Question

When water is added to ${H_2}S{O_4}$, the reaction is exothermic. It is because of the formation of:
A. Viscous nature of ${H_2}S{O_4}$
B. Hydrates of ${H_2}S{O_4}$
C. Corrosive nature of ${H_2}S{O_4}$
D. None of the above.

Answer 1

In a chemical reaction when the heat is absorbed during the process or the products formed are less stable than reactants, then the reaction is known as endothermic reaction whereas if the heat is released after the reaction, then it is said to be an exothermic reaction.

Complete answer:
When water is added to sulphuric acid i.e., a violent reaction takes place as it can boil and split dangerously. It is a highly exothermic reaction i.e., a high amount of energy is released in the form of heat in the hydration reaction of sulphuric acid. When water is added to sulphuric acid, formation of hydronium ions takes place along with the removal of hydrogen sulphate ions. The reaction takes place as follows:
${H_2}S{O_4} + {H_2}O \to {H_3}{O^ + } + HSO_4^ -$
On adding water in excess, the hydrogen sulphate ions further decompose into hydronium ions and sulphate ions. The reaction takes place as follows:
$HSO_4^ - + {H_2}O \to {H_3}{O^ + } + SO_4^{2 - }$
Because the hydration reaction of sulphuric acid is highly exothermic that means it is thermodynamically favourable, so it acts as an excellent dehydrating agent and is used in synthesis of chemical compounds in many organic as well as inorganic reactions. The affinity of sulphuric acid for water is strong enough that it will take the hydrogen atoms and oxygen atoms out of the other compound and thus show dehydration.
Hence, on addition of water to ${H_2}S{O_4}$, the reaction is exothermic. It is because of the formation of hydrates of ${H_2}S{O_4}$.
Thus, option (B) is the correct answer.

Note:
Remember that the water is less dense than sulphuric acid, if we pour water in sulphuric acid it floats on it and the reaction will occur at the top of the liquid. Conversely, if you add sulphuric acid to the water, it sinks due to its greater density. As the reaction is happening at the top of liquid, a person must be careful while performing this reaction as it may split over skin to cause severe burns.