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

Oxygen (O) is a non-metal in group VI with atomic number 8 and the electronic configuration 1s²2s²2p⁴. It is a colourless, odourless and tasteless diatomic gas that makes up 21% of the volume of atmospheric air. It occurs free and combined, is prepared in the laboratory and industry, and combines with other elements to form oxides.

Behavioural objectives

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

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

What is oxygen and where does it occur?

Oxygen is the most abundant element on earth. It constitutes 21% of the volume of atmospheric air.

Occurrence: It occurs as a free element in nature and in combined states.

What are the general properties of the oxygen family?

Elements in group VI include Oxygen (O), Sulphur (S), Selenium (Se), Tellurium (Te) and Polonium (Po). Their properties are as follows:

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 in any form. Oxygen and sulphur, however, combine directly with hydrogen to yield water and hydrogen sulphide respectively.

What is the electronic structure of oxygen?

Oxygen has an atomic number of 8; hence its electronic configuration is 1s²2s²2p⁴. This shows that oxygen needs two electrons to attain an octet configuration.

The oxygen atom has six valence electrons and can acquire a stable octet configuration by:

1. Accepting two electrons from electropositive elements like metals to form a negative oxide ion, O^2-. Example: Ca²⁺ + O^2- → CaO
2. Entering into covalent bond formation with non-metals by covalently sharing two out of its six outer electrons.
3. Forming covalent bonds with itself.

How is oxygen prepared in the laboratory?

Thermal decomposition of potassium trioxochlorate (V)

Potassium trioxochlorate (V) is heated in the presence of MnO₂ as a catalyst.

2KClO₃(s) → (heat) 2KCl(s) + 3O₂(g)

Decomposition of hydrogen peroxide

Hydrogen peroxide, H₂O₂, decomposes in the presence of MnO₂ as a catalyst.

2H₂O₂(aq) → (heat) 2H₂O(l) + O₂(g)

Hydrogen peroxide also reacts with acidified KMnO₄ in the cold to produce oxygen.

5H₂O₂(aq) + 2KMnO₄(aq) + 3H₂SO₄(aq) → K₂SO₄(aq) + 2MnSO₄(aq) + 8H₂O(l) + 5O₂(g)

Note: If the gas is required to be dry, it is passed through anhydrous calcium chloride or concentrated tetraoxosulphate (VI) acid and collected over mercury.

How is oxygen prepared industrially?

Oxygen is prepared industrially by:

1. Electrolysis of water
2. Fractional distillation of liquid air

Fractional distillation of liquid air

This preparation involves two main processes.

Liquefaction of air

Air is first passed through caustic soda, NaOH(aq), to remove CO₂. It is then subjected to a series of compressions, expansions and cooling until liquid air is obtained at -200 °C.

Fractional distillation of the liquid air

The liquid air is led to a fractional distillation column. On distillation, nitrogen, which has a lower boiling point of -196 °C, evolves first, leaving behind liquid oxygen. Further heating converts the liquid oxygen to a gas at -183 °C.

What are the physical properties of oxygen?

1. It is a colourless, odourless and tasteless diatomic gas.
2. It is neutral to litmus.
3. It is slightly soluble in water.
4. Gaseous oxygen is denser than air.
5. Gaseous oxygen liquefies at -183 °C.

What are the chemical properties of oxygen?

Reaction with metals

Oxygen combines directly with most metals to form basic oxides.

2Ca + O₂ → 2CaO

4K + O₂ → 2K₂O

The oxides of very electropositive metals, K, Na and Ca, dissolve in water to form alkalis.

2K₂O + 2H₂O → 4KOH

Reaction with non-metals

Non-metals burn in oxygen to form acidic oxides. These are known as acid anhydrides because they dissolve in water to form acids.

S(s) + O₂(g) → SO₂(g)

SO₂(g) + H₂O(l) → H₂SO₃(aq)

P₄(s) + 3O₂(g) → P₄O₆(g)

P₄O₆ + 6H₂O(l) → 4H₃PO₃(aq)

Reaction with hydrocarbons and other carbon compounds

Most hydrocarbons and compounds of carbon, hydrogen and oxygen burn in oxygen to give CO₂ and H₂O.

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

What are the uses of oxygen?

1. It is used in the oxy-acetylene flame.
2. It is required for respiration.
3. It is used in the steel industry for the removal of C, S and P impurities from pig iron.
4. Liquid oxygen and fuel are used as propellant for space rockets.
5. Oxygen is used in the manufacture of tetraoxosulphate (VI) acid, trioxonitrate (VI) acid and ethanoic acid.

How do you test for oxygen?

Insert a glowing splint into a gas jar containing an unknown gas. If the splint is rekindled, the gas is likely to be oxygen or nitrogen (I) oxide.

If the gas is colourless and reacts with nitrogen (II) oxide to produce reddish-brown fumes of nitrogen (IV) oxide, the gas is confirmed to be oxygen.

What are oxides?

Oxides are binary compounds formed when oxygen combines with other elements. They are classified as follows.

Basic oxides

These are oxides of metals, e.g. Na₂O, K₂O, MgO and CaO. They react with acids to form salt and water only.

Na₂O(s) + 2HCl(aq) → 2NaCl(s) + H₂O(l)

Acidic oxides

These are oxides of non-metals which dissolve in water to form acids, e.g. CO₂, SO₂ and NO₂. They react with alkalis to form a salt and water only.

CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l)

Amphoteric oxides

These are oxides of metals which can react with both acids and alkalis to form salt and water only. They include the oxides of Al, Zn, Pb and Sn.

ZnO(s) + H₂SO₄ → ZnSO₄(aq) + H₂O(l)

ZnO(s) + 2NaOH(aq) + H₂O → Na₂Zn(OH)₄(aq)

Neutral oxides

These are oxides of non-metals which are neither acidic nor basic. They are neutral to litmus. They include CO, H₂O and N₂O.

Peroxides

These are oxides which contain a higher proportion of oxygen than ordinary oxides, e.g. Na₂O₂, CaO₂ and BaO₂. They react with dilute acid to produce hydrogen peroxide, H₂O₂.

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

Why is oxygen collected over mercury?