In certain reactions a change of state is observed i.e. solid to liquid, liquid to gas etc. State the change of state of the reactants to give the respective products in each case.
(a) $\mathrm{C}+2\mathrm{S}\to {\mathrm{CS}}_2$
(b) ${\mathrm{NH}}_3+\mathrm{HCl}\rightleftharpoons {\mathrm{NH}}_4\mathrm{Cl}$
(c) $2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}$
(d) $2{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Electric} \mathrm{current}}{\to }2{\mathrm{H}}_2+{\mathrm{O}}_2$

Give examples of exothermic reactions i.e. heat evolved in each of the following:
a) Reaction between a solid & a liquid.
b) Reaction between two gases.

Give two examples of exothermic reactions seen in everyday life.

Give one example where supplying heat energy is necessary for a chemical reaction.