Preparation and Classification of Oxides
Senior 2 · Chemistry
Preparation and Classification of Oxides
Lesson 1 of 4
25 min
Most of the rock beneath your feet is made of oxides. Sand is silicon dioxide. The red colour of Rwanda's laterite soils comes from iron oxides. Cassiterite, the tin ore mined in several parts of Rwanda, is tin(IV) oxide. Rust, the carbon dioxide you breathe out and the water you drink are oxides too.
An oxide is a compound of oxygen with one other element. In this lesson you will learn to name oxides correctly and to prepare them in the laboratory by four standard methods.
Metal oxides take the name of the metal followed by "oxide": MgO is magnesium oxide, Al2O3 is aluminium oxide. Some metals form ions with different charges, so a Roman numeral in brackets gives the metal's charge (its oxidation state):
| Formula | Name | Metal ion |
|---|---|---|
| FeO | iron(II) oxide | Fe2+ |
| Fe2O3 | iron(III) oxide | Fe3+ |
| Cu2O | copper(I) oxide | Cu+ |
| CuO | copper(II) oxide | Cu2+ |
| PbO | lead(II) oxide | Pb2+ |
| SnO2 | tin(IV) oxide | Sn4+ |
Non-metal oxides are molecules, and prefixes (mono-, di-, tri-, tetra-, penta-) show how many atoms of each element there are: CO is carbon monoxide, CO2 carbon dioxide, SO2 sulfur dioxide, SO3 sulfur trioxide, NO nitrogen monoxide, NO2 nitrogen dioxide and N2O dinitrogen oxide ("laughing gas"). P4O10 is tetraphosphorus decaoxide, usually called phosphorus(V) oxide.
Worked example. Name Fe2O3. Oxide is O2−, and three oxide ions carry a total charge of 6−. The two iron ions must therefore carry 6+, which is 3+ each. The name is iron(III) oxide, not "iron trioxide", because the Roman numeral gives the charge on the iron, not the number of oxygen atoms.
Many elements combine directly with oxygen when heated:
Gold and platinum do not combine directly with oxygen, which is one reason they stay shiny for centuries.
Most metal carbonates decompose on heating to give the metal oxide and carbon dioxide:
Sodium and potassium carbonates do not decompose at the temperature of a Bunsen flame.
Most metal hydroxides lose water on heating:
Sodium hydroxide and potassium hydroxide do not decompose on heating.
Nitrates of most metals, from magnesium down to copper in the reactivity series, give the metal oxide, brown nitrogen dioxide and oxygen:
Sodium and potassium nitrates behave differently: they give the nitrite and oxygen only, as you saw when oxygen was prepared.
Ishimwe heats 24.7 g of green copper(II) carbonate in a boiling tube until the solid is completely black, passing the gas through limewater. (Cu = 63.5, C = 12, O = 16)
To be sure the decomposition is complete, heat, cool and weigh the solid several times until two readings agree (heating to constant mass).
Warning
Safety when preparing oxides
Tip
Four routes to an oxide: burn, or heat one of the "C-H-N" compounds
Burn the element; or heat a Carbonate (loses CO2), a Hydroxide (loses H2O) or a Nitrate (loses NO2 and O2). In names, the Roman numeral is the charge on the metal ion, worked out from O2−, never the number of oxygen atoms.
Name: (a) Cu2O (b) PbO2 (c) SO3.
Solution: (a) Two Cu ions balance one O2−, so each is Cu+: copper(I) oxide. (b) Two O2− make 4−, so the lead is Pb4+: lead(IV) oxide. (c) SO3 is a non-metal oxide, so it takes prefixes: sulfur trioxide.
Write the formula of aluminium oxide.
Solution: Al3+ and O2−. The lowest common multiple of 3 and 2 is 6, so we need 2 Al3+ (6+) and 3 O2− (6−): Al2O3.
Write a balanced equation for heating zinc carbonate. What colour is the residue while hot, and when cold?
Solution: ZnCO3 → ZnO + CO2. The residue is yellow while hot and white when cold.
Balance: __Pb(NO3)2 → __PbO + __NO2 + O2.
Solution: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. Check: Pb 2 = 2; N 4 = 4; O 12 = 2 + 8 + 2.
What mass of calcium oxide is obtained by heating 50 g of calcium carbonate? (Ca = 40, C = 12, O = 16)
Solution: M(CaCO3) = 100 g/mol, so 50 g is 0.50 mol. The ratio is 1 : 1, so 0.50 mol of CaO forms. M(CaO) = 56 g/mol, so the mass = 0.50 × 56 = 28 g.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
