Problem 125 - A Complete Resource Book in Chemistry for JEE Main

Question

The pair of compounds which cannot exist together in aqueous solution is 1\. \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) and \(\mathrm{Na}_{2} \mathrm{HCO}_{3}\) 2\. \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and \(\mathrm{NaHCO}_{3}\) 3\. \(\mathrm{NaOH}\) and \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) 4\. \(\mathrm{NaHCO}_{3}\) and \(\mathrm{NaOH}\) (a) \(1,2,3\) (b) 3,4 (c) 1,4 (d) 2,3

Step-by-Step Solution

Answer

The compounds that cannot coexist in aqueous solution are 3 (\( \mathrm{NaOH} \) and \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \)) and 4 (\( \mathrm{NaHCO}_{3} \) and \( \mathrm{NaOH} \)).

Step 1: Understand Compound Properties

First, identify the chemical properties and possible interactions of each compound. Analyze how each might react when mixed in aqueous solution. For example, compounds like \( \mathrm{NaOH} \) are strong bases, while \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) is a weak acid.

Step 2: Identify Potential Reactions

Consider potential reactions from mixing these compounds. \( \mathrm{NaOH} \) will react with weak acids like \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) to form salts and water. Similarly, analyze \( \mathrm{NaHCO}_{3} \), which can react with strong bases like \( \mathrm{NaOH} \) to produce \( \mathrm{Na}_{2} \mathrm{CO}_{3} \), water, and carbon dioxide.

Step 3: Determine Non-Coexistence

Evaluate which compounds cannot coexist due to direct reactions. For instance, \( \mathrm{NaOH} \) cannot stably coexist with \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) as they will react together, neutralizing the solution.

Step 4: Compound Pair Analysis

Analyze each pair of compounds: 1. \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) and \( \mathrm{Na}_{2} \mathrm{HCO}_{3} \) are both weak acids and bases, possibly coexisting without immediate reaction. 2. \( \mathrm{Na}_{2} \mathrm{CO}_{3} \) and \( \mathrm{NaHCO}_{3} \) coexist as they are related by a dissociation equilibrium. 3. \( \mathrm{NaOH} \) and \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) will not coexist as they undergo an acid-base reaction. 4. \( \mathrm{NaHCO}_{3} \) and \( \mathrm{NaOH} \) also react to form \( \mathrm{Na}_{2} \mathrm{CO}_{3} \).

Step 5: Conclusion on Correct Answer

The pairs that cannot coexist in aqueous solution, due to reactions that produce substantial changes or secondary compounds, are - \( \mathrm{NaOH} \) and \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) - \( \mathrm{NaHCO}_{3} \) and \( \mathrm{NaOH} \) Thus, the correct answer is option (b) 3,4.

Key Concepts

Acid-Base Reactions

In the realm of aqueous solution chemistry, acid-base reactions play a crucial role. They involve the transfer of a proton (H extsuperscript{+}) from an acid to a base. When dealing with compounds such as \( \mathrm{NaOH} \) and \( \mathrm{NaH}_2 \mathrm{PO}_4 \), understanding their behavior is pivotal.

• \( \mathrm{NaOH} \), known as sodium hydroxide, is a strong base. This means it can readily release \( \mathrm{OH}^- \) ions in solution.
• \( \mathrm{NaH}_2 \mathrm{PO}_4 \), or sodium dihydrogen phosphate, acts as a weak acid by releasing \( \mathrm{H}^+ \) ions, though not as readily as stronger acids.

When these two compounds are mixed in an aqueous solution, \( \mathrm{NaOH} \) will react with \( \mathrm{NaH}_2 \mathrm{PO}_4 \) through an acid-base reaction: \[ \mathrm{NaH}_2 \mathrm{PO}_4 (aq) + \mathrm{NaOH} (aq) \rightarrow \mathrm{Na}_2 \mathrm{HPO}_4 (aq) + \mathrm{H}_2 \mathrm{O} (l) \] This reaction results in the formation of a salt, \( \mathrm{Na}_2 \mathrm{HPO}_4 \), and water, demonstrating why these two substances cannot coexist stably without undergoing a reaction.

Compound Coexistence

In some instances, certain pairs of compounds can coexist in an aqueous solution without reacting. This is because they do not have complementary chemical properties that would cause a reaction. Consider \( \mathrm{Na}_2 \mathrm{CO}_3 \) and \( \mathrm{NaHCO}_3 \):

• \( \mathrm{Na}_2 \mathrm{CO}_3 \), sodium carbonate, and \( \mathrm{NaHCO}_3 \), sodium bicarbonate, exist in equilibrium in solution.
• Their ability to coexist stems from their chemical nature allowing for the maintenance of an equilibrium state.

In solution, they engage in the following reversible reaction:\[ \mathrm{Na}_2 \mathrm{CO}_3 (aq) + \mathrm{H}_2 \mathrm{O} (l) \leftrightarrow 2 \mathrm{NaHCO}_3 (aq) \]This balance means the compounds do not fully convert into one form. Instead, they can exist together without reacting completely, maintaining a stable solution as long as the solution conditions remain constant, such as pH and concentration.

Chemical Properties Analysis

Analyzing chemical properties is essential to predict how substances will interact in solution. Knowing whether compounds act as acids, bases, or salts helps to predict possible reactions.

• Strong bases like \( \mathrm{NaOH} \) dissociate completely in water, providing \( \mathrm{OH}^- \) ions which can neutralize acids.
• Weak acids, such as \( \mathrm{NaH}_2 \mathrm{PO}_4 \), partially dissociate, offering \( \mathrm{H}^+ \) ions but not as aggressively as strong acids.

Considering the interaction of \( \mathrm{NaHCO}_3 \) and \( \mathrm{NaOH} \):

• \( \mathrm{NaHCO}_3 \), while acting as a base, will react with strong bases like \( \mathrm{NaOH} \) to yield \( \mathrm{Na}_2 \mathrm{CO}_3 \), water, and carbon dioxide:

\[ \mathrm{NaHCO}_3 (aq) + \mathrm{NaOH} (aq) \rightarrow \mathrm{Na}_2 \mathrm{CO}_3 (aq) + \mathrm{H}_2 \mathrm{O} (l) + \mathrm{CO}_2 (g) \]This reaction illustrates that analyzing the chemical properties of compounds allows us to predict coexistence. Through this lens, one can determine possible reactions, stability, and non-coexistence in aqueous environments, contributing to a more thorough understanding of solution chemistry.