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. CO₂ is a colourless, odourless gas that is denser than air, dissolves in water to form a weak acid, and turns lime water milky. CO is a poisonous, colourless gas that burns in air and reduces metallic oxides to metals.

Behavioural objectives

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

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

What is carbon (IV) oxide?

Carbon (IV) oxide is about 0.03% by volume of atmospheric air and about 0.50% by volume of dissolved air.

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

1. Carbon (IV) oxide is prepared by the action of dilute acids on a trioxocarbonate (IV) or a hydrogen trioxocarbonate (IV). Usually CaCO₃, in the form of marble chips or shells, is used with hydrochloric acid or trioxonitrate (V) acid. The reaction between CaCO₃ and HCl can be carried out in a Kipp’s apparatus.

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

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

2. It is also prepared by heating metallic trioxocarbonates (IV) [except those of Na and K], or the hydrogen trioxocarbonate (IV) of Na or K.

   CuCO₃(s) → CuO(s) + CO₂(g)

Note: If the gas is required dry, it is passed first through potassium hydrogen trioxocarbonate (IV) solution to remove any acid fumes, and then through a U-tube containing fused calcium chloride to remove the water vapour. The dry gas is then collected by downward delivery as it is heavier than air.

How are gases collected?

The method of collection of a gas depends on its:

1. density;
2. solubility.

There are two methods of collecting gases:

How is carbon (IV) oxide prepared industrially?

CO₂ is obtained industrially as a by-product in fermentation processes and when limestone is heated to make quicklime.

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

1. CO₂ is a colourless, odourless gas with a sharp refreshing taste.
2. It is about 1.5 times denser than air.
3. It is soluble in water. At room temperature and standard pressure, water dissolves the volume of the gas.
4. It turns damp blue litmus paper pink because CO₂ dissolves in water to yield trioxocarbonate (IV) acid.
5. On cooling, it readily liquefies and solidifies at -78 °C to form a white solid known as dry ice.

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

1. Reaction with water: Carbon (IV) oxide is not very active chemically. It dissolves in water to form trioxocarbonate (IV) acid (soda water). This is a weak, dibasic acid which 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, trioxocarbonate (IV) acid decomposes to form H₂O(l) and CO₂(g).

2. Reaction with alkalis: It reacts directly with alkalis to yield a trioxocarbonate (IV). This happens when the amount of CO₂ is limited.

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

   Excess CO₂ reacts with alkalis to produce a hydrogen trioxocarbonate (IV) salt.

   CO₂(g) + NaOH(aq) → NaHCO₃(aq)

3. Reaction with burning Na, K or Mg: CO₂ is reduced to carbon by burning Na, K or Mg.

   CO₂(g) + 2Mg(s) → C(s) + 2MgO(s)

   Note: CO₂ does not support combustion.

4. Reaction with red-hot carbon: CO₂ is reduced to CO if the gas is passed over red-hot carbon.

   CO₂(g) + C(s) → 2CO(g)

   The reaction is of great importance in the blast furnace and the manufacture of gaseous fuels.

How do you test for carbon (IV) oxide?

Bubble the unknown gas through lime water (calcium hydroxide solution). If the lime water turns milky because insoluble calcium trioxocarbonate (IV) forms, the unknown gas is CO₂.

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

If the gas is bubbled in excess, the milkiness disappears and a clear solution forms, because soluble calcium hydrogen trioxocarbonate (IV) is formed.

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

Finally, if the clear solution is heated, the milkiness reappears because soluble Ca(HCO₃)₂ decomposes to form insoluble CaCO₃.

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

What are the uses of carbon (IV) oxide?

1. It is used as a fire extinguisher since it does not support combustion.
2. It gives carbonated (aerated) drinks their refreshing taste. Beer, cider and champagne contain CO₂.
3. It is used in the manufacture of Na₂CO₃ (washing soda) by the Solvay process.
4. It is used as a leavening agent in the baking of bread. Yeast and baking powder produce CO₂, which makes the dough rise.
5. It is used in the manufacture of fertilisers (such as urea and (NH₄)₂SO₄).
6. Solid CO₂ (dry ice) is used as a refrigerant for perishable goods, e.g. ice cream. It sublimes on warming and provides a lower temperature.
7. Gaseous CO₂ is used to preserve fruits.
8. CO₂ is also used as a coolant in nuclear reactors.

