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*Revision Materials and Past Papers* 1 Atomic Structure and the Periodic Table 2 Bonding and Structure 3 Redox I 4 Inorganic Chemistry and the Periodic Table 5 Formulae, Equations and Amounts of Substance 6 Organic Chemistry I 7 Modern Analytical Techniques I 8 Energetics I 9 Kinetics I 10 Equilibrium I 11 Equilibrium II 12 Acid-base Equilibria 13 Energetics II 14 Redox II 15 Transition Metals 16 Kinetics II 17 Organic Chemistry II 18 Organic Chemistry III 19 Modern Analytical Techniques II RP Required Practicals

3 Redox I

3.1 Oxidation Numbers and Rules 3.2 Oxidation, Reduction, and Electron Transfer 3.3 Disproportionation and Redox Classifications 3.4 Ionic Half-Equations and Redox Equations

Redox I: Oxidation Numbers and Formulae

Specification Reference Topic 3, points 1, 2, 9, 10 (Edexcel A-Level Chemistry)

Quick Notes

  • Oxidation number (state): Represents the "charge" an atom would have if the bonding in the compound was fully ionic.
    • We assume electrons in bonds 'belong' to the more electronegative atoms, even if no actual electron transfer has occurred.
  • Rules for oxidation states:
    • Elements in their natural state have an oxidation number of 0.
    • Oxygen is usually −2 (except in peroxides where it is −1).
    • Hydrogen is usually +1 (except in metal hydrides where it is −1).
    • Group 1 metals are +1, Group 2 metals are +2.
    • The sum of oxidation states in a neutral compound is 0.
    • The sum of oxidation states in an ion equals the charge of the ion.
  • Roman numerals in names indicate the oxidation number of an element
    (e.g. iron(III) = +3).

Full Notes

Oxidation Numbers

Oxidation numbers help track electron transfer in reactions. It is straightforward to see how atoms have lost or gained electrons when ions get formed, however it can be harder to see how atoms have lost or gained electron density when dealing with molecules.

Example Carbon combustion

Carbon is oxidised to form carbon dioxide when combusted. However, no ions get formed, meaning it isn’t immediately clear how electrons are involved.

OCR A A-Level Chemistry diagram showing oxidation of carbon to carbon dioxide in combustion.

To help, we consider each atom to have an ‘imaginary’ charge, described as its oxidation number (or state).

Rules for Assigning Oxidation States

Element / Case Oxidation State
Uncombined elements (e.g., O2, N2, Fe) 0
Group 1 metals +1
Group 2 metals +2
Oxygen (usual) −2
Oxygen in peroxides (O22−) −1
Oxygen in OF2 +2
Hydrogen (usual) +1
Hydrogen in metal hydrides (e.g. NaH) −1
Neutral compound Sum of oxidation states = 0
Polyatomic ion Sum of oxidation states = charge of ion

Using these rules, we can see how carbon gets oxidised from an oxidation state of 0 in C(s) to +4 in CO2(g).

OCR A A-Level Chemistry diagram showing electron transfer and change in oxidation state of carbon from 0 to +4.

An increase in oxidation number (gets more positive) means oxidation has occurred. A decrease in oxidation number (gets more negative) means reduction has occurred.

Worked Example

Assign oxidation states in H2SO4 (sulfuric acid).

  1. H = +1 (there are 2 H, total +2).
  2. O = −2 (there are 4 O, total −8).
  3. The total charge must be 0, so S must be +6 to balance the equation:
    2(+1) + S + 4(−2) = 0 → S = +6.

Roman Numerals in Names

Oxidation numbers are shown in Roman numerals in the names of compounds, particularly for transition metals and other elements with variable oxidation states.

Examples

Summary