10.04 Colour Tests for Oxidising and Reducing Agents
1. Introduction to Colour Tests for Oxidising and Reducing Agents
- Colour tests are simple and effective qualitative methods used in chemistry to identify the presence of specific oxidising or reducing agents in a solution. These tests rely on observable colour changes that occur due to redox reactions between the test substance and the reagent used.
1.1 Importance of Colour Tests
- Identification: Quickly identify the presence of oxidising or reducing agents in a sample.
- Qualitative Analysis: Determine the nature of substances without the need for complex instrumentation.
- Indicator of Redox Reactions: Visual confirmation of oxidation and reduction processes.
2. Colour Tests for Oxidising Agents
2.1 Potassium Iodide (KI) Test
Purpose: Detect the presence of oxidising agents.
Mechanism:
- Reaction: An oxidising agent oxidises the iodide ion (I⁻) to iodine (I₂).
- Colour Change:
- Before Reaction: The solution is typically colourless or pale.
- After Reaction: The solution turns yellow-brown due to the formation of iodine (I₂).
- With Starch Indicator: A dark blue colour appears, enhancing visibility.
Chemical Equations:
- Without Starch Indicator:
Oxidising Agent + 2I−→Reduced Form of Oxidising Agent + I2- Visual Change: Colourless to yellow-brown.
- With Starch Indicator: I2 + Starch → Blue Complex
- Visual Change: Yellow-brown to dark blue.
Example Reaction: Cl2(g)+2I−(aq) → 2Cl−(aq)+I2(s)
- Observation: Formation of iodine (I₂) leads to a yellow-brown or blue colour if starch is present.
Diagram Reference:
2.2 Acidified Potassium Manganate(VII) (KMnO₄) Test
Purpose: Detect the presence of reducing agents.
Mechanism:
- Reaction: Reducing agents reduce the manganate(VII) ion (MnO₄⁻) from a +7 oxidation state to the Mn²⁺ ion in a +2 oxidation state.
- Colour Change:
- Before Reaction: The solution is purple due to KMnO₄.
- After Reaction: The solution becomes colourless or pale pink as MnO₄⁻ is reduced to Mn²⁺.
Chemical Equations: MnO4−(aq)+8H+(aq)+5e−→Mn2+(aq)+4H2O(l)
- Observation: Purple solution fades to colourless or pale pink.
Example Reaction: SO32−(aq)+2MnO4−(aq)+6H+(aq)→SO42−(aq)+2Mn2+(aq)+3H2O(l)
- Observation: Purple KMnO₄ solution loses its colour, indicating the presence of a reducing agent.
Diagram:
3. Key Vocabulary
- Oxidising Agent: A substance that causes another substance to oxidise by accepting electrons; it is itself reduced in the process.
- Reducing Agent: A substance that causes another substance to reduce by donating electrons; it is itself oxidised in the process.
- Iodide Ion (I⁻): An ion of iodine with a -1 charge.
- Iodine (I₂): A diatomic molecule of iodine, which is brown in colour.
- Manganate(VII) Ion (MnO₄⁻): A strong oxidising agent with manganese in the +7 oxidation state.
- Manganese(II) Ion (Mn²⁺): A reduced form of manganese with a +2 oxidation state.
- Starch Indicator: A substance that forms a blue complex with iodine, enhancing the visibility of iodine in solution.
4. Practical Applications of Colour Tests
4.1 Detecting Oxidising Agents
Procedure:
- Add Potassium Iodide (KI): Introduce KI to the solution suspected of containing an oxidising agent.
- Observe Colour Change: Look for a change from colourless to yellow-brown.
- Add Starch Indicator (Optional): For better visibility, add starch to detect iodine through a blue colour.
Safety Tips:
- Handle oxidising agents with care as they can be reactive and potentially hazardous.
- Perform the test in a well-ventilated area or under a fume hood.
4.2 Detecting Reducing Agents
Procedure:
- Add Acidified Potassium Manganate(VII) (KMnO₄): Introduce KMnO₄ to the solution suspected of containing a reducing agent.
- Observe Colour Change: Look for a change from purple to colourless or pale pink.
Safety Tips:
- KMnO₄ is a strong oxidiser; handle with gloves and eye protection.
- Dispose of chemicals according to safety guidelines.
