Problem 92 - A Complete Resource Book in Chemistry for JEE Main

Question

In the detection of sulphur, sodium nitroprusside solution is added to the Lassaigne's test solution, a purple coloured is obtained. It is due to (a) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NOSNa}\right]\) (b) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (c) \(\mathrm{Fe}(\mathrm{CNS})_{3}\) (d) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NS}\right]\)

Step-by-Step Solution

Answer

The purple color is due to  ext{Na}_3[ ext{Fe}( ext{CN})_5 ext{NOS}], best approximated by option (a).

Step 1: Understanding Lassaigne's Test for Sulphur

Lassaigne's test is used to detect the presence of sulphur in an organic compound. In this test, the organic compound is fused with sodium, and then various reagents are added to detect different elements present, including sulphur.

Step 2: Forming the Lassaigne's Solution

When an organic compound containing sulphur is fused with sodium, it forms sodium sulphide ( ext{Na}_2 ext{S}). This is a key product used in the detection of sulphur.

Step 3: Reacting with Sodium Nitroprusside

The sodium sulphide solution obtained from Lassaigne's test is treated with sodium nitroprusside ( ext{Na}_2[ ext{Fe}( ext{CN})_5 ext{NO}]). If sulphide ions ( ext{S}^{2-}) are present, they react with sodium nitroprusside to produce a purple-colored complex.

Step 4: Identifying the Complex Formed

The reaction of sodium sulphide with sodium nitroprusside results in the formation of the complex  ext{Na}_3[ ext{Fe}( ext{CN})_5 ext{NOS}], which is responsible for the characteristic purple color observed. This signals the presence of sulphur.

Step 5: Selecting the Correct Option

Among the given options, option (a)  ext{Na}_3[ ext{Fe}( ext{CN})_5 ext{NOSNa}] is the closest representation. However, the correct form for the complex responsible for the purple color is  ext{Na}_3[ ext{Fe}( ext{CN})_5 ext{NOS}], which is slightly different but aligns with the selection.

Key Concepts

Sulphur Detection

Detecting sulphur is essential in chemistry, especially within Lassaigne's test. This fascinating test reveals how organic compounds interact with other elements. In Lassaigne's test, the organic compound undergoes fusion with sodium. This isn't merely for fun; it has a purpose!

• The fusion aims to convert covalently bonded sulphur into ionic form, making it easier to detect.
• When sodium combines with sulphur, it turns into sodium sulphide ( Na_2S ).
• This compound, sodium sulphide, acts as a landmark, indicating where sulphur is present.

The beauty of this test exists in this transformation. From organic mystery to clear evidence, Lassaigne's test unveils the hidden sulphur using straightforward chemical reactions.

Sodium Nitroprusside

Sodium nitroprusside is not only a mouthful to say, but it also plays a crucial role in detecting sulphur. It's a key reagent in Lassaigne's test for sulphur detection. This compound, sodium nitroprusside, has a specific purpose:

• It reacts specifically with sulphide ions ( S^{2-} ), which are released when sodium sulphide is mixed in water.
• One cannot perform the magic of turning invisible sulphides into a visible spectacle without it.
• It detects these sulphide ions and signals their presence via a color change.

Imagine this reagent as a detective seeking out sulphide's presence amidst a sea of compounds. Only sodium nitroprusside can reveal the sulphur, showing its presence with a vivid and observable change. This step is where the sodium nitroprusside takes center stage to highlight any sulphide ions lurking in the solution.

Purple Complex Formation

In the dramatic finale of the sulphur detection in Lassaigne's test, the formation of the purple complex occurs. When sodium nitroprusside meets the sulphide ions, a vibrant transformation takes place:

• The reaction between sodium nitroprusside and the sulphide ions results in the emergence of a purple-colored complex.
• This complex, Na_3[Fe(CN)_5NOS] , confirms the presence of sulphur conclusively.
• The color change to purple is a bright indicator of sulphur and is easy to identify even without sophisticated instruments.

This is what makes Lassaigne's test both reliable and visual. The purple hue is not just beautiful; it is proof positive of sulphur lying within the sample. Understanding this transformation allows one to appreciate how chemistry can visually convey reactions and results.