Explain the reason chromium complexes are coloured.
Four distinct compounds are produced when anhydrous chromium(III) chloride reacts with water under different conditions. When separate samples of each compound are treated with aqueous silver nitrate, different amounts of silver chloride are formed. The precipitation leaves the complex ions $P$, $Q$, $R$ and $S$ in solution.\n\nThe table gives:\n- $\text{CrCl}_3(\text{H}_2\text{O})_6$: 3 moles of $\text{AgCl}$ precipitated for each mole of complex ion, complex ion $P$, non-polar\n- $\text{CrCl}_2(\text{H}_2\text{O})_5$: 2 moles of $\text{AgCl}$ precipitated for each mole of complex ion, complex ion $Q$, polar\n- $\text{CrCl}(\text{H}_2\text{O})_4$: 1 mole of $\text{AgCl}$ precipitated for each mole of complex ion, complex ion $R$, polar\n- $\text{CrCl}_3(\text{H}_2\text{O})_4$: 1 mole of $\text{AgCl}$ precipitated for each mole of complex ion, complex ion $S$, non-polar
Draw three-dimensional diagrams of the complex ions $P$, $Q$, $R$ and $S$. Include the charge on each complex ion.
Suggest why complex ion $S$ is non-polar.
The structure of picolinic acid is shown. Its conjugate base acts as a bidentate ligand, $Z$.
Define the term bidentate ligand.
Draw the structure of $Z$.
$Z$ reacts with aqueous chromium(III) ions, $[\text{Cr(H}_2\text{O)}_6]^{3+}$, 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.
When heated, $(\text{NH}_4)_2\text{Cr}_2\text{O}_7$ decomposes to give $\text{Cr}_2\text{O}_3$, steam and an inert colourless gas.
Work out the oxidation numbers of chromium in $(\text{NH}_4)_2\text{Cr}_2\text{O}_7$ and in $\text{Cr}_2\text{O}_3$.
Construct an equation for the thermal decomposition of $(\text{NH}_4)_2\text{Cr}_2\text{O}_7$.