Chemistry · Lesson 12

Non-metals & compounds: H, O, halogens, N, S, C

JAMBWAECNECO

What the syllabus asks

JAMB: non-metals and their compounds: hydrogen, halogens, oxygen, nitrogen, sulphur, carbon

WAEC: hydrogen, oxygen (and oxides), halogens (chlorine, hydrogen chloride), nitrogen (ammonia, nitric acid, oxides of nitrogen, nitrogen cycle), sulphur (allotropes, SO₂, H₂S, sulphuric acid by the contact process), carbon (allotropes, CO, CO₂, carbonates): laboratory preparation, properties, tests and uses

NECO: hydrogen, oxygen (and oxides), halogens (chlorine, hydrogen chloride), nitrogen (ammonia, nitric acid, oxides of nitrogen, nitrogen cycle), sulphur (allotropes, SO₂, H₂S, sulphuric acid by the contact process), carbon (allotropes, CO, CO₂, carbonates): laboratory preparation, properties, tests and uses

What you will learn

Describe laboratory preparation, collection, drying and tests of H₂, O₂, Cl₂, HCl, NH₃, SO₂, H₂S and CO₂.
Classify oxides as acidic, basic, amphoteric or neutral.
Describe properties and uses of the halogens and the trend in reactivity.
Describe the Haber process and the manufacture and properties of nitric acid.
Describe the allotropes of sulphur and carbon and the contact process.
Explain the special properties of concentrated sulphuric acid.
Carry out simple calculations on gas preparation.

Key points

Collect over water if insoluble; upward delivery if light and soluble (NH₃); downward delivery if dense and soluble.
Dry most gases with conc. H₂SO₄ or CaCl₂; dry ammonia with CaO.
H₂: Zn + dilute acid; pops; reducing agent; Haber process and margarine.
O₂: H₂O₂ + MnO₂; relights glowing splint; oxides are acidic, basic, amphoteric or neutral.
Cl₂: MnO₂ + conc. HCl; damp Cl₂ bleaches by oxidation; reactivity of halogens falls down the group.
NH₃: NH₄Cl + Ca(OH)₂; the alkaline gas; Haber: Fe, 450 °C, 200 atm.
HNO₃: oxidising acid made by Ostwald process; NO₂ is brown; legumes fix nitrogen.
SO₂ reduces dichromate (orange → green) and bleaches by reduction; H₂S blackens lead acetate.
Contact process: V₂O₅, 450 °C; SO₃ absorbed in conc. H₂SO₄ to form oleum; conc. H₂SO₄ dehydrates.
Diamond hard, non-conducting; graphite layered, conducting; CO is poisonous; CO₂ turns limewater milky.

Start here: Collecting and drying gases

Before studying each gas, learn the general rules, because WAEC practical and theory questions use them again and again.

Methods of collection

Method	Used when the gas is	Examples
Over water	Insoluble or only slightly soluble in water	H₂, O₂, N₂, CO
Upward delivery (downward displacement of air)	Lighter than air and soluble	NH₃ (also H₂)
Downward delivery (upward displacement of air)	Denser than air and soluble	Cl₂, HCl, SO₂, CO₂, NO₂, H₂S
Gas syringe	Any gas when volume must be measured	All

To decide "lighter or denser than air", compare molar masses with air's average, about 29 g/mol. NH₃ (17) is lighter; CO₂ (44) is denser.

Drying agents

Drying agent	Use for	Do NOT use for
Concentrated H₂SO₄	Most gases: Cl₂, HCl, SO₂, CO₂, O₂, H₂	NH₃ (reacts, it is a base); H₂S (oxidised)
Fused (anhydrous) CaCl₂	Most gases	NH₃ (forms a complex)
Quicklime (CaO)	NH₃	Acidic gases (they react with it)

Worked example 12.1

Suggest a drying agent and collection method for (a) ammonia, (b) chlorine.

(a) Ammonia is basic, so avoid acidic drying agents: use quicklime. It is lighter than air (17 < 29) and very soluble: upward delivery.
(b) Chlorine is acidic: concentrated H₂SO₄ works. It is denser than air (71 > 29) and fairly soluble: downward delivery.

(a) CaO, upward delivery (b) conc. H₂SO₄, downward delivery

⚠️ Common mistake: drying ammonia with concentrated sulphuric acid. They react to form ammonium sulphate.

Try it yourself. Why can carbon(IV) oxide not be dried with quicklime?

Show solution

CO₂ is acidic and reacts with the basic CaO to form CaCO₃.

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Practice questions

1. Which gas turns lime water milky?

Carbon(IV) oxide
Oxygen
Hydrogen
Ammonia

Show answer

A: Carbon(IV) oxide
CO₂ forms insoluble CaCO₃.

2. Ammonia gas is best dried using

concentrated H₂SO₄
anhydrous CaCl₂
phosphorus(V) oxide
calcium oxide (quicklime)

Show answer

D: calcium oxide (quicklime)
NH₃ is alkaline and reacts with acidic drying agents and with CaCl₂.

3. The gas that bleaches damp litmus paper is

nitrogen
hydrogen
chlorine
oxygen

Show answer

C: chlorine
Chlorine forms HOCl which bleaches.

4. Lime water is used to test for

ammonia
carbon dioxide
oxygen
hydrogen

Show answer

B: carbon dioxide
CO₂ gives a white precipitate of CaCO₃.

5. Hydrogen burns with a

squeaky pop
bright white flame
green flame
smell of rotten eggs

Show answer

A: squeaky pop
The test for hydrogen.

Key words

Allotropy: Existence of an element in two or more forms in the same physical state.
Acidic oxide: Oxide of a non-metal that reacts with bases/dissolves to give acids, e.g. CO₂, SO₂.
Basic oxide: Metal oxide that reacts with acids to give salt and water, e.g. CaO.
Amphoteric oxide: Oxide that reacts with both acids and bases, e.g. ZnO, Al₂O₃.
Neutral oxide: Oxide neither acidic nor basic, e.g. CO, N₂O, H₂O.
Halogens: Group 17 elements: F, Cl, Br, I.
Bleaching: Removal of colour, e.g. by chlorine (oxidation) or SO₂ (reduction).
Haber process: Industrial synthesis of ammonia from nitrogen and hydrogen.
Contact process: Industrial manufacture of sulphuric acid via SO₃.
Dehydrating agent: Substance that removes water (or the elements of water) from compounds.
Nitrogen fixation: Conversion of atmospheric N₂ into compounds plants can use.
Drying agent: Substance used to remove water vapour from a gas.

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