Trends in Chemical Properties of Group 16 Elements and Their Compounds

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Senior 4 · Chemistry

Trends in Chemical Properties of Group 16 Elements and Their Compounds

4 lessons

Lesson 3 of 4

Sulphuric Acid: Manufacture and Properties

25 min

Three reactions in one bottle

Pour a little concentrated sulphuric acid onto white sugar and, after a minute, the mixture turns yellow, then brown, then black, and a steaming column of carbon rises out of the beaker. Drop a piece of copper into the hot concentrated acid and a choking gas, sulphur dioxide, is given off. Dilute the acid, and it behaves like any strong acid, fizzing with magnesium. The same bottle shows three different kinds of behaviour: the acid is a dehydrating agent, an oxidising agent and a strong acid. Sulphuric acid is one of the most produced chemicals in the world, about 260 million tonnes in 2022, much of it used to make phosphate fertilisers. This lesson explains how it is made and why it behaves in each of these ways.

Definition

Contact process: the industrial manufacture of sulphuric acid in which sulphur dioxide and oxygen react on the surface of (in contact with) a solid vanadium(V) oxide catalyst to form sulphur trioxide.

Oleum: "fuming sulphuric acid", $\mathrm{H_2S_2O_7}$ (disulphuric acid), made by dissolving sulphur trioxide in concentrated sulphuric acid.

Dehydration: removal of water, or of hydrogen and oxygen atoms in the ratio 2:1, from a substance. Example: $\mathrm{C_{12}H_{22}O_{11} \rightarrow 12C + 11H_2O}$ .

Manufacture: the Contact process

Making sulphur dioxide. Molten sulphur is burned in air, $\mathrm{S(l) + O_2(g) \rightarrow SO_2(g)}$ . (Roasting sulphide ores also gives $\mathrm{SO_2}$ .)
Cleaning. The gas is cleaned of dust and dried, because impurities would poison the catalyst.
Conversion. $\mathrm{2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)} \qquad \Delta H = -197.8\ \mathrm{kJ\,mol^{-1}}$
- A vanadium(V) oxide catalyst, which works between about 420 and 620 °C.
- A temperature of about 450 °C: the reaction is exothermic, so a lower temperature would raise the equilibrium yield, but it would be too slow and the catalyst would stop working; 450 °C is the compromise.
- A pressure of only about 1–2 atm: higher pressure would favour $\mathrm{SO_3}$ (three moles of gas become two), but the yield is already very high, so the cost of compression is not worthwhile.
- A slight excess of air pushes the equilibrium to the right; the gases pass over several beds of catalyst, cooled between beds, so almost all the $\mathrm{SO_2}$ is converted.
Absorption. Sulphur trioxide is absorbed in 98% sulphuric acid, giving oleum: $\mathrm{SO_3 + H_2SO_4 \rightarrow H_2S_2O_7}$ . It is not dissolved straight in water: that reaction is so exothermic that it forms a fog of acid droplets that is hard to collect.
Dilution. The oleum is diluted with the right amount of water: $\mathrm{H_2S_2O_7 + H_2O \rightarrow 2H_2SO_4}$ .

Figure 11.6: The Contact process.

Properties of sulphuric acid

1. As an acid (dilute). Sulphuric acid is a strong, dibasic acid: the first proton is lost completely and the second partly ( $\mathrm{p}K_{a2} = 2.0$ ), so it forms two series of salts, hydrogensulphates and sulphates.

\mathrm{H_2SO_4 \rightarrow H^+ + HSO_4^-} \qquad \mathrm{HSO_4^- \rightleftharpoons H^+ + SO_4^{2-}}

\mathrm{Mg + H_2SO_4 \rightarrow MgSO_4 + H_2} \qquad \mathrm{CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O} \qquad \mathrm{2NaOH + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O}

2. As a dehydrating agent (concentrated). Concentrated sulphuric acid has a very strong attraction for water. It removes the elements of water from many compounds:

\mathrm{C_{12}H_{22}O_{11}(s) \xrightarrow{\text{conc. } H_2SO_4} 12C(s) + 11H_2O} \qquad \mathrm{CuSO_4{\cdot}5H_2O\ (blue) \rightarrow CuSO_4\ (white) + 5H_2O}

It is used to dry gases that do not react with it (such as chlorine and sulphur dioxide, but not ammonia), and it chars paper, wood and skin.

