Nitrogen and Its Inorganic Compounds

Lessons

Senior 3 · Chemistry

Nitrogen and Its Inorganic Compounds

4 lessons

Lesson 2 of 4

Ammonia and the Haber Process

25 min

Bread from air

Every protein in the beans and maize you eat contains nitrogen. Plants cannot use the nitrogen gas that surrounds them; they need it as ammonium or nitrate ions from the soil. Until about 1910, the only large sources of such compounds were manure, guano from sea birds and mineral nitrates dug in Chile. Then the German chemist Fritz Haber found how to make nitrogen from the air combine with hydrogen, and Carl Bosch turned his method into a factory process. Today about half of the world's people are fed by crops grown with fertilisers made from their ammonia. This lesson starts with ammonia in a test tube and ends in that factory.

Preparing ammonia in the laboratory

Any ammonium salt gives off ammonia when it is warmed with an alkali; you used this as the test for ammonium ions in S2. To prepare the gas, a mixture of solid calcium hydroxide and ammonium chloride is heated (Figure 2.1):

$\mathrm{Ca(OH)_2(s) + 2NH_4Cl(s) \rightarrow CaCl_2(s) + 2H_2O(l) + 2NH_3(g)}$

The test tube slopes down towards its mouth, because water is also formed; if it ran back onto the hot glass, the tube could crack.
The gas is dried with calcium oxide (quicklime). The usual drying agents cannot be used: concentrated sulfuric acid is an acid and reacts with ammonia, and anhydrous calcium chloride combines with it.
Ammonia is collected by upward delivery (downward displacement of air) into an upside-down jar, because it is much less dense than air ( $M_{r} = 17$ , against 29) and far too soluble to collect over water.

Figure 2.1: Laboratory preparation of dry ammonia.

Warning

Safety. Ammonia is toxic and its sharp smell irritates the eyes, nose and lungs. Prepare it in a fume cupboard or a well-ventilated room, in small amounts, and never sniff it directly: waft a little towards your nose with your hand. Remove the delivery tube from any liquid before you stop heating, to avoid suck-back.

Tests for ammonia

It turns damp red litmus paper blue. It is the only common gas that is alkaline.
It forms dense white fumes of ammonium chloride with hydrogen chloride, for example when a glass rod dipped in concentrated hydrochloric acid is held in it: $\mathrm{NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)}$ .

Physical properties

Ammonia is a colourless gas with a sharp, choking smell. It is much less dense than air. It liquefies easily (at −33 °C), and it is extremely soluble in water: at room temperature one volume of water dissolves roughly 700 volumes of ammonia.

The fountain experiment shows this solubility. A dry flask full of ammonia is closed with a bung carrying a glass jet, and turned upside down with the jet in water containing a few drops of red litmus or phenolphthalein. When a little water is squirted in, the ammonia dissolves so fast that the pressure in the flask falls; the air pressure outside then pushes water up through the jet in a fountain, which changes colour as it enters the alkaline solution.

Chemical properties

An alkali in water. Ammonia reacts slightly with water, forming hydroxide ions: $\mathrm{NH_3(g) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}$ . The solution is a weak alkali.
Neutralises acids, forming ammonium salts, which are fertilisers: $\mathrm{2NH_3(g) + H_2SO_4(aq) \rightarrow (NH_4)_2SO_4(aq)}$ and $\mathrm{NH_3(g) + HNO_3(aq) \rightarrow NH_4NO_3(aq)}$ .
A reducing agent. Passed over hot copper(II) oxide it forms copper, nitrogen and water: $\mathrm{3CuO(s) + 2NH_3(g) \rightarrow 3Cu(s) + N_2(g) + 3H_2O(l)}$ .
Burns in pure oxygen (but not in air) with a yellowish flame: $\mathrm{4NH_3(g) + 3O_2(g) \rightarrow 2N_2(g) + 6H_2O(l)}$ . With a platinum catalyst it forms nitrogen monoxide instead, the first step in making nitric acid (Lesson 3).

The Haber process

Nitrogen and hydrogen combine to form ammonia in a reversible reaction: as ammonia forms, some of it breaks down again into nitrogen and hydrogen. The reaction gives out heat:

$\mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)}$

The raw materials are nitrogen from the air and hydrogen made from natural gas (methane) and steam. They are mixed in the ratio 1 volume of nitrogen to 3 volumes of hydrogen, the ratio in the equation, and then (Figure 2.2):

Pressure of about 200 atmospheres. Four molecules of gas become two, so a high pressure favours the side with fewer molecules and gives more ammonia. It also speeds the reaction up. Higher pressures would give still more, but the thick steel equipment and the energy for compressors become too expensive.
Temperature of about 450 °C. Because the reaction gives out heat, a lower temperature would give more ammonia, but the reaction would be far too slow. About 450 °C is a compromise between a good yield and a fast rate.
An iron catalyst speeds up the reaction without being used up, so the mixture reaches its final state quickly.
Only about 15% of the gases turn into ammonia on each pass through the converter. The mixture is cooled, the ammonia liquefies and is run off, and the unreacted nitrogen and hydrogen are recycled, so that in the end almost all of them are used.

Figure 2.2: The Haber process for making ammonia.

Definition

Reversible reaction: a reaction that can go in both directions, so that the products can react to re-form the reactants. It is shown by the sign $\rightleftharpoons$ . Example: $\mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)}$ in the Haber process.

Uses of ammonia

Fertilisers, by far the biggest use: ammonium nitrate, ammonium sulfate, urea and the ammonium phosphates (Lesson 4).
Nitric acid (Lesson 3), and from it explosives for mining and road building.
Cleaning products: dilute ammonia solution removes grease from glass.
Refrigeration in large cold stores: liquid ammonia takes in a lot of heat as it evaporates.

