Transition Metals

The properties and similarities in transition metals comes down to their electron configurations. The 3d sub-shell is closer to the nucleus, but higher in energy than the 4S; meaning that 4S fills up first (giving K and Ca). Therefore we define transition metals as those that have an incomplete d sub-level: in the element form or one of its ions. There are exceptions that arise as a result of this definition.

Scandium, in its element form, has a typical transition configuration with an incomplete d shell, yet its ion Sc³⁺ has no d shell electrons so strictly isn't a transition metal. Similarly, copper element doesn't have an incomplete 3d, but its most common ion (Cu²⁺) does, and so therefore it can be a transition metal. Zinc, on the other hand, has a complete 3d in both element and ion form so is not a transition metal.

The various common properties of transition metals are outlined in this section, and are: coloured ions, variable oxidation states, and catalytic activity.

Variable Oxidation States

Because the 4S and 3d energy levels are so similar, the transition elements can lose differing numbers of electrons and have a similar stability. This means they have variable oxidation states. Vanadium has 4 oxidation states (+2, +3, +4 and +5). These can be seen as 4 distinct colours when zinc is added to acidified ammonium vandate (V).

Similarly, chromium ions show different colours according to the oxidation state, when Cr₂O₇^2- is reduced in solution by zinc.

Oxygen from the air can act as an oxidizing agent on many compounds such as Co(OH)₂ to Co(OH)₃. The ions can be protected from this by acidification. If ammonia solution is added to cobalt (II) salt, the precipitate Co(OH)₂ is formed. If shaken it will oxidize to Co(OH)₃, which is brown.

Substances can also be oxidized in alkaline solution. For example, adding excess of NaOH to chromium (III) salt gives the chromate (III) ion. Now when this is treated with hydrogen peroxide it is readily oxidised to chromate (VI) ions.

2[Cr(OH)₆]^3- + 3H₂O₂ ==>> 2CrO₄^2- + 2OH- + 8H₂O

Redox Titrations

Titrations are important in analyzing solutions. For example, testing the amount of iron in an iron tablet. This can be done by reacting Fe²⁺ with either MnO^4- - manganate (VII) or Cr₂O^7- - dichromate (VI).

• First, the tablet is dissolved in acid. Dilute sulphuric acid is used because it is strong, isn't an oxidizing agent (as concentrated sulphuric acid is) and will not be oxidized.
• The manganate ion is added from the burette in the form of potassium manganate, which is dark purple, but the reaction product is colourless so when the end-point is reached the solution will be purple. The following reaction occurs.

MnO^4- + 8H+ + 5e- ==>> Mn²⁺ + 4H₂O Fe₂+ ==>> Fe³⁺ + e-

Overall this makes...

5Fe²⁺ + MnO^4- + 8H+ ==>> 5Fe³⁺ + Mn²⁺ + 4H₂O

Now take a look at the worked example below to see how to perform calculations given this information.

If potassium dichromate is used instead, an indicator must be used, this is commonly sodium diphenylamine sulphonate. The overall reaction for this is.

6Fe²⁺ + Cr₂O₇²- + 14H+ ==>> 6Fe³⁺ + 2Cr³⁺ + 7H₂O

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