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

Question

The pair of compounds which cannot exist together in aqueous solution is (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

The pair \( \mathrm{NaHCO}_3 \) and \( \mathrm{NaOH} \) cannot exist together in aqueous solution.

Step 1: Identify the Properties of Each Compound

First, recognize the chemical nature of each compound. \( \mathrm{NaH}_2 \mathrm{PO}_4 \) is a weak acid (sodium dihydrogen phosphate), \( \mathrm{Na}_2 \mathrm{HCO}_3 \) is a basic salt (sodium bicarbonate), \( \mathrm{Na}_2 \mathrm{CO}_3 \) is a strong base (sodium carbonate), \( \mathrm{NaOH} \) is a strong base, and \( \mathrm{NaHCO}_3 \) is a weak base (baking soda).

Step 2: Analyze Acid-Base Reactions

Evaluate potential acid-base reactions. \( \mathrm{NaH}_2 \mathrm{PO}_4 \) can react with strong bases like \( \mathrm{NaOH} \) to form salts, whereas some weak acids or bases may not significantly react with similar strength bases or salts.

Step 3: Evaluate Pair (a)

For \( \mathrm{NaH}_2 \mathrm{PO}_4 \) and \( \mathrm{Na}_2 \mathrm{HCO}_3 \), the phosphate, being weakly acidic, can react with sodium bicarbonate to form some intermediate compounds but they can coexist due to a weak base-weak acid combination.

Step 4: Evaluate Pair (b)

\( \mathrm{Na}_2 \mathrm{CO}_3 \) and \( \mathrm{NaHCO}_3 \) are part of the carbonate buffer system and will not undergo a reaction to invalidate co-existence in solution.

Step 5: Evaluate Pair (c)

In the pair \( \mathrm{NaOH} \) and \( \mathrm{NaH}_2 \mathrm{PO}_4 \), a strong base reacts with the weak acid phosphate to form \( \mathrm{Na}_2 \mathrm{HPO}_4 \) and water, enabling their coexistence.

Step 6: Evaluate Pair (d)

For \( \mathrm{NaHCO}_3 \) and \( \mathrm{NaOH} \), a strong base like \( \mathrm{NaOH} \) will convert \( \mathrm{NaHCO}_3 \) into \( \mathrm{Na}_2 \mathrm{CO}_3 \) and water, causing a chemical change which prevents original substances from coexisting.

Step 7: Conclusion

Based on these evaluations, the compounds that cannot exist together in a stable coexistent form in aqueous solution are \( \mathrm{NaHCO}_3 \) and \( \mathrm{NaOH} \).

Key Concepts

Chemical Properties

Understanding the chemical properties of compounds is fundamental when assessing their behavior in different solutions. Each compound has unique chemical characteristics that determine its reactivity and interactions with other substances. For instance, sodium dihydrogen phosphate ( NaH_2PO_4 ) is recognized as a weak acid. Its ability to donate a proton in solution allows it to react with bases. In contrast, substances like sodium carbonate ( Na_2CO_3 ) and sodium hydroxide ( NaOH ) are considered strong bases. They have a high capacity to accept protons and can significantly alter the pH of a solution.

On the other hand, sodium bicarbonate ( NaHCO_3 ) acts as a weak base, with less impact on pH compared to stronger bases. This difference in chemical properties influences how these compounds interact in aqueous solutions.

• Acids release hydrogen ions ( H^+ ), making the solution more acidic.
• Bases accept hydrogen ions, making the solution more basic.
• The strength of an acid or base is determined by its degree of dissociation in water.

Knowing these properties helps predict reactions and stability in mixed solutions, crucial for understanding conceptually complex scenarios like the coexistence of multiple compounds in an aqueous environment.

Buffer Systems

Buffer systems play a critical role in maintaining the pH of a solution, resisting significant changes despite the addition of stronger acids or bases. A buffer usually consists of a weak acid and its conjugate base or a weak base and its conjugate acid. This system works by neutralizing added acids or bases, absorbing the shock of sudden changes in pH.

For instance, the carbonate buffer system involving sodium bicarbonate ( NaHCO_3 ) and sodium carbonate ( Na_2CO_3 ) demonstrates how buffers operate. When an acid is added, the bicarbonate neutralizes it by forming carbonic acid, while adding a base would convert bicarbonate into carbonate, stabilizing pH levels. This resilience to pH change is indispensable in biological systems and many industrial and chemical processes.

• Buffers can only mitigate changes within a particular range, specific to the buffer pair.
• The effectiveness of a buffer is characterized by its buffer capacity.
• Buffers in living organisms are crucial for maintaining homeostasis.

Understanding buffer systems gives insight into how chemicals interact and coexist without radical alteration of their environment, making them a cornerstone of chemical equilibrium in aqueous solutions.

Aqueous Solutions

Aqueous solutions are solutions where water acts as the solvent. In such solutions, the dissociation or interaction of compounds often leads to chemical reactions. The nature of these reactions, particularly in acid-base chemistry, depends on the solutes' properties.

When a solution contains weak and strong acids or bases, reactions often occur that can alter the availability of the original substances. For example, a strong base like sodium hydroxide ( NaOH ) in water will almost completely dissociate, potentially affecting weak acids or bases in the solution. This interaction may cause a chemical change, preventing the coexistence of the initial compounds. In the case of sodium bicarbonate ( NaHCO_3 ) with sodium hydroxide, the latter would convert it into forming stronger bases, leading them to no longer coexist in their original forms.

• Aqueous solutions provide a medium for ionic interactions.
• The solubility and dissociation levels of compounds in water are crucial for predicting behavior in solutions.
• Accurate predictions of reactions and stability require an understanding of both the solute properties and the solvent’s characteristics.

Studying aqueous solutions is essential for grasping the behavior of chemicals in water, which directly impacts fields such as biology, chemistry, and pharmacology.