4.4.1.1Metal oxides
Metals react with oxygen to produce metal oxides. The reactions are oxidation reactions because the metals gain oxygen.
You should be able to explain reduction and oxidation in terms of loss or gain of oxygen. At this stage the definitions are simply:
• oxidation is the gain of oxygen • reduction is the loss of oxygen.
Both happen in the same reaction. When magnesium burns in air, 2Mg + O₂ → 2MgO, the magnesium is oxidised because it has gained oxygen — and the oxygen itself is not described as reduced here, because nothing has lost oxygen. It is in the extraction reactions below that the two appear together.
- Oxidation
- the gain of oxygen in a reaction; more widely, the loss of electrons
- Reduction
- the loss of oxygen in a reaction; more widely, the gain of electrons
4.4.1.2The reactivity series
When metals react with other substances the metal atoms form positive ions. The reactivity of a metal is related to its tendency to form positive ions — the more readily a metal loses its outer electrons, the more reactive it is.
Metals can be arranged in order of their reactivity in a reactivity series. The metals potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper can be put in order of their reactivity from their reactions with water and dilute acids. The non-metals hydrogen and carbon are often included in the reactivity series.
You should be able to recall and describe the reactions, if any, of potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper with water or dilute acids, and where appropriate place these metals in order of reactivity.
The two general equations are:
• metal + water → metal hydroxide + hydrogen • metal + acid → salt + hydrogen
You should also be able to explain how the reactivity of metals with water or dilute acids is related to the tendency of the metal to form its positive ion. Both reactions require the metal atom to lose electrons and become a positive ion, so a metal that loses electrons readily reacts vigorously, and one that does not — copper — does not react at all.
A more reactive metal can displace a less reactive metal from a compound. That gives a second way of ordering metals, and you should be able to deduce an order of reactivity of metals based on experimental results.
Worked example
Four metals W, X, Y and Z are tested. W and Y fizz in dilute acid but not in water; X fizzes in cold water; Z does nothing in either. W displaces Y from its sulfate solution. Deduce an order of reactivity.
- Reacting with cold water is the strongest evidence, so X is the most reactive.
- W and Y react with acid but not water, so both are below X.
- W displaces Y, so W is more reactive than Y.
- Z reacts with neither, so it is the least reactive.
- Assemble the order from the evidence, most reactive first.
Answer: X, W, Y, Z. Every step must be justified by an observation — deducing an order means quoting the evidence, not recalling the standard series.
Common mistake
Copper does not react with dilute acids, so "no observation" is a result, not a failed experiment. It places copper below hydrogen in the series.
- Reactivity series
- an order of metals by reactivity, determined by their reactions with water and dilute acids; the more readily a metal loses its outer electrons to form positive ions, the more reactive it is
4.4.1.3Extraction of metals and reduction
Unreactive metals such as gold are found in the Earth as the metal itself, but most metals are found as compounds that require chemical reactions to extract the metal.
Metals less reactive than carbon can be extracted from their oxides by reduction with carbon. Reduction involves the loss of oxygen.
Knowledge and understanding are limited to the reduction of oxides using carbon, and knowledge of the details of the processes used in the extraction of metals is not required.
You should be able to identify the substances which are oxidised or reduced in terms of gain or loss of oxygen, and interpret or evaluate specific metal extraction processes when given appropriate information.
In ZnO + C → Zn + CO, the zinc oxide has lost oxygen so it has been reduced, and the carbon has gained oxygen so it has been oxidised. Both happen at once, which is why these are called redox reactions.
Worked example
Explain why aluminium cannot be extracted by heating its oxide with carbon, but iron can.
- Reduction with carbon works only for metals less reactive than carbon.
- Aluminium is above carbon in the reactivity series.
- So carbon cannot take the oxygen away from aluminium oxide — aluminium holds its oxygen more strongly.
- Iron is below carbon in the series, so carbon can remove the oxygen from iron oxide.
Answer: Aluminium is more reactive than carbon, so carbon cannot reduce its oxide; iron is less reactive than carbon, so it can be. Aluminium must be extracted by electrolysis instead.
4.4.1.4Oxidation and reduction in terms of electronsHigher tier
Oxidation is the loss of electrons and reduction is the gain of electrons.
This is the wider definition, and it applies to reactions where no oxygen is involved at all. The word OILRIG is the usual memory aid: Oxidation Is Loss, Reduction Is Gain — of electrons.
You should be able to write ionic equations for displacement reactions, and identify in a given reaction, symbol equation or half equation which species are oxidised and which are reduced.
An ionic equation for a displacement reaction leaves out the spectator ions — the ones that are in solution before and after and take no part.
Worked example
Zinc displaces copper from copper sulfate solution: Zn + CuSO₄ → ZnSO₄ + Cu. Write the ionic equation and say which species is oxidised.
- Write the ions present: Zn(s), Cu²⁺(aq), SO₄²⁻(aq), Zn²⁺(aq), Cu(s).
- The sulfate ion is unchanged on both sides, so it is a spectator ion and is left out.
- What remains is Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s).
- The zinc atom has become a 2+ ion, so it has lost two electrons — oxidation.
- The copper ion has become an atom, so it has gained two electrons — reduction.
Answer: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Zinc is oxidised; copper ions are reduced.
Common mistake
Decide which species is oxidised by tracking the charge, not the appearance of the substance. Going from Zn to Zn²⁺ is a rise in charge, which means electrons were lost — oxidation — even though nothing burned and no oxygen was present.
- Redox reaction
- a reaction in which oxidation and reduction happen together, one species losing electrons while another gains them
Before you practise
- Metals react with oxygen to form metal oxides; oxidation is gain of oxygen, reduction is loss of oxygen.
- The reactivity of a metal is related to its tendency to form positive ions.
- Order from experiment: potassium, sodium, lithium, calcium, magnesium, zinc, iron, copper — with carbon and hydrogen often included.
- Reactions with water and acids are limited to room temperature; steam is not included.
- A more reactive metal can displace a less reactive metal from a compound.
- Metals less reactive than carbon can be extracted from their oxides by reduction with carbon; unreactive metals such as gold are found as the metal itself.
- Higher tier: oxidation is loss of electrons, reduction is gain of electrons — and ionic equations leave out the spectator ions.