7.4.2 How Can You Identify Oxidizing and Reducing Agents in a Chemical Reaction?

Consider the following reaction that takes place in the manufacture of steel:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Steps to identify oxidizing and reducing agents:

Step 1: Work out the oxidation states of all the elements involved in the reaction.

Step 2: Identify the element undergoing an increase in oxidation state — it is being oxidized. The reactant containing this element is the reducing agent.

• C is oxidized (oxidation state increases from +2 to +4).
• The reactant containing C is CO.
• So CO is the reducing agent.

Step 3: Identify the element undergoing a decrease in oxidation state — it is being reduced. The reactant containing this element is the oxidizing agent.

• Fe is reduced (oxidation state decreases from +3 to 0).
• The reactant containing Fe is Fe₂O₃.
• So Fe₂O₃ is the oxidizing agent.

Important note from the book: > "Oxidizing or reducing agent is the whole molecule or formula unit and not the atom that has undergone change in oxidation number."

Simple explanation: The oxidizing/reducing agent is the entire compound (like Fe₂O₃ or CO), not just the atom that changes.

Example 7.6: Identifying the oxidizing and reducing agents

Problem: Tungsten is used to make filaments for electric bulbs because it has the highest melting point and high electrical resistance. This metal is obtained from tungsten(VI) oxide, WO₃, by reducing it with hydrogen gas.

WO₃ + 3H₂ → W + 3H₂O

Identify the oxidizing and reducing agents in this reaction.

Problem Solving Strategy:

Step 1: Work out the oxidation states of all the elements involved in the reaction.

Step 2: Note the element that is undergoing an increase in oxidation state. Since it is being oxidized, the reactant that contains this element is the reducing agent.

• H is oxidized (0 → +1).
• The reactant containing H is H₂.
• So H₂ is the reducing agent.

Step 3: Note the element that is undergoing a decrease in oxidation state. Since it is being reduced, the reactant that contains this element is the oxidizing agent.

• W is reduced (+6 → 0).
• The reactant containing W is WO₃.
• So WO₃ is the oxidizing agent.

• Oxidizing agent: WO₃
• Reducing agent: H₂

Concept Assessment Exercise 7.5 (from the book)

1. Identify oxidizing and reducing agents in the following reactions:

(a) 2S + Cl₂ → S₂Cl₂

• S is oxidized (0 → +1) → S is the reducing agent.
• Cl is reduced (0 → −1) → Cl₂ is the oxidizing agent.

(b) 2Na + Br₂ → 2NaBr

• Na is oxidized (0 → +1) → Na is the reducing agent.
• Br is reduced (0 → −1) → Br₂ is the oxidizing agent.

2. Differentiate between oxidizing and reducing agents.

Intermediate Level

How to Identify Oxidizing and Reducing Agents

Steps:

• Write the balanced equation.
• Determine the oxidation state of each element on both sides.
• Identify the element that is oxidized (oxidation state increases). The reactant containing this element is the reducing agent.
• Identify the element that is reduced (oxidation state decreases). The reactant containing this element is the oxidizing agent.

Example: Fe₂O₃ + 3CO → 2Fe + 3CO₂

• Fe: +3 → 0 (reduced) → Fe₂O₃ is the oxidizing agent.
• C: +2 → +4 (oxidized) → CO is the reducing agent.

Common Oxidizing and Reducing Agents

Colour Changes in Redox Reactions

A colour change during a chemical reaction may indicate a redox reaction.

• Potassium permanganate(VII) is an oxidizing agent.
• Often used to test for the presence of reducing agents.
• When acidified KMnO₄ is added to a reducing agent, it changes from purple to colourless.
• This is because MnO₄⁻ (purple) is reduced to Mn²⁺ (colourless).
• Oxidation number of Mn changes from +7 to +2.

• Potassium iodide is a reducing agent.
• Often used to test for the presence of oxidizing agents.
• When KI solution is added to an acidified hydrogen peroxide solution, the solution turns reddish-brown.
• This is because iodide (I⁻) is oxidized to iodine (I₂).
• The appearance of this colour is due to the formation of iodine.