3. As an oxidising agent (hot and concentrated). Sulphur in $\mathrm{H_2SO_4}$ is in its highest oxidation state (+6), so the hot concentrated acid can accept electrons, usually being reduced to sulphur dioxide (+4):

\mathrm{Cu(s) + 2H_2SO_4(conc) \rightarrow CuSO_4(aq) + SO_2(g) + 2H_2O(l)}

\mathrm{C(s) + 2H_2SO_4(conc) \rightarrow CO_2(g) + 2SO_2(g) + 2H_2O(l)}

With bromide and iodide ions it is reduced further (to $\mathrm{SO_2}$ , S or $\mathrm{H_2S}$ ), which is why concentrated sulphuric acid cannot be used to make HBr or HI (Unit 12).

4. Involatile. Its boiling point (337 °C) is high, because of extensive hydrogen bonding, so it displaces more volatile acids from their salts: $\mathrm{NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl}$ .

Diluting concentrated sulphuric acid safely

Mixing the concentrated acid with water releases a great deal of heat. Always add the acid slowly to the water, stirring, never water to acid: a little water added to a large volume of acid can boil instantly and spray hot acid out of the container.

Worked example 1: the energy of conversion

Use $\Delta_f H^\circ(\mathrm{SO_2, g}) = -296.81\ \mathrm{kJ\,mol^{-1}}$ and $\Delta_f H^\circ(\mathrm{SO_3, g}) = -395.7\ \mathrm{kJ\,mol^{-1}}$ to calculate $\Delta H$ for $\mathrm{2SO_2 + O_2 \rightarrow 2SO_3}$ . Explain how a plant uses this heat.

Solution. $\Delta H = 2(-395.7) - 2(-296.81) = -791.4 + 593.62 = -197.8\ \mathrm{kJ\,mol^{-1}}$ The heat released (and the heat from burning sulphur) is used to raise steam in heat exchangers; the steam can drive turbines to generate electricity for the plant. Cooling between catalyst beds also keeps the temperature near 450 °C, which helps the yield.

Worked example 2: concentrated acid by numbers

A bottle of concentrated sulphuric acid is 98.0% $\mathrm{H_2SO_4}$ by mass and has a density of 1.84 g cm⁻³. (a) Calculate its concentration in mol dm⁻³. (b) What volume of it is needed to make 1.00 dm³ of 1.00 mol dm⁻³ acid? $M(\mathrm{H_2SO_4}) = 98.09\ \mathrm{g\,mol^{-1}}$ .

Solution. (a) Mass of 1.00 dm³ $= 1000 \times 1.84 = 1840\ \mathrm{g}$ ; mass of $\mathrm{H_2SO_4} = 0.980 \times 1840 = 1803\ \mathrm{g}$ ; $c = \frac{1803}{98.09} = 18.4\ \mathrm{mol\,dm^{-3}}$ (b) $V = \frac{1.00 \times 1.00}{18.4} = 0.0544\ \mathrm{dm^3} = 54.4\ \mathrm{cm^3}$ . Add the 54.4 cm³ of acid slowly, with stirring, to about 800 cm³ of water, cool, then make up to 1.00 dm³.

Warning

Common mistakes. (1) Saying a high pressure is used in the Contact process: about 1–2 atm is enough because the yield is already high. (2) Writing $\mathrm{SO_3 + H_2O \rightarrow H_2SO_4}$ as the industrial absorption step: $\mathrm{SO_3}$ is absorbed in concentrated acid to form oleum. (3) Calling the sugar reaction "burning": no oxygen is involved; it is dehydration. (4) Expecting copper to give hydrogen with concentrated sulphuric acid: the acid acts as an oxidising agent and $\mathrm{SO_2}$ is formed.

Safety. Concentrated sulphuric acid causes severe burns by dehydrating skin and eyes; wear goggles, gloves and a lab coat. If it splashes on the skin or into the eyes, rinse at once with plenty of running water and get medical help immediately. Always add acid to water. The sugar and copper reactions give off steam and toxic sulphur dioxide and are done in a fume cupboard.

Tip

Sulphuric acid wears three hats. Dilute: a strong acid (salts + $\mathrm{H_2}$ , water, $\mathrm{CO_2}$ ). Concentrated, cold: a dehydrating agent (sugar → carbon; blue → white copper sulphate). Concentrated, hot: an oxidising agent (Cu or C → $\mathrm{SO_2}$ ). Contact process in three words: V₂O₅, 450 °C, oleum.

FuseSchool: Manufacturing Sulphuric Acid (5 min). Watch for the stages of the Contact process, the conditions in the converter and why sulphur trioxide is absorbed in concentrated acid rather than water.

