Chemistry Sulphur Dioxide

SULPHUR DIOXIDE

Physical properties

Sulphur dioxide is:
(i) a colourless gas with an irritating smell.
(ii) Two and half times denser than air.
(iii) soluble in water forming an acidic solution.

Confirmatory test for sulphur dioxide

To confirm the presence sulphur dioxide, we use:
(i) Acidified orange potassium dichromate (VII) solution that changes from orange to green.
(ii) A purple solution of potassium manganate (VII) that turns to colourless.
Chemical properties (Reactions)

As a reducing agent:

Sulphur dioxide acts as a reducing agent by supplying electrons or gaining oxygen from other substances. Examples include:

The bleaching action: This is due to sulphurous acid which removes oxygen from the dye.
Equations of reaction

With acidified dichromate(VI) solution

Equation of reaction:

Observation:

Solution turns from orange to green.

With potassium manganate(VII) solution

Equation of reaction

Observation:
Solution turns from purple/pink to colourless

With iron (III) sulphate solution

Iron (III) ions in solution are reduced by sulphur dioxide to iron (II) ions.

Equation of reaction

Observation:

Solution turns from brown to green

With bromine solution

Bromine in reduced to bromide ions.

Equation of reaction

Observation:

Solution turns from brown to colourless

With iodine solution

Iodine is reduced to iodide ions.

Equation of reaction

Observation:

Solution turns from brown to colourless.

With sodium hydroxide solution

With a little sulphur dioxide bubbled through an excess of the alkali, a normal salt is formed but equimular quantities react to form an acid salt.

Equation of reaction

(ii) Examples in which sulphur dioxide acts as an oxidising agent include:

With burning magnesium ribbon

Equation of reaction

Observation:

Mixture of white and yellow solids deposited at the bottom and sides of the jar.

Explanation:

The burning metal gives out a lot of heat that decomposes the gas into its elements sulphur and oxygen. The metal then continues to burn in the oxygen formed.

With hydrogen sulphide gas

Equation of reaction

Observation:

Yellow solid of sulphur formed

Large scale production of sulphur dioxide gas

By burning sulphur in air

By burning iron (II) sulphide or zinc sulphide in air

SULPHUR DIOXIDE

ACTIVITY 1.

Experiment to investigate the properties of sulphur dioxide.

Procedure

(a) Place two spatula ends full of sodium sulphite crystals in a test-tube,
3cm³ addof dilute hydrochloric acid and then warm the mixture.
(i) Observe the colour and note the smell of the gas.
(ii) Place moist blue and red litmus papers at the mouth of the test tube and note the effect of the gas, if any on the litmus paper.
ACTIVITY 2.
Experiment to investigate the chemical properties of sulphur dioxide
Procedure
(a) Place two spatula ends full of sodium sulphite crystals into a test-tube, add to it 3cm³of dilute hydrochloric acid to cover the crystals and then warm.
(b) Dip a clean glass rod into a very dilute acidified solution of potassium manganate (VII) and lower it into gas above the liquid and note the observation (made).
(c) Now repeat (b) using each of the following:
(i) Acidified potassium dichromate (VI) solution.
(ii) Iron (III) sulphate solution.
(iii) Iodine solution.
(d) Record all your observations in the table below

Test reagent	Observation
Acidified potassium manganate (VII) solution
Acidified potassium dichromate (VI) solution
Iron (III) sulphate solution
Iodine solution

ACTIVITY 3.
Experiment to prepare sodium sulphite
Procedure
(a) Pour equal portions of a solution of sodium hydroxide into two small beakers labelled 1 and 2.
(b) Bubble sulphur dioxide gas into the solution in beaker 1 until it is saturated (smells strongly of the gas).
(c) Now add the solution in beaker 1 to that in beaker 2.
(d) Carefully heat to evaporate the solution and leave it to cool. What do you think you will obtain?

SULPHUR DIOXIDE

MULTIPLE CHOICE QUESTIONS

SECTION A

1. Which one of the following is not a property of sulphur dioxide?

It turns into dioxide gas. It reduces hot magnesium oxide. It is denser than air. It is collected by downward delivery method.

2. Which of one the following is best for drying sulphur dioxide?
Calcium oxide Anhydrous calcium chloride. Sodium hydroxide solution. Concentrated sulphuric acid.

3. Which one of the following is oxidised by sulphur dioxide?

Sodium hydroxide solution. Hydrogen sulphide. Acidified potassium dichromate. Iron(III) sulphate solution.

4. Which one of the following is responsible for the bleaching action of sulphur dioxide?

Sulphurous acid. Sulphuric acid. Water. Oxygen.

5. Which one of the following is not a product formed when sulphur dioxide gas reacts with acidified potassium dichromate (VI)?

Water. Potassium sulphate. Chromium(III) sulphate. Potassium hydroxide.

6. Which of the following is not a use of sulphur dioxide?

As a cooling agent in refrigerators. Manufacture of sulphuric acid. Making explosives. Preserving tinned food.

7. The product formed when excess sulphur dioxide gas is bubbled through sodium hydroxide solution is

sodium hydrogen sulphite. sodium sulphite. sodium sulphate. sodium sulphide.

Paper 2: STRUCTURED

SECTION B

8. Sulphur dioxide was bubbled through a solution of iron(III) sulphate
(a) (i) State what was observed

(ii) Write the chemical equation of reaction that takes place

(iii) Name the property being shown by sulphur dioxide in the reaction above

(b) (i) State what would be observed if sulphur dioxide was instead bubbled through blue litmus solution

(ii) Name one gas that would give a similar observation as in (b) (i) above

SECTION C

The diagram below show apparatus which was used by a teacher to prepare a sample of dry sulphur dioxide

(a) Name the chemicals/substances labeled
(i) Y (ii) W (iii) Z
(b) State the role of Z
(c) Sulphur dioxide is very soluble in water. Name the solution formed when sulphur dioxide dissolves in water.
(d) Sulphur dioxide was bubbled through acidified potassium manganate(VII) solution.
(i) State what is observed.
(ii) Write the equation of reaction that took place.
(e) Give one example in which sulphur dioxide acts as an oxidising agent
(f) Give two large scale uses of sulphur dioxide gas.

EVALUATION EXERCISE

(a) (i) Name the substances that can be used to prepare sulphur dioxide in the laboratory.
(ii) Write equation for the reaction that took place between the reagents you named in (i).
(b) Name one possible drying agent that can be used.
(c) State the method used to collect sulphur dioxide and why?
(d) Explain one confirmatory test for sulphur dioxide.

 

Definitions

Reduction:

• Is a process in which the oxidation number of a species decreases
• Is the removal of oxygen from a substance
• Is the gain of electrons by a metal ion (cation during electrolysis)
• Is the addition of hydrogen to a substance

Oxidation:

• Is a process in which the oxidation number of a species increases
• Is addition of oxygen to a substance
• Is the loss of electrons by a metal or ion (anion during electrolysis)
• Is the removal of hydrogen from a substance

A reducing agent:

Is a substance that removes oxygen from another or supplies electrons or donates hydrogen

An oxidising agent:

Is a substance that adds oxygen or accepts electrons or receives hydrogen from another substance