Explain why chromium complexes are coloured.

1. (a)Explain why chromium complexes are coloured.[4]
2. (b)(i)Four different compounds can be obtained when anhydrous chromium(III) chloride reacts with water under various conditions. When samples of each compound are reacted separately with aqueous silver nitrate, different amounts of silver chloride are precipitated. The precipitation leaves the complex ions P, Q, R and S in solution. formula of compound | moles of AgCl precipitated per mole of complex ion | complex ion | property of complex ion --------------------|--------------------------------------------------|-------------|---------------------- CrCl3(H2O)6 | 3 | P | non-polar CrCl3(H2O)5 | 2 | Q | polar CrCl3(H2O)4 | 1 | R | polar CrCl3(H2O)4 | 1 | S | non-polar Draw three-dimensional diagrams for the structures of complex ions P, Q, R and S. Include the charges for each complex ion.[4]
3. (b)(ii)Suggest why complex ion S is non-polar.[1]
4. (c)(i)The structure of picolinic acid is shown. [Figure showing picolinic acid structure] The conjugate base of picolinic acid is a bidentate ligand, Z. Define the term bidentate ligand.[2]
5. (c)(ii)Draw the structure of Z.[1]
6. (c)(iii)Z reacts with aqueous chromium(III) ions, [Cr(H2O)6]3+, in a 3:1 ratio to form a new neutral complex. State the coordination number and the geometry of the chromium(III) centre in the complex. coordination number geometry[1]
7. (d)(i)Deduce the oxidation numbers of chromium in (NH4)2Cr2O7 and in Cr2O3. (NH4)2Cr2O7 Cr2O3[1]
8. (d)(ii)Construct an equation for the thermal decomposition of (NH4)2Cr2O7.[1]

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