Non Metals: Oxgyen – SS2 Chemistry Lesson Note (Second Term)

Oxygen (O) is a colourless, odourless and tasteless non-metal gas in group VI, with atomic number 8 and electronic configuration 1s²2s²2p⁴.

Behavioural objectives

By the end of the lesson, students should be able to:

1. state the general properties of the oxygen family and describe the electronic structure of oxygen;
2. describe the laboratory and industrial preparation of oxygen;
3. state the physical and chemical properties and the uses of oxygen;
4. describe the test for oxygen;
5. classify oxides into basic, acidic, amphoteric, neutral and peroxides, with examples.

What is oxygen and where does it occur?

Oxygen is a diatomic gas (O₂) and the most abundant element in the Earth’s crust.

• Atomic number: 8, with six electrons in its outer shell.
• In air: it makes up about 21% of the volume of atmospheric air.
• Free state: it occurs as uncombined O₂ in the atmosphere.
• Combined state: it occurs in water, rocks, minerals (oxides, carbonates, silicates) and living things.
• Allotropes: oxygen exists as O₂ (ordinary oxygen) and O₃ (ozone).

What are the general properties of the oxygen family?

The oxygen family is group VI of the periodic table, made up of oxygen, sulphur, selenium, tellurium and polonium.

• Oxygen (O): a gas at room temperature.
• Sulphur (S): a yellow solid.
• Selenium (Se): a solid.
• Tellurium (Te): a solid.
• Polonium (Po): a solid.

Properties of the family

1. They are non-metals and exist as solids at room temperature, except oxygen.
2. They are electron acceptors and oxidising in nature.
3. They do not react with water.
4. Oxygen and sulphur combine directly with hydrogen to form water and hydrogen sulphide respectively.

What is the electronic structure of oxygen?

Oxygen has the electronic configuration 1s²2s²2p⁴, so it needs two more electrons to complete its octet.

The oxygen atom has six valence electrons. It attains a stable octet in three ways:

1. Electron gain: it accepts two electrons from electropositive elements such as metals to form the oxide ion, O^2-. Example: Ca²⁺ + O^2- → CaO.
2. Sharing with non-metals: it forms covalent bonds by sharing two of its six outer electrons.
3. Sharing with itself: it forms covalent bonds with another oxygen atom to give O₂.

How is oxygen prepared in the laboratory?

Oxygen is prepared in the laboratory by the decomposition of potassium trioxochlorate (V) or hydrogen peroxide.

Thermal decomposition of potassium trioxochlorate (V)

• Method: potassium trioxochlorate (V) is heated with manganese (IV) oxide, MnO₂, as a catalyst.
• Equation: 2KClO₃(s) → 2KCl(s) + 3O₂(g) (heat, MnO₂).

Decomposition of hydrogen peroxide

• With a catalyst: hydrogen peroxide, H₂O₂, decomposes in the presence of MnO₂.
• Equation: 2H₂O₂(aq) → 2H₂O(l) + O₂(g) (MnO₂).
• With acidified potassium manganate (VII): hydrogen peroxide reacts in the cold to give oxygen.
• Equation: 5H₂O₂(aq) + 2KMnO₄(aq) + 3H₂SO₄(aq) → K₂SO₄(aq) + 2MnSO₄(aq) + 8H₂O(l) + 5O₂(g).

How is the gas dried and collected?

• Drying: pass the gas through anhydrous calcium chloride or concentrated tetraoxosulphate (VI) acid.
• Dry collection: collect the dried gas over mercury, which does not dissolve oxygen and adds no water vapour.
• Over water: oxygen is only slightly soluble, so it can also be collected over water when dryness is not needed.
• Upward delivery: oxygen is denser than air, so it can be collected by upward delivery (downward displacement of air).

How is oxygen prepared industrially?

Oxygen is prepared industrially by the electrolysis of water and by the fractional distillation of liquid air.

Electrolysis of water

• Method: an electric current is passed through acidified water.
• Anode: oxygen is given off.
• Cathode: hydrogen is given off.

Fractional distillation of liquid air

Fractional distillation separates liquid air into its gases because they have different boiling points.

Stage 1: Liquefaction of air

1. Pass air through caustic soda solution, NaOH(aq), to remove CO₂.
2. Compress, cool and expand the air in a series of steps.
3. Continue until liquid air is obtained at -200 °C.

Stage 2: Fractional distillation of the liquid air

1. Lead the liquid air into a fractional distillation column.
2. Nitrogen has the lower boiling point, -196 °C, so it evolves first.
3. Liquid oxygen is left behind.
4. Further heating turns the liquid oxygen to gas at -183 °C.

