Edexcel A Level Chemistry:复习笔记5.5.4 Initial Rates Method

Initial Rates Methods

 

  • 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

Worked Example

The concentration-time graph for the reaction of calcium carbonate with three different concentrations of hydrochloric acid is shown

8.1.1-Initial-Rates-Method-Results-Graph

Calculate the initial rate of reaction for the reaction of calcium carbonate with 2.0 mol dm-3 hydrochloric acid

 

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

8.1.5-Iodine-clock-reaction-results-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

Exam Tip

  • You can be expected to work with experimental data to deduce the order of reaction with respect to specific reactants
  • This is done in the same that has been previously discussed in 5.5.2 Rate Equations

 

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