When sulphur dioxide (SO2) is passed through an aqueous solution containing an Iron(III) (Fe(III)) salt, a chemical reaction occurs where the Iron(III) ions are converted into Iron(II) (Fe(II)) ions.
The Reaction: Sulphur Dioxide and Iron(III)
Sulphur dioxide acts as a versatile chemical agent, capable of behaving as both a reducing agent and an oxidizing agent depending on the specific reaction conditions and the substance it interacts with. In the presence of an aqueous solution of an Fe(III) salt, such as iron(III) chloride (FeCl₃) or iron(III) sulfate (Fe₂(SO₄)₃), SO₂ primarily functions as a reducing agent.
Key Information from Reference:
Based on the provided information:
- SO2 acts as a reducing agent.
- It reacts with an aqueous solution containing Fe(III) salt.
- The reaction results in the reduction of Fe(III) to Fe(II).
- This means ferric ions are converted to ferrous ions.
This process involves the transfer of electrons from the sulphur dioxide molecule to the iron(III) ions. The Fe(III) ions, having a +3 oxidation state, gain an electron to become Fe(II) ions, which have a +2 oxidation state. Simultaneously, the sulfur in SO₂ (which has an oxidation state of +4) is typically oxidized, often forming sulfate ions (SO₄²⁻) with sulfur in a +6 oxidation state, especially in acidic or neutral solutions.
Chemical Equation
The overall reaction can be represented by the following balanced chemical equation in acidic conditions:
2Fe³⁺(aq) + SO₂(g) + 2H₂O(l) → 2Fe²⁺(aq) + SO₄²⁻(aq) + 4H⁺(aq)
- Fe³⁺(aq): Iron(III) ions (ferric ions) in aqueous solution.
- SO₂(g): Sulphur dioxide gas.
- H₂O(l): Water.
- Fe²⁺(aq): Iron(II) ions (ferrous ions) in aqueous solution.
- SO₄²⁻(aq): Sulfate ions in aqueous solution.
- H⁺(aq): Hydrogen ions (indicating acidic conditions, often present or generated during the reaction).
Effects of the Reaction
The observable effects of passing SO₂ through an Fe(III) salt solution include:
- Color Change: Solutions of Fe(III) salts are typically yellowish-brown or brown. As Fe(III) is reduced to Fe(II), the solution often becomes paler green, characteristic of Fe(II) ions.
- Change in Reactivity: The chemical properties of the solution change significantly as the iron species changes from Fe(III) to Fe(II). For example, Fe(II) salts are less acidic and have different solubility characteristics compared to Fe(III) salts in certain conditions.
Why SO₂ Acts as a Reducing Agent Here
Sulphur in SO₂ is in the +4 oxidation state. It can be oxidized further to +6, as in sulfates (SO₄²⁻), or reduced to lower oxidation states, such as 0 (elemental sulfur) or -2 (hydrogen sulfide, H₂S). When reacting with a strong oxidizing agent like Fe³⁺, which readily accepts electrons, SO₂ is oxidized and thus acts as a reducing agent.
In summary, passing sulphur dioxide through an aqueous solution of an Fe(III) salt causes the reduction of the iron from its +3 oxidation state to its +2 oxidation state, changing ferric ions to ferrous ions.