What are the physical properties of oxygen?

Oxygen is a colourless, odourless and tasteless gas that is slightly soluble in water.

• Appearance: colourless, odourless and tasteless diatomic gas.
• Litmus: neutral to litmus.
• Solubility: slightly soluble in water.
• Density: gaseous oxygen is denser than air.
• Liquefaction: it liquefies at -183 °C.

What are the chemical properties of oxygen?

Oxygen supports burning and combines with metals and non-metals to form oxides.

Reaction with metals

• Product: oxygen combines directly with most metals to form basic oxides.
• Calcium: 2Ca + O₂ → 2CaO.
• Potassium: 4K + O₂ → 2K₂O.
• With water: oxides of very electropositive metals (K, Na, Ca) dissolve in water to form alkalis.
• Example: K₂O + H₂O → 2KOH.

Reaction with non-metals

• Product: non-metals burn in oxygen to form acidic oxides.
• Acid anhydrides: acidic oxides dissolve in water to form acids.
• Sulphur: S(s) + O₂(g) → SO₂(g).
• Sulphur dioxide in water: SO₂(g) + H₂O(l) → H₂SO₃(aq).
• Phosphorus (limited oxygen): P₄(s) + 3O₂(g) → P₄O₆(g).
• Phosphorus (III) oxide in water: P₄O₆ + 6H₂O(l) → 4H₃PO₃(aq).

Reaction with hydrocarbons and other carbon compounds

• Product: hydrocarbons and compounds of carbon, hydrogen and oxygen burn in oxygen to give CO₂ and H₂O.
• Ethanol: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l).

What are the uses of oxygen?

Oxygen is used for breathing, for burning fuels at high temperature and in making chemicals.

1. Respiration: living things need it to release energy from food.
2. Oxy-acetylene flame: used for welding and cutting metals.
3. Steel industry: removes carbon, sulphur and phosphorus impurities from pig iron.
4. Rockets: liquid oxygen and fuel are used as propellant for space rockets.
5. Chemical manufacture: used to make tetraoxosulphate (VI) acid, trioxonitrate (V) acid and ethanoic acid.

How do you test for oxygen?

Oxygen rekindles a glowing splint and turns colourless nitrogen (II) oxide reddish-brown.

1. Insert a glowing splint into a gas jar of the unknown gas.
2. If the splint is rekindled, the gas is likely to be oxygen or nitrogen (I) oxide.
3. Add colourless nitrogen (II) oxide to the gas.
4. If reddish-brown fumes of nitrogen (IV) oxide form, the gas is confirmed as oxygen.

What are oxides?

Oxides are binary compounds formed when oxygen combines with another element.

There are five types of oxide: basic, acidic, amphoteric, neutral and peroxides.

Basic oxides

Basic oxides are oxides of metals that react with acids to form salt and water only.

• Examples: Na₂O, K₂O, MgO and CaO.
• Equation: Na₂O(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l).

Acidic oxides

Acidic oxides are oxides of non-metals that dissolve in water to form acids and react with alkalis to form salt and water only.

• Examples: CO₂, SO₂ and NO₂.
• Equation: CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l).

Amphoteric oxides

Amphoteric oxides are oxides of metals that react with both acids and alkalis to form salt and water only.

• Examples: oxides of Al, Zn, Pb and Sn.
• With acid: ZnO(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l).
• With alkali: ZnO(s) + 2NaOH(aq) + H₂O(l) → Na₂Zn(OH)₄(aq).

Neutral oxides

Neutral oxides are oxides of non-metals that are neither acidic nor basic.

• Examples: CO, H₂O and N₂O.
• Litmus: they are neutral to litmus.

Peroxides

Peroxides are oxides that contain a higher proportion of oxygen than ordinary oxides.

• Examples: Na₂O₂, CaO₂ and BaO₂.
• With dilute acid: they produce hydrogen peroxide, H₂O₂.

How do the types of oxide compare?

Evaluation

1. Write the electronic configuration of oxygen and state two ways in which it attains a stable octet.
2. Write the equation for the laboratory preparation of oxygen from potassium trioxochlorate (V) and name the catalyst used.
3. How is oxygen separated from liquid air?
4. Describe the test that confirms a gas is oxygen.
5. Name the five types of oxides and give one example of each.

Assignment

Explain why oxygen is collected over mercury when a dry gas is needed, and name one other way of collecting oxygen.