Sulphur and Its Inorganic Compounds

Lessons

Senior 3 · Chemistry

Sulphur and Its Inorganic Compounds

4 lessons

Lesson 3 of 4

Sulfuric Acid and the Contact Process

25 min

The chemical in almost everything

The battery that starts a car or stores solar power in many Rwandan homes and shops is a lead–acid battery filled with dilute sulfuric acid. The phosphate fertiliser in a sack of NPK, the detergent in a packet of washing powder and the white paint on a wall were all made with sulfuric acid at some stage. More sulfuric acid is made than any other industrial chemical. This lesson shows how it is made from sulfur, and why the concentrated acid behaves so differently from the dilute acid you met in S2.

Making sulfuric acid: the Contact process

Step 1: sulfur dioxide. Molten sulfur is sprayed into a furnace and burned in dry air. (Some plants roast sulfide ores such as iron pyrites instead.)

$\mathrm{S(l) + O_2(g) \rightarrow SO_2(g)}$

Step 2: cleaning and drying. Dust would poison the catalyst (stop it working), so the gas is cleaned; it is dried with concentrated sulfuric acid.

Step 3: sulfur trioxide, in the converter. Sulfur dioxide and more air pass over a catalyst of vanadium(V) oxide, $\mathrm{V_2O_5}$ , at about 450 °C and a pressure of only 1–2 atmospheres. The reaction is reversible and gives out heat:

$\mathrm{2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)}$

As in the Haber process, 450 °C is a compromise between a good yield and a fast rate. Here, however, the yield is already very high (about 99% after the gas passes through several layers of catalyst), so a high pressure is not worth its cost.

Step 4: absorption. Sulfur trioxide is not dissolved directly in water: the reaction gives out so much heat that it forms a fog of tiny acid droplets that is very hard to collect. Instead it is absorbed in 98% sulfuric acid, forming oleum, which is then diluted with the right amount of water:

$\mathrm{SO_3(g) + H_2SO_4(l) \rightarrow H_2S_2O_7(l)}$

$\mathrm{H_2S_2O_7(l) + H_2O(l) \rightarrow 2H_2SO_4(l)}$

Figure 3.1: The Contact process for making sulfuric acid.

Definition

Catalyst: a substance that speeds up a chemical reaction without being used up itself. Example: vanadium(V) oxide in the Contact process; iron in the Haber process. A catalyst is poisoned when impurities stick to its surface and stop it working.

Concentrated sulfuric acid

Concentrated sulfuric acid (about 98%) is a colourless, oily liquid, almost twice as dense as water ( $1.84\,\mathrm{g\,cm^{-3}}$ ). It boils only at about 338 °C, so it is involatile. It has a very strong attraction for water, and it gives out a great deal of heat when it mixes with water. Its properties follow from these facts.

A drying agent. It absorbs water vapour, so gases that do not react with it (oxygen, carbon dioxide, sulfur dioxide, chlorine, hydrogen chloride) are dried by bubbling them through it. Ammonia cannot be dried this way, because it is alkaline.
A dehydrating agent. It removes hydrogen and oxygen atoms, in the ratio 2 : 1, even from compounds that contain no water molecules at all, and it pulls the water of crystallisation out of hydrated crystals. Sugar turns into a black, steaming column of carbon, and blue copper(II) sulfate crystals turn white:
$\mathrm{C_{12}H_{22}O_{11}(s) \rightarrow 12C(s) + 11H_2O(l)}$
$\mathrm{CuSO_4 \cdot 5H_2O(s) \rightarrow CuSO_4(s) + 5H_2O(l)}$
This is also why it chars paper, wood and cloth, and burns skin so badly.
An oxidising agent, when hot. It oxidises copper, carbon and sulfur and is reduced to sulfur dioxide:
$\mathrm{Cu(s) + 2H_2SO_4(l) \rightarrow CuSO_4(aq) + SO_2(g) + 2H_2O(l)}$
$\mathrm{S(s) + 2H_2SO_4(l) \rightarrow 3SO_2(g) + 2H_2O(l)}$
An involatile acid. Heated with the salt of a volatile acid, it drives out that acid. This is how nitric acid (Unit 2) and hydrogen chloride (Unit 4) are made:
$\mathrm{NaCl(s) + H_2SO_4(l) \rightarrow NaHSO_4(s) + HCl(g)}$

Definition

Dehydrating agent: a substance that removes the elements of water (hydrogen and oxygen in the ratio 2 : 1) from a compound. A drying agent only removes water that is already present as water, for example from a moist gas. Concentrated sulfuric acid is both.

