11C2 Rates of Reaction

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Rates of Reaction; Introduction

  • Reactions take place when particles collide with a certain amount of energy.
  • The minimum amount of energy that is needed for this to happen, is called the activation energy.
  • The rate of reaction depends on two things; the frequency of collisions and the energy with which they collide
  • If the particles collide with less energy than the required activation energy, the particles will simply bounce of each other and no reaction will occur.
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How Temperature Affects the Rate of Reaction

As the temperature of a substance increases, the rate of reaction increases!!

This is because...

  • As the temperature of a substance increases, the energy in the particles increases.
  • This means they move much faster.
  • This increases the likeliness of the particles colliding with each other.
  • So when the particles do collide, they do so with much more energy, speeding up the rate of reaction.
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How Surface Area Affects the Rate of Reaction

Reactions involving solids only take place on the surface of the solid.

If the solid has a greater surface area (e.g being split up into several pieces), the reaction will speed up

This is because, there is an increased area for the reactant particles to collide with.

The smaller the pieces, the larger the surface area (contrary to what you may think, but it does make sense- smaller but less space, so likely to bump into each other).

This means more collisions and a greater chance of reaction.

The collisions are more frequent, but not harder!

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How Concentration Affects the Rate of Reaction

Low Concentration:

  • less particles
  • lower chance of collisions
  • slower rate of reaction

High Concentration

  • more particles (in the same volume)
  • higher chance of collisions
  • faster rate of reaction

This is because...

  • The higher the concentration, the higher the number of particles in the same volume.
  • This means that the chance of collision is higher, as there are more particles to bump into
  • The particles then collide much more frequently, speeding up the rate of reaction
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Catalysts, are substances which change the rate of reaction without being used up in the reaction.

Catalysts do not produce more products- they produce the same amount, just quicker

Different catalysts work in different ways, but most lower the reaction energy.

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Exothermic Reactions

Exothermic reactions release energy into its surroundings  - they get hotter!

think of it like being released from your ex, if they are your ex, you have been released from them!


  • making ice cubes
  • rusting iron 
  • reactions between water and strong acids
  • nuclear fission
  • condesation of rain from water vapour
  • crystalising liquid salts (e.g in hand warmers)
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Endothermic Reactions

Endothermic reactions absorb energy from their surroundings - they get colder!

examples include

  • melting ice cubes
  • baking bread
  • separating ion pairs
  • evaporation of water
  • photosynthesis
  • splitting apart a gas molecule
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