Reaction - Magnesium & Hydrochloric Acid (OCR A Level Chemistry A): Revision Note

PAG 9.3: Rate of reaction - magnesium & 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 choice of the size of the gas syringe needs to be considered and the quantities of reagents judged accordingly so that a reasonable volume of gas can be evolved and also recorded

Method

• Check that the apparatus is gas-tight before starting

  • Assemble the setup without magnesium or acid and gently push the plunger

  • If you feel resistance, the system is sealed

• Pour the acid into the conical flask first

  • This allows the magnesium ribbon to be dropped in quickly to start the reaction

• Prepare a range of acid concentrations by serial dilution of the stock acid:

  • Measure a set volume of acid (e.g. 40 cm³) in one measuring cylinder

  • Measure distilled water in another cylinder and mix to make serial dilutions (e.g. 40 cm³ acid + 0 cm³ water, 35 cm³ acid + 5 cm³ water, etc.)

• Use a suitable volume of acid for your flask size (e.g. 40 cm³)

• Start with a concentration no higher than 2.0 mol dm^-3 to ensure the reaction proceeds at a measurable rate

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:

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 

Practical skills reminder

• This practical develops essential skills in measuring the rate of reaction using gas volume.

• It also supports:

  • Setting up a gas syringe to collect data at regular time intervals

  • Using serial dilutions to vary acid concentration

  • Plotting multiple curves on one graph and calculating initial rate from tangents

  • Understanding how to judge suitable acid concentrations for safe, measurable rates