Copper(II) Sulfate

Copper(II) sulfate (CuSO4) is a versatile 0620 compound, appearing in salt preparation, electrolysis, the test for water and water-of-crystallisation calculations.

Key facts

Property	Value
Formula	CuSO4 (anhydrous); CuSO4.5H2O (hydrated)
Mr	160 (anhydrous); 250 (hydrated)
Bonding	Ionic — Cu2+ and SO4^2-
Key property	Hydrated: blue crystals; anhydrous: white powder; soluble
Transition metal	Coloured compound (blue), typical of a transition element
Main uses	Test for water; copper purification and electroplating demos

Where copper(II) sulfate appears in 0620

• Tests for water: anhydrous CuSO4 turns white to blue.
• Empirical and molecular formulae: finding x in CuSO4.xH2O by heating.
• Preparation of salts: the excess-solid method.
• Electrolysis and electroplating: inert and copper electrode cases.
• Transition elements: a coloured salt.

Hydrated and anhydrous

The reversible reaction linking the two forms:

CuSO4.5H2O(s) ⇌ CuSO4(s) + 5H2O(g)

Heating drives off the water of crystallisation (endothermic, blue to white); adding water reverses it (exothermic, white to blue). This reversibility is the basis of the test for water.

Salt preparation (excess-solid method)

CuO(s) + H2SO4(aq) -> CuSO4(aq) + H2O(l)

Add excess black copper(II) oxide to warm dilute sulfuric acid, filter off the unreacted excess, then evaporate and cool the blue filtrate to crystallise.

Electrolysis of CuSO4 solution

Inert (graphite) electrodes

• Cathode: Cu2+(aq) + 2e- -> Cu(s) (pink-brown coating)
• Anode: 4OH-(aq) -> 2H2O(l) + O2(g) + 4e-
• The blue colour fades as Cu2+ ions are removed.

Copper electrodes (purification)

• Cathode: Cu2+(aq) + 2e- -> Cu(s) (pure copper deposited)
• Anode: Cu(s) -> Cu2+(aq) + 2e- (impure copper dissolves)
• The blue colour stays the same because Cu2+ is replaced as fast as it is removed.

Displacement reactions

Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)

A more reactive metal displaces copper: the blue solution fades and a pink-brown deposit of copper forms on the metal. See displacement reactions.

Identifying the copper(II) ion

Adding aqueous sodium hydroxide gives a light-blue precipitate of copper(II) hydroxide, insoluble in excess:

Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s)

Aqueous ammonia also gives the blue precipitate, but it then redissolves in excess ammonia to a deep-blue solution.

A student heats 5.00 g of hydrated copper(II) sulfate crystals to constant mass. The anhydrous solid left has a mass of 3.20 g. Calculate the value of x in CuSO4.xH2O. (Ar: Cu = 64, S = 32, O = 16, H = 1) (4 marks)

Mark scheme

• Mass of water lost = 5.00 - 3.20 = 1.80 g [1]
• Moles of CuSO4 = 3.20 / 160 = 0.020 mol; moles of H2O = 1.80 / 18 = 0.10 mol [1]
• Ratio H2O : CuSO4 = 0.10 : 0.020 = 5 : 1 [1]
• x = 5 [1]

Examiner note: heat to constant mass so all the water is removed — stopping early makes the water mass too small and x below 5. Divide each mass by the correct relative mass (160 for CuSO4, 18 for water), not by an atomic mass.

Common exam mistakes

• Swapping the colours. Hydrated CuSO4 is blue and anhydrous is white; the water test is white turning blue.
• Claiming the water test proves a liquid is pure water. It only shows water is present; pure water is confirmed by a boiling point of 100 C and a freezing point of 0 C.
• Saying the blue colour fades during electrolysis with copper electrodes. It only fades with inert electrodes; copper electrodes keep the Cu2+ concentration constant.
• In the CuSO4.xH2O calculation, dividing the water mass by 16 or by 2 instead of 18, or forgetting to find the water mass by subtraction first.