Carbon and It’S Compounds III – SS1 Chemistry Lesson Note (Third Term)

Carbon forms two common oxides: carbon (IV) oxide, CO₂, and carbon (II) oxide, CO.

Behavioural objectives

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

1. describe the laboratory and industrial preparation of carbon (IV) oxide;
2. state the physical and chemical properties of carbon (IV) oxide and test for it;
3. state the uses of carbon (IV) oxide;
4. describe the laboratory preparation of carbon (II) oxide and explain why it is dangerous;
5. state the physical and chemical properties, the test and the uses of carbon (II) oxide.

What is carbon (IV) oxide?

Carbon (IV) oxide, CO₂, is a colourless, odourless gas denser than air, which turns lime water milky.

• Occurrence: it makes up about 0.03% by volume of atmospheric air.
• Formula: one carbon atom joined to two oxygen atoms.
• Nature: it is an acidic oxide, so it dissolves in water to form a weak acid.

How is carbon (IV) oxide prepared in the laboratory?

Carbon (IV) oxide is prepared in the laboratory by the action of dilute acid on a trioxocarbonate (IV), or by heating a trioxocarbonate (IV).

Method 1: action of dilute acid

• Reagents: dilute acid with a trioxocarbonate (IV) or hydrogen trioxocarbonate (IV).
• Common choice: marble chips or shells (CaCO₃) with dilute hydrochloric acid or trioxonitrate (V) acid.
• Apparatus: the reaction between CaCO₃ and HCl can be carried out in a Kipp’s apparatus.
• Acid to avoid: dilute tetraoxosulphate (VI) acid is not used, because insoluble CaSO₄ coats the marble and stops the reaction.

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

NaHCO₃(aq) + HNO₃(aq) → NaNO₃(aq) + H₂O(l) + CO₂(g)

Method 2: heating

• Metallic trioxocarbonates (IV): heating them gives the metal oxide and CO₂. Those of Na and K do not decompose this way.
• Hydrogen trioxocarbonates (IV) of Na or K: heating them also gives CO₂.

CuCO₃(s) → CuO(s) + CO₂(g)

2NaHCO₃(s) → Na₂CO₃(s) + H₂O(l) + CO₂(g)

How is dry carbon (IV) oxide obtained?

1. Pass the gas through potassium hydrogen trioxocarbonate (IV) solution to remove acid fumes.
2. Pass it through a U-tube containing fused calcium chloride to remove water vapour.
3. Collect the dry gas by downward delivery, because it is heavier than air.

How are gases collected?

The method used to collect a gas depends on its density and its solubility in water.

How is carbon (IV) oxide prepared industrially?

Carbon (IV) oxide is obtained industrially as a by-product of fermentation and of the heating of limestone.

• Fermentation: CO₂ is released when sugars are fermented to ethanol, for example in brewing.
• Making quicklime: CO₂ is released when limestone is heated.

CaCO₃(s) → CaO(s) + CO₂(g)

What are the physical properties of carbon (IV) oxide?

Carbon (IV) oxide is a colourless, odourless gas that is denser than air and slightly soluble in water.

• Appearance and smell: colourless and odourless, with a slightly sharp, refreshing taste.
• Density: about 1.5 times denser than air.
• Solubility: it dissolves in water. At room temperature and standard pressure, water dissolves about its own volume of the gas.
• Effect on litmus: it turns damp blue litmus paper red (pink), because CO₂ dissolves in water to give trioxocarbonate (IV) acid.
• Effect of cooling: under pressure it liquefies. On cooling to -78 °C it forms a white solid called dry ice, which sublimes on warming.

What are the chemical properties of carbon (IV) oxide?

Carbon (IV) oxide is not very active chemically, and it does not support combustion.

