Reactions and catalysts
A A substance that changes the rate of a chemical reaction without being changed by the reaction itself. is a substance that:
- speeds up reactions
- is not used up during the reaction (its mass is the same at the start and end of the reaction)
- is chemically unchanged after the reaction has finished
Only a very small amount of catalyst is needed to increase the rate of the reaction between large amounts of reactants. Different catalysts are needed to catalyse different reactions.
The table summarises some catalysts and the reactions they catalyse:
| Catalyst | Reaction catalysed |
| Iron | nitrogen + hydrogen → ammonia |
| Manganese dioxide | hydrogen peroxide → water + oxygen |
| Vanadium pentoxide | sulfur dioxide + oxygen → sulfur trioxide |
| Catalyst | Iron |
|---|---|
| Reaction catalysed | nitrogen + hydrogen → ammonia |
| Catalyst | Manganese dioxide |
|---|---|
| Reaction catalysed | hydrogen peroxide → water + oxygen |
| Catalyst | Vanadium pentoxide |
|---|---|
| Reaction catalysed | sulfur dioxide + oxygen → sulfur trioxide |
Catalytic converters
The exhaust systems of cars are fitted with catalytic converters. These help reduce the release of toxic gases from the exhaust pipe. They contain platinum and rhodium, which act as catalysts. The reactions in catalytic converters:
- convert carbon monoxide (which is toxic) into carbon dioxide
- convert nitrogen oxides (which cause acid rain) into nitrogen and oxygen
Platinum and rhodium are very expensive metals, but they are spread out very thinly in the catalytic converter – very little is needed and they are not used up.
Enzymes
A protein which catalyses or speeds up a chemical reaction. are biological catalysts.
They occur naturally in the body and help with The breakdown of large insoluble food molecules to smaller soluble ones..
They are used in the production of alcohol (zymase) and digestion of food (amylase). Enzymes are specific. This means that they can only catalyse one reaction.
