Among the 3d orbitals, two are the $3d_{xy}$ orbital and the $3d_{z^2}$ orbital. Sketch the shapes of these two orbitals.
The $\text{Ni}^{2+}$ ion can form many different complexes. A solution containing the $[\text{Ni}(\text{H}_2\text{O})_6]^{2+}$ complex ion is green. When an excess of 1,2-diaminoethane, en, $\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2$, is added, the solution turns blue. This happens because the $[\text{Ni}(en)_3]^{2+}$ complex ion is formed. Explain why the two solutions are coloured, and why the colours differ.
The $\text{[Ni(en)}_3]^{2+}$ complex can exist as a mixture of two stereoisomers. Complete the three-dimensional diagram so that it shows one stereoisomer. Each $en$ ligand may be represented by $\text{N-CH}_2\text{CH}_2\text{-N}$.
Name the geometry of the complex ion shown in (b)(ii) and the type of stereoisomerism displayed by $\text{[Ni(en)}_3]^{2+}$.
Iron(II) carbonate, $\text{FeCO}_3$, and nickel(II) carbonate, $\text{NiCO}_3$, both decompose on heating. $\text{FeCO}_3$ breaks down at a lower temperature than $\text{NiCO}_3$. Suggest one possible explanation for this difference. Explain your answer.
Describe the colour change observed in the conical flask at the end-point of this titration. The colour changes from ................ to ................ .
Calculate the percentage by mass of iron present in $A$. $[A_r: \text{Fe}, 55.8]$.