Question

In the following sequence of reactions, identify the end product (D): (1) \(\mathrm{Na}_{2} \mathrm{CO}_{3} \stackrel{\mathrm{SO}_{2}}{\longrightarrow}\) (B) \(\stackrel{\mathrm{Na}_{2} \mathrm{CO}_{3}}{\longrightarrow}\) (C) Element \(\mathrm{S}, \Delta \rightarrow\) (D) \(\stackrel{I_{2}}{\longrightarrow}\) (a) \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{Na}_{2} \mathrm{~S}_{4} \mathrm{O}_{6}\) (c) \(\mathrm{Na}_{2} \mathrm{~S}\) (d) \(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\)

Short answer

The end product (D) is sodium thiosulfate, bNa_2bS_2bO_3.

Step-by-step solution

Reaction between Na2CO3 and SO2

When sodium carbonate (bNa_2bCO_3) reacts with sulfur dioxide (bSO_2), it forms sodium sulfite (bNa_2bSO_3) and carbon dioxide (bCO_2). Therefore, compound (B) after this reaction is sodium sulfite, bNa_2bSO_3.

Reaction between Na2SO3 and Na2CO3

Sodium sulfite (bNa_2bSO_3) further reacts with sodium carbonate (bNa_2bCO_3) to form sodium sulfate (bNa_2bSO_4). Hence, compound (C) is sodium sulfate, bNa_2bSO_4.

Reaction with Elemental Sulfur

Sodium sulfate (bNa_2bSO_4) is heated with elemental sulfur (bS) to produce sodium thiosulfate (bNa_2bS_2bO_3). Thus, compound (D) becomes sodium thiosulfate, bNa_2bS_2bO_3.

Reaction with Iodine

When sodium thiosulfate (bNa_2bS_2bO_3) reacts with iodine (bI_2), no new compound is formed as sodium thiosulfate is commonly used to titrate iodine. Therefore, the end product remains sodium thiosulfate, bNa_2bS_2bO_3.

Key concepts

Sodium Carbonate and Sulfur Dioxide Reaction

When sodium carbonate (\(\mathrm{Na}_2\mathrm{CO}_3\)) meets sulfur dioxide (\(\mathrm{SO}_2\)), a chemical reaction occurs that results in the formation of sodium sulfite (\(\mathrm{Na}_2\mathrm{SO}_3\)). Here's why this is important:

• It is one of the primary methods to obtain sodium sulfite, a valuable compound in the manufacturing industry.
• This reaction also releases carbon dioxide (\(\mathrm{CO}_2\)) as a byproduct.

This occurs due to the neutralization of the acidic \(\mathrm{SO}_2\) by the basic \(\mathrm{Na}_2\mathrm{CO}_3\). The equation of the reaction is: \[\mathrm{Na}_2\mathrm{CO}_3 + \mathrm{SO}_2 \rightarrow \mathrm{Na}_2\mathrm{SO}_3 + \mathrm{CO}_2\]The production of sodium sulfite in this reaction is crucial, as it sets the path for further reactions in the sequence to form sodium thiosulfate.

Sodium Sulfite Formation

Sodium sulfite (\(\mathrm{Na}_2\mathrm{SO}_3\)) is not just a simple byproduct. Its formation has practical applications and engaging chemistry.

• Sodium sulfite acts as a preservative in many food products due to its ability to inhibit microbial growth.
• It also plays a role in the bleaching process of paper and textiles.

In our chemical series, sodium sulfite further reacts with sodium carbonate that we already have. This leads to the production of sodium sulfate (\(\mathrm{Na}_2\mathrm{SO}_4\)). This reaction can be summarized by the equation:\[\mathrm{Na}_2\mathrm{SO}_3 + \mathrm{Na}_2\mathrm{CO}_3 \rightarrow \mathrm{Na}_2\mathrm{SO}_4\]This step is vital as it transitions us from sodium sulfite to a compound that will react with elemental sulfur, which then contributes to the formation of sodium thiosulfate.

Sodium Sulfate and Elemental Sulfur Interaction

In an intriguing turn, when sodium sulfate (\(\mathrm{Na}_2\mathrm{SO}_4\)) is heated with elemental sulfur (\(\mathrm{S}\)), they react to form sodium thiosulfate (\(\mathrm{Na}_2\mathrm{S}_2\mathrm{O}_3\)). This is a cornerstone reaction that serves as an excellent example of how sulfur-based compounds can transform into different but related compounds.

• Sodium thiosulfate is widely used in photographic processing, cleaning iodine stains, and as an antidote to cyanide poisoning.
• The reaction highlights the versatility of sulfur in chemical transformations.

The reaction can be represented by the following equation:\[\mathrm{Na}_2\mathrm{SO}_4 + \mathrm{S} \rightarrow \mathrm{Na}_2\mathrm{S}_2\mathrm{O}_3\]This step underline the intricate interaction between sulfur compounds, ultimately leading to the versatile sodium thiosulfate.

Iodine Titration with Sodium Thiosulfate

The journey of our chemical series concludes with the presence of sodium thiosulfate (\(\mathrm{Na}_2\mathrm{S}_2\mathrm{O}_3\)) when it is used with iodine (\(\mathrm{I}_2\)). Although no new compound is formed, the interaction is crucial in analytical chemistry.

• Sodium thiosulfate serves as a titrant in iodometry, a common laboratory technique.
• It helps determine the concentration of iodine solutions precisely.

During the titration, sodium thiosulfate reacts with iodine according to the equation:\[2\mathrm{Na}_2\mathrm{S}_2\mathrm{O}_3 + \mathrm{I}_2 \rightarrow \mathrm{Na}_2\mathrm{S}_4\mathrm{O}_6 + 2\mathrm{NaI}\]This significant step allows chemists to calculate the amount of a substance by tracking how much iodine reacts with the thiosulfate. Even though sodium thiosulfate remains as the end product in the sequence, its utility in titrations is invaluable in quantifying chemical concentrations.