Collision Theory and Energy Transfer

Reaction rates are explained by collision theory. The rate of reaction depends on how often and how hard the reacting particles collide with each other. In order to react they have to collide hard enough (with enough energy)
More collisions increase the rate of reaction
1. The effects of temperature concentration and surface area on the rate of reaction can be explained in terms of how often the reacting particles collide successfully
  1. Higher temperature increases collisions
    1. When the temperature is increased the particles all move quicker. If there moving quicker they are going to collide more often
  2. Higher concentration (or pressure) increases collisions
    1. If a solution is made more concentrated it means there are more particles of reactant knocking about between the water molecules which makes collisions between the important particles more likely. In a gas increasing the pressure means the particles are more squashed up together so there will be more frequent collisions
  3. Larger surface area
    1. If one of the reactants is a solid then breaking it up into smaller pieces will increase the total surface area. This means the particles around it in the solution will have more area to work on so theyll be more frequent collisions
Collision theory and catalysts
1. Increasing the temperature causes faster reactions
  1. Reactions only happen if the particles collide with enough energy.
    1. The minimum amount of energy needed by the particles to react is known as the activation energy.
    2. At a higher temperature there will be more particles colliding with enough energy to make the reaction
2. Catalysts speed up reactions
  1. A catalyst is a substance which speeds up a reaction without being changed or used up
  2. A solid catalyst works by giving the reacting particles a surface to stick to. This increases the number of successful collisions and so speed the reaction up
  3. Catalysts help reduce cost in industrial reactions
    1. The plant doesnt need to operate for as long to produce the same amount of stuff
    2. Allows the reaction to operate at a much lower temperature this reduces the energy used up in the reaction (energy cost) which is good for sustainable development
    3. Disadvantages
      1. Very expensive to buy.
      2. Different reactions use different catalysts so if you make more than one product at your plant youll probably need to buy different catalysts
      3. Catalysts can be poisoned by impurities so they stop working. This means you have to keep the reaction very clean
Energy transfer in reactions
1. Whenever chemical reactions occur energy is transferred to or from the surroundings
2. In an exothermic reaction heat is given off
  1. An exothermic reaction is one which transfers energy to the surroundings usually in the form of heat and usually shown by a rise in temperature
    1. e.g. Combustion gives of a lot of heat
  2. Neutralisation reactions (acid + alkali) are also exothermic
  3. Many oxidation reactions are exothermic
  4. Have lots of everyday uses. E.g. Hand warmers use the exothermic oxidation of iron in air to generate heat.
3. In an endothermic reaction heat is taken in
  1. An endothermic reaction is one which takes in energy from the surroundings usually in the form of heat and is usually shown by a fall in temperature
  2. Less common
  3. Thermal decomposition
    1. Heat must be supplied to make calcium carbonate decompose to make quicklime CaCO3 to CaO + CO2
  4. Everyday uses include some sports injury packs use endothermic reactions they take heat in and the pack becomes very cold
4. Reversible Reactions can be endothermic and exothermic
  1. In reversible reactions if the reaction is endothermic in one direction it will be exothermic in one direction, it will be exothermic in the other direction. The energy absorbed by the endothermic reaction is equal to the energy released during the exothermic reaction. A good example is the thermal decomposition of hydrated copper sulfate

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	Created by sian.allison about 10 years ago