5.5.4 Initial Rates Method

• The initial rates method determines the rate at the very start of the reaction when t = 0
• The initial rate can be found by:
  • Drawing a concentration-time graph
  • Adding a tangent at t = 0
  • Calculating the gradient of the tangent
• One example of measuring the initial rate could be the reaction of calcium carbonate with hydrochloric acid 
  • The volume of carbon dioxide produced with different concentrations can be measured against time
  • The results are then plotted onto a graph 
  • The tangent at t = 0 is then added
  • The gradient of the tangent can then be calculated to give the initial rate of the reaction

Answer

• • Using the tangent drawn:
  • Gradient 1.05 mol dm^-3 s^-1 
  • Therefore, the initial rate of reaction is 1.05 mol dm^-3 s^-1 

Clock reactions

• Clock reactions are a more convenient way of measuring the initial rate of reaction using a single measurement
• The time taken, t, for a specific visual change in the reaction to occur is measured
  • These changes could be a colour change or formation of a precipitate
• The major assumption that clock reactions depend on is:
  • That there is no significant change in the rate of reaction between the start of the reaction and the time when the measurement is taken
• The initial rate is then proportional to  

• The iodine clock experiment is a common clock reaction
  • This is covered in more detail in the PAG notes
• Here is a set of typical results for the iodine clock reaction:

Specimen results for the iodine clock reaction table

• The initial rate is a relatively simple calculation
  • E.g. For a concentration of 0.01515 mol dm^-3, the rate is  = 0.025 s^-1 
• A closer look at the results shows that as the concentration doubles, the rate also doubles
  • This can be linked back to the reaction being first order 

• There are limitations to the accuracy of the clock reaction
  • These are, again, based on the assumption that the rate of reaction is constant
• As the reaction progresses, the concentration of the reactants decreases - typically on a curve
  • Therefore, when the time measured for the reaction to occur is short then there is a higher chance that the initial rate calculated will be closer to the true value
  • As the time taken for the reaction to occur, the value of the initial rate will become less accurate
• The initial rate measured during a clock reaction is an estimate