Trends in Chemical Properties of Group 15 Elements and Their Compounds

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Senior 4 · Chemistry

Trends in Chemical Properties of Group 15 Elements and Their Compounds

5 lessons

Lesson 3 of 5

Ammonia and Nitric Acid

25 min

Bread from air

In the 2024–25 farming year, Rwanda applied about 92,500 tonnes of mineral fertiliser to its food crops, and roughly a quarter of it was urea, a compound made from ammonia. Almost all the world's ammonia is made by the Haber process, which forces the unreactive nitrogen of the air to combine with hydrogen; part of that ammonia is then oxidised to nitric acid for fertilisers such as ammonium nitrate, and for explosives used in mining and road building. This lesson explains how the two compounds are made, and what they do.

Definition

Haber process: the industrial synthesis of ammonia from nitrogen and hydrogen over an iron catalyst at high pressure and moderate temperature: $\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}$ .

Ostwald process: the industrial manufacture of nitric acid by catalytic oxidation of ammonia to nitrogen monoxide, followed by oxidation to nitrogen dioxide and absorption in water.

Ligand: a molecule or ion that donates a lone pair to a metal ion to form a complex. Example: ammonia in $\mathrm{[Cu(NH_3)_4(H_2O)_2]^{2+}}$ .

Making ammonia: the Haber process

\mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)} \qquad \Delta H = -92\ \mathrm{kJ\,mol^{-1}}

The conditions are a compromise between yield, rate and cost (Figure 10.5):

Pressure, about 200 atm. Four moles of gas become two, so high pressure shifts the equilibrium to the right (Le Chatelier's principle) and also speeds the reaction. Higher pressures would give more ammonia but need stronger, more expensive plant and more energy for compression.
Temperature, about 400–500 °C. The forward reaction is exothermic, so a low temperature would give a higher equilibrium yield, but the reaction would be far too slow, because breaking into the N≡N bond has a very high activation energy. The temperature chosen gives an acceptable rate with a reasonable yield.
Iron catalyst (with promoters such as potassium oxide). It lowers the activation energy so equilibrium is reached quickly at this temperature; it does not change the position of equilibrium.
Recycling. Only about 15% of the gases react on each pass. The ammonia is removed by cooling it until it liquefies, and the unreacted gases go round again, so about 97% is converted overall.

Figure 10.5: The Haber process.

Ammonia in the laboratory

Ammonia is made by warming an ammonium salt with a base, and dried by passing it through calcium oxide (it reacts with the usual acidic drying agents):

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

Properties of ammonia

A colourless gas with a sharp smell, less dense than air. It is extremely soluble in water (about 530 g per litre at 20 °C) because it hydrogen-bonds with water molecules, enough to drive the "fountain experiment".
A weak base ( $\mathrm{p}K_b = 4.75$ ): $\mathrm{NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}$ . With acids it forms ammonium salts; with hydrogen chloride gas it gives a white smoke of ammonium chloride: $\mathrm{NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)}$ .
A ligand: it donates its lone pair to metal ions. With copper(II) ions, a little ammonia gives pale blue $\mathrm{Cu(OH)_2}$ ; excess gives a deep blue solution of $\mathrm{[Cu(NH_3)_4(H_2O)_2]^{2+}}$ .
A reducing agent when heated: $\mathrm{3CuO + 2NH_3 \rightarrow 3Cu + N_2 + 3H_2O}$ . It burns in pure oxygen to nitrogen and water, $\mathrm{4NH_3 + 3O_2 \rightarrow 2N_2 + 6H_2O}$ , but with a platinum catalyst it is oxidised to NO instead, the first step of the Ostwald process.

Test for ammonia: it turns damp red litmus paper blue (the only common alkaline gas), and gives white fumes with hydrogen chloride from a drop of concentrated hydrochloric acid.

Making nitric acid: the Ostwald process

Catalytic oxidation of ammonia. Ammonia and air pass over a platinum gauze alloyed with about 10% rhodium at about 900 °C: $\mathrm{4NH_3(g) + 5O_2(g) \rightarrow 4NO(g) + 6H_2O(g)}$
Oxidation of NO. The gases are cooled (to about 250 °C and below) and more air is added: $\mathrm{2NO(g) + O_2(g) \rightarrow 2NO_2(g)}$ .
Absorption. Nitrogen dioxide and more oxygen are absorbed in water: $\mathrm{4NO_2(g) + O_2(g) + 2H_2O(l) \rightarrow 4HNO_3(aq)}$

Properties of nitric acid

A strong acid: fully ionised in dilute solution, it reacts with bases, basic oxides and carbonates to give nitrates.
A strong oxidising agent, because nitrogen is in its highest oxidation state, +5. With metals it seldom gives hydrogen; the nitrate ion is reduced instead. Concentrated acid is reduced mainly to $\mathrm{NO_2}$ (+4), dilute acid mainly to NO (+2): $\mathrm{Cu(s) + 4HNO_3(conc) \rightarrow Cu(NO_3)_2(aq) + 2NO_2(g) + 2H_2O(l)}$ $\mathrm{3Cu(s) + 8HNO_3(dil) \rightarrow 3Cu(NO_3)_2(aq) + 2NO(g) + 4H_2O(l)}$ It also oxidises non-metals: hot concentrated nitric acid turns sulphur into sulphuric acid and carbon into carbon dioxide.
Passivation. Concentrated nitric acid makes iron, aluminium and chromium passive by thickening their oxide film.
Decomposition. In light or when warmed it slowly decomposes, $\mathrm{4HNO_3 \rightarrow 4NO_2 + O_2 + 2H_2O}$ ; dissolved $\mathrm{NO_2}$ turns it yellow, so it is stored in brown bottles.

