Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reactions written here are correct? (1) \(\mathrm{B}+\mathrm{NaOH} \longrightarrow 2 \mathrm{Na}_{3} \mathrm{BO}_{3}+\mathrm{H}_{2}\) (2) \(\mathrm{P}_{4}+\mathrm{NaOH}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{NaH}_{2} \mathrm{PO}_{2}+\mathrm{PH}_{3}\) (3) \(\mathrm{S}+\mathrm{NaOH} \longrightarrow \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}+\mathrm{Na}_{2} \mathrm{~S}+\mathrm{H}_{2} \mathrm{O}\) (a) 1 only (b) 3 only (c) 2 and 3 (d) 1,2 and 3

Step-by-Step Solution

Answer

Correct reactions are (2) and (3). Option (c) is the correct answer.

Step 1: Analyze Reaction 1

Check if the reaction is balanced and chemically feasible. The reaction is \( \mathrm{B}+\mathrm{NaOH} \longrightarrow 2 \mathrm{Na}_{3}\mathrm{BO}_{3}+\mathrm{H}_{2} \). For this to be balanced and feasible, the oxidation states must be consistent, and the reaction should conserve mass. Here, the reaction creates an incorrect stoichiometry and compound formation for boron with NaOH. Therefore, this reaction is not correct.

Step 2: Analyze Reaction 2

Evaluate \( \mathrm{P}_{4}+\mathrm{NaOH}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{NaH}_{2} \mathrm{PO}_{2}+\mathrm{PH}_{3} \). This reaction is typical for phosphorus(V) reaction in alkaline conditions where hypophosphite (\(\mathrm{NaH}_{2} \mathrm{PO}_{2}\)) and phosphine (\(\mathrm{PH}_{3}\)) are possible products. The stoichiometry can be balanced given appropriate amounts of reactants, making this a plausible and correct reaction.

Step 3: Analyze Reaction 3

Consider \( \mathrm{S}+\mathrm{NaOH} \longrightarrow \mathrm{Na}_{2} \mathrm{~S}_{2}\mathrm{O}_{3}+\mathrm{Na}_{2} \mathrm{~S}+\mathrm{H}_{2} \mathrm{O} \). Sulfur can react with sodium hydroxide to produce sodium thiosulfate (\(\mathrm{Na}_{2}\mathrm{S}_{2}\mathrm{O}_{3}\)) and sodium sulfide (\(\mathrm{Na}_{2}\mathrm{S}\)), along with water as a by-product, thus making this balanced and chemically valid. Therefore, this reaction is correct.

Key Concepts

Reaction Balancing

Chemical reactions must be balanced to conform to the law of conservation of mass. This means that the number of atoms of each element should be the same on both sides of the equation. If you encounter an equation such as \( \mathrm{B}+\mathrm{NaOH} \longrightarrow 2 \mathrm{Na}_{3}\mathrm{BO}_{3}+\mathrm{H}_{2} \), start by counting the atoms of each element. It's important to adjust the coefficients such that both sides of the equation have equal numbers of each type of atom. Remember:

• Each element must have the same amount before and after the reaction.
• Adjust coefficients, not subscripts, to balance equations.

To check if a reaction is balanced, count each atom carefully. For example, in Reaction 1, the stoichiometry of boron with sodium hydroxide is incorrect. Thus, the reaction cannot be balanced correctly, indicating the equation's infeasibility as written.

Oxidation States

Oxidation states, or oxidation numbers, represent the degree of oxidation of an atom within a compound. Understanding these is crucial for determining the electron transfer in reactions. A basic guide to assign oxidation states is:

• The oxidation state of a free element is always zero.
• The sum of oxidation states in a neutral compound is zero; in a polyatomic ion, it's equal to the ion's charge.
• Fluorine is always -1, oxygen usually -2 (except in peroxides), and hydrogen is +1.

Consider Reaction 2 where phosphorus reacts. Phosphorus in \( \mathrm{P}_{4} \) has an oxidation state of 0, yet it forms \( \mathrm{NaH}_{2} \mathrm{PO}_{2} \) and \( \mathrm{PH}_{3} \) with oxidation states of +1 and -3 respectively. This change indicates a transfer of electrons, underpinning the reaction's viability.

Stoichiometry

Stoichiometry involves the quantitative relationships within a chemical reaction. It calculates the reactants and products using balanced equations. For reactions to be possible, a precise ratio of reactants is essential. For example:

• In Reaction 2, the combination of \( \mathrm{P}_{4} \), \( \mathrm{NaOH} \), and \( \mathrm{H}_{2} \mathrm{O} \) forms \( \mathrm{NaH}_{2} \mathrm{PO}_{2} \) and \( \mathrm{PH}_{3} \) in a specific proportion.
• Reaction 3 uses sulfur to yield \( \mathrm{Na}_{2} \mathrm{S}_{2}\mathrm{O}_{3} \) and \( \mathrm{Na}_{2} \mathrm{S} \) with water produced, maintained by the correct stoichiometric coefficients.

Using stoichiometry ensures that all reactants convert optimally into products. It also helps to predict the quantities needed or produced, assisting with accurate and efficient chemical processes. Understanding stoichiometry is vital for solving any chemistry problem that involves reactions, especially in determining which equations are balanced correctly.