Question

6. Write one of the reaction for each of the following characteristics :

(e) Evolution of heat

Answer 1

Part 1: Chemical Reaction

• A chemical reaction may take place when two or more reactants come in contact with one another resulting in the formation of new substances called products accompanied by the transfer of energy among them.
• All chemical reaction takes place either by the absorption or the release of energy, generally in the form of heat energy.
• The chemical reactions which proceed with the release of heat energy are called exothermic reactions.
• The chemical reactions which proceed with the absorption of heat energy are called endothermic reactions.

Part 2: Types of reactions

• Hydrochloric acid reacts with sodium hydroxide solution to form sodium chloride (salt) and water with the evolution of a lot of heat.
• $\underset{\mathrm{Sodium}\mathrm{hydroxide}}{\mathrm{NaOH}\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydrochloric}\mathrm{acid}}{\mathrm{HCl}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Sodium}\mathrm{Chloride}}{\mathrm{NaCl}\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\mathrm{Heat}$
• This is a type of exothermic reaction.
• When sodium bicarbonate is heated strongly, it swells up to form sodium carbonate, steam, and carbon dioxide gas. However, the reaction takes place only on heating, i.e., with the absorption of energy.
• $\underset{\mathrm{Sodium}\mathrm{bicarbonate}\mathrm{sodium}\mathrm{carbonate}\mathrm{water}\mathrm{Carbon}\mathrm{dioxide}}{2{\mathrm{NaHCO}}_3\to {\mathrm{Na}}_2{\mathrm{CO}}_3+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2}$
• This is a type of endothermic reaction.
• In an exothermic reaction, heat is evolved in the reaction whereas in endothermic reaction heat is given for the reaction to proceed.