Catalysis of the reaction between sodium thiosulfate solution and iron(III) nitrate solution

Read our standard health & safety guidance

Lesson organisation

Students can do this experiment in pairs or small groups. If each pair of students attempts this experiment, large volumes of both the iron(III) nitrate solution and the sodium thiosulfate solution will be required.

Apparatus and chemicals

Eye protection

Each pair of students will require:

Stopclock or timer
Dropping pipette. Use the type of teat pipette usually fitted to Universal indicator bottles that does not allow squirting
Glass measuring cylinder (100 cm³)
Measuring cylinder (50 cm³)

Students need access to: (see note 1)

Copper(II) sulfate solution, 0.1 mol dm^-3 (Low hazard at this concentration), one drop
Nickel(II) sulfate solution, 0.1 mol dm^-3 (Harmful), one drop
Cobalt(II) chloride solution, 0.1 mol dm^-3 (Toxic), one drop
Iron(II) sulfate solution, 0.1 mol dm^-3 (Low hazard at this concentration), one drop
Iron(III) nitrate solution, 0.1 mol dm^-3 (Low hazard at this concentration), 250 cm³ (see note 1)
Sodium thiosulfate solution, 0.1 mol dm^-3 (Low hazard) 250 cm³

Technical notes

Copper(II) sulfate solution (Low Hazard at this concentration) Refer to CLEAPSS Hazcard 27C and Recipe card 19
Nickel(II) sulfate solution (Harmful) Refer to CLEAPSS Hazcard 65B and Recipe card 71
Cobalt(II) chloride solution (Toxic) Refer to CLEAPSS Hazcard 25 and Recipe card 71
Iron(II) sulfate (Low hazard at concentration used) Refer to CLEAPSS Hazcard 55B and Recipe card 40
Iron(III) nitrate (Low hazard at concentration used) Refer to CLEAPSS Hazcard 55C
Sodium thiosulfate solution (Low hazard) Refer to CLEAPSS Hazcard 95C and Recipe card 67

1 If iron(III) nitrate is not available, iron(III) chloride, 0.1 mol dm^-3 (Low hazard at this concentration), or iron(III) ammonium sulphate, 0.1 mol dm^-3 (Low hazard at this concentration), can be used instead.

2 It is important that the concentrations of the solutions are accurate. If higher concentrations are used the experiment proceeds too quickly. It is useful if each group of students has access to their own supply of solutions, this prevents contaminating the bulk supply. The catalyst solutions can be provided in dropping bottles and the iron(III) nitrate and sodium thiosulfate solutions in 500 cm³ beakers.

Procedure

HEALTH & SAFETY: Wear eye protection

a Draw a cross on a piece of scrap paper and put it underneath the 100 cm³ measuring cylinder so it can be seen when looking down the cylinder from the top.

b Using the 100 cm³ measuring cylinder, measure 50 cm³ of sodium thiosulfate solution. Place the cylinder back on top of the cross.

c Using a 50 cm³ measuring cylinder, measure 50 cm³ of iron(III) nitrate solution.

d Pour the iron(III) nitrate solution into the sodium thiosulfate solution, and start the timer.

e Look through the reaction mixture from above until the cross can first be seen. Stop the timer and record the time.

f Repeat this experiment, but add one drop of catalyst to the iron(III) nitrate solution before mixing. Test the various catalysts in the same way.

g Record the times for no catalyst and all the catalysts tested.

Teaching notes

If too much catalyst is used then the reaction proceeds instantaneously. It is important that students only use one drop of catalyst.

It is possible to set up this experiment using a light sensor and data logging. The data logging software should show the colour change occurring on a graph. This gives more information than the standard end point approach. The rate of change can be measured from the slope of the graph or the time taken for the reaction to occur.

Student questions
Here are some possible questions to ask students.

1 Which is the best catalyst?

2 Why were only very dilute solutions of the catalysts used?

3 Could you slow the reaction down? If so, how?

Health and safety checked, February 2008