Rates of reaction

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Chemical Ideas 10.1

OCR Salters B F332

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Collision theory

Reactions occur when particles of reactants collide with a certain minimum kinetic energy:

  • at higher concentratiosn and higher pressures, the particles are in closer proximity to each other encouraging more frequent collisions
  • at higher temperature, 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 the chance of successful collisions
  • heterogeneous catalysts provide a surface where reacting particles may break and make bonds

All of the above serve to increase the rate of a chemical reaction.

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Activation enthalpy

The activation enthalpy is the minimum kinetic energy required by a pair of colliding atoms or molecules before a reaction will occur.

An enthalpy profile shows hwo the enthalpy changes as a reaction proceeds.

(http://www.avogadro.co.uk/definitions/ae.gif)

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