Sulphur (iv) oxide (SO2)

What is Sulphur (IV) Oxide?

Sulphur (IV) oxide, also written as sulphur dioxide (SO₂), is one of two main oxides formed when sulphur burns in oxygen. The other is sulphur (VI) oxide (SO₃). SO₂ is more common and important for WAEC and NECO examinations.

This gas has a sharp, irritating smell that makes you cough. You may have noticed this smell near industrial areas in Lagos, Port Harcourt, or Kano where factories burn fossil fuels. It is also present in volcanic gases.

Laboratory Preparation of Sulphur (IV) Oxide

There are two main methods used in school laboratories to prepare SO₂ gas.

Method 1: Heating Sodium Sulphite with Dilute Acid

This is the most common method taught in Nigerian secondary schools. We react sodium trioxosulphate (IV) or potassium trioxosulphate (IV) with dilute hydrochloric acid or dilute tetraoxosulphate (VI) acid.

Using sodium sulphite and hydrochloric acid:

Na₂SO₃₍s₎ + 2HCl₍aq₎ → 2NaCl₍aq₎ + H₂O₍l₎ + SO₂₍g₎

Using potassium sulphite and hydrochloric acid:

K₂SO₃₍aq₎ + 2HCl₍aq₎ → 2KCl₍aq₎ + H₂O₍l₎ + SO₂₍g₎

Procedure:

1. Place sodium sulphite crystals in a round-bottom flask
2. Add dilute hydrochloric acid through a thistle funnel
3. Warm gently if reaction is slow
4. Collect the gas by downward delivery (SO₂ is denser than air)
5. The gas can also be collected over mercury (not water, because SO₂ dissolves in water)

Method 2: Heating Copper with Concentrated Tetraoxosulphate (VI) Acid

This method produces SO₂ by reacting copper metal with hot concentrated H₂SO₄.

Chemical Equation:

Cu₍s₎ + 2H₂SO₄₍conc₎ → CuSO₄₍aq₎ + 2H₂O₍l₎ + SO₂₍g₎

Procedure:

1. Place copper turnings in a round-bottom flask
2. Add concentrated tetraoxosulphate (VI) acid
3. Heat the mixture
4. Collect the colorless SO₂ gas by downward delivery

This method is less common in school practicals because concentrated H₂SO₄ is dangerous to handle. Your teacher will likely demonstrate rather than let you do it yourself.

Industrial Preparation

In factories, sulphur (IV) oxide is produced by burning sulphur in excess air or oxygen.

Equation:

S₍s₎ + O₂₍g₎ → SO₂₍g₎

This method is used in industries that make tetraoxosulphate (VI) acid by the Contact Process.

Physical Properties of Sulphur (IV) Oxide

• Color: Colorless gas
• Smell: Sharp, choking, irritating smell
• Density: Denser than air (can be collected by downward delivery)
• Solubility: Very soluble in water to form acidic solution
• Boiling point: -10°C (exists as gas at room temperature)
• Liquefaction: Can be liquefied under pressure and used as refrigerant

Chemical Properties of Sulphur (IV) Oxide

1. Acidic Nature

SO₂ is an acidic oxide. When dissolved in water, it forms trioxosulphuric (IV) acid (also called sulphurous acid).

Equation:

SO₂₍g₎ + H₂O₍l₎ → H₂SO₃₍aq₎

The solution turns blue litmus paper red, confirming it is acidic. This acidic solution reacts with bases to form salts called trioxosulphates (IV) or sulphites.

Reaction with sodium hydroxide:

SO₂₍g₎ + 2NaOH₍aq₎ → Na₂SO₃₍aq₎ + H₂O₍l₎

2. Bleaching Action

Sulphur (IV) oxide is a powerful bleaching agent. It removes color from materials by a chemical process that is different from chlorine bleaching.

• SO₂ bleaching is done in the presence of moisture
• The bleaching is often temporary – colors may return over time
• Used for bleaching wood pulp, straw, silk, and wool
• Cannot be used for materials damaged by acid

3. Reducing Properties

SO₂ acts as a reducing agent. It removes oxygen from other substances while itself getting oxidized to sulphur (VI) oxide (SO₃) or tetraoxosulphate (VI) acid (H₂SO₄).

Example – Reducing acidified potassium heptaoxodichromate (VI):

The orange dichromate (VI) solution turns green when SO₂ is passed through it.

K₂Cr₂O₇₍aq₎ + H₂SO₄₍aq₎ + 3SO₂₍g₎ → K₂SO₄₍aq₎ + Cr₂(SO₄)₃₍aq₎ + H₂O₍l₎

4. Reaction with Oxygen

In the presence of a catalyst (vanadium (V) oxide), SO₂ reacts with oxygen to form sulphur (VI) oxide.

Equation:

2SO₂₍g₎ + O₂₍g₎ ⇌ 2SO₃₍g₎

This reaction is reversible and used in the Contact Process for manufacturing tetraoxosulphate (VI) acid.