Important Points

• - Oxidizing agent: substance that causes another substance to oxidize by taking electrons from it; it is itself reduced.
• - Reducing agent: substance that causes another substance to be reduced by donating electrons to it; it is itself oxidized.
• - Oxidizing agent = electron acceptor.
• - Reducing agent = electron donor.
• - Common oxidizing agents: O₂, H₂O₂, Cl₂, KMnO₄.
• - Common reducing agents: H₂, NaBH₄, CO, Zn, Al.
• - Identifying agents: The reactant containing the element that is oxidized is the reducing agent; the reactant containing the element that is reduced is the oxidizing agent.
• - Colour changes: KMnO₄ (purple → colourless) for reducing agents; KI (colourless → reddish-brown) for oxidizing agents.
• - Examples:
• - 2Na + Cl₂ → 2NaCl (Na = reducing agent; Cl₂ = oxidizing agent)
• - Fe₂O₃ + 3CO → 2Fe + 3CO₂ (CO = reducing agent; Fe₂O₃ = oxidizing agent)
• - WO₃ + 3H₂ → W + 3H₂O (H₂ = reducing agent; WO₃ = oxidizing agent)

Remember

• - Oxidizing agent = takes electrons = itself reduced.
• - Reducing agent = gives electrons = itself oxidized.
• - Oxidizing agent = electron acceptor.
• - Reducing agent = electron donor.
• - Common oxidizing agents: O₂, H₂O₂, Cl₂, KMnO₄.
• - Common reducing agents: H₂, NaBH₄, CO, Zn, Al.
• - KMnO₄ test: purple → colourless for reducing agents.
• - KI test: colourless → reddish-brown for oxidizing agents.
• - Whole molecule or formula unit is the agent, not just the atom.

Common Mistakes

• 1. Confusing oxidizing and reducing agents.
• - Oxidizing agent takes electrons (is reduced).
• - Reducing agent gives electrons (is oxidized).
• 2. Thinking the oxidizing agent is oxidized. It is reduced.
• 3. Thinking the reducing agent is reduced. It is oxidized.
• 4. Confusing the colour changes. KMnO₄ (purple → colourless) tests for reducing agents; KI (colourless → reddish-brown) tests for oxidizing agents.
• 5. Forgetting that the whole compound is the agent. Fe₂O₃ is the oxidizing agent, not just Fe.
• 6. Forgetting to work out oxidation states first. You need oxidation states to identify which element is oxidized/reduced.

Quick Revision

• Oxidizing agent: a substance that causes another substance to oxidize by taking electrons from it; often called an electron acceptor; itself is reduced.
• Reducing agent: a substance that causes another substance to be reduced by donating electrons to it; often called an electron donor; itself is oxidized.
• Examples of oxidizing agents: O₂, H₂O₂, Cl₂, KMnO₄.
• Examples of reducing agents: H₂, NaBH₄, CO, Zn, Al.
• Example reaction: 2Na + Cl₂ → 2NaCl (Na = reducing agent; Cl₂ = oxidizing agent)
• Example reaction: Fe₂O₃ + 3CO → 2Fe + 3CO₂ (CO = reducing agent; Fe₂O₃ = oxidizing agent)
• Example reaction: WO₃ + 3H₂ → W + 3H₂O (H₂ = reducing agent; WO₃ = oxidizing agent)
• Colour changes: KMnO₄ (purple → colourless) tests for reducing agents; KI (colourless → reddish-brown) tests for oxidizing agents.
• Oxidizing or reducing agent is the whole molecule or formula unit, not the atom that has undergone change in oxidation number.

60-Second Revision

• Oxidizing agent = takes electrons = itself reduced.
• Reducing agent = gives electrons = itself oxidized.
• Common oxidizing agents: O₂, Cl₂, KMnO₄.
• Common reducing agents: H₂, CO, Zn.
• KMnO₄: purple → colourless (tests for reducing agents).
• KI: colourless → reddish-brown (tests for oxidizing agents).
• Warning: the oxidizing agent is reduced itself; the reducing agent is oxidized itself.