Preparation and Classification of Oxides
Senior 2 · Chemistry
Preparation and Classification of Oxides
Lesson 3 of 4
20 min
"Metal oxides are basic, non-metal oxides are acidic" is a useful rule, but chemistry has exceptions, and examiners love them. Aluminium cooking pans are protected by a thin skin of aluminium oxide. That skin is attacked by acidic foods and by strong alkaline cleaners alike, so it behaves as an acid and as a base. The carbon monoxide produced by a charcoal stove in a closed room has no acidic or basic properties at all, yet it is deadly. This lesson covers these two remaining classes of oxide.
An amphoteric oxide (from the Greek for "both") reacts with acids and with alkalis, forming a salt and water in each case. The common examples are the oxides of zinc, aluminium and lead (and tin). All are insoluble in water.
| Oxide | Acting as a base (with an acid) | Acting as an acid (with an alkali) |
|---|---|---|
| Zinc oxide, ZnO | ZnO + 2HCl → ZnCl2 + H2O | ZnO + 2NaOH → Na2ZnO2 + H2O (sodium zincate) |
| Aluminium oxide, Al2O3 | Al2O3 + 6HCl → 2AlCl3 + 3H2O | Al2O3 + 2NaOH → 2NaAlO2 + H2O (sodium aluminate) |
| Lead(II) oxide, PbO | PbO + 2HNO3 → Pb(NO3)2 + H2O | PbO + 2NaOH → Na2PbO2 + H2O (sodium plumbate(II)) |
In solution, the zincate and aluminate ions are really hydrated, for example [Zn(OH)4]2−, but the simpler formulas above are the ones expected at this level. The same metals' hydroxides, Zn(OH)2, Al(OH)3 and Pb(OH)2, are amphoteric too.
Byiringiro has a white oxide Z that is insoluble in water. He adds some to dilute hydrochloric acid and some to warm sodium hydroxide solution. It dissolves in both.
Compare magnesium oxide, which dissolves only in the acid (basic), and silicon dioxide, which reacts only with the hot alkali (acidic).
A neutral oxide reacts with neither acids nor alkalis to form a salt. The examples are all non-metal oxides:
| Class | Reacts with acid? | Reacts with alkali? | Examples |
|---|---|---|---|
| Basic | yes | no | Na2O, CaO, MgO, CuO, Fe2O3 |
| Acidic | no | yes | CO2, SO2, SO3, P4O10, SiO2 |
| Amphoteric | yes | yes | ZnO, Al2O3, PbO |
| Neutral | no | no | CO, NO, N2O, H2O |
Some compounds of oxygen fall outside these four classes. Peroxides such as hydrogen peroxide, H2O2, contain an O–O bond, and mixed oxides such as Fe3O4 contain the metal in two different oxidation states.
Tip
Memory hook: "ZAP" oxides go both ways
Zinc, Aluminium and lead (Pb) oxides are amphoteric. For neutral oxides, remember the three small nitrogen and carbon oxides that do nothing with acids or alkalis: CO, NO and N2O. In an exam, show amphoteric behaviour with two equations, one with an acid and one with an alkali.
Define an amphoteric oxide and give two examples.
Solution: An oxide that reacts with both acids and alkalis to form a salt and water, for example zinc oxide and aluminium oxide (lead(II) oxide is another).
Write equations to show that zinc oxide is amphoteric.
Solution: With an acid: ZnO + 2HCl → ZnCl2 + H2O. With an alkali: ZnO + 2NaOH → Na2ZnO2 + H2O.
Classify each oxide: SO2, K2O, CO, Al2O3.
Solution: SO2 acidic; K2O basic; CO neutral; Al2O3 amphoteric.
Carbon monoxide is a neutral oxide. Explain why it is still dangerous.
Solution: "Neutral" describes only its acid–base behaviour. Carbon monoxide is poisonous: it combines with haemoglobin in the blood, so the blood can no longer carry oxygen to the body.
Balance: Al2O3 + __NaOH → __NaAlO2 + H2O.
Solution: Al2O3 + 2NaOH → 2NaAlO2 + H2O. Check: Al 2 = 2; Na 2 = 2; O 3 + 2 = 5 and 4 + 1 = 5; H 2 = 2.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
