Carbon and its compounds

Carbon exists in 3 allotropic forms, diamond, graphite and amorphous carbon (charcoal, lampblack, soot, and coke) diamond and graphite are crystalline while amorphous carbon is noncrystalline.

Allotropy is the existence of an element in more than one form without change of state

A. Graphite

In graphite, each carbon atom is covalently bonded to 3 carbon atoms to form a layer of hexagons. Each layer is bonded to another by weak van der Waal forces.

Properties of graphite as a result of its structure

1. Has open structure and low density.
2. It is slippery and used as a lubricant.
3. Has un-bonded that is free to move about making graphite a good conductor of electricity and heat

Uses of graphite

1. Manufacture of lead pencil
2. As lubricant
3.  Diamond

Structure of diamond

Each carbon atom is bonded tetrahedrally to four carbon atoms to form a 3D compact structure by strong covalent bonds as shown below. As a result, diamond has a high density, melting and boiling point. It is the hardest substance known.

Diamond is used as an ornament, and to drill, glass cutters and cut other substances.

Differences between diamond and graphite

Graphite	Diamond
The density of graphite is lower 2.3 gcm-3	The density of diamond is3.5 g cm-3
Soft	hard
Slippery	Not slippery
Conducts electricity	Does not conduct electricity

Experiment to show that graphite and diamond are both allotropes of carbon

Equal masses of graphite and diamond burn in oxygen to give equal volume/mass of oxygen

Properties of carbon

1. Carbon burns in excess oxygen to form carbon dioxide

C (s) + O₂(g)        →           CO₂(g)

2. Carbon burns in limited oxygen to form carbon monoxide

2C (s) + O₂(g)        →               2CO (g)

3. Carbon reduce oxides of metals (lead, copper, zinc, iron) to metals

2PbO(s) + C (s)  →           2Pb(s) + CO₂(g)

2CuO(s) + C (s)  →           2Cu(s) + CO₂(g)

ZnO(s)  + C (s)    →           Zn (s) + CO(g)

3. At high temperature, carbon reacts with steam to form water gas

[(CO (g) + H₂(g)]

C(s) + H₂O (g)      →          CO(g) + H₂ (g)

Carbon dioxide

Preparation

By reaction between calcium carbonate (marble) and dilute hydrochloric acid.

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

Testing for carbon dioxide

Carbon dioxide forms white precipitates with lime water. The white precipitate turns colorless with excess carbon dioxide

Calcium hydroxide + carbon dioxide        give        calcium carbonate (white ppt.) + water

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

Calcium carbonate (white ppt) + carbon dioxide (excess)      give        calcium hydrogen carbonate   (colorless solution)

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

Properties of carbon dioxide

1. It is colorless
2. Denser than air
3. Ordorless
4. Slightly soluble in water to form an acid solution
5. Extinguishes burning splint
6. Reacts with magnesium to form black specks

2Mg (s) + CO₂(g)     →   2MgO(s) + C(s)

Uses of carbon dioxide

1. In soda
2. In fire extinguishers because it is nonflammable and denser than air thus displaces oxygen from burning item.
3. It is a coolant

Carbon monoxide

Preparation

By reacting sodium methanoate with concentrated sulphuric acid

2HCOONa (s) + H₂SO₄ (aq)       →           Na₂SO₄(aq) + 2H₂O(l) + 2CO (g)

Properties of carbon monoxide

1. Very poisonous, it is produced by incomplete combustion of carbon therefore cooking on nigiri  should be done when the door and the windows are open to allow in enough air.
2. Has no action on litmus paper
3. Burns with a blue flame
4. Reduce metallic oxides

For example, it reduces black copper oxide and orange lead (II) oxide to brown copper and grey lead respectively

CuO(s) + CO (g) →  Cu (s) + CO₂ (g)

Cu(s) + CO (g)    → Cu (s) + CO₂ (g)

Uses of  carbon monoxide

1. It is used in the extraction of iron; it reduces iron (III) oxide to Iron

Fe₂O₃ (s) + 3CO (g)          →           3Fe(s) + 3CO₂(g)

Carbonates

Carbonates are derivatives of carbonic acid, H₂CO₃.

Properties of carbonate are given in the table below

metal	Formula of carbonate	solubility	Effect of heat	Reaction with acid
K	K2CO3	soluble	Do not decompose	Produce carbon dioxide
Na	Na2CO3	soluble	Do not decompose	Produce carbon dioxide
Ca	CaCO3	Insoluble	Decompose into oxides and carbon dioxide	Produce carbon dioxide
Mg	Mg CO3	Insoluble	Decompose into oxides and carbon dioxide	Produce carbon dioxide
Al	does not form carbonate	–	–	–
Zn	ZnCO3	Insoluble	Decompose into oxides and carbon dioxide	Produce carbon dioxide
Fe	FeCO3	Insoluble	Decompose into oxides and carbon dioxide	Produce carbon dioxide
Pb Cu	PbCO3 CuO	Insoluble	Decompose into oxides and carbon dioxide	Produce carbon dioxide

Test for carbonate ion

All carbonates react with nitric acid to produce carbon dioxide with turn lime water milky.

Hydrogen carbonate

It is only group 1 elements (K, Na) that form solid hydro carbonates.

Hydro carbonates decompose on heating liberating carbon dioxide

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

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Compiled by Dr. Bbosa Science