2.6a: demonstrate an understanding of: (i) oxidation number — the rules for assigning oxidation numbers (ii) oxidation and reduction as electron transfer (iii) oxidation and reduction in terms of oxidation number changes (iv) how oxidation number is a useful concept in terms of the classification of reactions as redox and as disproportionation

OXIDATION NUMBERS

Not real values

Oxidation = increase in oxidation number (positive), reduction = decrease in oxidation number (decrease)

The oxidation number of an uncombined element (eg. H2, Mg) = 0 (zero)

In simple ions (eg. Mg2+, O2-), the oxidation number is the charge on the ion

The sum of oxidation numbers in a neutral compound is 0 (zero)

The sum of the oxidation numbers for an ion is the charge on the ion

Exceptions:

• Na, K = usually +1
• F = usually -1
• O = usually -2, but in peroxides like H2O2, is it -1, and in OF2, it is +2
• H = usually +1, but in metal hydrides like NaH, it is -1
• Cl = usually -1

OXIDATION AND REDUCTION

Oxidation: the gain of oxygen and loss of hydrogen or the loss of electrons

Reduction: the loss of oxygen and gain of hydrogen or the gain of electrons

Eg. Mg + Cl2 → MgCl2

• O = Mg → Mg2+ + 2e-
• R =  Cl2 + 2e- → 2Cl-

Oxidising and reducing agents:

• Oxidising agent: accepts electrons from another reactant, causing the agent itself to be reduced and the reactant oxidised
• Reducing agent: donates electrons to another reactant, causing the agent itself to be oxidised and the reactant reduced

CLASSIFICATION OF REACTIONS

A redox reaction is when one substance in a reaction is reduced, and another is oxidised

Disproportionation is when the same element is both oxidised and reduced

• eg. Cu2O(s) + H2SC4(aq) → CuSO4(aq) + Cu(s) + H2O(l)
• the Cu was reduced to Cu(s) and oxidised to CuSO4(aq)