Redox and Oxidation Numbers

Oxidation

LOSS of electrons

Reduction

GAIN of electrons

Reducing agent

The substance that loses electrons (is oxidized) and causes reduction of another species

Oxidizing agent

The substance that gains electrons (is reduced) and causes oxidation of another species

Do oxidation and reduction occur separately?

No – they always occur together; electrons lost by one species are gained by another

What is a redox reaction?

A reaction involving electron transfer (oxidation numbers change)

2Na(s) + Cl₂(g) → 2NaCl(s): what is oxidized and reduced?

Na is oxidized (0 → +1, reducing agent); Cl₂ is reduced (0 → −1, oxidizing agent)

Oxidation number (O.N.) of an element in elemental form

0 (e.g., Na, O₂, Cl₂)

O.N. of a monatomic ion

Equal to the ion's charge

Sum of O.N.s in a neutral compound

Zero

Sum of O.N.s in a polyatomic ion

Equal to the ion's charge

O.N. of Group 1A(1) elements

+1 in all compounds

O.N. of Group 2A(2) elements

+2 in all compounds

O.N. of hydrogen

+1 with nonmetals (−1 with metals, in hydrides)

O.N. of fluorine

−1 in combination with metals and boron (always −1)

O.N. of oxygen (usual)

−2 in most compounds (except with F)

O.N. of oxygen in peroxides

−1

O.N. of Group 7A(17) halogens

−1 in combination with metals, nonmetals (except O), and halogens lower in the group

Memory trick: LEO says GER

Lose Electrons = Oxidation; Gain Electrons = Reduction

Memory trick: OIL RIG

Oxidation Is Loss, Reduction Is Gain

Find O.N.s in zinc chloride (ZnCl₂)

Zn = +2, Cl = −1 each (Cl is −1 with metals; sum = +2 − 2 = 0)

Find O.N.s in sulfur trioxide (SO₃)

O = −2 each (total −6), so S = +6

Find O.N.s in nitric acid (HNO₃)

H = +1, O = −2 each (−6), so N = +5 (+1 + 5 − 6 = 0)

Redox? CaO(s) + CO₂(g) → CaCO₃(s)

NOT redox – Ca stays +2, C stays +4, O stays −2 (no change in any O.N.)

Redox? 4KNO₃(s) → 2K₂O(s) + 2N₂(g) + 5O₂(g)

YES redox – N: +5 → 0 (reduced); O: −2 → 0 (oxidized); K stays +1

Identify agents: 2Al(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂(g)

Al: 0 → +3 oxidized, so Al is the reducing agent. H: +1 → 0 reduced, so H₂SO₄ (H⁺) is the oxidizing agent

Identify agents: 2H₂(g) + O₂(g) → 2H₂O(g)

H₂ (0 → +1) is oxidized = reducing agent; O₂ (0 → −2) is reduced = oxidizing agent

Displacement reaction example: 2Li(s) + 2H₂O(l) → ?

2LiOH(aq) + H₂(g)

In 2Li + 2H₂O → 2LiOH + H₂, what is oxidized/reduced?

Li: 0 → +1 oxidized (reducing agent); H in water: +1 → 0 reduced (H₂O is the oxidizing agent)

Displacement reaction (general idea)

One element displaces another from a compound – a redox reaction involving electron transfer