Draw the Lewis (electron dot) structure of the superoxide and hydroxyl radicals. Explain what a radical is and the steps involved in a radical chain reaction. Explain how radioactivity can cause the initiation step.

A modern solution to the provision of power for remote places is the dye-sensitized solar cell (DSSC). A Gratzel DSSC contains an organic dye molecule on the surface of a titanium dioxide, ${\mathrm{TiO}}_2$, semiconductor and an electrolyte containing iodide ions.

Explain its operation, including the importance of nanotechnology in its construction and its advantage compared with silicon-based photovoltaic devices.

Discuss the use of silicon in photovoltaic cells, with reference to the following:

Why pure silicon is a better electrical conductor than non-metals such as phosphorus and sulphur.

Discuss the use of silicon in photovoltaic cells, with reference to the following:

How a $\mathrm{p}$-type semiconductor made from silicon is different from pure silicon.

Discuss the use of silicon in photovoltaic cells, with reference to the following:

How sunlight can produce an electric current in a photovoltaic cell.

Bromothymol blue is an acid-base indicator that is blue in base and yellow in acid:

$\left(\mathrm{Yellow}\right) \mathrm{HB}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{B}}^{-} \left(\mathrm{Blue}\right)$

 Deduce which of the two forms of the indicator has the higher degree of conjugation. Explain it in terms of the wavelength of light absorbed and molecular structure of an organic indicator.