Rate of Reaction

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Using Collision Theory to Explain Rate of Reaction

A chemical reaction occurs when two or more molecules collide

Not all collisions results in a reaction

For a reaction to result in a collision, it must have sufficient energy, known as the activation energy

The number of collisions that have sufficient energy for a reaction to take place is a small fraction of the overall number of collisions

Increasing how often these successful collisions take place increases the rate of reaction

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Factors which affect the rate of reaction

1. Temperature

2. Pressure

3. Presence of catalysts

4. Surface area to volume ratio of reactants

5. Concentration

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Catalysts

Catalyst: A substance that changes the rate of reaction but emerges chemically unchanged at the end of the reaction

Catalysts can speed up or slow down the rate of reaction

The catalyst does not change the overall energy change. However, since less energy is needed to start the reaction, more reactant molecules have enough energy and so more collisions are successful.

Examples of catalysts in everyday life:

Catalytic converters in car exhausts use platinum and palladium. They lower the activation energy needed to convert harmful gases to harmless gases

Enzymes are large complex protein molecules that act as catalysts in a biological reaction. Alcoholic drinks are produced using an enzyme called yeast

Each enzyme molecule has a part with a specific shape called the active site

The reactant molecules (substrates) fit the shape of the active sit, a bit like a lock and key

Therefore each enzyme catalyses only one reaction and we say that enzymes are specific

Enzymes are sensitive to changes in temperature and pH as these can denature their molecules (make them change shape)