Question

The below reaction is an example of which of the following reactions?
${H_2} + C{l_2} \to 2HCl$

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

Answer 1

We must first know the exact mechanism of preparation of hydrogen chloride in order to answer this question. Hydrogen chloride is prepared by the direct combination of moist hydrogen gas with chlorine in the presence of diffused sunlight. This reaction is accompanied by evolution of heat and appears to be accelerated by moisture.

Complete step-by-step answer:
${H_2} + C{l_2} \to 2HCl$
In the above reaction two molecules of hydrogen react with two chlorine molecules to produce a single substance which is hydrogen chloride or which is also known as hydrochloric acid.
Therefore we can say that it is a combination reaction because two elements are combined to form a single product.
Combination reaction can be simply represented as;
$X + Y \to XY$
Combination reactions are usually exothermic because when the bond forms between the reactants, heat is released.
Hence the correct option is A which is combination reaction.
Additional information:
Decomposition reaction - The opposite of a combination reaction which is a complex molecule breaks down to make simpler ones.
Displacement reaction - One element displaces with another element in the compound.

Note: Combination reaction is also known as a synthesis reaction this is because in these reactions new substances are being synthesized. Combination reaction can be of three types.
Reaction between two or more elements - the reaction between a metal and a non- metal.
Reaction between elements and compounds - carbon monoxide reacts with oxygen gas to form carbon dioxide.
Reaction between two compounds - Two compounds react with each other to form a new compound. Example, calcium oxide reacts with carbon dioxide gas to form calcium carbonate.