rate

• rate equations tell you how rate is affected by the concentrations of reactants
• Rate = k[A]^m [B]^n

  • rate = mol dm-3 s -1
  • K = constant
  • m, n = reaction orders
  • [A] and [B] = concentrations (mol dm-3)
• The total order for a reaction is worked out by adding all the individual orders together (m+n)
• 0 order in respect of reagent = rate is the same but concentrations change

  • when writing calculations you must always eliminate anything to power of 0 as it equals 1

  
• 1st order in respect to reagent = rate and concentration are proportional

  • when writing calculations eg [A]^1 you must not write the 1 so it would only be [A]

  
• 2nd order in respect to reagent = rate proportional to the square of the concentration of one reactant eg. [NO]^2 ........ 2nd order so rate x4

  
• The value of k refers to a specific temperature and it increases if we increase temperature - but it isn't effected by the concentrations
• the bigger k is the faster the reaction
• moles wouldnt effect reaction order as order can only be worked out by rate equation or carrying out an experiment
• Calculating a value for k using initial rate data

  k = r / [X]m [Y]n
• INITIAL RATES METHOD

  1. series of experiments is carried out at constant temperature each with a different combination of initial concentrations of reactants
  2. The experiments ensure the concentration of only one species varies - the rest stay the same.
  3. plot concentration time graph and gradient of tangent is initial rate
  4. By measuring the initial rate, the concentrations of all substances in the reaction mixture are known exactly at this time.
• iodine clock experiment = reaction where you can time how long it takes for a colour change to occur

  1. add sodium thiosulfate and starch (acts as an indicator)
  2. sodium thiosulfate reacts immediately with I2 that is produced
  3. when there is no more sodium thiosulfate left then I2 reacts with starch = blue/ black colour
  4. varying the concentration of I2 and keeping everything else constant will result in the time taken for blue/black colour to appear changing ---> work out order of the reaction