Exploring the chemistry of chromium, molybdenum and tungsten

In this unusual experiment, some features of the chemistry of three transition elements are examined. The experiment illustrates aspects of colour, precipitate formation, changes in oxidation state and equilibria. Students may be familiar with the properties of chromium, but will be able to explore the features of molybdenum and tungsten too.

This practical should take 10 minutes.

Equipment

Apparatus

• Student worksheet
• Clear plastic sheet (eg OHP sheet)

Chemicals

• Potassium chromate, 0.2 mol dm^–3
• Ammonium molybdate, 0.05 mol dm^–3
• Sodium tungstate, 0.2 mol dm^–3
• Hydrochloric acid, 1 mol dm^–3
• Sodium hydroxide, 1 mol dm^–3
• Iron(II) sulfate, 0.2 mol dm^–3

Health, safety and technical notes

• Read our standard health and safety guidance.
• Students must wear suitable eye protection (splash proof goggles to BS EN166 3).
• Potassium chromate, 0.2 M KCrO₄ is a carcinogen, mutagen and skin sensitiser as well as a skin/eye irritant (see CLEAPSS HazCard HC080).
• Explosive or vigourous-burning mixtures can be formed with aluminum and other metals and combustible materials (see CLEAPSS HazCard HC001a).
• Hydrochloric acid, 1 mol dm^–3 HCl(aq), ammonium molybdate, 0.05 mol dm^–3 (NH₄)₆Mo₇O₂₄.4H₂O(aq), sodium tungstate, 0.2 mol dm^–3 Na₂WO₄.2H₂O(aq) and iron(II) sulfate, 0.2 mol dm^–3, FeSO₄.7H₂O(aq) are of low hazard (see CLEAPSS HazCard HC047a, HC009a, HC089 and HC055B).
• Sodium hydroxide solution, 1 mol dm^–3 NaOH(aq), is corrosive (see CLEAPSS HazCard HC091a

Molybdenum and tungsten

These two transition metals, in the same group as chromium, are rarely encountered in experiments at secondary level, although students should be familiar with the use of tungsten metal as filaments in light bulbs (it has the highest melting point of any metal – 3422 ± 200 °C). Molybdenum is an essential element in animal biochemistry and occurs in the enzyme xanthine oxidase, which is involved in purine–adenine and guanine metabolism. Both metals are widely used in the steel industry, where they are essential in the manufacture of high-speed steels for cutting tools.

Their solution chemistry is very complex – students might be surprised by the formula of ammonium molybdate.

The procedures described here illustrate changes in oxidation state (molybdenum) and precipitation from acid solution (tungsten). The addition of a mild reducing agent (iron(II)) to a solution of molybdate produces a blue colour, generally known as molybdenum blue. These are non-stoichiometric compounds, containing both oxides and hydroxides, the mean oxidation number of molybdenum being between 5 and 6. It has been suggested that Mo₃ atom clusters might be responsible for the blue colour.

The acidification of a solution of a tungstate produces a precipitate of tungsten(VI) oxide. Tungsten is therefore extracted from its ore (wolframite (Fe, Mn) WO₄) under alkaline conditions.

Observations

1. Students should observe that the solution of chromium (as chromate(III)) is coloured, whereas those of molybdenum and tungsten are not. Discussion could ensue on whether this is significant, bearing in mind the general characteristic of transition elements that their compounds are coloured. Molybdenum and tungsten do form coloured compounds in other oxidation states, and students should be aware of the blue copper(II) and white copper(I) salts.
2. Acidification of the yellow chromate(VI) solution produces an orange colour due to the formation of dichromate ions as the position of equilibrium is shifted:
   
   2CrO₄^2– + 2H⁺ ⇄  2HCrO₄^– ⇄  Cr₂O₇^2– + H₂O
   
   yellow                 intermediate            orange
   
   The addition of alkali removes the hydrogen ions and shifts the position of equilibrium back to the left, and so the yellow colour is seen again.
3. Acidification of the molybdate(VI) solution produces a very interesting result. A white precipitate is formed at first. However, after a short while this precipitate starts to dissolve until eventually a colourless solution reforms. Molybdenum complexes are formed and, as these disproportionate, the species formed redissolve.
4. Acidification of the tungstate(VI) solution slowly produces a white precipitate of hydrated tungsten(VI) oxide, (WO₃). Whereas chromium and its compounds are soluble in acid solution, molybdenum and tungsten, and their compounds are precipitated at low pH but brought into solution at high pH. This is significant in the methods used to extract both molybdenum and tungsten from their ores.
5. The addition of iron(II) ions to chromate(VI) ions produces an orange colour. This is probably due to the effect of a change in pH. The iron(II) is oxidised to iron(III) and this, being slightly acidic, causes a shift in the position of equilibrium forming orange dichromate ions.
6. The addition of iron(II) ions – a mild reducing agent – to a molybdate solution produces a dark bluish colour sometimes known as molybdenum blue.
7. Addition of iron(II) ions to a tungstate solution produces a whitish precipitate of WO₃.