Warning

Safety: always add acid to water, never water to acid. Mixing concentrated sulfuric acid with water gives out so much heat that water poured into the acid boils where it lands and spits hot, concentrated acid out of the container (Figure 3.2). To dilute it, pour the acid slowly, down a glass rod, into a large volume of water while stirring. Wear eye protection and gloves. If it touches your skin, rinse it at once under plenty of running water for at least 15 minutes and tell the teacher.

Figure 3.2: The safe way to dilute concentrated sulfuric acid.

Dilute sulfuric acid

In dilute solution the acid is fully ionised, $\mathrm{H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)}$ , and shows all the typical properties of a strong acid that you met in S2. It is dibasic: each molecule can give two hydrogen ions, so it forms two kinds of salt, normal salts (sulfates) and acid salts (hydrogensulfates):

$\mathrm{2NaOH(aq) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + 2H_2O(l)}$

$\mathrm{NaOH(aq) + H_2SO_4(aq) \rightarrow NaHSO_4(aq) + H_2O(l)}$

Table 3.1: Dilute and concentrated sulfuric acid compared

Reagent	Dilute acid	Concentrated acid
zinc	hydrogen and zinc sulfate: $\mathrm{Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)}$	(when hot) sulfur dioxide, not hydrogen
copper	no reaction	(when hot) copper(II) sulfate, sulfur dioxide and water
sodium carbonate	fizzes: sodium sulfate, water and carbon dioxide	reacts, but it is never used this way
blue copper(II) sulfate crystals	dissolve to a blue solution	turn white (water removed)
sugar	dissolves; no charring	chars to black carbon

Uses of sulfuric acid

Fertilisers: ammonium sulfate and superphosphate (made from phosphate rock), the biggest use.
Lead–acid batteries in vehicles and solar-power systems.
Detergents, paints and pigments, and plastics.
Cleaning metals ("pickling") before they are painted or galvanised.
A drying agent in the laboratory and in industry (including the Contact process itself).

Worked example

Problem. A Contact plant burns 32 tonnes of sulfur a day. (a) What is the greatest mass of sulfuric acid it could make? (b) If the overall yield is 96%, what mass is actually made? (H = 1, O = 16, S = 32)

Solution. Every sulfur atom ends up in one $\mathrm{H_2SO_4}$ molecule: S → $\mathrm{SO_2}$ → $\mathrm{SO_3}$ → $\mathrm{H_2SO_4}$ . So 1 mol S (32 g) gives 1 mol $\mathrm{H_2SO_4}$ (98 g).

(a) $\text{mass} = 32\,\mathrm{t} \times \frac{98}{32} = 98\,\mathrm{t}$

(b) $0.96 \times 98 = 94\,\mathrm{t}$ (to 2 significant figures).

Worked example

Problem. 25.0 g of blue copper(II) sulfate crystals, $\mathrm{CuSO_4 \cdot 5H_2O}$ , are completely dehydrated by concentrated sulfuric acid. What mass of white anhydrous copper(II) sulfate is left? (Cu = 64, S = 32, O = 16, H = 1)

Solution. $M(\mathrm{CuSO_4 \cdot 5H_2O}) = 64 + 32 + 64 + 5 \times 18 = 250\,\mathrm{g\,mol^{-1}}$ ; $M(\mathrm{CuSO_4}) = 160\,\mathrm{g\,mol^{-1}}$ .

$n = \frac{25.0}{250} = 0.100\,\mathrm{mol} \qquad \text{mass of } \mathrm{CuSO_4} = 0.100 \times 160 = 16.0\,\mathrm{g}$

The acid has removed $25.0 - 16.0 = 9.0\,\mathrm{g}$ of water, which is $0.100 \times 5 \times 18 = 9.0\,\mathrm{g}$ . The numbers agree.

