Initial-Rate Method (Edexcel International A Level Chemistry): Revision Note

Initial-Rate 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

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 Core Practical 9b: Investigating a Clock Reaction

• 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