2.2 rates of reaction

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Collision theory

Collision theory states that for a reaction to occur between two molecules an effective collision must take place. An effective collision is a collision that leads to the formation of the products. In order to be an effective collision they must collide in the right way and have enough energy to react. Not all collisions are succesfull but the higher the number of collisions the likelyhood of effective collisions is increased. The rate of a reaction is the measurement of how frequently an effective collision takes place. Anything that increases the rate of collisions will also increase the rate of reaction

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Activation energy

The activation energy of a reaction is the ammount of energy required for a reaction to take place.

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How concentration effects rate

Concentration effects rate as if the concentration increases so does the rate of reaction. This is because with a higher concentration there is an increase in the number of molecules in a given volume. This means they are closer together and the ammount of collisions per unit time is increased.This increases the number of effective collisions.

For a gaseous reaction increasing the pressure has the same effect.

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How Temperature effects rate

As the temperature increases so does the rate. This is because as temperature increases so does the molecules Kinetic Energy so they will move faster and have enough energy to react on collision

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How particle size effects rate

As the particle size of a solid decreases the rate of reaction increases. This is because as the particles get smaller the surface area increases so the molecules are closer together. This increases the number of effective collisions.

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How Catalysts effect the rate of reaction

A catalyst increases the rate of reaction, without itself undergoing a change (stays the same shape and isnt used up). They do this by providing the reaction with an alternative pathway of lower reaction energy.  

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How light effects the rate of reaction

Some reactions like the photochlorination of methene are far more vigorous when carried out under bright light.

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measuring rate of reaction (gas produced)

There are two methods you can use:

1. You could use a gas suringe to measure a change in volume over time. This would have to be carried out at constant pressure and temperature.

2.You could use a pressure gadge to measure a change in pressure over time. Carried out at constant volume and temperature.

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Measuring the rate of reaction (colour change)

There are two methods you can use:

1. If there is a decrease or increase in colour, use a colorimiter to measure the increase/decrease in colour over time.

2. If there is an abrupt colour change use a stopwatch to measure the time it takes. 

Do these at constant temperature and pressure.

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Measuring the rate of reaction (mass decrease)

Use a mass balance to measure the decrease in mass over time

Do this at constant pressure and temperature.

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