Problem 19 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The pair of compounds which cannot exist together in solution is: (a) \(\mathrm{NaHCO}_{3}\) and \(\mathrm{NaOH}\) (b) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and \(\mathrm{NaHCO}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and \(\mathrm{NaOH}\) (d) \(\mathrm{NaHCO}_{3}\) and \(\mathrm{NaCl}\)

Step-by-Step Solution

Answer

Option (a) \((\mathrm{NaHCO}_3 \text{ and } \mathrm{NaOH})\) is the pair that cannot exist together in solution.

Step 1: Understand the Chemistry

To solve this problem, we need to consider acid-base chemistry and reaction compatibility between the compounds. Specifically, we are looking for pairs that react with each other in solution, meaning they cannot coexist without reacting.

Step 2: Analyze Option (a)

In option (a), sodium bicarbonate \((\mathrm{NaHCO}_3)\) is a weak acid, while sodium hydroxide \((\mathrm{NaOH})\) is a strong base. These two compounds will react in an aqueous solution as follows: \[\mathrm{NaHCO}_3 + \mathrm{NaOH} \rightarrow \mathrm{Na}_2\mathrm{CO}_3 + \mathrm{H}_2\mathrm{O}\].Since they react together, they cannot coexist without reacting.

Step 3: Analyze Option (b)

In option (b), sodium carbonate \((\mathrm{Na}_2 \mathrm{CO}_3)\) and sodium bicarbonate \((\mathrm{NaHCO}_3)\) can coexist in solution without reacting significantly, as they are part of the bicarbonate-carbonate buffer system.

Step 4: Analyze Option (c)

In option (c), sodium carbonate \((\mathrm{Na}_2 \mathrm{CO}_3)\) and sodium hydroxide \((\mathrm{NaOH})\) both provide basic conditions, and no notable reaction occurs. Thus, they can exist together in solution without reacting.

Step 5: Analyze Option (d)

In option (d), sodium bicarbonate \((\mathrm{NaHCO}_3)\) and sodium chloride \((\mathrm{NaCl})\) do not react with each other in solution. They can co-exist without any reaction.

Step 6: Conclusion

Reviewing the options, option (a) \((\mathrm{NaHCO}_3 \text{ and } \mathrm{NaOH})\) is the pair that cannot exist together in solution as they undergo an acid-base reaction immediately.

Key Concepts

Reaction Compatibility

When we talk about reaction compatibility in chemistry, we are referring to how well different substances can exist together without undergoing a chemical reaction. In the context of acid-base chemistry, this concept helps us understand why certain compounds cannot co-exist in solution. For example, in the given exercise, sodium bicarbonate \((\mathrm{NaHCO}_3)\) and sodium hydroxide \((\mathrm{NaOH})\) are not compatible in an aqueous solution. This is because they react immediately in the presence of water. Here’s what happens:

• Sodium bicarbonate acts as a weak acid, while sodium hydroxide is a strong base.
• These react to form sodium carbonate \((\mathrm{Na_2CO_3})\) and water \((\mathrm{H_2O})\).
• The reaction takes place quickly due to their differing properties.

Understanding reaction compatibility is crucial, as it can prevent unexpected reactions from occurring in chemical processes. It is important in fields such as chemistry, biology, and environmental science.

Aqueous Solutions

A solution is termed "aqueous" when water acts as the solvent. When discussing aqueous solutions in chemistry, it's all about understanding how substances dissolve and interact in water. Water is a polar solvent, making it exceptionally good at dissolving ionic compounds and polar molecules.
This property of water is key in chemical reactions because it allows ions and molecules to interact freely. In the case of our solution exercise, any reactions between sodium bicarbonate, sodium carbonate, sodium chloride, and sodium hydroxide all occur in an aqueous medium.

• Water interacts with the molecules by surrounding them, effectively "breaking" ionic bonds.
• Solubility in water determines whether reactions will occur spontaneously.

Aqueous solutions are pervasive in all chemical processes occurring in living organisms and the environment, thanks to water's ability to mediate reactions.

Buffer Systems

Buffer systems are fascinating components in chemistry. They help maintain a stable pH even when acids or bases are added to the solution. Buffer solutions typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid. They work by reacting with added acids or bases to prevent drastic changes in pH.
In our problem, sodium bicarbonate \((\mathrm{NaHCO}_3)\) and sodium carbonate \((\mathrm{Na_2CO_3})\) form such a buffer system. Here’s how it works:

• Sodium bicarbonate \((\mathrm{NaHCO}_3)\) can act as an acid, donating a proton \((\mathrm{H}^+)\).
• Sodium carbonate \((\mathrm{Na_2CO_3})\) acts as a base, accepting protons in solution.
• Together, they neutralize additional acids or bases, maintaining the solution's pH.

Buffer systems are vital in biological systems, ensuring that enzymes and other biochemical processes function optimally. Understanding them is crucial for students studying biochemistry, medicine, and environmental science.