Inorganic Chemistry of the Main-Group and Transition Elements
This domain tests descriptive and explanatory inorganic chemistry: trends across Period 3, the reactions of s-block metals and halogens, and the characteristic properties of transition metals and their complexes.
Period 3 trends
Oxides change from basic (Na2O, MgO) through amphoteric (Al2O3) to acidic (SO2, SO3, P4O10, Cl2O7). Chlorides change from ionic (NaCl, MgCl2) to covalent (AlCl3, SiCl4, PCl5); covalent chlorides hydrolyse to give acidic solutions. Al2O3 and Al(OH)3 dissolve in both acid and alkali.
s-Block and halogen chemistry
Group 1 metals react with water giving hydroxide and hydrogen, reactivity increasing down the group. Group 2 metals give the hydroxide and hydrogen; solubility of sulfates decreases down the group while hydroxide solubility increases. Halogens are oxidising agents, with oxidising power decreasing down Group 17; a halogen displaces a halide below it in the group. Concentrated H2SO4 oxidises bromide and iodide to Br2 and I2, whereas with chloride it only protonates.
Transition metals
Typical transition metals have variable oxidation states, form coloured complexes, act as catalysts and form complex ions with ligands. Colour arises from d–d transitions split by ligands; ligand substitution changes the splitting and hence the colour. Scandium (only +3, d0) and zinc (only +2, d10) are not typical. Coordination number is the number of dative bonds to the central ion.
Worked example
A pale-green solution gives a dirty-green precipitate with dilute NaOH, which turns brown at the surface on standing, and gives no red colour with KSCN. The cation is Fe2+: Fe2+ + 2OH− → Fe(OH)2, which is oxidised by air to brown Fe(OH)3. The absence of a blood-red colour with KSCN confirms Fe3+ is not initially present.
Traps that cost marks
- Describing MgO as amphoteric; only Al2O3 is.
- Claiming Zn2+ complexes are coloured; d10 ions are colourless.
- Mixing up the direction of solubility trends down Group 2.
- Forgetting that concentrated sulfuric acid is an oxidising acid only towards Br− and I−.
- Confusing ligand substitution with redox.