Study of Compounds: Nitric Acid notes: ICSE Class 10 Chemistry (Science Paper 2)

1. Laboratory preparation

Reactants: potassium nitrate (or sodium nitrate) and concentrated sulphuric acid, heated gently.

Principle: the less volatile (higher-boiling) sulphuric acid displaces the more volatile nitric acid from its salt.
Apparatus: a glass retort with a glass stopper; the neck leads into a receiver cooled by water. The apparatus is all glass because nitric acid vapour attacks cork and rubber.
Temperature: kept below 200 °C.

Table of the parts of the apparatus for preparing nitric acid and their purposes
Part	Purpose
Glass retort and stopper	Holds the mixture; resists the acid vapour
Receiver in cold water	Condenses the nitric acid vapour
Gentle heating	Keeps the temperature below 200 °C

Formulas

Below 200 °C
$\ce{KNO3 + H2SO4 -> KHSO4 + HNO3}$
With sodium nitrate
$\ce{NaNO3 + H2SO4 -> NaHSO4 + HNO3}$

Key terms

All-glass apparatus: Apparatus with no cork or rubber parts, used because hot nitric acid vapour attacks cork and rubber.
Volatile acid: An acid with a low boiling point that easily forms vapour, such as nitric acid.

2. Precautions, colour and physical properties

Why below 200 °C? At higher temperature KNO₃ + KHSO₄ → K₂SO₄ + HNO₃ would occur, but the strong heat needed would decompose nitric acid, may crack the glass, leaves a hard crust of K₂SO₄ and wastes fuel.

Yellow colour: heat or light decomposes some acid; the NO₂ formed dissolves in it. Remove it by bubbling dry air or CO₂ through the warm acid, or by adding water. Store the acid in brown bottles.

Physical properties: colourless fuming liquid (pure), choking smell, heavier than water, miscible with water in all proportions, corrosive (yellow stains on skin).

Table of three observations about nitric acid and their reasons
Observation	Reason
Laboratory acid is yellow	Dissolved NO₂ from decomposition
Stored in brown bottles	Light decomposes the acid
Yellow stain on skin	The acid attacks proteins in the skin

Formulas

Heat or sunlight
$\ce{4HNO3 -> 4NO2 + 2H2O + O2}$
Only at high temperature: avoided
$\ce{KNO3 + KHSO4 -> K2SO4 + HNO3}$

Key terms

Decomposition of nitric acid: The breakdown of nitric acid by heat or light into nitrogen dioxide, water and oxygen.

3. Ostwald’s process (equations and conditions)

Step 1 (catalytic oxidation of ammonia): dry ammonia and air, about 1 : 10 by volume, over platinum gauze at about 800 °C. Exothermic, so the catalyst stays red-hot.

Step 2: nitric oxide combines with more oxygen on cooling.

Step 3: nitrogen dioxide is absorbed in water in the presence of excess air (oxygen).

Overall, one NH₃ gives one HNO₃ (the nitrogen atom is carried through).

Table of the three steps of Ostwald’s process with conditions
Step	Change	Condition
1	NH₃ → NO	Pt gauze, about 800 °C, NH₃ : air = 1 : 10
2	NO → NO₂	Cooling, more oxygen
3	NO₂ → HNO₃	Water and excess air

Formulas

Platinum gauze, about 800 °C
$\ce{4NH3 + 5O2 -> 4NO + 6H2O}$
On cooling
$\ce{2NO + O2 -> 2NO2}$
Absorption in water with air
$\ce{4NO2 + 2H2O + O2 -> 4HNO3}$

Key terms

Ostwald’s process: The industrial manufacture of nitric acid by the catalytic oxidation of ammonia over platinum.
Catalytic oxidation: Oxidation that takes place only in the presence of a catalyst, here platinum.

4. Oxidising agent: copper

Nitric acid gives oxygen on decomposition (conc.: 2HNO₃ → H₂O + 2NO₂ + [O]; dil.: 2HNO₃ → H₂O + 2NO + 3[O]), so it oxidises copper, which ordinary dilute acids cannot attack.

Concentrated: reddish-brown NO₂; solution green, blue on dilution.
Dilute (about 1 : 1): colourless NO, which turns reddish brown at the mouth of the tube (2NO + O₂ → 2NO₂); blue solution.

Table of the gases formed when copper reacts with concentrated and dilute nitric acid
Acid	Gas	Mole ratio Cu : gas
Concentrated	NO₂ (reddish brown)	1 : 2
Dilute	NO (colourless)	3 : 2

Formulas

Concentrated acid
$\ce{Cu + 4HNO3 -> Cu(NO3)2 + 2H2O + 2NO2}$
Dilute acid
$\ce{3Cu + 8HNO3 -> 3Cu(NO3)2 + 4H2O + 2NO}$
Brown fumes at the mouth of the tube
$\ce{2NO + O2 -> 2NO2}$

Key terms

Oxidising agent: A substance that gives oxygen to (or removes electrons from) another substance.
Nitric oxide: NO, a colourless gas that turns reddish brown in air as it forms nitrogen dioxide.

