Sulfuric Acid

Sulfuric acid (H2SO4) is the most industrially important acid and one of the most examined in 0620. It makes sulfate salts, is manufactured by the Contact process, and is the catalyst for esterification.

Key facts

Property	Value
Formula	H2SO4
Mr	2(1) + 32 + 4(16) = 98
Acid strength	Strong (fully dissociated) and diprotic (donates 2 H+)
Salt produced	Sulfate (SO4^2-)
Key property	Dense, oily liquid when concentrated; always add acid to water
Main use	Making ammonium sulfate fertiliser; car battery electrolyte

Where sulfuric acid appears in 0620

• Acids, bases and indicators and preparation of salts: making soluble sulfates.
• Titration and solubility: the 2:1 mole ratio in titration calculations.
• Reversible reactions and equilibrium: the Contact process (Supplement).
• Esters: concentrated H2SO4 as the esterification catalyst.

Key reactions

Always forming a sulfate salt:

With metals

Mg(s) + H2SO4(aq) -> MgSO4(aq) + H2(g)

Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)

Effervescence of hydrogen; the metal dissolves. Copper, silver and gold do not react with the dilute acid.

With bases and carbonates

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

2NaOH(aq) + H2SO4(aq) -> Na2SO4(aq) + 2H2O(l)

Na2CO3(aq) + H2SO4(aq) -> Na2SO4(aq) + H2O(l) + CO2(g)

With calcium carbonate the fizzing quickly stops: the calcium sulfate produced is insoluble and coats the solid, blocking the acid (CaCO3 + H2SO4 -> CaSO4 + H2O + CO2).

As an esterification catalyst

CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O (concentrated H2SO4, warm)

Testing for the sulfate ion

Acidify with dilute nitric acid, then add aqueous barium nitrate. A white precipitate of barium sulfate confirms sulfate:

Ba2+(aq) + SO4^2-(aq) -> BaSO4(s)

The nitric acid removes carbonate or sulfite ions that would otherwise give their own precipitates.

The Contact process (Supplement)

1. S(s) + O2(g) -> SO2(g)
2. 2SO2(g) + O2(g) ⇌ 2SO3(g) — vanadium(V) oxide catalyst, about 450 C and 2 atm
3. SO3 is absorbed into concentrated sulfuric acid, then diluted to give more H2SO4

Step 2 is a reversible reaction run at a compromise temperature: lower temperature would raise the yield but the rate would be too slow. See equilibrium and Le Chatelier.

Uses

Use	Detail
Fertiliser manufacture	Making ammonium sulfate: 2NH3 + H2SO4 -> (NH4)2SO4
Car batteries	Dilute sulfuric acid as the electrolyte
Cleaning metals (pickling)	Removing oxide layers before plating
Detergents, paints and dyes	Raw material in manufacture

In a titration, 25.0 cm3 of 0.10 mol/dm3 sodium hydroxide is exactly neutralised by 12.5 cm3 of sulfuric acid. Calculate the concentration of the sulfuric acid in mol/dm3. (3 marks)

Mark scheme

• Moles of NaOH = 0.10 x (25.0/1000) = 0.0025 mol [1]
• From 2NaOH + H2SO4 -> Na2SO4 + 2H2O, moles of H2SO4 = 0.0025 / 2 = 0.00125 mol [1]
• Concentration = 0.00125 / (12.5/1000) = 0.10 mol/dm3 [1]

Examiner note: the whole question turns on H2SO4 being diprotic. Candidates who forget to divide by 2 report 0.20 mol/dm3 and lose two marks. Always read the mole ratio from the balanced equation.

Common exam mistakes

• Treating sulfuric acid as monoprotic in calculations. It is diprotic — 2 mol NaOH react with 1 mol H2SO4 — so the mole ratio is 2:1.
• Giving the sulfate ion the wrong formula. SO4^2- is 2-, so the salts are Na2SO4, MgSO4 and Al2(SO4)3.
• Expecting calcium carbonate to keep reacting with sulfuric acid. Insoluble calcium sulfate coats the marble and the reaction soon stops; use hydrochloric acid for a steady reaction.
• Acidifying the sulfate test with the wrong acid. Use dilute nitric acid; sulfuric acid would add sulfate ions and give a false positive.