Kevlar^® is a lyotropic liquid crystal. Explain the strength of Kevlar^® and its solubility in concentrated sulfuric acid.

Describe the use of silicon in photovoltaic cells. Include the following in your description:

• why pure silicon is a better conductor than non-metals such as sulfur and phosphorus

• how a p-type semiconductor made from silicon is different from pure silicon

• how sunlight interacts with semiconductors.

strong intermolecular hydrogen bonds between the chains;

intermolecular bonds can be broken (by concentrated sulfuric acid) as O and N atoms

are protonated (breaking the hydrogen bonds) / hydrolysis of amide linkage;

Si has a lower ionization energy (than P or S);

so electrons can flow through the material more easily;

(p-type) has small amount of/is doped with a group 3 element/B/In/Ga;

which produces electron holes/positive holes;

sun/photons cause release of electrons;

electrons move from n-type to p-type material;

In (b) many candidates omitted the location of Hydrogen bonding in Kevlar and frequently vague responses were given for the effect of concentrated ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$.

In (c) candidates showed a poor understanding of the better conduction of Si compared to S and P, very few answered how a p-type semiconductor worked correctly, but most showed a reasonable understanding of the role of light. The movement of electrons from n to p was also poorly understood.