secondary chemistry – sulphuric (VI) acid

Sulphuric (VI) acid
Large scale manufacture of sulphuric (VI) acid


Sulphur is burnt in air to form sulphur (IV) oxide

S(s) +O2 (g)       →      SO2 (g)

Dust particle which would poison the catalyst are removed by electrostatic precipitation.
The impurities would reduce the surface are of the catalyst reducing its efficiency
The gases are dried by passing them through concentrated sulphuric (VI) acid

The mixture is then heated in the heat exchanger by hot gases from the catalytic chamber
Sulphur (IV) oxide is then mixed with excess air and passed through cleaners
The mixture is then heated to a temperature of 450°C and a pressure of 2 to 3 atmospheres in presence of vanadium (V) oxide catalyst.
Platinum catalyst may be used however it is more expensive and easily poisoned by impurities.
Sulphur (IV) oxide reacts with oxygen forming sulphur (VI) oxide

2SO2(g) +O2 (g)      →       2 SO3 (g)

Sulphur (VI) oxide is not dissolved directly in water since the reaction would be very exothermic making the acid to boil and the factory would be filled by acid droplets
Instead it is dissolved in concentrated sulphuric (VI) acid to form oleum

SO3 (g) +H2SO4 (l)   →     2H2S2O7 (l)

Oleum is then diluted with the collect amount of water to form concentrated sulphuric (VI) acid

H2S2O7 (l) +H2O (l)  →     2H2SO4 (l)

Unreacted sulphur (IV) oxide is passed through chimneys lined with calcium hydroxide in a process known as scrubbing the gas
This reduces the pollution effects of sulphur (IV) oxide

SO2 (g) +Ca (OH) 2 (aq)  →  CaSO3 (aq) +2H2O (l)

In some industries filters fitted with strong alkalis are installed to remove any traces of acid or mist from exhaust gases

Physical properties of sulphuric (VI) acid

• Colourless oily liquid
• Density 1.84g/cm3
• Boiling point 338oC
• Hygroscopic hence it is used as a drying agent for gases that don’t react with it.
• When added to water steam is produced since the reaction is highly exothermic
Therefore when diluting the acid small portions of the acid are added to water
Note: water should never be added to the acid

Dehydrating property

A dehydrating agent is a substance which is capable of removing combined water or elements of from a compound.
a) When added to blue hydrated copper (II) sulphate crystals, the crystals turn into a white powder

This is because copper (II) sulphate crystals have been dehydrated by concentrated sulphuric (VI) acid

CuSO4.5H2O (s)      →       CuSO4 (s) + 5H2O (l)


Also removes elements of water from compounds that contain elements of water (hydrogen and oxygen)

b) When added to sugar crystals the crystals are charred to a black mass of carbon

C12H22O11(s)       →     12C (s) + 11H2O (l)

c) Methanoic acid is dehydrated to carbon (II) oxide

HCOOH(s)       →     CO (s)    +    H2O (l)


d) Dehydrates alcohols to alkenes

C2H5OH (aq)    →    C2H4 (g) + H2O (l)

Oxidising property

a) Reaction with some metals
Hot concentrated sulphuric (VI) acid oxidises copper and zinc. The acid is reduced to sulphur (IV) oxide.

Zn(s) +2H2SO4 (l)   →     ZnSO4 (aq) +2H2O (l) +SO2 (g)

Cu(s) +2H2SO4 (l)    →   CuSO4 (aq) +2H2O (l) +SO2 (g)

b) It also oxidises non-metals
i. Oxidise sulphur to sulphur (IV) oxide

S(s) +2H2SO4 (l)   →    3SO2 (g) +2H2O (l)

ii. Oxidises carbon to carbon (IV) oxide.
Sulphuric (VI) acid is reduced to sulphur (IV) oxide.

C(s) +2H2SO4 (l)   →    2SO2 (g) + CO2 (g) +2H2O (l)

Displacement reactions
Concentrated sulphuric (VI) acid is less volatile and displaces the less volatile hydrochloric acid and nitric (V) acid from their salts.

KNO3 (s) +H2SO4 (l)   →    HNO3 (g) + KHSO4 (s)

NaCl (s) +H2SO4 (l)   →    HCl (g) + NaHSO4 (s)

Reactions of dilute sulphuric (VI) acid

a) Reaction with metals
Reacts with some metals higher in reactivity series than hydrogen forming hydrogen gas

Mg (s) +H2SO4 (aq)   →    MgSO4 (aq) + H2 (g)

Zn (s) +H2SO4 (aq)   →    ZnSO4 (aq) + H2 (g)

Reaction with very reactive metals like potassium, sodium and calcium is violent and should not be attempted

Copper is below hydrogen in the reactivity series hence does not react with dilute sulphuric (VI) acid since it cannot displace hydrogen from dilute sulphuric (VI) acid

b) Reaction with metal oxides and hydroxides (neutralisation reaction)
Forms salt and water only

c) Reaction with carbonates and hydrogen carbonates
Forms carbon (IV) oxide, salt and water