Problem 12 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

In an iodine-thiosulfate titration, thiosulfate is oxidized to tetrathionate \(\left(\mathrm{S}_{4} \mathrm{O}_{6}^{2-}\right) .\) What are the average oxidation states of sulfur in thiosulfate and tetrathionate? (Section 27.6 )

Step-by-Step Solution

Answer

Average oxidation states: Sulfur in thiosulfate is +2; in tetrathionate is +2.5.

Step 1: Understand Oxidation States

The oxidation state is the hypothetical charge an atom would have if all bonds to atoms of different elements were 100% ionic. We calculate it based on known rules for oxidation states.

Step 2: Determine the Oxidation State in Thiosulfate

Thiosulfate is given by \[\mathrm{S}_{2}\mathrm{O}_3^{2-}.\] Since oxygen has an oxidation state of \(-2\) and there are three oxygens, the total contribution from oxygen is \(-6\). The overall charge of the ion is \(-2\), so the sum of the oxidation states of the sulfurs plus \(-6\) must equal \(-2\). Let the oxidation state of sulfur be \(x\). Thus, we have: \[2x - 6 = -2.\]Solving for \(x\), we find that \(2x = 4\) which gives \(x = +2.\) Therefore, the average oxidation state of sulfur in thiosulfate is \(+2\).

Step 3: Determine the Oxidation State in Tetrathionate

Tetrathionate is given by \[\mathrm{S}_{4}\mathrm{O}_6^{2-}.\]Following the same method, oxygen contributes \(-12\) because its oxidation state is \(-2\) and there are six oxygens. The overall charge of the ion is \(-2\), so the sum of the oxidation states of the sulfurs plus \(-12\) must equal \(-2\). We set up the equation: \[4x - 12 = -2.\]Solving for \(x\), we have that \(4x = 10\), which gives \(x = +2.5\). Therefore, the average oxidation state of sulfur in tetrathionate is \(+2.5\).

Key Concepts

Iodine-Thiosulfate Titration

Iodine-thiosulfate titration is a common redox titration method used in chemistry. Thiosulfate ions are oxidized by iodine, allowing us to analyze the concentration of a solution. What’s fascinating about this titration is its use of a starch indicator, which produces a deep blue color when iodine is present.

• The titration involves the reaction between iodide, which turns to iodine, and thiosulfate ions.
• As thiosulfate is added, it reacts with iodine, turning it into iodide.
• The process continues until all iodine is consumed, indicated by the disappearance of the blue color.

This color change is crucial because it signifies the endpoint of the titration. The reaction is specific and clear, making it an excellent method for determining iodine concentration in various solutions.

Thiosulfate Ion

The thiosulfate ion (\[\text{S}_2\text{O}_3^{2-}\]) is an important sulfur-containing species encountered in redox chemistry. It's especially known for its role in iodine-thiosulfate titration. In this ion:

• Two sulfur atoms are bonded, with one sulfur atom being part of a sulfate group and the other bonded to the first sulfur atom directly.
• The ion provides two negative charges, which affects its interactions with other species.
• In the titration process, thiosulfate acts as a reducing agent.

In terms of oxidation states, each sulfur atom does not contribute equally to the overall charge, which simplifies to giving an average oxidation state of (\[+2\]). Understanding the behavior and structure of thiosulfate is critical in redox reactions involving sulfur.

Tetrathionate Ion

The tetrathionate ion (\[\text{S}_4\text{O}_6^{2-}\]) is formed in the iodine-thiosulfate titration as thiosulfate ions are oxidized. Key points:

• This ion contains four sulfur atoms and six oxygen atoms.
• The tetrathionate ion carries a (\[-2\]) charge, similar to thiosulfate, but its structure is more complex.
• Each of the sulfur atoms connects through multiple bonds in a complex arrangement.

The average oxidation state of sulfur in tetrathionate is (\[+2.5\]), showcasing the increase from thiosulfate. This progression highlights the shift in sulfur oxidation states during the titration process.

Average Oxidation State

The average oxidation state is a critical concept in understanding reactions like the iodine-thiosulfate titration. It offers a simplified view of the oxidation level of elements in a compound or ion.

• This is especially useful when dealing with polyatomic ions containing the same element, such as sulfur in thiosulfate or tetrathionate.
• The calculation involves setting up an equation based on the total charges contributed by all atoms in the molecule.
• By solving equations, like those in the original exercise, we can determine that the average oxidation state is (\[+2\]) for sulfur in thiosulfate and (\[+2.5\]) for sulfur in tetrathionate.

Understanding this concept makes it easier to predict and comprehend redox processes that involve oxidation state changes.