5.3.1 (c) Properties of Transition Elements

(c) illustration, using at least two transition elements, of:

(i) the existence of more than one oxidation state for each element in its compounds

(ii) the formation of coloured ions

(iii) the catalytic behaviour of the elements and their compounds and their importance in the manufacture of chemicals by industry

{No detail of how colour arises required.}

{Practical examples of catalytic behaviour include: Cu²⁺ for reaction of Zn with acids; MnO₂ for decomposition of H₂O₂.}

{No detail of catalytic processes required.}

{Benefits of reduced energy usage; risks from toxicity of many transition metals.}

You should already know that Iron commonly exists as both Fe²⁺ and Fe³⁺.

You should also know that Copper exists as Cu²⁺ and may also know that is less frequently Cu⁺.

If you think back to the Redox topics you should know that Dichromate ions are Cr₂O₇^2- and easily becomes Cr³⁺ ions

This means it exists in the +6 and +3 state, although Chromium can exist in several other Oxidation States.

Similarly, Manganate is MnO₄^- and easily becomes Mn²⁺, meaning it exists in the +7 and +2 states, as well as several others.

As the syllabus states, you don't have to explain how colours are created, much less why the colours are different.

The exams are unlikely to expect you to remember much more than:

Fe²⁺ --- grey-green

Fe³⁺ --- orange/rust

Cu²⁺ --- blue

Mn²⁺ --- pink

MnO₄^- --- purple

Cr₂O₇^2- --- orange

Cr³⁺ --- green

The colours for ions can vary according to concentration of the solution and, if crystalised into solids, what they are bonded to.

The reason the Transition Elements and their compounds make good catalysts is the variability of their oxidation states but you don't have to be able to explain how they work.

You are required to know the catalysts that you learned for GCSE however, and the processes that they catalyse.

Haber Process

The GCSE examiner's favourite

Catalysed (as far as we're concerned) by either beds of finely divided Iron powder or fine meshes of Iron through which the reacting gases pass.

As long as the reacting gases are pure the catalyst should last forever, although impurities in the gases may eventually coat the catalyst and stop it working as it no longer has a surface for the gases to stick to.

Contact Process

The GCSE examiner's least favourite

Only the second equation requires a calatyst as Sulphur burns well and Sulphur Trioxide reacts with water easily .

Catalysed by Vanadium Oxide.

Hydrogenation of alkenes (and margarine).

The examiners answer is Nickel, although other metals do much the same job.

Decomposition of Hydrogen Peroxide.

The examiners answer is Manganese Oxide (MnO₂), although you may have seen it done with carrots or bits of liver (mmm!).

If you wanted to decompose very quickly, like in the Elephant's Toothpaste demo, then a Homogeneous catalyst (one which is also aqueous) would be your best choice.

All the above examples are expected knowledge and are all examples of Heterogeneous Catalysis - the catalysts are solids when the reactants are gases or aqueous solutions.