Explain why pure hydrogen cyanide does not conduct electricity, but become a conductor when it is dissolved in water? (at room temp, pure hcn exists as a volatile liquid)

Generally we have to kinds of conductors

a) metallic conductors: which conduct electricity due to free movement of electrons

b) electrolytic conductors: which conduct electricity due to movement of ions.

HCN is an electrolyte and is non conducting in pure state as it has no free ions or electrons

When HCN is dissolved in water it dissociates as

These ions obtained due to dissociation of HCN makes it conducting in nature.

the potential energy of the reactants is 200j.

from the energy diagram, the energy of the product formed is 350j; this means that, this reaction is an endothermic reaction, because it absorbs energy from its environment.