9.3

Question 1

9. 3 The rate of reaction of magnesium and hydrochloric acid- To use appropriate measurements to monitor the rate of reaction using mass lost, and gas collection.
   - To use appropriate software to process data

Answer 1

You will investigate the rate of the reaction between magnesium and different concentrations of hydrochloric acid.
Mg(s) + 2HCl (aq) → MgCl2(aq) + H2(g)

In each experiment, the hydrochloric acid is in excess. From the results, you will be able to determine the order with respect to the acid.

Question 2

Magnesium ribbon

Answer 2

Flammable

Question 3

2 mol dm–3 HCl(aq)

Answer 3

Currently no hazard classification at this concentration

Question 4

Procedure

Answer 4

1. Using a measuring cylinder, measure 20 cm3 of 2 mol dm–3 hydrochloric acid. Add the acid to the side-arm boiling tube.
2. Place about 0.08 g of Mg in the side-arm boiling tube and quickly put the stopper in place. Start timing immediately.
3. Taking readings of volume every 10 seconds until the reaction is over and the volume no longer changes.
4. Plot a graph of volume H2/cm3 on the y axis against time/s, on the x axis. Draw a suitable curve on the graph to summarise the results.
5. After you have carried out the experiment once,
6. Repeat the procedure a further three times to see the effect of changing the concentration of the hydrochloric acid. You can easily do this by halving the concentration of the HCl (aq) before it is added.
7. Add your new results to the graph.
8. Clearly label each curve.

Question 5

Analysis of results1.

Answer 5

Work out the initial rate, in terms of the increase in H2 volume, at each HCl concentration using a tangent.

2. Deduce the order of reaction with respect to HCl.
3. Deduce the rate equation for the reaction

Question 6

1. One problem with this experiment is that the reaction between Mg and HCl is exothermic.
   (a) What effect will this have on the results and the curves that you have plotted?
   (

Answer 6

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

Question 7

1. One problem with this experiment is that the reaction between Mg and HCl is exothermic.b) Suggest how the experiment could be modified to reduce this problem.

Answer 7

b) Increase the volume of HCl used. As HCl is in excess, the energy change will be spread over a greater volume and the temperature increase will be less.

Question 8

2. Suggest another way of analysing the results to determine the order with respect to HCl.

Answer 8

2. Measure the half-life. If the half-life is constant, the reaction is first order with respect to HCl.

Question 9

3. In reactions that form a gas, the rate of reaction can also be following by measuring the mass of the reactants with time. Why would this reaction be unsuitable for this method

Answer 9

3. Hydrogen has such a small mass that the mass loss will be very small.

Question 10

note

Answer 10

In each experiment, the hydrochloric acid is in excess. From the results, you will be able to determine the order with respect to the acid

Question 11

Equipment

Answer 11

Each student or group will require: 
•	Balance to 2 decimal places
•	25 cm3 measuring cylinder
•	stopwatch
•	boiling tube with side arm and bung
•	Glass syringe
•	Clamp stand, boss and clamp