Carbon and its Compounds II – SS1 Chemistry Lesson Note (Third Term)

Amorphous carbon is carbon with no definite crystalline structure, such as charcoal, carbon black and lamp black.

Behavioural objectives

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

1. name the amorphous forms of carbon and state their uses;
2. state the physical properties of carbon;
3. write equations for the complete and incomplete combustion of carbon;
4. write equations for the combination of carbon with sulphur, hydrogen, calcium and aluminium;
5. explain why carbon is a reducing agent and say which metal oxides it can reduce;
6. write equations for the reaction of carbon with concentrated nitric acid and concentrated sulphuric acid.

What is amorphous carbon?

Amorphous carbon is a non-crystalline form of carbon whose particles have no regular, repeating arrangement.

• Not true allotropes: these forms are not considered to be true allotropes of carbon.
• Examples: charcoal, carbon black and lamp black.
• Crystalline carbon: diamond and graphite have a definite, regular structure.

What are the types of amorphous carbon?

There are three common amorphous forms of carbon: charcoal, carbon black and lamp black.

Charcoal

• Source: made by burning wood, bones, sugar and similar materials in a limited amount of air.
• Wood charcoal: used for adsorbing poisonous gases.
• Animal charcoal: used for adsorbing colours, so it removes colour from substances.

Lamp black

• Source: formed when the wick of an oil lamp is burnt excessively.
• Deposit: it leaves soot on the lamp-glass and cover.

Carbon black

• Source: obtained from burning coal gas, natural gas or petroleum.

What are the uses of carbon black and lamp black?

• Rubber tyres: added to the rubber as an additive.
• Printer ink: gives the black colour.
• Carbon paper and typewriter ribbons: supply the black marking material.
• Black shoe polish: gives the black colour.

What are the physical properties of carbon?

Carbon, apart from diamond, is a black or greyish-black solid with a high melting point that does not dissolve in common solvents.

• Appearance: all allotropes of carbon, except diamond, are black or greyish-black solids.
• Odour and taste: odourless and tasteless.
• Melting point: high, about 3500 °C.
• Solubility: insoluble in all common solvents, such as water, alkalis, acids, petrol and carbon (iv) sulphide (CS₂).

What is decarbonisation of motor engines?

Decarbonisation is the mechanical removal of carbon deposits from inside a motor engine.

• Reason: carbon does not dissolve in any common solvent.
• Method: the deposits must be scraped off, not washed out with a solvent.

What are the chemical properties of carbon?

Carbon burns in oxygen, combines directly with some elements, reduces the oxides of less active metals and is oxidised by hot concentrated acids.

1. What happens when carbon burns?

Carbon burns in oxygen to form carbon (iv) oxide in plenty of oxygen or carbon (ii) oxide in a limited supply.

• Complete combustion: burning in excess oxygen gives carbon (iv) oxide gas.
• Incomplete combustion: burning in a limited supply of air gives carbon (ii) oxide gas.

• Safety note: carbon (ii) oxide is a poisonous gas.

2. What are the combination reactions of carbon?

Carbon combines directly with sulphur, hydrogen, calcium and aluminium at very high temperatures.

3. Why is carbon a reducing agent?

A reducing agent is a substance that removes oxygen from another substance and is itself oxidised.

• Action: carbon is a strong reducing agent. It reduces the oxides of the less active metals to the metals.
• Carbon is oxidised: it changes to carbon (iv) oxide or carbon (ii) oxide, depending on the conditions.

Examples of reduction by carbon

• Fe₂O₃(s) + 3C(s) → 2Fe(s) + 3CO(g)
• 2CuO(s) + C(s) → 2Cu(s) + CO₂(g)
• H₂O(g) + C(s) → CO(g) + H₂(g)
• CO₂(g) + C(s) → 2CO(g)

Which metal oxides can carbon reduce?

Carbon reduces the oxides of metals from zinc downwards in the activity series, but not those of K, Na, Ca, Mg and Al.

The activity series is: K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, H, Cu, Hg, Ag, Au.

Examples

• ZnO(s) + C(s) → Zn(s) + CO(g)
• PbO(s) + C(s) → Pb(s) + CO(g)

4. What happens when carbon reacts with strong oxidising agents?

Carbon heated with concentrated nitric acid or concentrated sulphuric acid is oxidised to carbon (iv) oxide.

Evaluation

1. Name two amorphous forms of carbon and state one use of each.
2. Why do carbon deposits in motor engines have to be removed mechanically?
3. Write balanced equations for the complete and incomplete combustion of carbon.
4. Write an equation to show carbon acting as a reducing agent on iron (iii) oxide.
5. Write the equation for the reaction of carbon with concentrated sulphuric acid.

Assignment

1. Answer all parts:
   1. Write an equation for the reduction of zinc oxide by carbon.
   2. Explain why carbon cannot reduce magnesium oxide.
   3. Write the equation for the reaction of carbon with concentrated nitric acid.