Gases

Oxygen and Oxides

• Many elements react with oxygen to produce oxides.

Reactions of oxygen with magnesium, iron, copper, carbon, and sulphur

Method

1. Place a small coil of magnesium ribbon into a deflagrating spoon and heat in a roaring Bunsen flame until glowing.

2. Plunge into a gas jar containing oxygen.

3. Record observations.

4. Once cooled slightly, add water and a few drops of universal indicator.

5. Record observations.

6. Repeat with other elements.

Observations / Results

• Metals react with oxygen to form basic oxides, which may dissolve in water to form alkaline solutions.

• Non-metals react with oxygen to form acidic oxides, which may dissolve in water to form acidic solutions.

Balanced Symbol Equations

• Magnesium: 2Mg + O2 → 2MgO

• Iron: 4Fe + 3O2 → 2Fe2O3

• Copper: 2Cu + O2 → 2CuO

• Carbon: C + O2 → CO2

• Sulphur: S + O2 → SO2

% Composition of Dry Air

Experiment to Determine Percentage by Volume of Oxygen in Air

Method

1. Set up the apparatus and draw 100 cm³ of air into one syringe.

2. Heat the copper at one end of the silica tube with a roaring Bunsen flame while moving air from one syringe to the other.

3. Move the flame along the tube so all copper turnings are heated, continuing to move the syringes.

4. Allow the apparatus to cool, continuing to move the syringes.

5. Push the air into a syringe and measure the final volume.

Results

Questions / Answers

1. Why was the apparatus allowed to cool before the final volume of gas was measured?

   • To prevent expansion of the gas from affecting the final volume reading.

2. Why did most of the copper turn black during the experiment?

   • Copper reacted with oxygen to form copper(II) oxide.

3. The copper was in excess – some copper was unchanged at the end. Why was this important?

   • Ensures all oxygen reacts, giving an accurate measurement of oxygen volume.

4. What happened to the mass of the contents of the silica tube during the experiment? Explain.

   • Mass increased because copper combined with oxygen to form solid copper(II) oxide.

Percentage of Oxygen in Air Using Phosphorus

Method

1. Set up the apparatus as shown in the diagram.

2. Remove the bell jar (marked with 5 equal divisions) and light the phosphorus in the bottle top using a roaring Bunsen flame.

3. Once lit, replace the bell jar and insert the stopper.

4. Observe until the water level stops changing.

Observations

• Water rises inside the bell jar as phosphorus reacts with oxygen.

• Reaction slows until no further change in water level.

Conclusion

• Oxygen reacts with phosphorus to form phosphorus oxide, reducing gas volume.

• Remaining gas is mainly nitrogen.

Calculations / Answers

1. a) Start: 500 cm³, End: 400 cm³

   • Volume of oxygen reacted = 500 – 400 = 100 cm³

   • % oxygen = (100 ÷ 500) × 100 = 20%

   b) Solid left: phosphorus oxide

2. a) Start: 850 cm³, End: 710 cm³

   • Volume of oxygen reacted = 850 – 710 = 140 cm³

   • % oxygen = (140 ÷ 850) × 100 ≈ 16.5%

   b) Solid left: magnesium oxide

   c) % oxygen lower than expected because magnesium was not fully reacted or experimental errors (gas leaks, incomplete reaction).

3. a) Start: 120 cm³, End: 96 cm³

   • Volume of oxygen reacted = 120 – 96 = 24 cm³

   • % oxygen = (24 ÷ 120) × 100 = 20%

   b) Solid left: copper(II) oxide

4. a) Start: 79.0 cm³, End: 62.4 cm³

   • Volume of oxygen reacted = 79.0 – 62.4 = 16.6 cm³

   • % oxygen = (16.6 ÷ 79.0) × 100 ≈ 21%

   b) Colour of solid left: black (copper(II) oxide)
   c) Main gas left: nitrogen

Carbon Dioxide

• Carbon dioxide has a simple molecular structure and is a gas at room temperature.

• Dot-and-cross diagram: carbon shares 4 electrons with 2 oxygens (O=C=O), forming double covalent bonds.

• Carbon dioxide is a gas at room temperature because it has weak intermolecular forces, so little energy is needed to separate molecules.

Reactions Producing Carbon Dioxide

• Combustion of hydrocarbons and reaction of metal carbonates with acids produce CO₂.

• Some metal carbonates decompose on heating to form metal oxides and carbon dioxide:

  • Metal carbonate → Metal oxide + Carbon dioxide

Method

1. Heat metal carbonate in a test tube.

2. Observe changes in colour and gas produced.

Results

Balanced Equations

• CuCO₃(s) → CuO(s) + CO₂(g)

• CaCO₃(s) → CaO(s) + CO₂(g)

Properties of Carbon Dioxide

Reactions of Carbon Dioxide

• Carbon dioxide reacts with water to form the weak acid carbonic acid:

  • CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq)

• Carbonic acid decomposes on heating to release carbon dioxide:

  • H₂CO₃(aq) → CO₂(g) + H₂O(l)

• Rainwater is naturally acidic (pH ~6.5) due to dissolved CO₂ forming carbonic acid.

• Question: You collect CO₂ over water but get less gas than expected.

  • Answer: Some CO₂ dissolves in water, so the volume collected is lower.

• Carbon dioxide reacts with alkalis to form metal carbonates:

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

• Test for CO₂ with lime water:

  • CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) (white precipitate)

  • Excess CO₂ reacts to form soluble calcium hydrogen carbonate:

    • CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)

Uses of Carbon Dioxide

• Carbonates drinks: CO₂ dissolves to form carbonic acid, giving a sharp taste.

• Fire extinguishers: CO₂ is denser than air and does not support combustion.

• Greenhouse gas: Contributes to global warming.

  • Increasing levels caused by burning fossil fuels, deforestation, and industrial processes.