Problem 11 - Chemistry in 30 days

Question

\(\mathrm{Na}_{2} \mathrm{SO}_{3}\) and \(\mathrm{Na}_{2} \mathrm{~S}\) can be distinguished from each other by using (a) concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (b) an acidified \(\mathrm{KMnO}_{4}\) solution (c) an acidified \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) solution (d) a sodium nitroprusside solution

Step-by-Step Solution

Answer

Option (d), a sodium nitroprusside solution, specifically distinguishes \(\mathrm{Na}_{2} \mathrm{S}\) by forming a purple color.

Step 1: Identify Chemical Properties

Determine the key chemical properties of \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) and \(\mathrm{Na}_{2} \mathrm{S}\). \(\mathrm{Na}_{2} \mathrm{S}\) will release hydrogen sulfide gas \(\mathrm{H}_{2}\mathrm{S}\) when reacting with an acid, which has a rotten egg smell. \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) will release sulfur dioxide gas \(\mathrm{SO}_{2}\), which can decolorize potassium permanganate.

Step 2: Analyze Reaction with Concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Both sodium sulfide and sodium sulfite react with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\). \(\mathrm{Na}_{2} \mathrm{S}\) produces \(\mathrm{H}_{2}\mathrm{S}\) gas, while \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) produces \(\mathrm{SO}_{2}\) gas. Therefore, the smell of \(\mathrm{H}_{2}\mathrm{S}\) can be a distinguishing factor.

Step 3: Examine Reaction with Acidified \(\mathrm{KMnO}_{4}\) Solution

Acidified \(\mathrm{KMnO}_{4}\) solution can oxidize \(\mathrm{SO}_{2}\) to sulfate ions and will decolorize as the permanganate ion is reduced. \(\mathrm{Na}_{2} \mathrm{S}\) does not cause decolorization, so \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) can be distinguished by this reaction.

Step 4: Reaction with Acidified \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) Solution

Similar to the \(\mathrm{KMnO}_{4}\) test, \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) will reduce \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) to \(\mathrm{Cr}^{3+}\), changing the color from orange to green. \(\mathrm{Na}_{2} \mathrm{S}\) does not cause this color change.

Step 5: Use Sodium Nitroprusside Solution

When \(\mathrm{Na}_{2} \mathrm{S}\) reacts with sodium nitroprusside, it forms a deep purple complex known as a 'prussian blue' color due to the presence of sulfide ions, whereas \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) does not produce this reaction.

Key Concepts

Sodium Sulfide

Sodium sulfide, with the chemical formula \( \mathrm{Na}_2 \mathrm{S} \), is a compound that exhibits some interesting behaviors in the presence of acids. This compound is known for producing hydrogen sulfide gas \( \mathrm{H}_2 \mathrm{S} \) when it comes into contact with various acids. This gas has a characteristic rotten egg smell, which is often used as an indicator for the presence of sulfide ions.
Sodium sulfide is quite reactive and must be handled with care to avoid unpleasant odors and potential irritation. Its behavior in chemical reactions makes it a useful compound in laboratories for qualitative analysis, allowing chemists to confirm the presence of sulfide ions through distinctive and observable reactions.

Sodium Sulfite

Sodium sulfite, with the formula \( \mathrm{Na}_2 \mathrm{SO}_3 \), is another significant compound in the field of qualitative analysis. Unlike sodium sulfide, sodium sulfite reacts with acids to release sulfur dioxide gas \( \mathrm{SO}_2 \).
This gas does not carry any offensive smell like \( \mathrm{H}_2 \mathrm{S} \), but it can be identified through its property of decolorizing potassium permanganate solutions. Chemists use this property to distinguish sodium sulfite from other similar compounds. Additionally, \( \mathrm{SO}_2 \) can reduce \( \mathrm{Cr}_2 \mathrm{O}_7^{2-} \) to \( \mathrm{Cr}^{3+} \), causing a color change from orange to green, another hallmark feature that helps in its identification.

Qualitative Analysis

Qualitative analysis is a field of chemistry focused on identifying the constituents of a chemical substance using various techniques.
Rather than measuring the quantity of a component, qualitative analysis is concerned with determining what substances are present. For compounds like sodium sulfide and sodium sulfite, qualitative analysis involves recognizing distinct reactions:

• Production of gases with defining properties when reacted with specific chemicals
• Observable color changes in the presence of certain reagents
• Reaction characteristics that are unique enough to serve as identifiers

These methods allow chemists to discern one chemical from another accurately, based on their unique reactivity. A classic example includes the use of reagents like potassium permanganate or dichromate, which react distinctly with specific substances to provide visual proof of their identity.

Chemical Reactions

Chemical reactions are the backbone of chemistry, where substances interact to form new compounds. These reactions often result in energy changes, color changes, gas production, or precipitate formation. They can be used to identify compounds, such as sodium sulfide and sodium sulfite, based on their unique responses to other chemicals.
For instance, sodium sulfide releases \( \mathrm{H}_2 \mathrm{S} \) gas when reacting with acids, creating a distinct smell, while sodium sulfite releases \( \mathrm{SO}_2 \), which causes potassium permanganate solutions to lose their color. Additionally, \( \mathrm{Na}_2 \mathrm{SO}_3 \) can cause a color shift from orange to green in a reaction with dichromate ions. Another reaction includes the formation of a prussian blue color when sodium sulfide reacts with sodium nitroprusside.
Through such distinctive reactions, chemists can effectively carry out qualitative analyses, confirming the presence of specific compounds based on their characteristic chemical behavior.