Try this microscale practical investigating the transition elements, complex formation and change in oxidation state

In this experiment, students conduct a series of tests to examine the chemistry of the transition elements on a microscale. In particular, they identify evidence of complex formation and change in oxidation state – two important general characteristics of transition elements.

The practical should take approximately 20 minutes.

Equipment

Apparatus

  • Eye protection
  • Student worksheet (available for download below)
  • Clear plastic sheet (eg ohp sheet)
  • Magnifying glass

Chemicals

Note

Solutions should be contained in plastic pipettes – see the accompanying guidance on apparatus and techniques for microscale chemistry.

  • Potassium chromate, 0.2 mol dm–3
  • Potassium manganate(VII), 0.2 mol dm–3
  • Cobalt nitrate, 0.5 mol dm–3
  • Ammonia solution, 3 mol dm–3
  • Ammonium vanadate(V), 0.2 mol dm–3
  • Hydrochloric acid, 1 mol dm–3
  • Sodium hydroxide, 1 mol dm–3
  • Copper(II) sulfate, 0.2 mol dm–3
  • Iron(II) sulfate, 0.2 mol dm–3
  • Iron(III) nitrate, 0.2 mol dm–3
  • Silver nitrate, 0.2 mol dm–3
  • Potassium thiocyanate, 0.1 mol dm–3
  • Potassium iodide, 0.2 mol dm–3
  • Starch solution (freshly made)
  • Zinc metal granules

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection throughout (splash-resistant goggles to BS EN166 3).
  • Potassium chromate, K2CrO4, 0.2 mol dm–3 is a carcinogen, mutagen and skin sensitiser. It is also toxic to aquatic life. Wear splash-proof eye protection if transferring large amounts. Avoid skin contact.
  • Potassium manganate(VII), 0.2 mol dm–3 is hazardous to the aquatic environment. Avoid direct contact and store in the dark, stains glass, plastic, clothing and skin.
  • Ammonia solution, NH3(aq), 3 mol dm–3 is CORROSIVE and a respiratory irritant.
  • Ammonium vanadate(V), NH4VO3, 0.2 mol dm–3 (acidified with sulfuric acid) is a mutagen and very TOXIC if inhaled (but not by other routes).
  • Sodium hydroxide solution, NaOH(aq), 1 mol dm–3 is CORROSIVE.
  • Cobalt nitrate, 0.5 mol dm–3 is a carcinogen, mutagen, reproductive toxin, skin and respiratory sensitiser and hazardous to the aquatic environment.
  • Copper(II) sulfate solution, CuSO4(aq), 0.2 mol dm–3 causes eye damage and is hazardous to the aquatic environment.
  • Silver nitrate, AgNO3(aq), 0.1 mol dm–3 is a skin/eye irritant. Keep separate from organic waste containers.
  • Zinc powder, Zn(s) is FLAMMABLE and hazardous to the aquatic environment.
  • The following are of low hazard:
    • Hydrochloric acid, HCl(aq), 1 mol dm–3
    • Potassium thiocyanate, KSCN(aq), 0.1 mol dm–3
    • Potassium iodide, 0.2 mol dm–3
    • Starch solution
    • Iron(III) nitrate, Fe(NO3)3.9H2O(aq), 0.2 mol dm–3
    • Iron(II) sulfate, FeSO4.7H2O(aq), 0.2 mol dm–3

Procedure

  1. Cover the worksheet with a clear plastic sheet.
  2. Put two separate drops of the solutions of each of the elements from vanadium to zinc in the appropriate boxes. Observe and comment.
  3. Do the experiments for each solution of each element as described below on one of the drops in each box only (the other drop will act as a reference). In each case, observe carefully and try to give explanations for your observations.
    • Vanadium (V): add one drop of dilute hydrochloric acid and a small piece of zinc.
    • Chromium (Cr): add one drop of silver nitrate solution.
    • Manganese (Mn): add one drop of iron(II) solution.
    • Iron (Fe): add one drop of potassium iodide solution. After one minute, add one drop of starch test solution.
    • Cobalt (Co): add one drop of ammonia solution.
    • Copper (Cu): add one drop of ammonia solution.
    • Zinc (Zn): add two drops of sodium hydroxide solution.

Questions for students

  1. Which element among the ones that you have tested does not behave as a transition element?
  2. Why is this?

Teaching notes and expected observations

Vanadium

Bubbles (of hydrogen) are seen. The yellow colour of the ammonium vanadate (vanadium(V) ions) gradually changes (as the vanadium is reduced) to blue due to the formation of the vanadium(IV) ions (VO2+). The colour changes to green due to vanadium(III) ions (V3+) and possibly to lilac due to vanadium(II) ions (V2+) (although this species is a strong reducing agent and is very air-sensitive).

Chromium

A red precipitate of silver chromate(VI) is seen. This is an interesting example of the modification of the colour of a coloured anion (yellow chromate(VI) by a colourless cation (silver(I)).

Manganese

The deep purple colour of the potassium manganate(VII) gradually fades, first to the brown manganese(IV) oxide then to the very pale pink manganese(II) ions.

(Manganese(II) compounds in solution usually appear virtually colourless. However, a bottle of a solid manganese(II) salt – eg the sulfate – is pink.)

Iron

A yellowish colour (due to iodine) starts to form as the iron(II) oxidises the iodide. Addition of starch produces the characteristic intense blue-black colour of the starch– iodine complex.

Cobalt

The addition of one drop of ammonia gives a deep green precipitate. Addition of further ammonia gives a green or brown solution.

Copper

The addition of ammonia gives a light blue precipitate of copper(II) hydroxide together with the deep blue tetra-amminocopper(II) ion.

Zinc

A white precipitate of zinc hydroxide is observed. (Zinc is not a transition metal because it only has one oxidation state in its compounds and the Zn2+ ion has a full d-sub-shell.)

 

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