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Redox Reactions revision notes

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Oxidation and reduction

Oxidation and reduction
Zn→ZnX2++2 eX−  (oxidation),CuX2++2 eX−→Cu  (reduction)\ce{Zn -> Zn^{2+} + 2e-}\;(\text{oxidation}),\qquad \ce{Cu^{2+} + 2e- -> Cu}\;(\text{reduction})
Oxidation: loss of e−e^-, rise in ON (classically, gain of O or loss of H). Reduction is the reverse; both occur together.
Zn (0)ZnX2+\ce{Zn^{2+}} (+2)ZnCu (0)CuX2+\ce{Cu^{2+}} (+2)Culoses 2e⁻: oxidationgains 2e⁻: reduction
Oxidation raises the oxidation number, reduction lowers it; the two happen together
Is it a redox reaction?
4× asked, last 2024
Redox only if at least one element changes its ON. Check each element; a colour change alone proves nothing.
  • No ON change, so not redox: neutralisation, precipitation (AgX++ClX−\ce{Ag+ + Cl-}), OX2→OX3\ce{O2 -> O3}, CrX2OX7X2−⇌CrOX4X2−\ce{Cr2O7^{2-} <=> CrO4^{2-}} with pH, and forming CrOX2ClX2\ce{CrO2Cl2} (Cr stays +6). Two free elements combining is always redox.
Oxidising and reducing agents
6× asked, last 2024
oxidant+ne−→reduced form,reductant→oxidised form+ne−\text{oxidant} + n e^- \to \text{reduced form},\qquad \text{reductant} \to \text{oxidised form} + n e^-
An ion reaching its stable state by losing e−e^- is a reductant (EuX2+\ce{Eu^{2+}}); by gaining e−e^-, an oxidant (CeX4+\ce{Ce^{4+}}).
SX2OX3X2−\ce{S2O3^{2-}} (+2)SX4OX6X2−\ce{S4O6^{2-}} (+2.5)SOX4X2−\ce{SO4^{2-}} (+6)SIX2\ce{I2}: weakerBrX2\ce{Br2}: stronger
Same reductant, two oxidants: the one that drives S higher is the stronger oxidant
  • A salt cannot exist if its cation oxidises its own anion. FeX3+\ce{Fe^{3+}} and CuX2+\ce{Cu^{2+}} oxidise IX−\ce{I-}, so FeIX3\ce{FeI3} and CuIX2\ce{CuI2} are unknown, while FeIX2\ce{FeI2} and FeClX3\ce{FeCl3} exist.
  • Quick method. Compare two oxidants by how far each takes the same reductant: the higher final ON means the stronger oxidant. BrX2\ce{Br2} takes SX2OX3X2−\ce{S2O3^{2-}} to SOX4X2−\ce{SO4^{2-}} (+6); IX2\ce{I2} stops at SX4OX6X2−\ce{S4O6^{2-}} (+2.5).
Highest and lowest oxidation number
3× asked, last 2024
highest ON=v,lowest ON=v−8\text{highest ON} = v,\qquad \text{lowest ON} = v - 8
Groups 14–17 non-metals (not O, F) with vv valence e−e^-. Highest: oxidant only; lowest: reductant only; between: both.
-2-10+1+2+3+4+5+6HX2S\ce{H2S} (−2): reductant onlyS (0)SOX2\ce{SO2} (+4): bothHX2SOX4\ce{H2SO4} (+6): oxidant onlyS
Group 16: highest v=+6v=+6, lowest v−8=−2v-8=-2; in between it can go either way
  • Not 'too stable to be an oxidant'. The place in the ON range decides. S at +6 in SOX4X2−\ce{SO4^{2-}} can only be reduced, so it is an oxidant; an atom at its lowest ON, like NX3−\ce{N^{3-}}, can never be one.
  • Not v−8v-8 for a metal. It would give Na −7. A main-group metal's lowest ON is 0 (the free metal); its highest is still vv (Na +1, Al +3).
Hydrogen peroxide both ways
9× asked, last 2023
HX2OX2→OX2+2 HX++2 eX−  (reductant),HX2OX2+2 HX++2 eX−→2 HX2O  (oxidant)\ce{H2O2 -> O2 + 2H+ + 2e-}\;(\text{reductant}),\qquad \ce{H2O2 + 2H+ + 2e- -> 2H2O}\;(\text{oxidant})
O in HX2OX2\ce{H2O2} is −1, between −2 and 0. OX2\ce{O2} given off: it was the reductant. Only water formed: it was the oxidant.
HX2O\ce{H2O} (−2)HX2OX2\ce{H2O2} (−1)OX2\ce{O2} (0)Oreductant: OX2\ce{O2} given offoxidant: only water
O in HX2OX2\ce{H2O2} is −1, between −2 and 0, so it can go either way
  • Oxidant: FeX2+\ce{Fe^{2+}} to FeX3+\ce{Fe^{3+}} in acid. Reductant towards MnOX4X−\ce{MnO4-}, which it decolourises while giving off OX2\ce{O2}.

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Practise Redox Reactions

15 more concepts in 4 topics

Oxidation number, Types of redox reactions, Balancing redox reactions and Redox titrations and stoichiometry. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Oxidation number

  • Oxidation number rules9×
  • Oxygen and hydrogen exceptions2×
  • Metals in complexes and carbonyls4×
  • Average versus structural ON2×

Types of redox reactions

  • Combination and decomposition3×
  • Displacement reactions4×
  • Disproportionation14×
  • Comproportionation2×

Balancing redox reactions

  • Ion-electron (half-reaction) method12×
  • Oxidation-number method6×

Redox titrations and stoichiometry

  • Mole ratio from the balanced equation7×
  • n-factor and equivalents8×
  • Permanganate: product depends on medium19×
  • Dichromate in acid9×
  • Iodometry and iodimetry4×
  • Also remember1

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