(4) Orders of Reaction

• changing the concentration doesn’t have a simple effect on the rate of reaction because the reactions usually take place in more than one step - some steps are fast and other slow
• how fast the reaction happens depends on the slowest step
• speeding up the fast step makes no difference/ affect on the rate of reaction
• only the slow step will have a significance effect on the rate of reaction
• the slow step of a reaction is known as the rate - determining step
• chemists try to speed up the slow step (rate - determining step)
• mechanism of the reaction = the process by which the reaction occurs, the steps involved in the reaction
• • mechanisms are deduced experimentally - by finding which concentration effects rate of reaction
  • need to do experiments to find which steps are fast which ones slow
  • only change concentration of one reactant at a time
• K is the rate constant - stays the same for every reaction in that experiment
• rate constant (K) can be affected by temperature, pressure, presence of catalyst, surface area
• the rate equation always starts as 'Rate = K[][]'
• square brackets in rate equations mean the concentration of that reactant
• order of rate equations = the superscript number which helps to relate the change in concentration to the actual change in rate of reaction
• overall order of reaction = the sum of all the orders of the reaction
• through experiments you’ll find that the rate of reaction is related to the concentration of the reactants
• Zero Order :

  • an order of 0
  • the concentration of the substance (reactant) does not affect the rate of reaction - is the fast step
  • this reactant does not appear in the rate equation
  • x1
• First Order :

  • an order of 1
  • change to the concentration of the reactant will have the same change on the rate of reaction (eh conc tripled = rate tripled)
  • written in the rate equation
  • xn (number the conc of the reactant has changed by)
• Second Order :

  • an order of 2
  • change in the concentration of the reactant, the change will be square of what will happen to the rate (the reactant is doubles = rate is x4 as 2 is squared)
  • written in the rate equation
  • xn^2