40 cm³ of fuming nitric acid in a bottle. Its yellow colour comes from dissolved nitrogen dioxide, formed as the acid slowly decomposes (4HNO₃ → 4NO₂ + O₂ + 2H₂O); that is why nitric acid is stored in the dark. Photo: W. Oelen, CC BY-SA 3.0, via Wikimedia Commons

Worked example 1: nitrogen for Rwanda's fields

In 2024–25, Rwanda applied 92,520.2 t of mineral fertiliser to food crops, of which 24.5% was urea, $\mathrm{CO(NH_2)_2}$ . Calculate (a) the mass of urea and (b) the mass of nitrogen it supplied. $A_{r}$ : C 12.01, O 16.00, N 14.01, H 1.01.

Solution. (a) Mass of urea $= 0.245 \times 92\,520.2 = 22\,667\ \mathrm{t}$ .
(b) $M(\mathrm{CO(NH_2)_2}) = 12.01 + 16.00 + 2(14.01 + 2.02) = 60.07\ \mathrm{g\,mol^{-1}} \qquad \%\,\mathrm{N} = \frac{2 \times 14.01}{60.07} \times 100 = 46.6\%$ Mass of nitrogen $= 0.466 \times 22\,667 = 1.06 \times 10^4\ \mathrm{t}$ , about 10,600 tonnes. Urea is popular because it has the highest nitrogen content of the common solid fertilisers (ammonium nitrate, $\mathrm{NH_4NO_3}$ , is 35.0% N).

Worked example 2: copper and dilute nitric acid

1.27 g of copper dissolves completely in excess dilute nitric acid. (a) Assign oxidation numbers to show what is oxidised and what is reduced. (b) Calculate the volume of NO formed at room temperature and pressure (24.0 dm³ mol⁻¹). $A_r(\mathrm{Cu}) = 63.55$ .

Solution. $\mathrm{3Cu + 8HNO_3 \rightarrow 3Cu(NO_3)_2 + 2NO + 4H_2O}$ .
(a) Copper: 0 → +2 (oxidised). Nitrogen: +5 in $\mathrm{HNO_3}$ → +2 in NO (reduced) for two of the eight nitrate groups; the other six stay as nitrate (+5).
(b) $n(\mathrm{Cu}) = \frac{1.27}{63.55} = 0.0200\ \mathrm{mol} \qquad n(\mathrm{NO}) = 0.0200 \times \frac{2}{3} = 0.0133\ \mathrm{mol}$ $V = 0.0133 \times 24.0 = 0.320\ \mathrm{dm^3} = 320\ \mathrm{cm^3}$ The colourless NO turns brown at the mouth of the tube as it meets air: $\mathrm{2NO + O_2 \rightarrow 2NO_2}$ .

Warning

Common mistakes. (1) Saying a high temperature is used in the Haber process "to increase the yield": it lowers the equilibrium yield; it is used for rate. (2) Saying the catalyst increases the yield: it only helps equilibrium to be reached faster. (3) Writing hydrogen as the gas from copper and nitric acid: the products are NO or $\mathrm{NO_2}$ . (4) Drying ammonia with concentrated sulphuric acid or calcium chloride: both react with it; use calcium oxide.

Safety. Concentrated ammonia solution and ammonia gas irritate the eyes and lungs: work in a ventilated room. Concentrated nitric acid causes severe burns and stains skin yellow, and nitrogen dioxide is toxic: reactions of nitric acid with metals are done in a fume cupboard. If acid or ammonia splashes on skin or into the eyes, rinse with plenty of running water for several minutes and get medical help.

Tip

Haber in four numbers: 200 atm, 450 °C, iron, 15% per pass. Ostwald in three steps: NH₃ → NO (Pt/Rh, 900 °C) → NO₂ → HNO₃. Nitric acid attacks metals as an oxidising agent: concentrated gives NO₂, dilute gives NO, never H₂ (with copper).

Professor Dave Explains: The Haber–Bosch Process, Industrial Ammonia Synthesis (11 min). Watch for how Le Chatelier's principle and reaction rate together decide the pressure and temperature, and why the unreacted gases are recycled.

