Rates of reaction

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The effect of catalysts

Catalysts change the rates of chemical reactions. Most catalysts are used to speed up reactions.

The catalyst is left at the end of the reaction and so it can be used over and over again.

Catalysts work by lowering the activation energy of a reaction so that more collisions result in a reaction.

Although some catalysts are expensive, they can be economical because they do not need replacing very often.

They can also reduce the energy costs and time needed for a reaction. Catalysts often work with only one type of reaction and so different reactions need different reactions need different catalysts.

Finding new and better catalysts is a major area of research for the chemical industry.

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The effect of concentration

If the concentration of a solution is increased there are more particles dissolved in the same volume.

This means the dissolved particles are closer together and so they collide more often.

Increasing the concentration of a reactant therefore increases the rate of a reaction.

In a similar way, increasing the pressure of a gas puts more molecules into the same volume, and so they collide more frequently.

This increases the rate of reactions that have gases as reactants.

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The effect of temperature

Increasing the temperature increases the speed of the particles in a reaction mixture.This means they collide more often, which increases the rate of reaction. As well as colliding more frequently they collide with more energy, which also increases the rate of reaction.

Therefore, a small change in temperature has a large effect on reaction rates. At ordinary temperatures a rise of 10°C will roughly double the rate of many reactions, so they go twice as fast.

A decrease in temperature will slow reactions down, and a change of 10°C will double the time that many reactions take. This is why we refrigerate or freeze food so it stays fresh for longer.

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Collision Theory

The collision theory states that reactions can only happen if particles collide. The particles have to collide with enough energy to change into new substances. The minimum energy they need is called the 'activation energy'.

Factors that increase the chance of collisions or the energy of the particles will increase the rate of the reaction. Increasing the temperature, concentration of solutions, pressure of gases, surface area of solids and using a catalyst will increase the rate of a reaction.

Reactions happen when particles collide with enough energy to bring about a change.

Factors that affect the frequency or energy of collisions will change the rate of a reaction.

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How Fast?

The rate of a reaction measures the speed of a reaction or how fast it is. The rate can be found by measuring how much of a reactant is used up or how much of a product is formed in a certain time.

You could also measure the time for a certain amount of rectant to be used or product to be formed.

rate=amount of reactant used / time


amount of product formed / time

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Amritpal Mundra


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