10.2

Question 1

10.2 Rates – Thiosulfate and acid STUDENT

• To use appropriate measurements to monitor the rate of reaction using an initial rate method.

Answer 1

Aqueous thiosulfate ions, S2O32–(aq), react with aqueous acid, H+(aq):

S2O32–(aq) + 2H+(aq)  S(s) + SO2(aq) + H2O(l)

The rate of this reaction can be followed by observing the precipitation of sulfur.

From the start of each experiment, you will time how long it takes for a fixed amount of sulfur to appear. The initial rate of the reaction can be determined from this time.

There are two parts to this activity.

• Part 1 You will investigate the effect on the rate of this reaction of changing the concentration of thiosulfate ions, S2O32–, in an aqueous solution of sodium thiosulfate, Na2S2O3(aq).
• Part 2 You will investigate the effect on the rate of this reaction of changing the concentration of hydrogen ions, [H+], in hydrochloric acid, HCl(aq).

Question 2

Sodium thiosulfate, Na2S2O3(aq), 0.150 mol dm–3

Answer 2

Currently no hazard classification

Question 3

Hydrogen chloride, HCl(aq), 2.00 mol dm–3

Answer 3

Currently no hazard classification at this concentration

Question 4

Part 1 – Changing the concentration of thiosulfate ions, S2O32–(aq)

Answer 4

1. On a piece of paper, clearly mark a cross. Alternatively, use the supplied paper with cross.
   You will view this cross through the solution that reacts.
2. Measure out 5.00 cm3 of HCl(aq) into a suitable container.
3. Measure out 50.0 cm3 of Na2S2O3(aq).
4. Add the 50.0 cm3 of Na2S2O3(aq) to the HCl(aq) and immediately start timing.
   • Swirl the mixture thoroughly and place the container over the cross on the paper.
   • Observe the cross from above the container (looking down through the solution).
   • Stop timing when the cross is just no longer visible.
   • Record the time, t, and the temperature.
   CAUTION! SO2 gas is produced in this reaction.
   The contents of the container should be disposed of as directed by your teacher.
5. Carry out further experiments in the same way but varying the concentration of Na2S2O3(aq).
6. For each experiment, the initial rate of reaction can be represented by 1/t.
   The concentration of S2O32–(aq) is directly proportional to the volume of S2O32–(aq) used in each experiment

Question 5

Part 2 – Changing the concentration of hydrogen ions, H+(aq)

Answer 5

Carry out further experiments to find the effect on the reaction rate of changing the concentration of H+(aq) ions.
You should record your methods in an appropriate format.
Record all your readings in an appropriate format.

Question 6

Analysis of your results

1/t  proportional to rate
volume of reactant  proportional to concentration.

Answer 6

1. From your results, plot graphs that would enable you to determine the orders with respect to each reactant.
2. Deduce the rate equation for the reaction between S2O32–(aq) ions and H+(aq) ions.
3. Measure the gradient of the graph for S2O32–(aq) to give a value which is proportional to the rate constant.

Question 7

1. The overall equation for this reaction is
   S2O32–(aq) + 2H+(aq)  S(s) + SO2(aq) + H2O(l)
   From your results, suggest the experimental evidence that this reaction proceeds by more than one step

Answer 7

1. The rate equation includes just one S2O32– ion, which reflects the rate-determining step. As the stoichiometry of the overall equation is different, further steps must take place.

Question 8

Equipment

Answer 8

Learners will be selecting appropriate apparatus for the experiment. It is likely they will require
• beakers (range of sizes)
• test tubes/ boiling tubes
• conical flasks, 100 cm3
• measuring cylinders, range of sizes
• burettes or graduated pipettes, 1 cm3, 2 cm3 and 5 cm3
• stopwatch or stop clock
• They may opt to use colorimeters as an alternative to the cross

Question 9

note

Answer 9

The most suitable glassware to use for measuring out solutions is a burette or a graduated pipette but some learners may initially choose a measuring cylinder. The solutions are best measured out into a beaker or a conical flask. After learners have had an opportunity to choose glassware it might be helpful for them to share the use of burettes or pipettes. Learners calculate the rate of reaction and plot a graph against the volume or concentration of thiosulfate used.