Worked Example 1
5.1 Example Reaction: Chlorine Gas and Potassium Iodide
Reaction: Cl2(g)+2KI(aq)→2KCl(aq)+I2(s)
Procedure:
- Mix Cl₂ Gas with KI Solution:
- Initially, the KI solution is colourless.
- Observe Colour Change:
- Formation of I₂ causes the solution to turn yellow-brown.
- Add Starch Indicator:
- The solution turns dark blue, confirming the presence of I₂.
Conclusion:
- Chlorine (Cl₂) acts as an oxidising agent by oxidising I⁻ to I₂.
- KI serves as the reducing agent by reducing Cl₂ to Cl⁻.
5.2 Example Reaction: Iron(II) Sulfate and Potassium Permanganate
Reaction: 5FeSO4(aq)+KMnO4(aq)+8H2SO4(aq)→5Fe2(SO4)3(aq)+MnSO4(aq)+8H2O(l)
Procedure:
- Add Acidified KMnO₄ to FeSO₄ Solution:
- Initially, the KMnO₄ solution is purple.
- Observe Colour Change:
- The purple colour fades as KMnO₄ is reduced to Mn²⁺, which is pale pink.
Conclusion:
- KMnO₄ acts as an oxidising agent by oxidising Fe²⁺ to Fe³⁺.
- FeSO₄ serves as the reducing agent by reducing KMnO₄ to Mn²⁺.
Worked Example 2
6. Answering Example Questions
Question 7: What is the oxidation number of the element underlined in the following elements, compounds, and ions?
a. Al(s)
- Element: Aluminium in its elemental form.
- Oxidation Number: 0
b. Cl₂(g)
- Element: Chlorine in its elemental form.
- Oxidation Number: 0
c. I⁻(aq)
- Ion: Iodide ion.
- Oxidation Number: -1
d. Cl⁻(aq)
- Ion: Chloride ion.
- Oxidation Number: -1
e. Cr₂O₇²⁻(aq)
- Ion: Dichromate ion.
- Oxidation Number of Cr:
- Let the oxidation number of Cr = xxx
- 2x+7(−2)=−22x + 7(-2) = -22x+7(−2)=−2
- 2x−14=−22x – 14 = -22x−14=−2
- 2x=+122x = +122x=+12
- x=+6x = +6x=+6
- Oxidation Number of Cr: +6
Worked Example 3
Question 8: Which is the oxidising agent in each of the following reactions?
a. Mg(s)+ZnSO4(aq)→MgSO4(aq)+Zn(s)
Analysis:
- Assign Oxidation Numbers:
- Mg: 0 → +2 (oxidation)
- Zn: +2 → 0 (reduction)
Oxidising Agent: Zn²⁺ (from ZnSO₄) as it gains electrons (is reduced).
b. Br2(aq)+2KI(aq)→2KBr(aq)+I2(aq)
Analysis:
- Assign Oxidation Numbers:
- Br₂: 0 → -1 (reduction)
- I⁻: -1 → 0 (oxidation)
Oxidising Agent: Br₂ as it gains electrons (is reduced).
c. 5Fe2+(aq)+MnO4−(aq)+8H+(aq)→5Fe3+(aq)+Mn2+(aq)+4H2O(l)
Analysis:
- Assign Oxidation Numbers:
- Fe²⁺: +2 → Fe³⁺: +3 (oxidation)
- MnO₄⁻: +7 → Mn²⁺: +2 (reduction)
Oxidising Agent: MnO₄⁻ (permanganate ion) as it gains electrons (is reduced).
Question 9: Describe the colour change you would see in reactions b and c in Question 8.
Reaction b: Br2(aq)+2KI(aq)→2KBr(aq)+I2(aq)
Colour Change:
- Before Reaction: The solution contains Br₂, which has a reddish-brown colour.
- After Reaction: Formation of I₂ imparts a yellow-brown colour to the solution.
Summary:
- Colour Change: Reddish-brown (Br₂) → Yellow-brown (I₂)
Reaction c: 5Fe2+(aq)+MnO4−(aq)+8H+(aq)→5Fe3+(aq)+Mn2+(aq)+4H2O(l)
Colour Change:
- Before Reaction: The solution contains MnO₄⁻ ions, which impart a purple colour.
- After Reaction: MnO₄⁻ is reduced to Mn²⁺, which is pale pink or colourless in solution.
Summary:
- Colour Change: Purple (MnO₄⁻) → Pale Pink/Colourless (Mn²⁺)
Quizzes
Quiz 1