Question

${\mathrm{H}}_2+{\mathrm{Cl}}_2\to 2\mathrm{HCl}$

The reaction is an example of which of the following reactions?

• A. Decomposition reaction
• B. Combination reaction
• C. Double Displacement reaction
• D. Displacement reaction

Answer 1

The correct option is B

Combination reaction

Explanation of Correct option

(B) Addition/Combination Reactions: It is a union of two or more substances to form a new substance. It may be brought about by the application of heat, light, electricity, or pressure.

$\underset{\mathrm{Hydrogen}}{{\mathrm{H}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Chlorine}}{{\mathrm{Cl}}_2\left(\mathrm{g}\right)}\to \underset{\mathrm{Hydrochloric}\mathrm{Acid}}{2\mathrm{HCl}\left(\mathrm{l}\right)}$

In this reaction, two elements are combined to form a single product. This is a combination reaction.

Explanation of Incorrect option

(A) Decomposition Reaction: It is the breaking up of a substance into simpler compounds and it may be brought about by the application of heat, light, electricity etc.
A decomposition reaction brought by heat is known as thermal decomposition.
Example: $\underset{\mathrm{Calcium}\mathrm{Carbonate}}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}\to \underset{\mathrm{Calcium}\mathrm{oxide}}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

(D) Displacement Reactions: It involves the displacement of one of the constituents of a compound by another substance and may be regarded as a displacement reaction.

When zinc reacts with sulfuric acid, hydrogen gas is liberated and zinc sulfate is formed.

$\underset{\mathrm{Zinc}}{\mathrm{Zn}\left(\mathrm{s}\right)}+\underset{\mathrm{Sulfuric}\mathrm{acid}}{{\mathrm{H}}_2{\mathrm{SO}}_4(\mathrm{aq}.)}\to \underset{\mathrm{Zinc}\mathrm{sulfate}}{{\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydrogen}\mathrm{gas}}{{\mathrm{H}}_2\left(\mathrm{g}\right)}$

(C) Double Displacement: It is a mutual exchange of the radicals of two compounds taking part in the reaction and results in the formation of two new compounds.
$\underset{\mathrm{Sodium}\mathrm{Chloride}}{\mathrm{NaCl}\left(\mathrm{aq}\right)}+\underset{\mathrm{Silver}\mathrm{Nitrate}}{{\mathrm{AgNO}}_3\left(\mathrm{aq}\right)}\to \underset{\mathrm{Silver}\mathrm{chloride}}{\mathrm{AgCl}\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{Nitrate}}{{\mathrm{NaNO}}_3(\mathrm{aq}.)}$

Hence, option(B) is correct.