Unit 16 Reduction and oxidation reactions
Key unit competence: Apply the concepts of oxidation and reduction to balance redox equations.
Key ideas
- Electrochemistry studies the link between chemical reactions and electricity; it is based on redox reactions (electron transfer).
- Definitions: oxidation = gain of oxygen, loss of hydrogen, loss of electrons, or increase in oxidation number; reduction = loss of oxygen, gain of hydrogen, gain of electrons, or decrease in oxidation number (OIL RIG). Oxidation and reduction always occur together.
- An oxidising agent accepts electrons and is itself reduced (e.g. O2, Cl2, MnO4−/H+, Cr2O72−/H+, H2O2); a reducing agent donates electrons and is itself oxidised (e.g. metals, H2, C, CO, Fe2+, I−, SO2).
- Oxidation number rules: elements 0; monatomic ion = its charge; O usually −2 (−1 in peroxides); H usually +1 (−1 in metal hydrides); group 1 +1, group 2 +2; F −1; sum = 0 for a compound or = charge for an ion.
- Balancing redox equations: (1) half-equation method: balance atoms other than O and H, then O with H2O, H with H+, charge with electrons; multiply and add so electrons cancel. (2) oxidation-number method: make the total increase equal the total decrease.
- Reactivity series of metals (K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, H, Cu, Ag, Au): a more reactive metal is a stronger reducing agent and displaces a less reactive metal from its compounds (Zn + Cu2+ → Zn2+ + Cu).
Common misconceptions
- "Oxidation always involves oxygen." No: Oxidation is any loss of electrons; Na + Cl₂ is a redox reaction without oxygen.
- "The oxidising agent is the substance that is oxidised." No: The oxidising agent is REDUCED; it oxidises something else.
- "Oxidation number and charge are always the same." No: In covalent compounds oxidation numbers are assigned by rules; there is no real charge.
Worked examples
Example 1. Assign oxidation numbers and identify the substances oxidised and reduced in Fe2O3 + 3CO → 2Fe + 3CO2.
Solution
Fe in Fe2O3: +3 → Fe: 0 (decrease): iron(III) oxide is reduced.
C in CO: +2 → C in CO2: +4 (increase): carbon monoxide is oxidised.
CO is the reducing agent; Fe2O3 is the oxidising agent.
Example 2. Write the half-equations and the overall equation for the reaction of acidified dichromate(VI) with iron(II) ions.
Solution
Reduction: Cr2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O.
Oxidation: Fe2+ → Fe3+ + e− (× 6).
Overall: Cr2O72− + 14H+ + 6Fe2+ → 2Cr3+ + 7H2O + 6Fe3+.
Colour change: orange to green.
Practice questions
Define oxidation in terms of electrons and oxidation number.
Show answer
Loss of electrons; increase in oxidation number.
Common mistake: Saying gain of electrons.
Find the oxidation number of S in H2SO3.
Show answer
+4.
Common mistake: Saying +6.
Find the oxidation number of Fe in Fe3O4 (average).
Show answer
+8/3 (about +2.67).
Common mistake: Saying +3.
Identify the oxidising agent in 2Al + Fe2O3 → Al2O3 + 2Fe.
Show answer
Fe2O3 (iron is reduced).
Common mistake: Saying aluminium.
Write the half-equation for zinc becoming zinc ions.
Show answer
Zn → Zn2+ + 2e−.
Common mistake: Writing electrons on the left.
Explain why chlorine is an oxidising agent.
Show answer
It readily gains electrons to form Cl−.
Common mistake: Saying it loses electrons.
Give the oxidation number of Mn in MnO2.
Show answer
+4.
Common mistake: Saying +2.
Name two reducing agents used in industry.
Show answer
Carbon/coke, carbon monoxide, hydrogen (any two).
Common mistake: Naming oxygen.
Is NaOH + HCl → NaCl + H2O a redox reaction? Explain.
Show answer
No; no oxidation numbers change.
Common mistake: Saying yes because water forms.
Balance Al + O2 → Al2O3 and state which is oxidised.
Show answer
4Al + 3O2 → 2Al2O3; aluminium is oxidised.
Common mistake: Saying oxygen is oxidised.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The oxidation number of Cr in K2Cr2O7 is: a. +6 2. A disproportionation reaction is one in which: b. the same element is both oxidised and reduced 3. Which is a reducing agent? c. Iron 4. Which metal can displace copper from copper(II) sulfate? d. Carbon monoxide Show answer
1 → a (+6) · 2 → b (the same element is both oxidised and reduced) · 3 → d (Carbon monoxide) · 4 → c (Iron)
- 2(+1) + 2Cr − 14 = 0.
- e.g. Cl2 + 2NaOH.
- CO removes oxygen from metal oxides.
- Iron is above copper.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
zinc · 0 · loss of electrons · −1- The oxidation number of oxygen in H2O2 is .
- In Zn + CuSO4 → ZnSO4 + Cu, the reducing agent is .
- Oxidation is .
Show answer
1: −1 · 2: zinc · 3: loss of electrons
- Peroxide.
- Zinc donates electrons.
- OIL: oxidation is loss.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. In 2Mg + O2 → 2MgO, magnesium is: a. 0 2. The oxidation number of an element in its free state is: b. Oxidation Is Loss, Reduction Is Gain (of electrons) 3. The oxidation number of hydrogen in NaH is: c. oxidised 4. OIL RIG stands for: d. −1 Show answer
1 → c (oxidised) · 2 → a (0) · 3 → d (−1) · 4 → b (Oxidation Is Loss, Reduction Is Gain (of electrons))
- Mg loses electrons: 0 → +2.
- Uncombined elements are 0.
- Metal hydrides contain H−.
- Memory aid.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
gain of electrons · +7 · reduced · gain of oxygen- Reduction is .
- In the half-equation Fe3+ + e− → Fe2+, iron(III) is .
- The oxidation number of Mn in KMnO4 is .
Show answer
1: gain of electrons · 2: reduced · 3: +7
- RIG: reduction is gain.
- It gains an electron.
- +1 + Mn − 8 = 0.