Question

Assertion :
Concentrated and not dilute ${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$ behaves as a dehydrating agent.
Reason :
Concentrated sulphuric acid behaves as a strong dehydrating agent as the hydration reaction is highly exothermic. It has strong affinity for water removing water molecules from the other compounds

Answer 1

Hint: A dehydrating agent is a chemical substance that will remove water or the element of water from other chemical substances. Sulphuric acid is represented by ${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$. In sulphuric acid sulphur atoms present its maximum oxidation state, so it acts as a strong oxidizing agent.
- Concentrated sulphuric acid is obtained by the Contact process which contains 96-98 % pure sulphuric acid.

Complete Solution :
Concentrated ${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$ acts as a strong dehydrating agent because ${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$ forms hydrate with water, that’s why it has a great affinity toward water. It will take water from a variety of organic substances, one such substance is cane sugar or sucrose.
$\begin{align}
& \text{sucrose(sugar)}\,\text{+}\,\text{conc}\text{.}\,{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\to \,\text{carbon}\,\text{+}\,\text{water} \\
& {{\text{C}}_{\text{12}}}{{\text{H}}_{\text{22}}}{{\text{O}}_{\text{11}}}\text{(s)}\,\,\,\,\,\to \,\,\text{12C(s)}\,\,\text{+}\,\,\text{11}{{\text{H}}_{\text{2}}}\text{O(l)} \\
\end{align}$

- Concentrated sulphuric acid will also take water from blue hydrated copper (II) sulphate crystals, leaving only white crystal of anhydrous copper (II) sulphate. Many gases are also dried by passing them through concentrated sulphuric acid because the hydration reaction of sulphuric acid is a highly exothermic (energy releasing) process.
So, given both the statements are correct and the second statement is the correct explanation of the first statement.

Note: Concentrated sulphuric acid should be treated very carefully, because it will also remove water from flash. Diluting concentrated sulphuric acid must be done with great care because of its affinity for water. The concentrated sulphuric acid should always be added to the water, not the other way.