5.1.1 How fast?

• Rate of reaction is the change in concentration of a substance in a unit of time, usually given in mol dm3s-1
• Initial rate is the rate at the start of the reaction where it is the fastest.
• Reaction rates can be calculated from concentration graphs by drawing a tangent to the curve (at different times) and calculating the gradient.
• Rate equation r= k[A]m [B]n

  • r is the rate.
  • Square brackets with [A] is the concentration.
  • k is the rate constant.
  • m and n are reaction orders. The total order of a reaction is worked out by adding all the individual orders together.
• Orders have nothing to do with stochiometric coefficients in the balanced equation. They are worked out experimentally.
• Calculating orders from initial rate data:

  • Zero order - Concentration of A has no effect on the rate of reaction.
  • First order - Rate of reaction is directly proportional to the concentration of A.
  • Second order - Rate of reaction is proportional to the concentration of A squared.

  
• Continuous rate data is the data from one experiment where the concentration of a substance is followed throughout the experiment.

  • Calculated by plotting data and calculating successive half-lives (half-lives always constant = first order reaction, half-lives rapidly increase then the order is second)
• Rate constant:

  • Units ok k depend on the overall order of reaction - must be worked out by the rate equation.
  • Value of k is independent of concentration and rime - its at a constant fixed temperature.
  • Value of k refers to a specific temperature and it increases if we increase the temperature.
• Rate constants:

  • First order, units of k is s-1
  • Second order, units of k is mol-1 dm3 s-1
  • Third order, units of k is mol-2 dm6 s-1