Topic 8 · page 4 of 29

Oxidation and reduction

Oxidation is loss of electrons and a rise in oxidation number; reduction is gain of electrons and a fall in oxidation number. Metals form positive ions this way, non-metals negative ions.

12 min read8.58.9

Read first: Roman numerals and writing formulae

Electron transfer 8.5

A memory aid: Oxidation Is Loss, Reduction Is Gain.

Electrons are not created or destroyed, so oxidation and reduction always happen together: electrons lost by one species are gained by another. A reaction with both is a redox reaction. When magnesium burns in chlorine, each magnesium atom loses two electrons and each chlorine atom gains one.

Mg→MgX2++2 eX−ClX2+2 eX−→2 ClX− \ce{Mg -> Mg^2+ + 2e-} \qquad\qquad \ce{Cl2 + 2e- -> 2Cl-}

The magnesium is oxidised and the chlorine is reduced. One magnesium atom supplies the two electrons that one Cl₂ molecule takes.

Oxidation numbers follow the electrons 8.5

Three panels. Left: sodium and chlorine. A curved arrow carries one electron from Na to Cl; Na goes from 0 to plus 1 (oxidised) and Cl from 0 to minus 1 (reduced). Middle: magnesium and oxygen. Two electrons move from Mg to O; Mg goes from 0 to plus 2 and O from 0 to minus 2. Right: hydrogen chloride, where the bonding pair is shared but pulled towards Cl, which is delta minus; by the oxidation-number rules H is plus 1 and Cl is minus 1.
Two ionic reactions and one covalent one. The oxidation number tracks the electrons in all three.

The covalent case matters. In H₂ + Cl₂ → 2HCl no ions form, but the bonding pair in HCl lies closer to chlorine. The oxidation numbers say hydrogen is oxidised (0 → +1) and chlorine is reduced (0 → −1). This is why we use oxidation numbers: they classify the reaction even when there is no clean transfer.

Worked exampleReading a reaction

Identify the oxidation and the reduction in 2 Na+ClX2→2 NaCl\ce{2Na + Cl2 -> 2NaCl}.

  1. Write the oxidation numbers: Na 0 → +1; Cl 0 → −1.
  2. Sodium's number increases by 1, so sodium is oxidised (each atom loses 1 electron).
  3. Chlorine's number decreases by 1, so chlorine is reduced (each atom gains 1 electron).
  4. Check the electrons: 2 sodium atoms lose 2 electrons in total; 1 Cl₂ molecule (2 atoms) gains 2.

Metals and non-metals 8.9

Most metals have few outer electrons, and lose them easily. Most non-metals are short of one or two electrons to fill their outer shell, and gain them. So, in general:

Reaction Metal: change Non-metal: change
2 Na+ClX2→2 NaCl\ce{2Na + Cl2 -> 2NaCl} Na 0 → +1 (loses 1 e⁻) Cl 0 → −1 (gains 1 e⁻)
2 Mg+OX2→2 MgO\ce{2Mg + O2 -> 2MgO} Mg 0 → +2 (loses 2 e⁻) O 0 → −2 (gains 2 e⁻)
2 Al+3 ClX2→2 AlClX3\ce{2Al + 3Cl2 -> 2AlCl3} Al 0 → +3 (loses 3 e⁻) Cl 0 → −1 (gains 1 e⁻)

Check yourself

  1. Define oxidation and reduction in terms of electrons.
  2. In 2 Mg+OX2→2 MgO\ce{2Mg + O2 -> 2MgO}, state the oxidation number change for each element and say what is oxidised and what is reduced.
  3. How many electrons does each aluminium atom lose when it forms Al³⁺, and what is the change in oxidation number?
  4. Explain why H₂ + Cl₂ → 2HCl is a redox reaction even though no ions form.
  5. A student writes "Mg + 2HCl → MgCl₂ + H₂ is not redox because there is no oxygen". Correct this using oxidation numbers.
  6. State the oxidation number change when each atom of (a) oxygen forms O²⁻, (b) calcium forms Ca²⁺.

Answers cover these until you have tried the questions

  1. Oxidation is the loss of electrons. Reduction is the gain of electrons.
  2. Mg: 0 → +2, oxidised. O: 0 → −2, reduced. (Each Mg loses 2 e⁻, each O atom gains 2 e⁻.)
  3. Each Al atom loses 3 electrons. The oxidation number changes from 0 to +3, an increase of 3.
  4. Hydrogen goes 0 → +1 (oxidised) and chlorine goes 0 → −1 (reduced). The electrons in the covalent bond lie closer to the more electronegative chlorine, so by the oxidation-number rules electrons have moved from H to Cl. There are changes in oxidation number, so it is redox.
  5. Mg: 0 → +2 (oxidised, loses electrons). H: +1 → 0 (reduced, gains electrons). The oxidation numbers change, so it is redox, even though no oxygen is involved. Chlorine stays −1.
  6. (a) O: 0 → −2, a decrease of 2 (reduction). (b) Ca: 0 → +2, an increase of 2 (oxidation).