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Practice

State the conditions of the Haber process and explain why each is chosen.
Use $\Delta_f H^\circ(\mathrm{NH_3}) = -46\ \mathrm{kJ\,mol^{-1}}$ to calculate the heat released when 1.00 tonne of ammonia is made. $M(\mathrm{NH_3}) = 17.03$ .
Write equations for: (a) the laboratory preparation of ammonia; (b) ammonia reacting with hydrogen chloride; (c) ammonia reducing copper(II) oxide.
Describe a test for ammonia gas.
Write the three equations of the Ostwald process.
Why does nitric acid turn yellow on standing, and how is it stored?
Calculate the volume of $\mathrm{NO_2}$ , at room temperature and pressure, formed when 1.27 g of copper reacts with excess concentrated nitric acid. $A_r(\mathrm{Cu}) = 63.55$ .
Calculate the percentage by mass of nitrogen in ammonium sulphate, $\mathrm{(NH_4)_2SO_4}$ , and compare it with urea (46.6%). Which fertiliser delivers more nitrogen per kilogram?

Show answersHide answers

About 200 atm (fewer gas moles on the right, so higher yield and rate; higher pressure too costly); about 400–500 °C (low temperature favours yield of the exothermic reaction, but the rate would be too slow: a compromise); iron catalyst (faster attainment of equilibrium); ammonia removed by liquefaction and unreacted gases recycled.
$n = \frac{1.00 \times 10^6}{17.03} = 5.87 \times 10^4$ mol; heat $= 5.87 \times 10^4 \times 46 = 2.70 \times 10^6\ \mathrm{kJ}$ (2.7 GJ).
(a) $\mathrm{2NH_4Cl + Ca(OH)_2 \rightarrow CaCl_2 + 2H_2O + 2NH_3}$ ; (b) $\mathrm{NH_3 + HCl \rightarrow NH_4Cl}$ ; (c) $\mathrm{3CuO + 2NH_3 \rightarrow 3Cu + N_2 + 3H_2O}$ .
It turns damp red litmus paper blue; it gives dense white fumes with HCl from a glass rod dipped in concentrated hydrochloric acid.
$\mathrm{4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O}$ ; $\mathrm{2NO + O_2 \rightarrow 2NO_2}$ ; $\mathrm{4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3}$ .
It decomposes in light: $\mathrm{4HNO_3 \rightarrow 4NO_2 + O_2 + 2H_2O}$ ; the dissolved $\mathrm{NO_2}$ is yellow-brown. It is kept in brown bottles, away from light.
$\mathrm{Cu + 4HNO_3 \rightarrow Cu(NO_3)_2 + 2NO_2 + 2H_2O}$ ; $n(\mathrm{Cu}) = 0.0200$ mol, $n(\mathrm{NO_2}) = 0.0400$ mol; $V = 0.0400 \times 24.0 = 0.960\ \mathrm{dm^3}$ .
$M = 132.17$ ; % N $= \frac{28.02}{132.17} \times 100 = 21.2\%$ . Urea delivers more than twice as much nitrogen per kilogram.

Quick check

Why is the Haber process run at about 450 °C rather than at a lower temperature?

Key points

Haber process: $\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}$ , $\Delta H = -92\ \mathrm{kJ\,mol^{-1}}$ ; about 200 atm, 400–500 °C, iron catalyst; about 15% per pass, about 97% overall with recycling.
Ammonia: very soluble, weak base ( $\mathrm{p}K_b$ 4.75), forms ammonium salts and white $\mathrm{NH_4Cl}$ smoke, acts as a ligand (deep blue copper complex) and as a reducing agent; it turns damp red litmus blue.
Ostwald process: $\mathrm{NH_3 \rightarrow NO}$ (Pt/Rh, about 900 °C) $\rightarrow \mathrm{NO_2 \rightarrow HNO_3}$ .
Nitric acid is a strong acid and a strong oxidising agent: with copper, concentrated acid gives $\mathrm{NO_2}$ , dilute acid NO; it passivates Fe and Al and decomposes in light.

Sources and credits

Image: Tetramminecopper sulfate.JPG by Chemicalinterest, via Wikimedia Commons, public domain.
Image: Fuming nitric acid 40ml.jpg by W. Oelen, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Rwanda's fertiliser use in 2024–25: How Rwanda's Fertiliser Subsidy Scheme Works, The New Times (Kigali), 14 July 2026, via allAfrica, citing the Ministry of Agriculture and Animal Resources (MINAGRI) 2024–25 Annual Report.
Process conditions from Wikipedia, Haber process and Ostwald process; solubility, $\mathrm{p}K_b$ and enthalpy of formation of ammonia from Ammonia; all CC BY-SA 4.0, adapted by Ganzaa.
First aid for chemical splashes: NIOSH Pocket Guide, first aid, CDC.
Video: The Haber-Bosch Process: Industrial Ammonia Synthesis, Professor Dave Explains, 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.

1Bread from air

2Making ammonia: the Haber process

3Ammonia in the laboratory

4Making nitric acid: the Ostwald process

5Properties of nitric acid

Bread from air

Making ammonia: the Haber process

Ammonia in the laboratory

Making nitric acid: the Ostwald process

Properties of nitric acid