AQA A Level Chemistry

Revision Notes

8.1.1 Required Practical 7

Test Yourself

Required Practical 7

Required Practical 7: Measuring the rate of a reaction

  • There are two ways to measure the rate of reaction
    • By an initial rate method
    • By a continuous monitoring method

An initial rate method: The reaction between magnesium and hydrochloric acid

  • This reaction can be used to investigate the effect of varying the concentration of the acid while keeping the temperature constant
  • When a gas is released in a reaction you can either try to measure the volume of gas given off or the mass change in the reaction flask
    • However, in this case the gas hydrogen is too low in density so the mass change will be far too small to register on a laboratory balance

  • Volume can be measured either by displacement of water into an inverted measuring cylinder or by using a gas syringe

 Gas collection Set Up, downloadable IB Chemistry revision notes

The rate of reaction between magnesium and dilute hydrochloric acid can be measured using a gas syringe and stopwatch

Steps in the procedure

  • Before you begin you need to check your apparatus is gas tight
    • This can be done by assembling everything without the acid or magnesium and trying to move the plunger
    • If you feel resistance the apparatus is gas tight

  • The acid will go in first because it is quicker to drop a piece of magnesium ribbon in than to pour in the acid
  • To vary the concentration of the acid you need to dilute it by measuring portions of acid in a measuring cylinder and then portions of distilled water in another measuring cylinder and adding them to the conical flask
  • Choose a suitable volume of acid to match the size of the flask, e.g. 40 cm3
  • You don't want to use acid that is stronger than 2 mol dm-3 because the reaction will be too fast, so its best to start with 2 mol dm-3 and perform a serial dilution, e.g. 40 cm3(acid) + 0 cm(water), 35 cm(acid) + 5 cm3 (water), etc

Practical tips

  • Make sure the plunger is fully inserted before you start the experiment otherwise you will have a volume error
  • If the magnesium does not look new and shiny, you may need to clean the surface with a bit of sandpaper
  • Make sure the plunger is secure and does not fall out of the barrel if the volume exceeds 100 cm3

Specimen Results

  • Here is a set of typical results for this experiment

Rate of reaction between magnesium and acid results table

Initial Rates Method- Results Table, downloadable AS & A Level Chemistry revision notes

Graphing the resultsInitial Rates Method - Results Graph, downloadable AS & A Level Chemistry revision notes

Graph showing typical results for the rate of reaction between hydrochloric acid and magnesium at different concentrations

Analysis

  • The lines of best fit are drawn for each concentration on the same graph
  • A tangent is then drawn starting from (0,0) since this method is to find the initial rate of reaction
  • The gradient of the tangent is determined which gives the rate of reaction
  • In the example above, the rate of reaction for 2.0 mol dm-3 acid is
    • Gradient equals space fraction numerator straight capital delta y over denominator straight capital delta x end fraction space equals space 40 over 38 space equals1.05 mol dm-3 s-1 

A continuous monitoring method: 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 beings 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:

Visible Spectroscopy Calibration Curve, downloadable AS & A Level Chemistry revision notes

A calibration curve showing the relationship between colour absorbance and concentration

 

Steps in the procedure

  • 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

Continuous Monitoring - Volumes Table, downloadable AS & A Level Chemistry revision notes 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

  • Here is a set of typical results for this experiment

Continuous Monitoring - Results Table, downloadable AS & A Level Chemistry revision notes

Graphing the resultsContinuous Monitoring -Results Graph, downloadable AS & A Level Chemistry revision notes

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

Analysis

  • 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 equals space fraction numerator capital delta y over denominator capital delta x end fraction space equals space fraction numerator 0.0069 over denominator 580 end fraction space equals 1.19 x 10-5 mol dm-3 s-1 
    • The gradient is the rate of reaction at that point

Exam Tip

Whichever rates experiments you carry out, make sure you can

  • describe the steps in the procedure
  • name all the apparatus used
  • draw data tables which include headings and units
  • draw graphs showing labels, units and best fit lines
  • determine an initial gradient or at any point in the curve

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