Required Practical 7 (AQA A Level Chemistry): Revision Note

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 iodination of propanone

• The iodination of propanone is 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:

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, 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 several 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 stopwatch

• 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 is started

• 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

• Here is a set of typical results for this experiment

Graphing the results

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  1.19 x 10^-5 mol dm^-3 s^-1 

  • The gradient is the rate of reaction at that point

A continuous monitoring 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, 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 the displacement of water into an inverted measuring cylinder or by using a gas syringe

Steps in the procedure

• Before you begin, you need to check that 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 it's best to start with 2 mol dm^-3 and perform a serial dilution, e.g. 40 cm³(acid) + 0 cm³ (water), 35 cm³ (acid) + 5 cm³ (water), etc

Practical tips

• Make sure the plunger is fully inserted before you start the experiment, or 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

Graphing the results

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 used 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 1.05 mol dm^-3 s^-1