Comparison of SO₂ Preparation Methods

Uses of Sulphur (IV) Oxide

• Bleaching agent: Used for bleaching wood pulp for paper making, straw, wool, and silk
• Preservative: Used in preserving fruits, fruit juices, and wines by killing bacteria
• Manufacturing H₂SO₄: Raw material in Contact Process for making tetraoxosulphate (VI) acid
• Refrigerant: Liquefied SO₂ was used in old refrigerators before modern refrigerants
• Fumigant: Used to kill insects and rats in storage facilities
• Disinfectant: Kills germs when released in enclosed spaces

Environmental Impact – Acid Rain

Sulphur (IV) oxide is a major air pollutant. When released from factories, power stations, and vehicle exhausts, it contributes to acid rain formation.

How acid rain forms:

1. SO₂ is released into the atmosphere from burning fossil fuels
2. In the atmosphere, SO₂ dissolves in rain water
3. Forms trioxosulphuric (IV) acid (H₂SO₃)
4. Can be further oxidized to tetraoxosulphate (VI) acid (H₂SO₄)
5. This acidic rain falls to earth

Effects of acid rain in Nigeria:

• Damages buildings and monuments (like the National Theatre in Lagos)
• Kills fish in rivers and lakes
• Damages crops and forest trees
• Corrodes iron roofing sheets on houses
• Makes soil acidic, reducing crop yields

Common Exam Mistakes

WAEC examiners commonly report these errors:

• Wrong collection method: Students write “collect over water” but SO₂ dissolves readily in water – use downward delivery or over mercury
• Confusing oxides: Writing SO₃ instead of SO₂, or mixing up their names and formulas
• Incomplete preparation equations: Forgetting to include H₂O as a product when acid reacts with sulphite
• Wrong acid concentration: Using “dilute H₂SO₄” with copper method – must be concentrated
• Bleaching explanation: Simply stating “SO₂ bleaches” without mentioning moisture is needed
• Property confusion: Stating SO₂ is a basic oxide when it is actually acidic
• Missing state symbols: Not indicating (s), (l), (g), (aq) in equations

Always read questions carefully. If asked to “describe the laboratory preparation,” include apparatus, procedure, equation, and collection method – not just the equation.

Practice Questions

Multiple Choice Questions

1. Which gas is produced when sodium sulphite reacts with dilute hydrochloric acid?

a) Sulphur (VI) oxide
b) Sulphur (IV) oxide ✓
c) Hydrogen sulphide
d) Chlorine gas

2. The correct method for collecting sulphur (IV) oxide in the laboratory is:

a) Over water
b) By upward delivery
c) By downward delivery ✓
d) Over dilute acid

3. When SO₂ dissolves in water, it forms:

a) Tetraoxosulphate (VI) acid
b) Trioxosulphuric (IV) acid ✓
c) Hydrogen sulphide
d) Sulphuric acid

4. In the industrial preparation of SO₂, sulphur is burned in:

a) Nitrogen
b) Carbon dioxide
c) Oxygen ✓
d) Hydrogen

5. Which of the following is NOT a use of sulphur (IV) oxide?

a) Bleaching wood pulp
b) Preserving fruit juices
c) Making fertilizers ✓
d) Manufacturing tetraoxosulphate (VI) acid

Essay/Theory Questions

1. Describe the laboratory preparation of sulphur (IV) oxide using sodium sulphite. Your answer should include the equation and method of collection. (6 marks)

Examiner’s tip: State reactants (1 mark), write balanced equation with state symbols (2 marks), describe procedure briefly (1 mark), state collection method (1 mark), give reason for collection method (1 mark).

Sample Answer: Sodium sulphite reacts with dilute hydrochloric acid. Equation: Na₂SO₃₍s₎ + 2HCl₍aq₎ → 2NaCl₍aq₎ + H₂O₍l₎ + SO₂₍g₎. Place sodium sulphite in a flask, add dilute HCl through a thistle funnel, and warm gently. Collect the gas by downward delivery because SO₂ is denser than air. Do not collect over water because SO₂ is very soluble in water.

2. State three physical properties and three chemical properties of sulphur (IV) oxide. (6 marks)

Examiner’s tip: Each property = 1 mark. Physical properties should be observable characteristics. Chemical properties should involve reactions or changes.

3. Explain with equations how sulphur (IV) oxide contributes to acid rain formation and state two effects of acid rain. (5 marks)

4. Compare the laboratory preparation of SO₂ using: (a) sodium sulphite and dilute acid; (b) copper and concentrated tetraoxosulphate (VI) acid. (8 marks)

Examiner’s tip: Give equation for each method (2 marks each = 4 marks), state conditions required (2 marks), explain which is safer for school use (2 marks).

Memory Aids

Remember “SO₂ CHOKES” for key properties:

• S – Soluble in water
• O₂ – Oxidized by oxygen to SO₃
• C – Colorless gas
• H – Heavy (denser than air)
• O – Oxide that is acidic
• K – Kills germs (disinfectant)
• E – Environmental pollutant (acid rain)
• S – Sharp choking smell

Lab Preparation Memory: “Sodium Sulphite + Hydrochloric acid = Salt + Water + SO₂ gas”

Collection Method: “Down-ward for Denser” (SO₂ is denser than air, so collect by downward delivery)

Related Topics

• Physical Properties of SO₂ – Detailed characteristics
• Chemical Properties of SO₂ – Reactions and behavior
• Tests for SO₂ – How to identify this gas
• Uses of SO₂ – Industrial and commercial applications
• Oxides of Sulphur – Comparison of SO₂ and SO₃
• Sulphur – Properties of the element