Lesson 11 of 11 · 14 min
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Must-know facts
20 facts
- 01Oxidation: loss of electrons or rise in oxidation number. Reduction: gain of electrons or fall in oxidation number.
- 02The oxidising agent is itself reduced; the reducing agent is itself oxidised.
- 03Oxidation and reduction always occur together in a redox reaction.
- 04A free element has oxidation number 0 in every allotrope.
- 05Oxygen: −2 normally, −1 in peroxides, −½ in superoxides, +2 in OF₂, +1 in O₂F₂.
- 06Hydrogen: +1 normally, −1 in active metal hydrides such as NaH and CaH₂.
- 07Fluorine is −1 in every compound and never disproportionates.
- 08Oxidation numbers add to zero in a molecule and to the charge in an ion.
- 09Zn displaces Cu from Cu²⁺ solution and Cu displaces Ag from Ag⁺ solution, but Cu does not displace Zn.
- 10Four types of redox reaction: combination, decomposition, displacement and disproportionation.
- 11CaCO₃ → CaO + CO₂ is a decomposition that is not redox.
- 12In disproportionation the same element in one state is both oxidised and reduced.
- 13ClO₄⁻ cannot disproportionate because Cl is at its highest state, +7.
- 14Halogen oxidising power: F₂ > Cl₂ > Br₂ > I₂.
- 15Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O; each dichromate takes six electrons.
- 16KMnO₄ acts as a self-indicator; dichromate titrations use diphenylamine.
- 17Iodometry: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻ with starch as indicator.
- 18In the Daniell cell electrons flow from Zn to Cu in the external wire; the salt bridge completes the circuit.
- 19E° of the standard hydrogen electrode is 0.00 V by convention.
- 20More negative E° means a stronger reducing agent; more positive E° means a stronger oxidising agent.
Common traps
Where marks are lost
Formulas
6 to know
Sum rule
Σ (oxidation numbers) = charge on the species
Zero for a neutral molecule.
Balancing by oxidation number
total increase in O.N. = total decrease in O.N.
Fixes the coefficients of oxidant and reductant.
Dichromate half reaction
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
Acidic medium; Cr +6 → +3.
Permanganate half reaction (acid)
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Mn +7 → +2; E° = +1.51 V.
Iodometry
I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
Two thiosulphate per iodine molecule.
Daniell cell reaction
Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
Zn is oxidised, Cu²⁺ reduced.
Key terms
11 terms
- Redox reaction
- A reaction in which oxidation and reduction take place together.
- Half reaction
- The oxidation part or the reduction part of a redox reaction, written with electrons shown.
- Oxidising agent
- A species that accepts electrons and is reduced.
- Reducing agent
- A species that donates electrons and is oxidised.
- Oxidation number
- The charge an atom would carry if all its bonding pairs went to the more electronegative atom.
- Stock notation
- A Roman numeral in brackets giving a metal's oxidation number, as in Fe(III).
- Disproportionation
- A reaction in which one element in one state is both oxidised and reduced.
- Redox couple
- The oxidised and reduced forms of a species in a half reaction, oxidised form first.
- Salt bridge
- A tube of electrolyte in agar that joins two half cells without mixing them.
- Standard electrode potential
- The electrode potential with all species at unit concentration or 1 atm and 298 K.
- Iodometric titration
- A titration in which liberated iodine is measured with thiosulphate using starch.