Required Practical 7 (AQA A Level Chemistry)

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

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 cm³ (water), 35 cm³ (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

Graphing the results

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 1.05 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:

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

• 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

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  1.19 x 10^-5 mol dm^-3 s^-1 
  • The gradient is the rate of reaction at that point