Rates of Reactions

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  • Rates of Reactions
    • Things that affects rates of energy
      • Temperature
      • Pressure of a gas
      • Concentration of a solution
      • Surface area of a solid
      • Catalyst
      • Intensity of radiation (photodissociation reactions)
    • Collision theory
      • At higher concentrations and pressures, the particles are in closer proximity to each other, encouraging more frequent collisions.
      • At higher temperatures, a much higher proportion of colliding particles have sufficient energy to react and more particles are able to overcome the activation enthalpy barrier.
      • With smaller particles of reactant there is a larger surface area on which the reactions can take place, so the greater chance of successful collisions.
      • Heterogeneous catalysts provide a surface where reacting particles may break and make bonds.
    • The effect of temperature
      • Colliding particles must have a total KE at or above Ea if reaction is to occur
      • As the temperature increases, the rate of a chemical reaction also increases. This is because of the distribution of energies among the reacting particles- this distribution is called the Maxwell-Boltzmann distribution.
      • Reactions go faster at higher temperatures because a larger proportion of the colliding particles have the minimum activation enthalpy needed to react.
    • Activation enthalpy
      • Ea is the minimum kinetic energy required by a pair of colliding atoms or molecules before a reaction will occur.
      • An enthalpy profile shows how the enthalpy changes as a reaction proceeds.
    • Catalysts
      • Catalysts work by providing an alternative reaction route with a lower activation enthalpy.
        • Successful collisions can take place at a lower energy- this is called lowering the activationenergy barrier.
      • Heterogeneous catalysts provide a surface on which a reaction may take place, lowering the energy needed for a successful collision.
      • Homogeneous catalysts work by forming an intermediate compound with the reactants.


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