Reaction Rates

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  • Reaction Rates
    • The rate of reaction is the rate of change of the concentration of a reactant or product with time
    • Effect of Concentration
      • The rate of reaction increases as the concentration increases
        • The reason is that for chemical reactions to occur, an effective collision has to take place between the particles of the reactants
          • If the concentration of one or more reactants increases, then the particles are closer together and so they collide more often
      • For gases, increasing the pressure increases the concentration and therefore increases the rate of reaction for the same reason
    • The Boltzmann distribution and activation energy
      • Even at the same temperature, the energy of particles varies considerably. Some are moving slowly with low kinetic energy, but some are moving faster with more kinetic energy
        • The Boltzmann distribution is a way of showing the number of particles that have particular energies
    • The effect of temperature changes on rates of reaction
      • Raising the temperature increases the rate of reaction. This is because;
        • As the temperature increases, so does the proportion of molecules that have an energy greater than the activation evergy
          • Consequently, when molecules collide, it is more likely that this will successfully lead to a reaction. This is the main reason for the rate increasing with temperature
        • As the temperature increases, the molecules are, on average, moving faster and therefore collide more often. This is generally a much smaller effect
      • We can therefore conclude that as the temperature increases, there is an increased frequency of successful collisions
    • Catalysts
      • A catalyst increases the rate of reaction, but is not used up in the process. It does this by providing an alternative pathway of lower activation energy
      • The effect is that a higher proportion of molecules have an energy greater than the activation energy, and hence, successful sollisions are more successful

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