How do the following help in bringing about a chemical change? Explain each with a suitable example.

(a) Pressure

(b) Light

(c) Catalyst

(d) Heat

(a) Some chemical reactions take place when the reactants are subjected to high pressure.

When nitrogen and hydrogen gas are subjected to high pressure, ammonia gas is produced.

$\text{N}$2 + 3\text{H}2 \xrightleftharpoons[\underset{\text{pressure}}{\text{200 atmos.}}]{\text{450 \degree C }} \text{2NH}_3

(b) Some chemical reactions can take place only in the presence of light and are called photochemical reactions.

Photosynthesis is a chemical reaction in which glucose is prepared by the green leaves of a plant but light is necessary for the reaction to take place.

$\underset{\text{[carbon dioxide]}}{6\text{CO}$2} + 6\text{H}2\text{O} \xrightarrow[\text{Sunlight}]{\text{Chlorophyll}} \underset{\text{[glucose]}}{\text{C}6\text{H}{12}\text{O}6} + \underset{\text{[oxygen]}}{6\text{O}2}

(c) Some chemical reactions need a catalyst to change the rate of reaction, if the reaction is too slow or too fast. Positive catalyst increases the rate of chemical reaction whereas negative catalyst decreases the rate of reaction.

Finely divided iron is used as a positive catalyst in the manufacture of ammonia from hydrogen and oxygen.

     $\text{N}$2 + 3\text{H}2 \xrightleftharpoons[\text{200-900 atmos.}]{\text{Fe catalyst/450\degree C }} \text{2NH}_3

(d) Some chemical reactions take place only in the presence of heat.

If iron powder and sulphur powder are mixed, they do not react. But they are heated, they react to form iron sulphide.

Fe + S $\xrightarrow{\enspace\Delta\enspace}$ FeS