State the conditions required for the following reactions to occur :

(a) Decomposition of water into hydrogen and oxygen.

(b) Formation of iron sulphide from iron and sulphur.

(c) Reaction between zinc and dilute hydrochloric acid to produce hydrogen gas.

(d) Decomposition of potassium chlorate into potassium chloride and oxygen.

(a) Water decomposes into hydrogen and oxygen (gases) when electric current is passed through it.

$\text{2H}$2\text{O} \xrightarrow[{\text{current}}]{\text{electric}} \text{2H}2 + \text{O}_2

(b) Iron powder and sulphur powder react to form a new substance, iron sulphide only when they are heated together.

$\text{Fe} + \text{S} \xrightarrow{\text{heat}} \text{Fe} \text{S}$

(c) A dilute solution of hydrochloric acid is needed because concentrated acids may cause side reactions and excessive heat generation.

$\text{Zn} + \text{H}$2 \text{SO}4 (dil.) \xrightarrow{} \text{Zn} \text{SO}4 + \text{H}2

(d) Usually, potassium chlorate decomposes to evolve oxygen gas on being heated at 700°C. However, when manganese dioxide is mixed with potassium chlorate as a catalyst (in the ratio 1 : 4), the decomposition takes place at a much lower temperature, i.e. at about 300°C.

$\text{2KClO}$3 \xrightarrow[{\text{300°C}}]{\text{MnO}2} \text{2KCl} + \text{3O}_2