• Reaction with water: it dissolves to form trioxocarbonate (IV) acid (soda water), a weak, dibasic acid that ionises slightly.
  (a) CO₂(g) + H₂O(l) → H₂CO₃(aq)
  (b) H₂CO₃(aq) + H₂O(l) → H₃O⁺(aq) + HCO₃^–(aq)
  (c) HCO₃^–(aq) + H₂O(l) → H₃O⁺(aq) + CO₃^2-(aq)
  On heating, the acid decomposes to H₂O(l) and CO₂(g).
• Reaction with alkalis (limited CO₂): it reacts directly with alkalis to give a trioxocarbonate (IV).
  CO₂(g) + 2NaOH(aq) → Na₂CO₃(aq) + H₂O(l)
• Reaction with alkalis (excess CO₂): it gives a hydrogen trioxocarbonate (IV) salt.
  CO₂(g) + NaOH(aq) → NaHCO₃(aq)
• Reaction with burning Na, K or Mg: CO₂ is reduced to carbon.
  CO₂(g) + 2Mg(s) → C(s) + 2MgO(s)
• Reaction with red-hot carbon: CO₂ is reduced to CO.
  CO₂(g) + C(s) → 2CO(g)
  This reaction is important in the blast furnace and in making gaseous fuels.

How do you test for carbon (IV) oxide?

Carbon (IV) oxide turns lime water (calcium hydroxide solution) milky when bubbled through it.

1. Bubble the gas through lime water. It turns milky because insoluble calcium trioxocarbonate (IV) forms.
   Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
2. Continue bubbling the gas in excess. The milkiness disappears because soluble calcium hydrogen trioxocarbonate (IV) forms.
   CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)
3. Heat the clear solution. The milkiness returns because Ca(HCO₃)₂ decomposes to insoluble CaCO₃.
   Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)

What are the uses of carbon (IV) oxide?

Carbon (IV) oxide is used in fire fighting, drinks, baking, industry and refrigeration.

• Fire extinguishers: it does not support combustion.
• Carbonated (aerated) drinks: it gives soft drinks their refreshing taste. Beer, cider and champagne also contain CO₂.
• Washing soda: it is used to make Na₂CO₃ by the Solvay process.
• Baking: yeast and baking powder produce CO₂, which makes dough rise.
• Fertilisers: it is used in making fertilisers such as urea and (NH₄)₂SO₄.
• Refrigerant: solid CO₂ (dry ice) keeps perishable goods such as ice cream cold. It sublimes on warming and gives a lower temperature than ice.
• Preservation: gaseous CO₂ is used to preserve fruits.
• Nuclear reactors: it is used as a coolant.

What is carbon (II) oxide?

Carbon (II) oxide, CO, is a poisonous, colourless gas that burns in air and reduces metallic oxides to metals.

How is carbon (II) oxide prepared in the laboratory?

Carbon (II) oxide is prepared in the laboratory from carbon (IV) oxide, or by dehydrating methanoic or ethanedioic acid.

Method 1: from carbon (IV) oxide

1. Pass carbon (IV) oxide over red-hot carbon. The CO₂ is reduced to CO.
   CO₂(g) + C(s) → 2CO(g)
2. Pass the gaseous mixture through concentrated NaOH to remove excess CO₂.
3. Collect the pure CO over water.

Method 2: dehydration of an acid

1. Add concentrated tetraoxosulphate (VI) acid to methanoic (formic) acid or ethanedioic (oxalic) acid. The acid removes water.
   HCOOH(l) → CO(g) + H₂O(l) (conc. H₂SO₄)
   (COOH)₂(s) → CO(g) + CO₂(g) + H₂O(l) (conc. H₂SO₄)
2. Pass the gaseous mixture through concentrated NaOH to remove CO₂.
3. Collect the CO over water.

Caution: prepare CO in a fume cupboard, because the gas is poisonous.

Why is carbon (II) oxide dangerous?

Carbon (II) oxide is dangerous because it prevents the blood from carrying oxygen, and it gives no warning.

• Source: it forms when carbon fuels burn in a limited supply of air, for example in smoky vehicles, engines and generators.
• Pollutants: smoky vehicles release CO and carbon particles into the air.
• Action in the body: it combines with haemoglobin to form carboxyhaemoglobin, so the red blood cells carry less oxygen.
• No warning: it is colourless, tasteless and odourless, so its effect builds up unnoticed.
• Danger level: even about 1% of CO in air breathed for some time may cause death.
• Everyday risk: never run a car engine in a closed garage or a generator in a closed room. The victim often collapses without warning.

