ChemistryICSE Class 10 · Nitric Acid

Ostwald's Process: Manufacture of Nitric Acid from Ammonia (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

Ostwald's process makes nitric acid from ammonia in three steps. Ammonia and air are passed over platinum gauze at about 800 °C to give nitric oxide; nitric oxide is oxidised by more air to nitrogen dioxide on cooling; nitrogen dioxide with air is absorbed in water to give nitric acid.

Understand Ostwald's Process

Nitric acid is manufactured on a large scale from ammonia (itself made by Haber's process). For ICSE, the syllabus asks only for the equations with their conditions.

Step 1: Catalytic oxidation of ammonia. A mixture of dry ammonia and air (about 1 : 10 by volume) is passed over platinum gauze heated to about 800 °C:

$4 N H_{3} + 5 O_{2} P t, 800^{\circ} C 4 N O + 6 H_{2} O$ + heat

The reaction is exothermic, so once started it keeps the platinum hot without further heating. Ammonia is oxidised to nitric oxide (not nitrogen, as when it simply burns in oxygen); the catalyst makes this the main reaction.

Step 2: Oxidation of nitric oxide. The hot gases are cooled and mixed with more air. Nitric oxide combines with oxygen to form reddish-brown nitrogen dioxide:

$2 N O + O_{2} \to 2 N O_{2}$

Step 3: Absorption in water. Nitrogen dioxide, with more air, is passed up an absorption tower down which water trickles. It dissolves to form nitric acid:

$4 N O_{2} + 2 H_{2} O + O_{2} \to 4 H N O_{3}$

The acid obtained is dilute; it is concentrated by further treatment.

Overall bookkeeping. Each NH₃ molecule ends up as one HNO₃ molecule: 4NH₃ → 4NO → 4NO₂ → 4HNO₃. Adding the oxygen used in the three steps (5 + 2 + 1 = 8 O₂ per 4 NH₃) gives the overall change $N H_{3} + 2 O_{2} \to H N O_{3} + H_{2} O$ . This 1 : 1 link between ammonia and nitric acid makes mass calculations quick.

Links with other topics. Step 1 is the same reaction as the catalytic oxidation of ammonia in the Ammonia chapter. The ammonia comes from Haber's process (iron catalyst), and the nitric acid is used for fertilisers such as ammonium nitrate, explosives and dyes.

Ostwald's Process: formulas and key facts

Step 1: catalytic oxidation of ammonia: $4 N H_{3} + 5 O_{2} P t, 800^{\circ} C 4 N O + 6 H_{2} O$
Step 2: oxidation of nitric oxide: $2 N O + O_{2} \to 2 N O_{2}$
Step 3: absorption in water: $4 N O_{2} + 2 H_{2} O + O_{2} \to 4 H N O_{3}$
Overall: $N H_{3} + 2 O_{2} \to H N O_{3} + H_{2} O$; 1 mol NH₃ gives 1 mol HNO₃.
Conditions: Ammonia to air about 1 to 10 by volume
Catalyst platinum gauze
Temperature about 800 °C
Step 2 cooling, more air
Step 3 absorption tower with water and air

Ammonia to air	about 1 to 10 by volume
Catalyst	platinum gauze
Temperature	about 800 °C
Step 2	cooling, more air
Step 3	absorption tower with water and air

How to solve Ostwald's Process questions

For Ostwald's process you must produce three balanced equations with conditions. For numericals, use the 1 : 1 link between NH₃ and HNO₃.

1Write step 1 with its catalyst and temperature: platinum, about 800 °C.
2Write step 2: nitric oxide + oxygen gives nitrogen dioxide.
3Write step 3: nitrogen dioxide + water + oxygen gives nitric acid.
4Check each equation balances (4-5-4-6; 2-1-2; 4-2-1-4).
5For mass questions, moles of HNO₃ = moles of NH₃; for gas volumes in step 1, use 4 : 5 : 4.
6If asked why the platinum keeps glowing, say the reaction is exothermic.

Ostwald's Process in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Nitric acid output from ammonia

Suppose a plant oxidises 3.4 tonnes of ammonia a day by Ostwald's process and every molecule of ammonia ends up as nitric acid. (N = 14, H = 1, O = 16)

What mass of nitric acid (as pure HNO₃) is produced each day?

Try it first, then show the solutionSolution

Moles of NH₃ $= 3400 000/17 = 200000$ mol.
NH₃ : HNO₃ = 1 : 1 (4NH₃ → 4NO → 4NO₂ → 4HNO₃).
Mass of HNO₃ $= 200000 \times 63 = 12600000$ g $= 12.6$ tonnes.

Answer: 12.6 tonnes of HNO₃ per day.

Memory hook: One ammonia in, one nitric acid out.

Tricks to remember Ostwald's Process

Short memory aids. Say each one aloud once, then use it on a question the same day.