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Practice

Write equations for the four chemical steps of the Contact process.
State and explain the temperature, pressure and catalyst used in the converter.
Why is sulphur trioxide not absorbed directly in water?
Describe and explain what happens when concentrated sulphuric acid is added to (a) sugar; (b) blue copper(II) sulphate crystals.
Write equations for the reactions of hot concentrated sulphuric acid with copper and with carbon, and state the change in oxidation number of sulphur.
Why can concentrated sulphuric acid be used to dry chlorine but not ammonia?
Calculate the mass of sulphur needed to make 1.00 tonne of 100% sulphuric acid, assuming 100% conversion. $A_r(\mathrm{S}) = 32.07$ ; $M(\mathrm{H_2SO_4}) = 98.09$ .
A student adds water to a beaker of concentrated sulphuric acid to dilute it. Evaluate the danger and say what should have been done.

Show answersHide answers

$\mathrm{S + O_2 \rightarrow SO_2}$ ; $\mathrm{2SO_2 + O_2 \rightleftharpoons 2SO_3}$ ; $\mathrm{SO_3 + H_2SO_4 \rightarrow H_2S_2O_7}$ ; $\mathrm{H_2S_2O_7 + H_2O \rightarrow 2H_2SO_4}$ .
About 450 °C: a compromise between rate and yield for an exothermic reaction, and the range where $\mathrm{V_2O_5}$ works. About 1–2 atm: higher pressure would raise the yield only slightly and cost more. $\mathrm{V_2O_5}$ : speeds up the reaction; it is cheaper and less easily poisoned than platinum.
The reaction is very exothermic and forms a mist of acid droplets that is hard to absorb and escapes.
(a) Sugar turns yellow, brown, then black, and a column of carbon rises, with steam: the acid removes H and O as water, $\mathrm{C_{12}H_{22}O_{11} \rightarrow 12C + 11H_2O}$ . (b) Blue crystals turn white: water of crystallisation is removed, $\mathrm{CuSO_4{\cdot}5H_2O \rightarrow CuSO_4 + 5H_2O}$ .
$\mathrm{Cu + 2H_2SO_4 \rightarrow CuSO_4 + SO_2 + 2H_2O}$ ; $\mathrm{C + 2H_2SO_4 \rightarrow CO_2 + 2SO_2 + 2H_2O}$ . Sulphur goes from +6 to +4.
Chlorine does not react with it; ammonia is a base and would react to form ammonium sulphate.
1 mol S gives 1 mol $\mathrm{H_2SO_4}$ : $n = \frac{1.00 \times 10^6}{98.09} = 1.02 \times 10^4$ mol; mass of S $= 1.02 \times 10^4 \times 32.07 = 3.27 \times 10^5\ \mathrm{g} = 0.327\ \mathrm{t}$ .
Dangerous: the first water added heats up so much it can boil and spray hot concentrated acid. The acid should be added slowly to the larger volume of water, with stirring and cooling.

Quick check

Why is the Contact process run at about 1–2 atm instead of a much higher pressure?

Key points

Contact process: $\mathrm{S \rightarrow SO_2}$ ; $\mathrm{2SO_2 + O_2 \rightleftharpoons 2SO_3}$ ( $\Delta H = -197.8\ \mathrm{kJ\,mol^{-1}}$ , $\mathrm{V_2O_5}$ , about 450 °C, 1–2 atm); $\mathrm{SO_3}$ absorbed in 98% acid as oleum, then diluted.
Dilute $\mathrm{H_2SO_4}$ is a strong dibasic acid. Concentrated: a dehydrating agent (sugar → C; $\mathrm{CuSO_4{\cdot}5H_2O}$ → white $\mathrm{CuSO_4}$ ); hot concentrated: an oxidising agent ( $\mathrm{Cu \rightarrow CuSO_4 + SO_2}$ ).
Being involatile, it displaces HCl from chlorides; it oxidises HBr and HI, so it cannot make them.
Always add acid to water.

Sources and credits

Image: Carbon snake2.jpg by Thee.p, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Process conditions from Wikipedia, Contact process; enthalpies of formation from Sulfur dioxide and Sulfur trioxide; boiling point, $\mathrm{p}K_{a2}$ and world production from Sulfuric acid; all CC BY-SA 4.0, adapted by Ganzaa.
First aid for chemical splashes: NIOSH Pocket Guide, first aid, CDC.
Video: Manufacturing Sulphuric Acid, FuseSchool - Global Education, embedded from YouTube.
The text, flow diagram, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Three reactions in one bottle

2Manufacture: the Contact process

3Properties of sulphuric acid

4Diluting concentrated sulphuric acid safely

Three reactions in one bottle

Manufacture: the Contact process

Properties of sulphuric acid

Diluting concentrated sulphuric acid safely