The chemical equation for the reaction between sulphur dioxide gas and acidified potassium dichromate solution is:
K₂Cr₂O₇ + 2H₂SO₄ + 3SO₂ → Cr₂(SO₄)₃ + K₂SO₄ + 2H₂O
This reaction is a classic example of a redox reaction where dichromate acts as an oxidizing agent and sulphur dioxide acts as a reducing agent.
Understanding the Reaction Components
Let's break down the key players in this chemical process:
- Reactants: The substances that react together.
- Potassium Dichromate (K₂Cr₂O₇): An orange crystalline solid, often used as an oxidizing agent in various applications.
- Sulphuric Acid (H₂SO₄): A strong acid used here to acidify the solution, providing the necessary hydrogen ions (H⁺) for the reaction to proceed effectively.
- Sulphur Dioxide (SO₂): A pungent gas produced by the burning of sulfur-containing materials.
- Products: The new substances formed by the reaction.
- Chromium(III) Sulphate (Cr₂(SO₄)₃): A green solution containing chromium ions in the +3 oxidation state. This distinct color change from orange to green is a visual indicator that the reaction has occurred and chromium has been reduced.
- Potassium Sulphate (K₂SO₄): A soluble salt.
- Water (H₂O): A common product in many acid-base and redox reactions.
The Balanced Chemical Equation
As provided in the reference, the balanced equation representing this reaction is:
K₂Cr₂O₇(aq) + 2H₂SO₄(aq) + 3SO₂(g) → Cr₂(SO₄)₃(aq) + K₂SO₄(aq) + 2H₂O(l)
(Note: State symbols have been added for clarity: (aq) for aqueous solution, (g) for gas, (l) for liquid.)
Redox Chemistry Explained
In this reaction, chromium in potassium dichromate starts in the +6 oxidation state (Cr⁶⁺) and is reduced to the +3 oxidation state (Cr³⁺) in chromium(III) sulphate. Simultaneously, sulphur in sulphur dioxide starts in the +4 oxidation state (S⁴⁺) and is oxidized to the +6 oxidation state (S⁶⁺), typically ending up as sulphate ions (SO₄²⁻).
This electron transfer process is the core of the redox reaction. The colour change from the characteristic orange of dichromate ions to the green of chromium(III) ions is a key visual identifier of this specific redox reaction.
Summary Table
| Component | Formula | Role | Initial Colour (in solution) | Final Colour (if changing) |
|---|---|---|---|---|
| Potassium Dichromate | K₂Cr₂O₇ | Oxidizing Agent | Orange | - |
| Sulphuric Acid | H₂SO₄ | Acidifier | Colourless | - |
| Sulphur Dioxide | SO₂ | Reducing Agent | Colourless (gas) | - |
| Chromium(III) Sulphate | Cr₂(SO₄)₃ | Product | - | Green |
| Potassium Sulphate | K₂SO₄ | Product | Colourless | - |
| Water | H₂O | Product | Colourless | - |
This reaction is often used in qualitative analysis to test for the presence of sulphur dioxide gas, as the orange-to-green color change is quite dramatic and easily observable.
