products during a chemical reaction. and products change during a reaction.
Enzymes
An A protein which catalyses or speeds up a chemical reaction. is a biological catalyst. Enzymes are important for controlling reactions in cells. They are also important in industry. The use of enzymes allows some industrial reactions to happen at lower temperatures and pressures than traditionally needed.
A unicellular fungus used in the brewing and baking industries. is a single-celled A large group of eukaryotic organisms that contain single celled yeasts, moulds and mushrooms.. The enzymes in yeast are used to produce wine, beer and other alcoholic drinks by Type of anaerobic respiration carried out by yeast. of sugars.