Sketch a graph of resistance versus temperature for a conductor and a superconductor.

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.

Heavy metals are often removed from solutions by precipitation. Magnesium ion concentrations can be determined by precipitation as magnesium hydroxide. Given that the solubility product, ${\mathrm{K}}_{\mathrm{sp}}$ of magnesium hydroxide is $1.20\times {10}^{-11}$. Calculate the concentration, in $\mathrm{mol} {\mathrm{dm}}^{-3}$ of magnesium ions required to form a precipitate in a solution where the final hydroxide ion concentration is $2.00 \mathrm{mol} {\mathrm{dm}}^{-3}$?