Oxygen: preparation, properties and oxides: WASSCE Chemistry

Oxygen keeps every fire burning and every living cell breathing. It is the most abundant element in the earth's crust and makes up about a fifth of the air. This lesson covers the laboratory and industrial preparation of oxygen, its test and properties, the oxides it forms, and its uses in hospitals, welding and steelmaking.

Learning objectives

By the end of this lesson you should be able to:

describe the laboratory preparation of oxygen, with equations and a diagram
describe the test for oxygen
state the physical and chemical properties of oxygen
classify oxides as acidic, basic, amphoteric, neutral, peroxide or mixed
describe the industrial production of oxygen from liquid air
state the uses of oxygen

Laboratory preparation

Hydrogen peroxide is dripped from a dropping funnel onto manganese(IV) oxide in a flask; the oxygen is collected over water

Figure 1. Preparing oxygen from hydrogen peroxide. WASSCE Tutor diagram.

Method 1 (no heating): decomposing hydrogen peroxide with manganese(IV) oxide as catalyst:

2\text{H}_2\text{O}_2\text{(aq)} \xrightarrow{\text{MnO}_2} 2\text{H}_2\text{O(l)} + \text{O}_2\text{(g)}

Method 2: heating potassium chlorate(V) with a little manganese(IV) oxide as catalyst:

2\text{KClO}_3\text{(s)} \rightarrow 2\text{KCl(s)} + 3\text{O}_2\text{(g)}

\begin{gathered}2\text{KClO}_3\text{(s)} \\[6pt] \rightarrow 2\text{KCl(s)} + 3\text{O}_2\text{(g)}\end{gathered}

Other sources: heating potassium manganate(VII), or sodium or potassium nitrate (which gives a little oxygen).

Collection: over water, because oxygen is only slightly soluble. It can be dried with concentrated sulphuric acid and collected by downward delivery (it is slightly denser than air).

Worked example

Proving MnO₂ is a catalyst. Weigh 1.0 g of manganese(IV) oxide and add it to hydrogen peroxide. Oxygen bubbles off rapidly. When the fizzing stops, filter off the black solid, wash, dry and weigh it again: still 1.0 g. Add it to fresh hydrogen peroxide: it again causes rapid decomposition. So it speeds up the reaction without being used up.

Test for oxygen

A glowing splint placed in the gas relights (bursts into flame). No other common gas does this.

Physical properties

Property	Oxygen
Colour, smell, taste	Colourless, odourless, tasteless
Density	Slightly denser than air
Solubility in water	Slightly soluble (enough for fish to breathe)
Boiling point	−183 °C (liquid oxygen is pale blue)
Effect on litmus	Neutral

Chemical properties

Oxygen supports combustion and reacts with most elements, especially when heated, forming oxides.

Element	Observation	Equation	Oxide
Magnesium	Burns with a dazzling white flame; white ash	2Mg + O₂ → 2MgO	Basic
Sodium	Burns with a yellow flame	4Na + O₂ → 2Na₂O	Basic
Iron (wool)	Burns with sparks	3Fe + 2O₂ → Fe₃O₄	Mixed oxide
Carbon (charcoal)	Glows red	C + O₂ → CO₂	Acidic
Sulphur	Burns with a blue flame; choking gas	S + O₂ → SO₂	Acidic
Phosphorus	Burns with a bright yellow flame; white smoke	P₄ + 5O₂ → P₄O₁₀	Acidic

Classes of oxides

Class	Behaviour	Examples
Basic oxides (metals)	React with acids to form salts and water; soluble ones form alkalis	Na₂O, CaO, MgO, CuO
Acidic oxides (non-metals)	React with alkalis; dissolve in water to form acids	CO₂, SO₂, SO₃, P₄O₁₀, NO₂
Amphoteric oxides	React with both acids and alkalis	ZnO, Al₂O₃, PbO
Neutral oxides	React with neither	CO, NO, N₂O, H₂O
Peroxides	Contain the O–O bond; give H₂O₂ with dilute acid	Na₂O₂, BaO₂
Mixed (compound) oxides	Behave as a mixture of two oxides	Pb₃O₄ (red lead), Fe₃O₄

Industrial production: fractional distillation of liquid air

Air is filtered to remove dust.
Carbon(IV) oxide and water vapour are removed (they would freeze and block the pipes).
The air is compressed and cooled repeatedly until it liquefies at about −200 °C.
The liquid air is fractionally distilled. Nitrogen (boiling point −196 °C) boils off first at the top of the column. Oxygen (−183 °C) stays as a liquid at the bottom. Argon is collected separately.

Uses of oxygen

Use	Detail
Medicine	Oxygen cylinders and concentrators for patients with breathing difficulty, pneumonia, COVID-19
Welding and cutting	Oxy-acetylene flame (about 3000 °C)
Steelmaking	Blown into molten iron to burn off carbon and other impurities
Breathing apparatus	Divers, mountaineers, fire-fighters, astronauts
Rocket fuel	Liquid oxygen as the oxidiser
Sewage and water treatment	Speeds up the breakdown of waste by bacteria

In Sierra Leone

During the COVID-19 pandemic, Sierra Leone expanded oxygen plants at major hospitals to supply medical oxygen. Many of these plants separate oxygen from air using special filters (pressure swing adsorption) rather than liquid air. Oxygen cylinders must be kept away from oil, grease and flames, because oxygen makes anything burn far more fiercely.

Common mistake

Oxygen does not burn: it supports burning. The test is that a glowing splint relights, not that the gas catches fire.

Key points

Lab preparation: 2H₂O₂ → 2H₂O + O₂ (MnO₂ catalyst) or heating KClO₃ with MnO₂; collected over water.
Test: relights a glowing splint.
Oxygen supports combustion; metals form basic oxides and non-metals acidic oxides.
Oxides: basic, acidic, amphoteric, neutral, peroxide, mixed.
Industrially from fractional distillation of liquid air (N₂ boils off first).
Uses: medicine, welding, steelmaking, breathing apparatus, rockets.

WASSCE examination focus

WAEC tests this

Expect to describe the preparation of oxygen with a labelled diagram and equation, to show that MnO₂ is a catalyst, to classify oxides with examples, and to describe the fractional distillation of liquid air. Questions often give burning reactions and ask you to name and classify the oxide formed.

Quick check

Write an equation for the preparation of oxygen from hydrogen peroxide.
What is the test for oxygen?
Classify each oxide: CaO, SO₂, ZnO, CO.
Why does nitrogen boil off before oxygen in the fractional distillation of liquid air?

Answers:

2H₂O₂ → 2H₂O + O₂ (MnO₂ catalyst).
It relights a glowing splint.
CaO basic; SO₂ acidic; ZnO amphoteric; CO neutral.
Nitrogen has a lower boiling point (−196 °C) than oxygen (−183 °C).

Oxygen: preparation, properties and oxides