5. Oxidising agent: carbon and sulphur

Hot concentrated nitric acid oxidises non-metals and gives reddish-brown NO₂:

Carbon → carbon dioxide.
Sulphur → sulphuric acid.

Table of the products when carbon and sulphur are oxidised by concentrated nitric acid
Non-metal	Product	Molecules of HNO₃ per atom
Carbon	CO₂	4
Sulphur	H₂SO₄	6

Formulas

Carbon
$\ce{C + 4HNO3 -> CO2 + 2H2O + 4NO2}$
Sulphur
$\ce{S + 6HNO3 -> H2SO4 + 2H2O + 6NO2}$

Key terms

Oxidation of a non-metal: Conversion of a non-metal into its oxide or oxyacid, here by hot concentrated nitric acid.

6. Acidic properties

Nitric acid is a strong, monobasic acid: it turns blue litmus red and forms nitrates.

Metals: hydrogen is given only by very dilute (about 1%) acid with magnesium or manganese; with other metals the acid oxidises the hydrogen to water.
Basic oxides and hydroxides: nitrate + water.
Carbonates and hydrogen carbonates: nitrate + water + CO₂.
Sulphites and hydrogen sulphites: nitrate + water + SO₂.

Table of four reactions showing the acidic properties of nitric acid
Reactant	Products
Mg (very dilute acid)	Mg(NO₃)₂ + H₂
NaOH	NaNO₃ + H₂O
K₂CO₃	2KNO₃ + H₂O + CO₂
NaHSO₃	NaNO₃ + H₂O + SO₂

Formulas

Very dilute acid
$\ce{Mg + 2HNO3 -> Mg(NO3)2 + H2}$
Very dilute acid
$\ce{Mn + 2HNO3 -> Mn(NO3)2 + H2}$
Basic oxide
$\ce{CuO + 2HNO3 -> Cu(NO3)2 + H2O}$
Carbonate
$\ce{CaCO3 + 2HNO3 -> Ca(NO3)2 + H2O + CO2}$
Hydrogen carbonate
$\ce{NaHCO3 + HNO3 -> NaNO3 + H2O + CO2}$
Sulphite
$\ce{Na2SO3 + 2HNO3 -> 2NaNO3 + H2O + SO2}$

Key terms

Monobasic acid: An acid with one replaceable hydrogen atom per molecule, such as nitric acid.

7. Tests for nitric acid

Copper turnings, warmed: reddish-brown fumes of NO₂ and a blue-green solution.
Brown ring test: add freshly prepared iron(II) sulphate solution, then pour conc. H₂SO₄ slowly down the side of the tube. A brown ring (of nitroso iron(II) sulphate, FeSO₄·NO) forms at the junction of the layers.

Table of two tests for nitric acid and their observations
Test	Observation
Warm with copper turnings	Reddish-brown fumes, blue-green solution
Brown ring test	Brown ring at the junction of the layers

Formulas

Nitric oxide formed in the brown ring test
$\ce{6FeSO4 + 3H2SO4 + 2HNO3 -> 3Fe2(SO4)3 + 4H2O + 2NO}$

Key terms

Brown ring test: A test for nitric acid or a nitrate: a brown ring forms at the junction when concentrated sulphuric acid is layered under a mixture of the test solution and fresh iron(II) sulphate solution.

Exam tips

Common mistakes

Writing K₂SO₄ instead of KHSO₄ for the laboratory preparation.
Using iron as the catalyst in Ostwald’s process (it is platinum).
Saying dilute nitric acid gives NO₂ with copper (it gives NO).
Writing hydrogen as a product of copper and nitric acid.

How marks are usually awarded

Equations (1 mark each): correct products and balancing, with “conc.” or “dil.” stated.
Conditions (1 mark): catalyst and temperature together.
Reasons (1 mark each): all-glass apparatus, below 200 °C, yellow colour.

1. Laboratory preparation

Formulas

Key terms

2. Precautions, colour and physical properties

Formulas

Key terms

3. Ostwald’s process (equations and conditions)

Formulas

Key terms

4. Oxidising agent: copper

Formulas

Key terms

5. Oxidising agent: carbon and sulphur

Formulas

Key terms

6. Acidic properties

Formulas

Key terms

7. Tests for nitric acid

Formulas

Key terms

Exam tips

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