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

Option (b) 3,4: \(\mathrm{NaOH}\) with \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) and \(\mathrm{NaHCO}_{3}\) with \(\mathrm{NaOH}\) cannot coexist.

Step 1: Understanding compound interactions

When determining if compounds can coexist, consider their chemical nature in water. Compounds that react together significantly may not be stable together in an aqueous solution.

Step 2: Analyze each pair

Consider each pair:1. \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) is a mild acid while \(\mathrm{Na}_{2} \mathrm{HCO}_{3}\) contains weak alkali properties. They are unlikely to react strongly together in normal conditions.2. \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) is a stronger base, and \(\mathrm{NaHCO}_{3}\) is a mild acid; they are different forms of the same carbonate ion.3. \(\mathrm{NaOH}\) is a strong base and \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) is a moderate acid. Mixing \(\mathrm{NaOH}\) with an acid can result in neutralization.4. \(\mathrm{NaHCO}_{3}\) and \(\mathrm{NaOH}\) have a bicarbonate and a strong base, leading to further ionization or decomposition.

Step 3: Identifying reactive pairs

Reactive pairs that cannot coexist: - \(\mathrm{NaOH}\) and \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\) (Pair 3) would neutralize due to \(\mathrm{NaOH}'s\) strong base nature.- \(\mathrm{NaHCO}_{3}\) and \(\mathrm{NaOH}\) (Pair 4) can react further due to the presence of the strong base \(\mathrm{NaOH}\).

Step 4: Select the correct option

Based on the analysis, the pairs that cannot coexist together in a solution are the ones listed in option (b) 3,4.

Key Concepts

Compound Interaction

In aqueous chemistry, understanding how compounds interact is vital to predicting their behavior when mixed in solution. When compounds are combined in water, they dissociate into ions that can interact in various ways depending on their chemical nature. Some interactions are mild and result in no significant changes, while others can lead to chemical reactions that disrupt the stability of the compounds in solution.

• For example, strong acids and bases will generally react with each other, neutralizing each other and forming water and salts.
• Weak acids and bases might only partially react and can often coexist without significant changes.

Understanding these interactions helps predict whether compounds can coexist stably in aqueous solutions or if they will undergo reactions.

Acid-Base Reactions

Acid-base reactions are common and highly significant in aqueous chemistry. These reactions generally occur when an acid and a base are mixed, leading to a neutralization reaction. The strength of the acid and base involved plays a critical role in determining the outcome.

• Strong acids and strong bases, like \( ext{NaOH}\) (a strong base) and many acids, will typically react completely, neutralizing to form salts and water.
• In the case of mild acids and bases, such as \( ext{NaHCO}_3\), reactions might not go to completion, allowing them to coexist with each other in certain conditions.

This is why understanding the nature and strength of the acids and bases involved helps predict whether a stable aqueous system can be achieved.

Solubility

Solubility plays a significant role in determining how compounds interact and react in aqueous solutions. Solubility is the ability of a substance to dissolve in a solvent, which in our topic, is water. For ionic compounds, solubility involves dissociation into ions, which are then free to move and interact:

• Highly soluble compounds dissociate completely in water, increasing the likelihood of interactions with other ions present.
• Compounds with low solubility may precipitate out or not fully dissociate, limiting interactions.
• For instance, if both a strong acid and base like \( ext{NaOH}\) and \( ext{NaH}_2 ext{PO}_4\) are fully soluble, they are more likely to react completely, highlighting the importance of solubility knowledge.

Understanding solubility ensures the prediction of reaction outcomes in aqueous solutions.

Chemical Stability

Chemical stability refers to the ability of a compound to retain its composition and resist chemical change in a given environment, such as an aqueous solution. Stability is influenced by several factors including the nature of the ions and the strength of any acids or bases involved.

• Compounds that are more reactive tend to be less stable in solution due to their propensity to react with other ions or molecules.
• For example, mixing a strong base like \( ext{NaOH}\) with a moderate acid such as \( ext{NaH}_2 ext{PO}_4\) may result in a neutralization reaction that disrupts the original composition.
• Conversely, stable compounds typically do not react easily and maintain their form in solution.

Assessing the chemical stability of compounds helps in understanding which solutions will remain unchanged and which may undergo transformations.