Question

Dilution of sulfuric acid is a/an:
A. synthesis reaction
B. exothermic reaction
C. endothermic reaction
D. none of these

Answer 1

Sulfuric acid (${H_2}S{O_4}$ ) also known as oil of vitriol, is a mineral acid which is composed of the elements sulfur, oxygen and hydrogen. It is a colorless, odorless and viscous liquid that is soluble in water and is synthesized in reactions that are highly exothermic and release a large amount of heat energy.

Complete step by step answer:

As the hydration reaction of sulfuric acid is highly exothermic and releases a huge amount of heat energy, dilution should always be performed by adding the sulfuric acid to the water rather than the water to the acid. As the reaction is in an equilibrium that favors the rapid protonation of water (due to high electronegativity of the oxygen atom, the proton is attracted towards it and forms the hydronium ion), addition of acid to the water ensures that the acid is the limiting reagent and controls the reaction. This reaction is best understood by the method of formation of hydronium ions. The reaction is as follows:
${H_2}S{O_4} + {H_2}O \to {H_3}{O^ + } + HSO_4^ -$
The bisulphate ion formed can further undergo a reaction with water as the water is in excess quantity to control the reaction. Thus, the final reaction occurs as:
$HSO_4^ - + {H_2}O \to {H_3}{O^ + } + SO_4^{2 - }$
Thus, the correct option is: B. exothermic reaction.

Note:
As we can observe from the above reactions, the hydration of sulfuric acid is thermodynamically favorable and the affinity of it for water is sufficiently strong which makes the sulfuric acid an excellent dehydrating agent. Concentrated sulfuric acid has a very powerful dehydrating property and removes the water from various compounds such as sugars and other carbohydrates.