Aluminium reacts with hydrochloric acid to form aluminium chloride, ${\mathrm{AlCl}}_3$, and hydrogen. This is a redox reaction. Explain in terms of electrons, what is meant by a redox reaction.

When a salt containing iodide ions is warmed with concentrated sulphuric acid and ${\mathrm{MnO}}_2$, iodine is evolved:

$2{\mathrm{I}}^{-}+{\mathrm{MnO}}_2+6{\mathrm{H}}^{+}+2{{\mathrm{SO}}_4}^{2-}\stackrel{ }{\to }{\mathrm{I}}_2+{\mathrm{Mn}}^{2+}+2{{\mathrm{HSO}}_4}^{-}+2{\mathrm{H}}_2\mathrm{O}$

State the systematic name for ${\mathrm{MnO}}_2$.