Rates of Reaction

Edited by Jamie (ScienceAid Editor)

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Collision Theory

This is the theory as to why chemical reactions occur.

It says that for a reaction to take place the particles must collide with sufficient energy to bond. Imagine lots of particles whizzing about, and bouncing into the container and each other. Those that bump into each other with enough energy will bond.

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The amount of energy they need to react is the activation energy which is the minimum energy required for a reaction to occur.

Maxwell-Boltzmann Distribution

Maxwell-Boltzmann distribution curve

This is a Maxwell-Boltzmann distribution curve. On the y (vertical) axis is the number or fraction of molecules and on the x (horizontal) axis is the amount of energy. Therefore the curve shows the proportion of particles that have a particular energy. The area underneath the curve show the total number of molecules with energy.

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Marked on a particular point on the graph is the activation energy, therefore the area shaded in green shows the fraction of particles that have energy more than or equal to the activation energy. Hence, these are the particles that will react.

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The blue curve (T1) is at a lower temperature and T2 at a higher. This graph shows that when the heat in a reaction is increased the number of particles with the activation energy or higher is greater. So the rate of reaction increases.

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Factors Affecting Rate

As well as temperature there are other factors that affect the rate of reaction. These are concentration, surface area and catalysts.

concentration

The graph above shows how the concentration of the products and reagents varies over the course of a reaction: as the concentration of the reactants drops, the concentration of the products climbs since they are directly related.

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surface and rate of reaction

The graph above shows how surface area affects rate of reaction. The top line shows the reaction when a powder is used. The rate of reaction is greater because there are more available particles to collide with. However when a lump of substance is used the reaction rate is less because not as many particles are available.

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A catalysts is a bit different, because rather than make collisions more likely it makes successful collisions more likely by lowering the activation energy without itself being used up. If you think of it in terms of the Maxwell-Boltzmann distribution then the Ea is moved to the left and more particles are in the shaded region.

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Categories : Physical

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