What is carbon (II) oxide?

Carbon (II) oxide, CO, is a poisonous, colourless gas. It is prepared in the laboratory from carbon (IV) oxide or from methanoic or ethanedioic acid.

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

1. Carbon (II) oxide can be prepared by passing carbon (IV) oxide through red-hot carbon. The carbon (IV) oxide is itself reduced to carbon (II) oxide. The gaseous mixture is passed through concentrated NaOH to remove the excess carbon (IV) oxide, and the pure carbon (II) oxide is collected over water.

   CO₂(g) + C(s) → 2CO(g)

2. Carbon (II) oxide can also be prepared by the dehydration of methanoic (formic) acid or ethanedioic (oxalic) acid, using concentrated tetraoxosulphate (VI) acid.

   HCOOH(l) → CO(g) + H₂O(l) (conc. H₂SO₄)

   (HCOOH is methanoic acid.)

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   Note: The gaseous mixture is passed through concentrated NaOH to remove the CO₂.

Caution: The preparation of CO must be done in a fume cupboard as the gas is poisonous.

Why is carbon (II) oxide dangerous?

The major air pollutants that can result from smoky vehicles are carbon (II) oxide and carbon particles.

When CO is breathed in for any length of time, even 1% of it in the air may cause death. This makes it clear how dangerous it can be to run a car engine in a closed garage or a generator in a closed room. Very often the victim collapses without warning, because the effect of the gas builds up unnoticed.

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

1. CO is a poisonous, colourless, tasteless and odourless gas.
2. It is insoluble in water but dissolves in a solution of ammoniacal copper (I) chloride.
3. It is neither lighter nor heavier than air.
4. It is neutral to litmus.

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

1. As a reducing agent: CO is a strong 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)

2. Combination reactions:

   (a) With oxygen: CO burns in air with a faint pale blue flame to form CO₂.

   2CO(g) + O₂(g) → 2CO₂(g)

   (b) With haemoglobin: CO combines irreversibly with haemoglobin in the red blood cells to form carboxyhaemoglobin. This prevents the red corpuscles from acting as oxygen carriers.

3. With NaOH: CO mixed with NaOH at 150 °C under 50 atm pressure forms sodium formate.

   CO(g) + NaOH(aq) → HCOONa(aq)

   At 400 °C, sodium formate decomposes into sodium oxalate and H₂.

   2HCOONa → (COONa)₂ + H₂(g)

4. With certain metals: CO combines with certain metals (e.g. Ni, Fe, Co) when hot to form a volatile carbonyl.

   Ni(s) + 4CO(g) → Ni(CO)₄(g)

5. With copper (I) chloride: CO is absorbed readily by a solution of copper (I) chloride (CuCl) in aqueous NH₃ or conc. HCl, forming the compound CuCl.CO.2H₂O. This reaction is used to separate CO from a mixture of gases.

   CO(g) + CuCl(s) + 2NH₄OH(aq) → CuCl.CO.2H₂O(aq) + 2NH₃(g)

6. With chlorine: CO combines with chlorine gas when exposed to ultraviolet light, or when passed over a catalyst of activated charcoal at 150 °C, to form carbonyl chloride.

   CO(g) + Cl₂(g) → COCl₂(g)

   This product, COCl₂, is also known as phosgene. It was used as a poisonous gas in the First World War and is now used in the manufacture of dyestuff.

How do you test for carbon (II) oxide?

Insert a lighted splint into a test tube containing CO(g), then add some lime water and shake. The CO(g) burns with a pale blue flame, and the lime water turns milky after burning but not before burning.

What are the uses of carbon (II) oxide?

1. CO is used in the extraction of metals from their ores.
2. It is an important constituent of gaseous fuels like producer gas and water gas.
3. CO gas is used in the manufacture of methyl alcohol, synthetic petrol, carbonyl chloride, oxalate and formate.

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

1(a) Why is the laboratory preparation of carbon (II) oxide done in a fume cupboard?

(b) State the property of CO₂ that makes it suitable for use in (i) carbonated drinks, (ii) fire extinguishers.

2(a) Why is it not advisable to stay in a closed garage for a long time when racing a car engine?

(b) State what is observed when (i) excess CO₂ is bubbled through lime water, (ii) the solution in (b)(i) above is heated.