Worked example

Problem. A student needs $2.40\,\mathrm{dm^3}$ of ammonia at room temperature and pressure. What masses of ammonium chloride and calcium hydroxide react to give this? (N = 14, H = 1, Cl = 35.5, Ca = 40, O = 16; molar volume $24\,\mathrm{dm^3\,mol^{-1}}$ )

Solution. $n(\mathrm{NH_3}) = \frac{2.40}{24} = 0.100\,\mathrm{mol}$ .

From the equation, 2 mol $\mathrm{NH_4Cl}$ give 2 mol $\mathrm{NH_3}$ : $n(\mathrm{NH_4Cl}) = 0.100\,\mathrm{mol}$ , mass $= 0.100 \times 53.5 = 5.35\,\mathrm{g}$ .

1 mol $\mathrm{Ca(OH)_2}$ gives 2 mol $\mathrm{NH_3}$ : $n(\mathrm{Ca(OH)_2}) = 0.0500\,\mathrm{mol}$ , and $M = 40 + 2 \times 17 = 74\,\mathrm{g\,mol^{-1}}$ , so the mass is $0.0500 \times 74 = 3.70\,\mathrm{g}$ .

Worked example

Problem. $100\,\mathrm{dm^3}$ of nitrogen and $300\,\mathrm{dm^3}$ of hydrogen enter a Haber converter, and 15% of the nitrogen reacts. (a) What volume of ammonia forms? (b) What volumes of nitrogen and hydrogen are recycled? (All volumes at the same temperature and pressure.)

Solution. Equal volumes of gases contain equal numbers of molecules, so the volumes react in the ratio of the equation, 1 : 3 : 2.

(a) Nitrogen reacting $= 0.15 \times 100 = 15\,\mathrm{dm^3}$ ; it uses $3 \times 15 = 45\,\mathrm{dm^3}$ of hydrogen and forms $2 \times 15 = 30\,\mathrm{dm^3}$ of ammonia.

(b) Recycled: nitrogen $100 - 15 = 85\,\mathrm{dm^3}$ and hydrogen $300 - 45 = 255\,\mathrm{dm^3}$ , still in the ratio 1 : 3, ready for the next pass.

Tip

The Haber numbers: 1, 3, 200, 450, iron. One volume of nitrogen, three of hydrogen, about 200 atmospheres, about 450 °C, an iron catalyst. Then remember the two tricks that make it economic: liquefy the ammonia and recycle the rest.

FuseSchool: What Is The Haber Process (4 min). Watch for the raw materials, the conditions in the converter and the recycling loop, and check them against Figure 2.2.

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Practice

Write the equation, with state symbols, for the laboratory preparation of ammonia from calcium hydroxide and ammonium chloride.
Why is ammonia (a) dried with calcium oxide rather than concentrated sulfuric acid, (b) collected by upward delivery?
Describe two tests for ammonia and their results.
Explain how the fountain experiment shows that ammonia is very soluble in water.
Write equations for the reactions of ammonia with (a) sulfuric acid, (b) hot copper(II) oxide. In (b), what does ammonia act as?
State the conditions of the Haber process and explain why a temperature of about 450 °C is used even though a lower temperature would give more ammonia.
How is the ammonia separated from the unreacted gases, and what happens to those gases?
What volume of hydrogen is needed to make $50\,\mathrm{dm^3}$ of ammonia, and how much nitrogen does it react with? (Same temperature and pressure.)

Show answersHide answers

$\mathrm{Ca(OH)_2(s) + 2NH_4Cl(s) \rightarrow CaCl_2(s) + 2H_2O(l) + 2NH_3(g)}$ .
(a) Ammonia is alkaline and would react with the acid (forming ammonium sulfate); calcium oxide is a base and does not react with it. (b) It is much less dense than air and very soluble in water.
It turns damp red litmus blue; it forms dense white fumes (ammonium chloride) with hydrogen chloride from concentrated hydrochloric acid.
A little water dissolves so much ammonia that the pressure inside the flask drops far below the air pressure outside; the air pressure then forces water up the jet as a fountain.
(a) $\mathrm{2NH_3(g) + H_2SO_4(aq) \rightarrow (NH_4)_2SO_4(aq)}$ ; (b) $\mathrm{3CuO(s) + 2NH_3(g) \rightarrow 3Cu(s) + N_2(g) + 3H_2O(l)}$ : ammonia is a reducing agent.
About 200 atm, about 450 °C, iron catalyst, $\mathrm{N_2}$ and $\mathrm{H_2}$ in the ratio 1 : 3. At a lower temperature the reaction would be too slow; 450 °C is a compromise between yield and rate.
The gas mixture is cooled; ammonia liquefies (it condenses much more easily than nitrogen or hydrogen) and is run off. The unreacted nitrogen and hydrogen are recycled to the converter.
From $\mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)}$ : hydrogen $= \frac{3}{2} \times 50 = 75\,\mathrm{dm^3}$ ; nitrogen $= \frac{1}{2} \times 50 = 25\,\mathrm{dm^3}$ .

Sources and credits

Physical Sciences Grade 12, 14.4: The fertiliser industry (the Haber process, its raw materials and conditions), Siyavula Education and volunteers, Siyavula, licensed under CC BY 3.0. Adapted by Ganzaa.
Photo: Ammoniak Reaktor BASF.jpg, Drahkrub, licensed under CC BY-SA 3.0, via Wikimedia Commons.
Video: What Is The Haber Process, FuseSchool – Global Education, embedded from YouTube.
The diagrams, worked examples and Practice items were written and drawn by Ganzaa.

1Bread from air

2Preparing ammonia in the laboratory

3Tests for ammonia

4Physical properties

5Chemical properties

6The Haber process

7Uses of ammonia