Problem 139 - A Complete Resource Book in Chemistry for JEE Main

Question

The pair of compounds which cannot exist together in aqueous solution is [IIT 1992] (a) \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) and \(\mathrm{Na}_{2} \mathrm{HCO}_{3}\) (b) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and \(\mathrm{NaHCO}_{3}\) (c) \(\mathrm{NaOH}\) and \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) (d) \(\mathrm{NaHCO}_{3}\) and \(\mathrm{NaOH}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{NaOH}\) and \(\mathrm{NaH}_{2}\mathrm{PO}_{4}\).

Step 1: Understanding the Problem

We need to find the pair of compounds that cannot coexist in an aqueous solution. This incompatibility generally arises when two compounds react with each other in water.

Step 2: Analyzing Each Pair

(a) Both \(\mathrm{NaH}_{2}\mathrm{PO}_{4}\) and \(\mathrm{Na}_{2}\mathrm{HCO}_{3}\) are acidic salts. They can coexist in the solution without reacting with each other. (b) \(\mathrm{Na}_{2}\mathrm{CO}_{3}\) is a basic salt, and \(\mathrm{NaHCO}_{3}\) is a weak acid. Since they respectively contain the \(\mathrm{CO}_{3}^{2-}\) ion and \(\mathrm{HCO}_{3}^{-}\) ion, they can interact, but won't lead to a reaction that alters the properties of the solution fundamentally.(c) \(\mathrm{NaOH}\) is a strong base, while \(\mathrm{NaH}_{2}\mathrm{PO}_{4}\) is a weak acid. They can react with one another to form \(\mathrm{Na}_{2}\mathrm{HPO}_{4}\) and water, thus they cannot coexist without reacting.(d) Both \(\mathrm{NaHCO}_{3}\) and \(\mathrm{NaOH}\) contain \(\mathrm{OH}^{-}\) which would likely lead to a neutralizing reaction but won't necessarily make them incompatible to coexist.

Step 3: Conclusion

The correct answer is option (c) \(\mathrm{NaOH}\) and \(\mathrm{NaH}_{2}\mathrm{PO}_{4}\). They cannot exist together in aqueous solution due to the acid-base neutralization reaction that will occur.

Key Concepts

Aqueous Solution Chemistry

Aqueous solution chemistry is a fascinating area of chemistry primarily focusing on reactions that occur in water. In an aqueous solution, water acts as a solvent, dissolving the solute to form a uniform mixture. This large role of water provides a unique environment, allowing for different types of chemical reactions.
A key aspect of aqueous solutions is the way ions dissociate. Positive and negative ions separate when a compound dissolves in water, enabling a wide variety of interactions and reactions.
Often, these reactions include acid-base reactions, precipitation reactions, or redox reactions.
Understanding these reactions requires a strong grasp of how compounds behave in water, and this is central to many chemistry applications.
A particularly interesting outcome occurs when ions from different dissolved substances interact, sometimes forming new compounds or being neutralized. Thus, aqueous solution chemistry is foundational for understanding how and why certain compounds might be incompatible in solution.

Incompatibility of Compounds

The incompatibility of compounds in solution arises when two dissolved substances react with each other, altering the solution's properties. Incompatibility may prevent them from coexisting without reacting. This phenomenon often occurs with acids and bases due to their natural tendency to neutralize each other.
Several factors can contribute to incompatibility, including the nature of the ions involved and the strength of the acids and bases.
For instance, a strong base like sodium hydroxide may not remain unreacted if paired with a weak acid like sodium dihydrogen phosphate in solution.
This reactive nature stems from their potential to combine, typically producing water and another substance, thus rendering them incompatible together. Classes of reactions indicative of incompatibility include acid-base neutralization and precipitation.
Recognizing incompatibility in solution involves understanding the potential for these reactions to occur upon mixing specific compounds.

Neutralization Reaction

Neutralization reactions are a subtype of chemical reactions where an acid reacts with a base to produce water and a salt. This process is fundamental in acid-base chemistry and highlights why some compounds cannot coexist.
During neutralization, hydrogen ions (\(\mathrm{H}^+\)) from the acid combine with hydroxide ions (\(\mathrm{OH}^-\)) from the base, forming water (\(\mathrm{H}_2\mathrm{O}\)).
This simultaneously eliminates the acidic and basic properties of the reactants.
The result of a neutralization reaction is typically less extreme in terms of pH compared to the original reactants. When strongly contrasting acids and bases meet, the reaction can be significant, reinforcing the concept of incompatibility in certain scenarios. Examples:

• When sodium hydroxide (\(\mathrm{NaOH}\)) and hydrochloric acid (\(\mathrm{HCl}\)) react, they form sodium chloride (\(\mathrm{NaCl}\)) and water.
• This is a classical example where something initially too acidic or too basic ends up neutral.

Understanding neutralization not only helps in grasping why certain compounds cannot coexist but also in predicting what happens when these reactions occur.