Question

Explain why \(\mathrm{SO}_{2}\) is used as a reducing agent but \(\mathrm{SO}_{3}\) is not.

Short answer

SO_2 acts as a reducing agent because it can be further oxidized from an oxidation state of +4 to +6. In contrast, SO_3 does not act as a reducing agent because sulfur is already at its maximum stable oxidation state of +6.

Step-by-step solution

Understanding oxidation states

To understand why sulfur dioxide (SO_2) acts as a reducing agent, evaluate the oxidation state of sulfur in this compound. In SO_2, sulfur has an oxidation state of +4. The possibility to increase its oxidation number to +6 allows SO_2 to act as a reducing agent by getting oxidized itself and reducing other substances.

Examining the stability and possibility of further oxidation

However, in sulfur trioxide (SO_3), sulfur already has an oxidation number of +6, which is the highest oxidation state sulfur can achieve in its stable compounds. Since it cannot be further oxidized, it does not act as a reducing agent.

Understanding the role of a reducing agent

A reducing agent is a substance that donates electrons to another substance in a chemical reaction. Sulfur dioxide can donate electrons since it can be oxidized from +4 to +6. Sulfur trioxide cannot donate electrons in a typical reaction because it is already in the highest stable oxidation state for sulfur.

Key concepts

Oxidation States

Understanding oxidation states is fundamental in explaining the behavior of different chemical species in reactions. In essence, the oxidation state, also known as oxidation number, is an indicator of the degree of oxidation of an atom in a chemical compound. It represents the number of electrons that an atom can lose, gain, or share when participating in chemical reactions.

Oxidation states help us track electron movement in redox (reduction-oxidation) reactions, where one species is oxidized by losing electrons and another is reduced by gaining electrons. Familiarity with the periodic table and the typical oxidation states of its elements allows chemists to predict and explain the reactivity of compounds. For instance, elements can often have multiple oxidation states, which can influence the role they play in a chemical reaction, as exhibited with sulfur in sulfur dioxide (SO2) and sulfur trioxide (SO3).

Tips for Understanding Oxidation States:

• Each element has its own range of possible oxidation states that can be found using established rules or the periodic table.
• Different oxidation states determine if a compound can be an oxidizing or reducing agent.
• Neutral molecules sum to an oxidation number of zero, while in ions the sum equals the charge of the ion.

Sulfur Dioxide (SO2)

Sulfur dioxide (SO2) is a notable compound with a variety of applications, including its use as a reducing agent. This gaseous substance is composed of sulfur and oxygen, where sulfur exhibits an oxidation state of +4. The lower oxidation state allows this molecule the potential to be further oxidized, which is a key characteristic of reducing agents.

A reducing agent is often involved in redox reactions, where it essentially donates electrons to another substance, thereby becoming oxidized itself. This is what sulfur dioxide can do - it can increase its oxidation number from +4 to +6, corresponding to the oxidation state of sulfur in sulfur trioxide (SO3). This electron-donating ability is what makes SO2 a valuable reducing agent in industrial processes, such as the purification of metal ores.

Applications of Sulfur Dioxide:

• Acts as a preservative in the food industry due to its anti-bacterial properties.
• Used in wine-making to prevent oxidation and maintain flavor.
• Utilized in the production of sulfuric acid through the contact process.

Sulfur Trioxide (SO3)

Sulfur trioxide (SO3) is distinguished from sulfur dioxide by the oxidation state of sulfur, which is at its maximum of +6 in SO3. This highly reactive compound is a crucial ingredient in the synthesis of sulfuric acid, one of the most important industrial chemicals. The oxidation state of +6 implies that the sulfur atom in SO3 is fully oxidized and, therefore, lacks the ability to undergo further oxidation to act as a reducing agent.

Since the reducing ability of a substance correlates with its potential to donate electrons, sulfur trioxide, being in the highest stable oxidation state for sulfur, cannot perform this role. It remains a strong oxidizing agent, which instead has the capacity to accept electrons.

Industrial Importance of Sulfur Trioxide:

• Primary use in the manufacture of sulfuric acid via the contact process, where SO3 reacts with water.
• Acts as a precursor for the production of detergents, explosives, and pharmaceuticals.
• Useful in research as a reagent to introduce the sulfate group into organic molecules.