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

This resource accompanies the article Teaching transition metals and complex ions at post-16 in Education in Chemistry, which provides classroom tips, activities and ideas to prevent misconceptions relating to transition elements.

In this practical activity, learners conduct a series of microscale experiments to model sustainable practices in the laboratory as they investigate the chemistry of the transition elements. Learners will use experimental evidence to identify reactions involving complex formation and changes in metal oxidation states. See the Teaching notes and expected observations for each reaction and find the answers to the student questions in the teacher notes download. The experiment will take approximately 20 minutes.

Learning objectives

  1. Develop an understanding of sustainable practices by carrying out a microscale experiment to minimise the use and disposal of toxic substances.
  2. Relate experimental observations to the oxidation state, ligand type and coordination number of transition element compounds.
  • Previews of the Transition metals microscale student sheets, teacher and technician notes

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    Conduct a series of microscale experiments to model sustainable practices in the laboratory using the student sheet, teacher notes (with answers) and technician notes.

Health and safety

Read our standard health and safety guidance and carry out a risk assessment before running any practical. Do the experiment in a well-ventilated laboratory, instruct learners to wear eye protection throughout the practical and see the technician notes for hazards, preparation (including CLEAPSS recipe books) and disposal instructions.

Equipment

  • Safety goggles (to BS EN166 3)
  • Chemical resistant gloves (optional – none of the chemicals require gloves but learners should avoid skin contact with some of the solutions)
  • Student worksheet (page 3 laminated)
  • Clear plastic sheet (eg, acetate sheet overlay if you are not using a laminated worksheet)
  • Magnifying glass

Chemical reagents

  • Acidified potassium dichromate, 0.2 mol dm–3  (if not acidified, add excess sulfuric acid 1 mol dm–3  to the stock solution bottle)
  • Potassium manganate(VII), 0.2 mol dm–3
  • Cobalt nitrate, 0.5 mol dm–3
  • Ammonia solution, 2 mol dm–3
  • Ammonium vanadate(V), 0.1 mol dm–3
  • Hydrochloric acid, 1 mol dm–3
  • Sulfuric acid, 1 mol dm–3 (to acidify reactions where necessary)
  • Hydrogen peroxide, 5% solution
  • Sodium hydroxide, 1 mol dm–3
  • Copper(II) sulfate, 0.2 mol dm–3
  • Iron(II) sulfate, 0.2 mol dm–3 (ensure solution is acidified with sulfuric acid for manganate reaction)
  • Iron(III) nitrate, 0.2 mol dm–3
  • Potassium iodide, 0.2 mol dm–3
  • Starch solution (freshly made)
  • Zinc metal granules (or metal foil pieces)
  • Zinc(II) sulfate solution, 0.2 mol dm–3

Instructions

  1. Read the Health, safety and technical notes and Chemical hazards table in the student worksheet before you begin.
  2. Cover page 3 with a clear plastic sheet (if it is not laminated).
  3. Draw a suitable table to record your observations from each experiment.
  4. Place a drop of each solution from vanadium to zinc in the appropriate boxes in the Microscale table. (Note: one drop will act as a reference).
  5. For each transition element solution, add the reactants described in the Experimental methods table to the appropriate box in the experiment row of the Microscale table. Observe each box carefully and compare it to the reference box, noting down any colour changes for each reaction in your results table.

Lab notes

Once the class have noted their initial observations in their tables, ask them to:

  • Write possible explanations for each experimental observation they have made.
  • Discuss their findings with their neighbour and the teacher (they will provide further detail for each experiment).
  • Complete their experimental notes by adding any final conclusions.

Teaching notes and expected observations

Vanadium

Colour change = change in oxidation state

Bubbles (of hydrogen) are seen. The yellow colour of the ammonium vanadate’s vanadium(V) ions (VO2+) gradually changes (as the vanadium is reduced) to blue due to the formation of 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

Colour change = change in oxidation state

The orange dichromate solution is reduced by hydrogen peroxide. The resulting solution turns deep blue with bubbling due to oxygen gas produced. After a short time, the colour fades to reveal the pale blue/green colour of the chromium(III) ions. The dichromate solution must be acidified to ensure enough H+ is present for the reaction.

Manganese

Colour change = change in oxidation state

As it is reduced, the deep purple colour of the oxidising agent, potassium manganate(VII), gradually fades, first to the brown manganese(IV) oxide then to the very pale pink manganese(II) ions.

The iron(II) solution is the reducing agent and is oxidised to Fe2+ (aq). You must acidify the manganate solution to ensure enough H+ is present for the reaction.

Iron

Colour change = change in oxidation state and complex formation

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

Note: iron(II) should give no reaction unless it contains some iron(III).

More resources

Cobalt

Colour change = ligand exchange and change in oxidation state

For the cobalt nitrate solution, Co(NO3)2.6H2 O, the addition of one drop of ammonia acts as a base to remove hydrogen ions from the hexaaqua cobalt(II) ion, [Co(H2 O)6]2+, to give the green hydroxide precipitate [Co(H2 O)4 (OH)2]. Addition of further ammonia dissolves the precipitate and the ammonia acts as a ligand to replace water (ligand exchange) to give hexaamminecobalt(II) ions, [Co(NH)3]2+, (greenish brown) which will further oxidise in air to produce [Co(NH)3]3+ (darker brown).

Copper

Colour change = ligand exchange

The copper sulfate solution provides the hexaaqua copper(II) ions, [Cu(H2 O)6]2+, which upon addition of ammonia solution forms a light blue precipitate of tetraaquadihydroxocopper(II), [Cu(H2 O)4 (OH)2](s), together with the developing deep blue solution colour change due to formation of tetraamminediaquacopper(II) ions, [Cu(NH3)4 (H2 O)2](aq).

Zinc

Colour change = not observed

A gelatinous white precipitate of zinc hydroxide, Zn(OH)2, 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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Additional information

This resource is part of our Microscale chemistry  collection, which brings together smaller-scale experiments to engage your students and explore key chemical ideas. The practical originally appeared in the book Microscale chemistry: experiments in miniature, published by the Royal Society of Chemistry in 1998.

This resource was updated and health and safety checked in 2023, including additional questions written by Nicola Kiernan and technician notes by Sandrine Bouchelkia.

© Royal Society of Chemistry