When sulfur dioxide gas is passed through an acidified solution of potassium dichromate, a noticeable chemical reaction occurs, resulting in a distinct color change and the formation of new chemical substances.
Specifically, the acidified potassium dichromate solution, which is typically orange, changes to green. This color transformation is a key indicator of the reaction taking place and is often used as a test for the presence of sulfur dioxide.
The Chemical Reaction
The reaction between sulfur dioxide ($\text{SO}_2$), acidified potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$), and sulfuric acid ($\text{H}_2\text{SO}_4$) is a redox (reduction-oxidation) reaction. In this reaction:
- Sulfur dioxide acts as a reducing agent. It gets oxidized from sulfur in the +4 oxidation state to sulfur in the +6 oxidation state (in the sulfate ions).
- Dichromate ions ($\text{Cr}_2\text{O}_7^{2-}$) act as an oxidizing agent. The chromium in the dichromate ion is in the +6 oxidation state and gets reduced to chromium(III) ions ($\text{Cr}^{3+}$), which are responsible for the green color.
The acidification is crucial, providing the necessary hydrogen ions ($\text{H}^+$) for the reaction to proceed effectively. Sulfuric acid is commonly used for this acidification.
Products Formed
As per the provided reference, the reaction of sulfur dioxide with potassium dichromate and sulfuric acid produce potassium sulfate, chromium(III) sulfate and water and the solution turns from orange to green.
The main products formed are:
- Potassium sulfate ($\text{K}_2\text{SO}_4$)
- Chromium(III) sulfate ($\text{Cr}_2(\text{SO}_4)_3$)
- Water ($\text{H}_2\text{O}$)
The overall balanced chemical equation for this reaction is:
$\text{K}_2\text{Cr}_2\text{O}_7\text{(aq)} + \text{H}_2\text{SO}_4\text{(aq)} + 3\text{SO}_2\text{(g)} \rightarrow \text{K}_2\text{SO}_4\text{(aq)} + \text{Cr}_2(\text{SO}_4)_3\text{(aq)} + \text{H}_2\text{O(l)}$
Visual Summary
Here is a summary of the key changes observed:
| Substance Involved | Initial State / Appearance | Final State / Appearance | Role |
|---|---|---|---|
| Acidified Potassium Dichromate Solution | Orange liquid | Green liquid (due to Cr³⁺ ions) | Oxidizing Agent |
| Sulfur Dioxide Gas | Colorless gas | Reacts, converted to sulfate | Reducing Agent |
| Chromium(VI) ions (in Dichromate) | +6 oxidation state | +3 oxidation state (in Cr³⁺) | Reduced |
| Sulfur (in SO₂) | +4 oxidation state | +6 oxidation state (in Sulfate) | Oxidized |
Practical Applications
The characteristic color change from orange to green when sulfur dioxide reacts with acidified potassium dichromate makes this reaction useful in analytical chemistry.
- Test for Sulfur Dioxide: This reaction serves as a sensitive test for the presence of sulfur dioxide gas. Passing a gas through an orange acidified potassium dichromate solution and observing a color change to green confirms the presence of $\text{SO}_2$.
Summary of the Process
In essence, when sulfur dioxide encounters acidified potassium dichromate, a redox reaction occurs where sulfur dioxide reduces the orange dichromate ions to green chromium(III) ions, itself being oxidized to sulfate ions. This results in the formation of potassium sulfate, chromium(III) sulfate, and water, accompanied by a visible color change from orange to green.