What are the physical properties of carbon (II) oxide?

Carbon (II) oxide is a poisonous, colourless, tasteless and odourless gas that is insoluble in water.

• Appearance: colourless, tasteless and odourless.
• Solubility: insoluble in water, but it dissolves in a solution of ammoniacal copper (I) chloride.
• Density: about the same as air (very slightly less dense).
• Effect on litmus: it is neutral to litmus.

What are the chemical properties of carbon (II) oxide?

Carbon (II) oxide is a strong reducing agent that burns in air to form carbon (IV) oxide.

• As a reducing agent: it reduces some metallic oxides to the metals and is oxidised to CO₂.
  PbO(s) + CO(g) → Pb(s) + CO₂(g)
  Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)
  CuO(s) + CO(g) → Cu(s) + CO₂(g)
• With oxygen: CO burns in air with a faint pale blue flame to form CO₂.
  2CO(g) + O₂(g) → 2CO₂(g)
• With haemoglobin: CO combines very firmly with haemoglobin in the red blood cells to form carboxyhaemoglobin. The red blood cells can no longer carry oxygen.
• With NaOH: CO and NaOH at 150 °C under 50 atm pressure form sodium formate. At 400 °C, sodium formate decomposes into sodium oxalate and H₂.
  CO(g) + NaOH(aq) → HCOONa(aq)
  2HCOONa → (COONa)₂ + H₂(g)
• With certain metals: CO combines with metals such as Ni, Fe and Co when hot to form a volatile carbonyl.
  Ni(s) + 4CO(g) → Ni(CO)₄(g)
• With copper (I) chloride: a solution of CuCl in aqueous NH₃ or conc. HCl absorbs CO readily, forming CuCl.CO.2H₂O. This reaction separates CO from a mixture of gases.
  CO(g) + CuCl(s) + 2NH₄OH(aq) → CuCl.CO.2H₂O(aq) + 2NH₃(g)
• With chlorine: in ultraviolet light, or over activated charcoal at 150 °C, CO and chlorine form carbonyl chloride (phosgene).
  CO(g) + Cl₂(g) → COCl₂(g)
  Phosgene was used as a poisonous gas in the First World War. It is now used to make dyestuff.

How do you test for carbon (II) oxide?

Carbon (II) oxide burns with a pale blue flame and forms a gas that turns lime water milky.

1. Insert a lighted splint into a test tube containing CO.
2. Observe the pale blue flame as the gas burns.
3. Add some lime water to the tube and shake.
4. The lime water turns milky after burning, but not before burning, because CO₂ has formed.

What are the uses of carbon (II) oxide?

Carbon (II) oxide is used to extract metals, as a fuel gas and to make other chemicals.

• Metal extraction: it reduces metal ores to the metals.
• Gaseous fuels: it is an important constituent of producer gas and water gas.
• Chemical manufacture: it is used to make methyl alcohol, synthetic petrol, carbonyl chloride, oxalate and formate.

What is the difference between carbon (IV) oxide and carbon (II) oxide?

Carbon (IV) oxide is an acidic gas that does not burn, while carbon (II) oxide is a neutral, poisonous gas that burns.

Evaluation

1. State two ways of preparing carbon (IV) oxide in the laboratory.
2. Write the equation for the reaction between CaCO₃ and dilute hydrochloric acid.
3. Describe the test for CO₂.
4. State two uses of carbon (II) oxide.
5. Explain why carbon (II) oxide is dangerous to breathe.

Assignment

Answer all parts in your notebook:

1. Why is carbon (II) oxide prepared in a fume cupboard?
2. State the property of CO₂ that makes it suitable for use in (i) carbonated drinks, (ii) fire extinguishers.
3. Why is it unsafe to stay in a closed garage for a long time while racing a car engine?
4. State what is observed when (i) excess CO₂ is bubbled through lime water, (ii) the solution in (i) is heated.