Outline why is $\mathrm{PCB}$ considered “dioxin like”, but not a dioxin.

Kevlar can be made by reacting $1,4-$diaminobenzene, ${\mathrm{H}}_2\mathrm{N}-{\mathrm{C}}_6{\mathrm{H}}_4-{\mathrm{NH}}_2$, with $1,4-$benzenedicarbonyl chloride, $\mathrm{Cl}-\left(\mathrm{O}\right)\mathrm{C}-{\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{C}\left(\mathrm{O}\right)-\mathrm{Cl}$. Write the equation for the reaction of $\mathrm{n}$ molecules of $1,4-$diaminobenzene reacting with $\mathrm{n}$ molecules of $1,4-$benzenedicarbonyl chloride.

Hydroxyl free-radicals can be generated naturally in the body. This process is catalysed by iron in the following two steps:

Reaction 1: ${\mathrm{Fe}}^{3+}+·{{\mathrm{O}}_2}^{-}\to {\mathrm{Fe}}^{2+}+{{\mathrm{O}}_2}^{-}$.
Reaction 2: ${\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{3+}+·\mathrm{OH}+\mathrm{OH}$.

Show that reactions 1 and 2 are redox equations by writing the oxidation half-equation and the reduction half-equation for each.

Heavy metals are often removed from solutions by precipitation. Use the Data booklet to calculate the concentration of sulphuric acid necessary to precipitate mercury($\mathrm{I}$) ions at a concentration of $3 \mathrm{\mu mol} {\mathrm{dm}}^{-3}$? [${\mathrm{Hg}}_2{\mathrm{SO}}_4, {\mathrm{K}}_{\mathrm{sp}}=6.5\times {10}^{-7}$].

Heavy metals are often removed from solutions by precipitation. Evaluate the effectiveness of this method for mercury removal and suggest improvements.