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 is the correct answer.

Step 1: Identify the Nature of Each Compound

Understand the chemical nature of each compound by identifying if it is acidic, basic, or neutral. For example, \( \mathrm{NaOH} \) is a strong base, and \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) is an acidic salt. \( \mathrm{NaHCO}_{3} \) and \( \mathrm{Na}_{2} \mathrm{CO}_{3} \) are basic in nature, while \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) is weakly acidic.

Step 2: Determine Reaction Possibilities

Consider possible reactions between the compounds if mixed in an aqueous solution. Generally, an acidic compound can react with a basic compound to form water and a salt, possibly leading to the compounds not coexisting as written.

Step 3: Check Compatibility of Each Pair

Evaluate each pair for reactions: 1. \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) (acidic) and \( \mathrm{Na}_{2} \mathrm{HCO}_{3} \) (basic) - No significant reaction.2. \( \mathrm{Na}_{2} \mathrm{CO}_{3} \) and \( \mathrm{NaHCO}_{3} \) both are basic - Compatible.3. \( \mathrm{NaOH} \) (strong base) and \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) (acidic) - Likely to react.4. \( \mathrm{NaHCO}_{3} \) and \( \mathrm{NaOH} \) both basic - Likely to react, forming \( \mathrm{Na}_{2} \mathrm{CO}_{3} \).

Step 4: Eliminate Logical Inconsistencies

Single out the pairs with potential chemical reactions that prevent their coexistence in solution. In this case, the pairs \( \mathrm{NaOH} \) with \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) (3) and \( \mathrm{NaHCO}_{3} \) with \( \mathrm{NaOH} \) (4) might not coexist in solution due to neutralization reactions, confirming their incompatibility.

Step 5: Select the Correct Answer Choice

From the analysis, \( \mathrm{NaOH} \) and \( \mathrm{NaH}_{2} \mathrm{PO}_{4} \) as well as \( \mathrm{NaHCO}_{3} \) and \( \mathrm{NaOH} \) cannot exist together. Option (b) 3,4 is the correct answer.

Key Concepts

Compound Compatibility

In chemistry, understanding compound compatibility is crucial, especially when dealing with solutions. Not all compounds can coexist peacefully in an aqueous environment. This incompatibility largely depends on the chemical nature of the compounds involved. Some common reasons include:

• Reactivity: Compounds may react with each other, creating new substances.
• Solubility: Sometimes, a compound will precipitate out of solution, making it incompatible with others.
• pH Effects: Acidic and basic compounds can drastically change the pH of a solution, affecting other substances present.

Considering these factors helps chemists predict the behavior of compounds when mixed, ensuring that no unwanted reactions occur during experiments or practical applications. Studying these compatibilities provides insight into the practical and theoretical aspects of aqueous solution chemistry.

Acid-Base Reactions

Acid-base reactions are a central concept in chemistry, often occurring when acidic and basic compounds are mixed. In aqueous solutions, these reactions are vital because they typically result in neutralization, where the acid and base react to form water and salt. This can significantly alter the outcome of a reaction or experiment. Common Scenarios:

• An acid reacting with a strong base will usually neutralize, potentially making coexistence impossible.
• Weak acids reacting with strong bases (or vice versa) lead to incomplete neutralization, sometimes allowing them to coexist briefly.
• The presence of an acid can cause a basic compound to form unexpected precipitates.

To predict acid-base reactions, it’s essential to understand the nature of the involved compounds, particularly their strength (strong vs. weak), which influences their reactivity. Familiarity with these principles is necessary for troubleshooting during chemical processes or predicting outcomes.

Chemical Reactions in Solution

Chemical reactions in solutions form the basis of much of aqueous chemistry. Solutions, which usually involve water as a solvent, allow for various reactions, including precipitation, redox, and acid-base interactions. These reactions depend on numerous factors, including concentration, temperature, and the specific properties of the solutes. Key Reaction Types:

• Precipitation Reactions: Occur when two soluble salts react to form an insoluble product, or precipitate.
• Redox Reactions: Involves the transfer of electrons between reactants, usually in specific redox-active species like metals and nonmetals.
• Acid-Base Reactions: As discussed, these result in neutralization, producing water and salt.

Understanding how these reactions occur in solution helps in laboratory settings and industrial applications. Chemists can manipulate conditions such as concentration to drive reactions in their desired direction, making solution chemistry a vital field of study.