Ammonia → NO → NO₂ → HNO₃
Each step adds oxygen: 'Oxidise, oxidise, dissolve'.
Use it when: Order of the three steps.
Platinum at 800
Pt glows at 800: the catalytic oxidation step.
Use it when: Catalyst and temperature for step 1.
4-5-4-6
4NH₃ + 5O₂ → 4NO + 6H₂O.
Use it when: Balancing step 1.

Ostwald's Process: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Catalyst · 1 mark · MCQBoard level

The catalyst used in the first step of Ostwald's process is:
(a) iron
(b) platinum
(c) nickel
(d) vanadium pentoxide

Try it first, then show the solutionSolution

Step 1 is the catalytic oxidation of ammonia.
It uses platinum gauze at about 800 °C.

Answer: (b) platinum.

Example 2: The first step · 2 marks · Short answerBoard level

Write the balanced equation, with conditions, for the catalytic oxidation of ammonia in Ostwald's process.

Try it first, then show the solutionSolution

$4 N H_{3} + 5 O_{2} \to 4 N O + 6 H_{2} O$ .
Conditions: platinum gauze catalyst, about 800 °C, ammonia and air about 1 : 10.

Answer: 4NH₃ + 5O₂ → 4NO + 6H₂O over Pt at about 800 °C.

Example 3: All three equations · 3 marks · Short answerHarder board level

Give the three balanced equations by which ammonia is converted to nitric acid in Ostwald's process.

Try it first, then show the solutionSolution

$4 N H_{3} + 5 O_{2} \to 4 N O + 6 H_{2} O$ (Pt, about 800 °C).
$2 N O + O_{2} \to 2 N O_{2}$ (on cooling, with air).
$4 N O_{2} + 2 H_{2} O + O_{2} \to 4 H N O_{3}$ (absorption in water).

Answer: The three equations above.

Example 4: Gas volumes in step 1 · 3 marks · Short answerHarder board level

20 L of ammonia is oxidised in step 1 of Ostwald's process. (a) What volume of oxygen is needed in step 1, and what volume of NO forms? (b) What total volume of oxygen is needed for all three steps to turn this ammonia into nitric acid? (Gases at the same temperature and pressure.)

Try it first, then show the solutionSolution

(a) $4 N H_{3} : 5 O_{2} : 4 N O$ : O₂ $= 20 \times \frac{5}{4} = 25$ L; NO $= 20$ L.
(b) Steps 2 and 3 for 20 L NO need $10 + 5 = 15$ L O₂ (2NO : O₂; 4NO₂ : O₂).
Total O₂ $= 25 + 15 = 40$ L (overall NH₃ : O₂ = 1 : 2).

Answer: (a) 25 L O₂, 20 L NO. (b) 40 L O₂.

Example 5: Mass of acid from ammonia · 3 marks · Short answerHOTS / competency

Calculate the mass of nitric acid that can be made from 6.8 g of ammonia by Ostwald's process, and the percentage of nitrogen in nitric acid. (N = 14, H = 1, O = 16)

Try it first, then show the solutionSolution

Moles of NH₃ $= 6.8/17 = 0.4$ ; NH₃ : HNO₃ = 1 : 1.
Mass of HNO₃ $= 0.4 \times 63 = 25.2$ g.
% N $= \frac{14}{63} \times 100 = 22.22%$ .

Answer: 25.2 g; 22.22%.

Example 6: Explaining the conditions · 4 marks · Long answerHOTS / competency

In Ostwald's process: (a) Why does the platinum gauze stay red-hot after the heater is switched off? (b) Why is ammonia oxidised to NO here, but to N₂ when it simply burns in oxygen? (c) 92 g of NO₂ is fully absorbed: what mass of HNO₃ forms? (N = 14, O = 16, H = 1)

Try it first, then show the solutionSolution

(a) The oxidation of ammonia is exothermic; the heat released keeps the gauze hot.
(b) The platinum catalyst directs the reaction to give nitric oxide; without it, burning gives nitrogen and water ( $4 N H_{3} + 3 O_{2} \to 2 N_{2} + 6 H_{2} O$ ).
(c) Moles of NO₂ $= 92/46 = 2$ ; $4 N O_{2} : 4 H N O_{3} = 1 : 1$ , so 2 mol HNO₃ $= 126$ g.

Answer: (a) Exothermic reaction. (b) Platinum catalyst gives NO. (c) 126 g.

Where students lose marks in Ostwald's Process

The slips examiners see most often, and what to write instead.

