PAG 9 Rates of reaction - continuous monitoring method

Investigating the rate of the reaction between magnesium and different concentrations of hydrochloric acid

Give the equation for the reaction of magnesium and HCl

Mg_(s) + 2HCl_(aq) → MgCl_2(aq) + H_2(g)

Describe the method for the practical

1. Set up a side-arm boiling tube connected to a gas syringe

2. Measure an excess amount of a certain concentration of HCl. Add the acid to the boiling tube

3. Place a piece of magnesium ribbon of set mass in the boiling tube and quickly stopper in place

4. Start the stopwatch immediately.

5. Record the volume of gas every 10 seconds until the reaction is over and the volume no longer changes

6. Repeat with other concentrations of HCl

Explain how the data for this experiment is processed

1. Set up a spreadsheet with appropriate column headings to record time (s) and volume of H₂ (cm³) using each concentration fo HCl

2. Enter data recorded from the experiments

3. Use the spreadsheet program to plot the graphs of gas volume against time. Plot all experiments on the same graph

Explain how this data is analysed

1. Work out the initial rate (H₂ volume increase) at each HCl concentration

2. Deduce the order of reaction with respect to HCl

3. Deduce the rate equation for the reaction

One problem with this experiment is that the reaction between Mg and HCl is exothermic.

1. What effect will this have on the results and the curves that you have plotted?

2. Suggest how the experiment could be modified to reduce this problem

1. The reaction rate increases (more frequent collisions) so more gas would be released with time than if the temperature didn’t change

2. Increase the volume of HCl used - as HCl is in excess, the energy change will be spread over a greater volume and temperature increase will be less

Why would measuring the mass of reactants over time be unsuitable for this reaction?

Hydrogen’s mass is far too small