Tip

Contact process in four words: burn, clean, convert, absorb. The converter numbers are $\mathrm{V_2O_5}$ , 450 °C and 1–2 atm. And for safety: "Do what you oughta, add acid to water."

Aaron Keller: Exothermic dehydration of table sugar by sulfuric acid (8 min). Watch the sugar turn yellow, brown and black, and notice the steam: which property of the concentrated acid does each observation show?

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Practice

Write the equations for the four chemical steps of the Contact process.
State the catalyst, temperature and pressure used in the converter.
Why is sulfur trioxide absorbed in concentrated sulfuric acid rather than in water?
Explain the difference between a drying agent and a dehydrating agent, with one example of each action of concentrated sulfuric acid.
Why can ammonia not be dried with concentrated sulfuric acid? Write an equation.
Write equations for the reactions of dilute sulfuric acid with (a) magnesium, (b) copper(II) oxide, (c) potassium hydroxide (forming the normal salt).
A student adds water to concentrated sulfuric acid and the mixture spits. Explain, and describe the correct method.
What mass of sulfuric acid can be made from 60 tonnes of iron pyrites, $\mathrm{FeS_2}$ , if all its sulfur is used? (Fe = 56, S = 32, $\mathrm{H_2SO_4} = 98$ )

Show answersHide answers

$\mathrm{S(l) + O_2(g) \rightarrow SO_2(g)}$ ; $\mathrm{2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)}$ ; $\mathrm{SO_3(g) + H_2SO_4(l) \rightarrow H_2S_2O_7(l)}$ ; $\mathrm{H_2S_2O_7(l) + H_2O(l) \rightarrow 2H_2SO_4(l)}$ .
Vanadium(V) oxide, about 450 °C, 1–2 atmospheres.
With water the reaction is so exothermic that a fog of acid droplets forms, which is hard to collect; oleum formation is controlled.
A drying agent removes water that is present as water, e.g. drying moist chlorine. A dehydrating agent removes H and O atoms (2 : 1) from a compound, e.g. turning sugar into carbon.
Ammonia is alkaline and reacts with the acid: $\mathrm{2NH_3(g) + H_2SO_4(l) \rightarrow (NH_4)_2SO_4(s)}$ .
(a) $\mathrm{Mg(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + H_2(g)}$ ; (b) $\mathrm{CuO(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l)}$ ; (c) $\mathrm{2KOH(aq) + H_2SO_4(aq) \rightarrow K_2SO_4(aq) + 2H_2O(l)}$ .
Mixing releases a lot of heat; the small amount of water added boils at once and throws hot acid out. Add the acid slowly to a large volume of water, down a glass rod, stirring.
Each $\mathrm{FeS_2}$ ( $M_{r} = 120$ ) contains 2 S, so it gives 2 $\mathrm{H_2SO_4}$ ( $2 \times 98 = 196$ ). Mass $= 60 \times \frac{196}{120} = 98\,\mathrm{t}$ .

Sources and credits

Physical Sciences Grade 12, 14.4: The fertiliser industry (the Contact process and the use of sulfuric acid in fertilisers), Siyavula Education and volunteers, Siyavula, licensed under CC BY 3.0. Adapted by Ganzaa.
Photo: Dehydration of sugar by sulfuric acid.png, Capaccio, licensed under CC BY-SA 3.0, via Wikimedia Commons.
Video: Exothermic Dehydration of Sugar by Sulfuric Acid, Aaron Keller, embedded from YouTube.
The diagrams, table, worked examples and Practice items were written and drawn by Ganzaa.

1The chemical in almost everything

2Making sulfuric acid: the Contact process

3Concentrated sulfuric acid

4Dilute sulfuric acid

5Uses of sulfuric acid

The chemical in almost everything

Making sulfuric acid: the Contact process

Concentrated sulfuric acid

Dilute sulfuric acid

Uses of sulfuric acid