Question

What will happen when dilute sulphuric acid is added to sodium sulfite?
(A) $N{{a}_{2}}S{{O}_{3}}+{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{4}}+S{{O}_{3}}+{{H}_{2}}O$
(B) $N{{a}_{2}}S{{O}_{3}}+{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{3}}+S{{O}_{2}}+{{H}_{2}}O$
(C) $N{{a}_{2}}S{{O}_{3}}+{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{4}}+S{{O}_{2}}+{{H}_{2}}O$
(D) $N{{a}_{2}}S{{O}_{4}}+{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{4}}+S{{O}_{2}}+{{H}_{2}}O$

Answer 1

Hint: The reaction between dilute sulphuric acid and sodium sulfite is a double displacement reaction. Double displacement reactions are the reactions in which two compounds react by an exchange of ions to form two new compounds.

Complete step by step solution:
-In double displacement reactions, the positive ions exchange with negative ions. The general reaction is represented by the following general equation-
${{A}^{+}}{{B}^{-}}+{{C}^{+}}{{D}^{-}}\to {{A}^{+}}{{D}^{-}}+{{C}^{+}}{{B}^{-}}$
-The reaction between sulphuric acid and sodium sulfite occurs in the following two steps-
(i) The first step involves the reaction of sulphuric acid with sodium sulfite giving sodium sulfite and sulfurous acid as products.
${{H}_{2}}S{{O}_{4}}+N{{a}_{2}}S{{O}_{3}}\to N{{a}_{2}}S{{O}_{4}}+{{H}_{2}}S{{O}_{3}}$
(ii) In the second step, sulfurous acid formed in the first step being highly unstable, decomposes spontaneously into sulfur dioxide and water.
${{H}_{2}}S{{O}_{3}}\to S{{O}_{2}}+{{H}_{2}}O$

Thus, the overall reaction can be given as-
${{H}_{2}}S{{O}_{4}}+N{{a}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{4}}+S{{O}_{2}}+{{H}_{2}}O$

So, the correct answer is option C.

Note: You should not get confused between sodium sulfate and sodium sulfite. Sodium sulfate is an inorganic salt of sodium with chemical formula as $N{{a}_{2}}S{{O}_{4}}$, which exists in three forms. The anhydrous form sodium sulfate is known as thenardite, a heptahydrate form is a rare form, and the decahydrate form is known as mirabilite. Sodium sulfite is an inorganic compound which a pale yellow, water-soluble solid with the chemical formula sodium sulfate minus one oxygen, that is $N{{a}_{2}}S{{O}_{3}}$. Sodium sulfite is used commercially as a preservative and an antioxidant. Another form of sodium sulfite is also known which is heptahydrate but it is not of much use due to its greater susceptibility towards oxidation by air. Sodium sulfite is used commercially in the pulp and paper industry, as an oxygen scavenger agent and is used to treat water being fed to steam boilers to avoid corrosion issues. It is also used as a reducing agent in the textile industry as a bleaching, desulfurizing, and dechlorinating agent. In the production of sodium thiosulphate, it is used as a reagent in sulfonation and sulfomethylation agents.