Mistake: Platinum confused with other catalysts
In 2025 (Q2(iv)) examiners noted that candidates confused platinum with nickel when matching $4 N H_{3} + 5 O_{2} \to 4 N O + 6 H_{2} O$ with its catalyst.
Do this: Platinum for ammonia oxidation; nickel for hydrogenation; iron for Haber; V₂O₅ for Contact.
Usually costs: 1 mark.
Mistake: Burning equation instead of catalytic oxidation
Students write N₂ as the product of step 1.
Wrong working
4NH₃ + 3O₂ → 2N₂ + 6H₂O
Right working
4NH₃ + 5O₂ → 4NO + 6H₂O (Pt, 800 °C)
Do this: With a platinum catalyst the product is NO.
Usually costs: 1 to 2 marks.
Mistake: Absorption step without oxygen
Students write 3NO₂ + H₂O → 2HNO₃ + NO, which is not the form expected here.
Do this: Use $4 N O_{2} + 2 H_{2} O + O_{2} \to 4 H N O_{3}$ , with air supplying oxygen.
Usually costs: 1 mark.
Mistake: Missing conditions
The syllabus asks for equations with conditions; equations alone lose the condition mark.
Do this: Write 'Pt, about 800 °C' with step 1.
Usually costs: ½ to 1 mark.

Exam tips

Learn Ostwald's process as three equations with conditions only; plant diagrams are not required.
Step 1 is identical to the catalytic oxidation of ammonia in the Ammonia chapter; revise both together.
For mass questions, remember NH₃ : HNO₃ = 1 : 1.
Balance step 3 carefully: 4 : 2 : 1 → 4.

Practice questions on Ostwald's Process

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In Ostwald's process, nitric oxide is converted to nitrogen dioxide by:
(a) heating with platinum
(b) reacting with oxygen of air
(c) dissolving in water
(d) passing over iron
Hint
2NO + O₂ → 2NO₂.
Answer
(b).
Q2 · 1 mark · Very short answerBoard level
State the temperature at which ammonia is catalytically oxidised in Ostwald's process.
Hint
Glowing platinum.
Answer
About 800 °C.
Q3 · 1 mark · Very short answerBoard level
Write the equation for the absorption step in Ostwald's process.
Hint
Nitrogen dioxide, water and oxygen.
Answer
$4 N O_{2} + 2 H_{2} O + O_{2} \to 4 H N O_{3}$ .
Q4 · 1 mark · Very short answerHarder board level
Why is extra air added in the absorption step?
Hint
Look at the equation.
Answer
Oxygen is a reactant: it converts all the NO₂ into nitric acid without leaving nitric oxide.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The platinum gauze continues to glow during the catalytic oxidation of ammonia.
Reason (R): The oxidation of ammonia to nitric oxide is exothermic.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Heat released.
Answer
(a).
Q6 · 2 marks · Short answerHarder board level
Name the catalyst and give the equation for (a) Haber's process and (b) the first step of Ostwald's process.
Hint
Iron and platinum.
Answer
(a) Iron: $N_{2} + 3 H_{2} ⇌ 2 N H_{3}$ . (b) Platinum: $4 N H_{3} + 5 O_{2} \to 4 N O + 6 H_{2} O$ .
Q7 · 2 marks · Short answerHarder board level
What mass of ammonia is needed to make 31.5 g of nitric acid by Ostwald's process? (N = 14, H = 1, O = 16)
Hint
1 : 1.
Answer
Moles of HNO₃ $= 31.5/63 = 0.5$ ; NH₃ $= 0.5 \times 17 = 8.5$ g.
Q8 · 3 marks · Short answerHOTS / competency
Suppose the platinum gauze in an Ostwald plant is damaged and ammonia burns in the air without the catalyst. Predict the main nitrogen product, write the equation, and explain why the plant's output of nitric acid would fall.
Hint
Burning versus catalytic oxidation.
Answer
Without the catalyst, ammonia burns to nitrogen: $4 N H_{3} + 3 O_{2} \to 2 N_{2} + 6 H_{2} O$ . Nitrogen is unreactive and does not form NO₂ or nitric acid, so the nitrogen of the ammonia is lost and acid output falls.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Product of step 1?Show answer
Nitric oxide (NO) and water.
2.Colour of NO₂?Show answer
Reddish-brown.
3.Ratio of ammonia to air in step 1?Show answer
About 1 : 10 by volume.
4.Moles of HNO₃ from 1 mol NH₃?Show answer
1 mol.
5.Where is NO₂ absorbed?Show answer
In water in an absorption tower, with air.

Ostwald's Process: frequently asked questions

What are the steps of Ostwald's process?

Catalytic oxidation of ammonia to NO over platinum at about 800 °C; oxidation of NO to NO₂ with air; absorption of NO₂ with air in water to give nitric acid.

Which catalyst is used in Ostwald's process?

Platinum gauze, in the first step.

Why does the catalyst stay hot?

The oxidation of ammonia is exothermic, so the heat it gives out keeps the platinum at the working temperature.

Do I need to draw the Ostwald plant for ICSE?

No. The 2027 syllabus asks only for the equations with conditions where applicable.