Category: Redox

Add a few drops of acidified potassium manganate(VII) solution to a solution containing an unknown agent.

If a reducing agent is present in the solution, the KMnO₄ solution will turn from purple to colorless.

In the presence of a reducing agent, a redox reaction occurs, turning the Mn⁷⁺ ion in KMnO₄ (which is purple) to Mn²⁺ ion (which is colorless or slightly pinkish).

Add a few drops of aqueous potassium iodide to a solution containing an unknown agent.

If an oxidizing agent is present in the solution, aqueous potassium iodide will turn from colorless to brown.

In the presence of an oxidizing agent, a redox reaction occurs, turning the iodide ion in KI (which is colorless) to iodine, I₂ (which is brown).

PbS (s) + 4H₂O₂ (aq) -> PbSO₄ (s) + 4H₂O (l)

PbS has been oxidized to PbSO₄.

The oxidation state of sulfur increased from -2 in PbS to +6 in PbSO₄.

H₂O₂ has been reduced to H₂O.

The oxidation state of oxygen decreased from -1 in H₂O₂ to -2 in H₂O.

Since oxidation and reduction occurred simultaneously in the reaction, it is a redox reaction.

(The reactant substance) has been oxidized / reduced to (the product substance).

The oxidation state of (the element) increased / decreased from (o.s. no.) in (the reactant substance) to (o.s. no.) in (the product substance).

Examples:

Explain whether the underlined substance has been oxidized or reduced, in terms of oxidation state.

1.   ZnO (s) + C (s) -> Zn (s) + CO (g)

Zinc oxide has been reduced to zinc.

The oxidatation state of zinc decreased from +2 in zinc oxide to 0 in zinc.

2. PbS (s) + 4H₂O₂ (aq) -> PbSO₄ (s) + 4H₂O (l)

Lead(II) sulfide has been oxidized to lead(II) sulfate.

The oxidation state of sulfur increased from -2 in lead(II) sulfide to +6 in lead(II) sulfate.

3. PbO (s) + H₂ (g) -> Pb (s) + H₂O (g)

Lead(II) oxide has been reduced to lead.

The oxidation state of lead decreased from +2 in lead(II) oxide to 0 in lead.

4. 2FeCl₂ (aq) + Cl₂ (g) -> 2FeCl₃ (aq)

FeCl₂ has been oxidized to FeCl_3.

The oxidation state of iron increased from +2 in iron(II) chloride to +3 in iron(III) chloride.

Reducing agents:

carbon (C)

carbon monoxide (CO)

hydrogen (H₂)

hydrogen sulfide (H₂S)

metals

potassium iodide (KI)

sulfur dioxide (SO₂)

ammonia (NH₃)

hydrogen peroxide (H₂O₂)
(can act as oxidizing agent as well)

Oxidizing agents:

fluorine (F₂)

chlorine (Cl₂)

bromine (Br₂)

iodine (I₂)

concentrated sulfuric acid (H₂SO₄)

nitric acid (HNO₃)

oxygen (O₂)

potassium manganate(VII) (KMnO₄)

potassium dichromate(VI) (K₂Cr₂O₇)

hydrogen peroxide (H₂O₂)
(can act as reducing agent as well. It is stronger as an oxidizing agent, especially in acidic solutions)