Problem 20 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Sodium salt of an organic acid ' \(X\) ' produces effervescences with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\). \(' \mathrm{X}^{\prime}\) reacts with the acidified aqueous \(\mathrm{CaCl}_{2}\) solution to give a white precipitate which decolourises acidic solution of \(\mathrm{KMnO}_{4} .{ }^{\prime} \mathrm{X}^{\prime}\) is : (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COONa}\) (b) \(\mathrm{HCOONa}\) (c) \(\mathrm{CH}_{3} \mathrm{COONa}\) (d) \(\mathrm{Na}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\)

Step-by-Step Solution

Answer

The organic acid 'X' is Na2C2O4 (sodium oxalate).

Step 1: Analyze the Reaction with H2SO4

The sodium salt of an organic acid 'X' produces effervescence with conc. H2SO4. This suggests the release of a gas, such as CO2. This is a typical reaction of carbonates and bicarbonates, but in this context of organic acids, oxalates (sodium oxalate, Na2C2O4) can also produce CO2 with H2SO4.

Step 2: Analyze the Reaction with CaCl2

The product 'X' is said to react with acidified CaCl2 solution to give a white precipitate. Sodium oxalate, Na2C2O4, reacts with CaCl2 to form calcium oxalate, which is a white precipitate: \[ Na_2C_2O_4 + CaCl_2 \rightarrow CaC_2O_4 \ (white \ precipitate) + 2 NaCl \]

Step 3: Analyze the Reaction with KMnO4

The resulting compound must also decolorize acidified KMnO4 solution. Oxalic acid or oxalates can reduce KMnO4 to Mn2+, thus decolorizing the purple solution, which matches the given condition in the exercise.

Step 4: Determine the Identity of 'X'

Considering all the reactions and properties given (producing CO2 with H2SO4, forming a white precipitate with CaCl2, and decolorizing KMnO4), 'X' must be sodium oxalate (Na2C2O4), which is option (d).

Key Concepts

Sodium oxalate

Sodium oxalate, with the chemical formula \( \text{Na}_2\text{C}_2\text{O}_4 \), is the sodium salt of oxalic acid. In organic chemistry, sodium oxalate plays a vital role due to its unique reactions. It acts as a reducing agent and can engage in various notable chemical processes. One important aspect of sodium oxalate is its ability to produce carbon dioxide gas when it reacts with strong acids. This reaction is due to its origin from oxalic acid, a dicarboxylic acid capable of forming carbon dioxide when protonated.

Sodium oxalate is often used in analytical chemistry environments as well. It's especially helpful in volumetric analysis, where it helps indicate changes in reactions through observable events. In this context, we see sodium oxalate reacting with different agents to form noticeable products, aiding in the identification of chemical compounds.

Effervescence reaction

Effervescence is an important and observable reaction in chemistry. It involves the release of gas bubbles from a liquid due to a chemical reaction. When organic compounds like sodium oxalate react with strong acids such as concentrated sulfuric acid \((\text{H}_2\text{SO}_4)\), it results in the release of carbon dioxide gas \((\text{CO}_2)\).

This release of gas creates the visible fizzing or bubbling effect, known as effervescence. In practical terms, the gas evolved in this scenario, like carbon dioxide, provides a confirmation of the chemical nature of a compound. For the sodium oxalate, this effervescence confirms the presence of a carbonate-like structure in the compound, as carbonates and their analogs often release \(\text{CO}_2\) upon reacting with acids. Effervescence is helpful in demonstrating the decomposition of compounds into their gaseous counterparts.

Calcium oxalate precipitation

Calcium oxalate is a white precipitate formed in specific reactions with sodium oxalate. When sodium oxalate \((\text{Na}_2\text{C}_2\text{O}_4)\) reacts with calcium chloride \((\text{CaCl}_2)\) in an acidified solution, it forms calcium oxalate \((\text{CaC}_2\text{O}_4)\). This is a noteworthy reaction in identifying oxalates.

• Precipitate Formation: The precipitation of calcium oxalate is due to the low solubility of \(\text{CaC}_2\text{O}_4\) in water, which causes it to separate from the solution as a solid.
• Significance: The formation of a white precipitate visually confirms the presence of oxalate ions in the solution, serving as a diagnostic indicator in various chemical analyses.

While fine and powdery, the white appearance of calcium oxalate is telling of its solid form. This property leverages the difference in solubility between the reactants and the product.

KMnO4 reduction

Potassium permanganate \((\text{KMnO}_4)\) is a strong oxidizing agent, commonly used in redox reactions. When sodium oxalate is present in an acidic medium, it can effectively reduce \(\text{KMnO}_4\). The permanganate ions \((\text{MnO}_4^- )\) themselves are usually deep purple and visible in solution. However, when reduced, they turn colorless as manganese is converted to \(\text{Mn}^{2+}\).

• This reaction is significant for qualitative analysis as it visibly changes the solution's color, indicating the reduction process is occurring.
• The decolorization confirms the presence of a reducing agent, like sodium oxalate. This shift is not only useful in analytical chemistry for detecting compounds but also illustrates the underlying principles of redox chemistry, where electrons are transferred between species.

It provides a vivid and clear example of how chemical reactions can be observed and measured based on color changes, making KMnO4 reduction an exceptionally educational demonstration of chemical theory in action.