5.1.4 Investigating Reaction Rates

• 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 (similar to one of the PAG9 practicals)
  • 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

• Continuous monitoring involves collecting experimental data throughout the course of a reaction to plot a concentration-time graph 
• Two of the most common ways to collect this data are by:
  • Measuring the volume / amount of gas evolved over time
  • Measuring the mass of reactants lost over time
• Another alternative method involves the use of colorimetry:

The iodination of propanone

• The iodination of propanone provides a suitable experiment in which the rate of reaction can be measured throughout the reaction by using a colorimeter
• The reaction is carried out using a catalyst of dilute sulfuric acid
• The iodine decolourises during the reaction as it turns into iodopropanone and hydrogen iodide:

CH₃COCH₃   +   I₂  → CH₃COCH₂I   + HI

• The colorimeter measures colour absorbance which is proportional to the concentration of the coloured species
• Before the investigation begins it is necessary to  measure the absorbance of a set of standard solutions of iodine and obtain a calibration curve
• For example, here is a calibration curve for a transition metal ion that allows you to convert colorimeter readings into concentrations:

A calibration curve showing the relationship between colour absorbance and concentration

• The colorimeter uses very small volumes of solutions, so four burettes can be filled with solutions of 0.02 mol dm^-3 iodine, 1.0 mol dm^-3 propanone and 1.0  mol dm^-3 sulfuric acid and distilled water
• By varying the volumes of solutions while maintaining a constant total volume with the use of distilled water, you can obtain a number of different concentrations
• The solutions are measured into a small beaker, leaving the iodine in a separate beaker - this starts the reaction, so it can be added when you start a timer or stop watch
• The iodine is added to the other liquids, the contents mixed and then quickly transferred into the cuvette (small receptacle) and the colorimeter / data logger started

The set up for using a colorimeter and data logger to continuously measure the rate of reaction

• A typical set of volume compositions could be as follows:

Volume Compositions Table

 Practical tip

• Choose a filter that gives the strongest absorbance for the solution you are using - this will be the complementary colour to the colour of the solution under investigation

Specimen results table for the iodination of propanone

Graph showing the change in concentration of iodine during the course of the reaction

Measuring rate

• To find the rate of reaction at any point, a tangent is drawn and the gradient is determined
• The gradient gives the rate of reaction
• For example, in the graph above, the rate of reaction at 300 seconds can be found
  • A vertical line is drawn from the 300 s mark until it meets the curve, then a tangent is drawn
  • Gradient 1.19 x 10^-5 mol dm^-3 s^-1 

• • The gradient is the rate of reaction at that point