When sulfur dioxide gas is passed through an acidified potassium dichromate solution, a distinct chemical reaction occurs, resulting in the formation of chromium sulphate and water, alongside potassium sulfate.
Understanding the Reaction
This chemical reaction is a classic example of a redox (reduction-oxidation) reaction. Potassium dichromate ($K_2Cr_2O_7$) acts as a strong oxidizing agent, while sulphur dioxide ($SO_2$) acts as a reducing agent in an acidic medium. Acidification is typically achieved by adding sulfuric acid ($H_2SO_4$).
The Chemical Transformation
As sulfur dioxide gas is bubbled through the orange acidified potassium dichromate solution, the potassium dichromate undergoes reduction, and the sulfur dioxide undergoes oxidation.
According to the reference, "when the sulphur dioxide gas is passed in the acidified potassium dichromate then it yields chromium sulphate and water." This confirms the primary products formed from the transformation of the original reactants.
Specifically:
- Sulphur dioxide ($SO_2$), where sulfur has an oxidation state of +4, is oxidized to the sulfate ion ($SO_4^{2-}$), where sulfur has an oxidation state of +6.
- Potassium dichromate ($K_2Cr_2O_7$), containing the dichromate ion ($Cr_2O_7^{2-}$), where chromium is in the +6 oxidation state, is reduced to chromium(III) ions ($Cr^{3+}$).
Why the Colour Change?
One of the most striking observations during this reaction is the dramatic colour change. The initial solution is the characteristic orange colour of the dichromate ion ($Cr_2O_7^{2-}$). As the reaction proceeds, the solution turns green.
As the reference states, "In the given reaction, the potassium dichromate is reduced to the chromium sulphate... which is responsible for giving the green colour." While the reference mentions a specific oxidation state change that may differ from the standard understanding (+6 to +3), it correctly identifies that the reduction to chromium sulphate is the cause of the green colour. The green colour is due to the formation of chromium(III) ions ($Cr^{3+}$) in the chromium sulphate product.
The Balanced Chemical Equation
The overall balanced chemical equation for the reaction between sulfur dioxide gas and acidified potassium dichromate solution is:
$K_2Cr_2O_7\ (aq) + 3SO_2\ (g) + H_2SO_4\ (aq) \rightarrow Cr_2(SO_4)_3\ (aq) + K_2SO_4\ (aq) + H_2O\ (l)$
This equation shows the reactants (Potassium dichromate, Sulfur dioxide, Sulfuric acid) reacting to produce the products (Chromium(III) sulphate, Potassium sulphate, Water).
Key Outcomes & Observations
| Aspect | Description | Observation |
|---|---|---|
| Reactants | Acidified Potassium Dichromate Solution ($K_2Cr_2O_7$ + $H_2SO_4$) & $SO_2$ gas | Orange solution, Colourless gas |
| Products | Chromium(III) Sulphate ($Cr_2(SO_4)_3$), Potassium Sulphate ($K_2SO_4$), Water ($H_2O$) | Green solution, Colourless solid (dissolved) |
| Reaction Type | Redox (Reduction-Oxidation) | Colour change indicates redox reaction |
| Colour Change | From Orange to Green | Distinct visual confirmation |
| Oxidation States | Cr: +6 $\rightarrow$ +3; S: +4 $\rightarrow$ +6 | Fundamental chemical transformation |
This reaction is often used as a test for the presence of sulfur dioxide, as the colour change is very characteristic.
