School Science Lessons
UNChem2a
Please send comments to: J.Elfick@uq.edu.au
2018-07-12

Table of contents
3.51 Sulfur dioxide, SO2

3.51 Sulfur dioxide, SO2
13.1.35 Sulfur dioxide
12.6.0.1 Acid rain, SOx, from burning sulfur or sulfur compounds
3.51.5 Bleach flowers with sulfur dioxide
3.51.0 Prepare sulfur dioxide
3.51.3 Reduce iron (III) chloride with sulfur dioxide
3.51.4 Reduce potassium manganate (VII) with sulfur dioxide
3.51.6 Tests for sulfur dioxide

3.51.0 Prepare sulfur dioxide
8.2.15 Heat sulfur to form sulfur dioxide
3.51.0 Prepare sulfur dioxide by burning sulfur
3.51.1 Prepare sulfur dioxide with sulfuric acid and sodium sulfite
3.51.2 Prepare sulfur dioxide with sulfuric acid and copper

3.51.0 Prepare sulfur dioxide by burning sulfur
| See diagram 3.2.75.1: Prepare sulfur dioxide by burning
| See diagram 3.2.75.2: Sulfur dioxide generator
| See diagram 3.51.1: Prepare sulfur dioxide by burning
| See diagram 3.51.2: Prepare sulfur dioxide in a generator
| See diagram 1.13a: Simple fume hood
Be careful! Prepare sulfur dioxide only in a fume hood or fume cupboard.
Sulfur dioxide, SO2, is a colourless gas that irritates the lungs.
Sulfur dioxide dissolves in water to form mainly, sulfurous acid, H2SO3.
Sulfur dioxide is one component of acid rain.
SO2 (g) + H2O (g) --> H2SO3 (l)
Be careful! Do not inhale the gas.
Do the following preparations in a fume cupboard, fume hood.

1. Ignite sulfur in an evaporating basin and collect the sulfur dioxide formed under a filter funnel.
Connect the filter funnel to a receiving container with a two-holes stopper containing water.
Use a filter pump to suck the sulfur dioxide into the receiving container.
S + O2 --> SO2

2. Pour some water into a screw cap container.
Heat a small amount of sulfur in a combustion spoon over a Bunsen burner until it melts then ignites.
Immediately put the burning sulfur into the screw cap container and block the opening.
When the burning stops, screw on the screw cap.

3. Add dilute sulfuric acid or hydrochloric acid to sodium sulfite crystals.
Collect the gas by upward displacement of air in a fume hood.
Na2SO3 (s) + H2SO4 (l) --> Na2SO4 (aq) + H2O (l) + SO2 (g)

4. Add dilute hydrochloric acid to 10 g sodium sulfite, or sodium bisulfite.
Collect the gas by upward displacement of air in a fume hood.
Na2SO3 (s) + 2HCl (l) --> 2NaCl (aq) + H2O (l) + SO2 (g)

5. Add dilute sulfuric acid or hydrochloric acid to sodium metabisulfite (Na2S2O5), crystals.
Collect the gas by upward displacement of air.
Na2S2O5 (s) + 2HCl (l) --> 2NaCl (aq) + H2O (l) + 2SO2 (g)

6. Be careful! Add hot concentrated sulfuric acid to copper to form copper (II) sulfate, water, and sulfur dioxide.
Cu (s) + 2H2SO4 (l) --> CuSO4 (aq) + 2H2O (l) + SO2 (g)

7. Dip a wire loop into sulfur powder.
Ignite the sulfur in a burner flame and then put it into a test-tube of oxygen.
The sulfur burns with a bright blue flame to form the colourless gas sulfur dioxide.
S (s) + O2 (g) --> SO2
sulfur + oxygen --> sulfur dioxide
Some sulfur trioxide may also form in this reaction.

3.51.1 Prepare sulfur dioxide with sulfuric acid and sodium sulfite
See diagram 13.13.3: Prepare sulfur dioxide
Na2SO3 (s) + H2SO4 (l) --> Na2SO4 (aq) + H2O (l) + SO2 (g)

3.51.2 Prepare sulfur dioxide with sulfuric acid and copper
Be careful!
Add hot concentrated sulfuric acid to copper to form copper (II) sulfate, water and sulfur dioxide.
Cu (s) + 2H2SO4 (l) --> CuSO4 (aq) + 2H2O (l) + SO2 (g)

3.51.3 Reduce iron (III) chloride with sulfur dioxide
Pass sulfur dioxide through brown-yellow iron (III) chloride solution. Sulfur dioxide reduces iron (III) chloride to Fe2+.
The reaction turns red.
Pour off some solution and boil.
The solution turns green.
Add drops of sodium hydroxide solution.
The reaction forms a brown-red precipitate of iron (III) hydroxide.
FeCl3 (aq) + 3NaOH (aq) --> Fe(OH)3 (s) + NaCl (aq)

3.51.4 Reduce potassium manganate (VII) with sulfur dioxide
Add 10 mL of 0.1M potassium manganate (VII) solution and 10 mL of 3M dilute sulfuric acid solution to 200 mL of water containing
sulfur dioxide.
The solution will gradually become colourless as the sulfur dioxide reacts with the potassium permanganate.
The colour of the manganate ion is lost as the sulfurous acid is oxidized to sulfuric acid.
Add a 0.25M barium chloride solution when the solution becomes "milky" because of the formation of barium sulfate
SO2 (g) + H2O (l) --> H2SO3 (aq)
2MnO4- (aq) + 6H+ (aq) + 5SO32- (aq) --> 2Mn2+ (aq) + 3H2O (l) + 5SO42- (aq)

3.51.5 Bleach flowers with sulfur dioxide
See diagram 3.2.76: Bleaching flowers
Add coloured flowers or fruit peel to a solution of sulfur dioxide in water.
Replace the stopper and shake.
The colour is bleached.
Restore the colour by placing the plant material in a dilute solution of hydrogen peroxide.

3.51.6 Tests for sulfur dioxide
1. Smell test
Notice its choking smell.
Be careful!
2. Litmus test
Sulfur dioxide dissolves in water to turn blue litmus paper red.
Shake water in a container of sulfur dioxide to form sulfurous acid solution.
3. Burning splint test
Sulfur dioxide extinguishes a burning splint.
4. Potassium permanganate test
Pass sulfur dioxide through a dilute solution of potassium permanganate until it loses its colour.