Carbon and its compounds for secondary schools

CARBON AND ITS COMPOUNDS

Occurrence

  • In living things- animals and plants
  • Combined form such as carbon (IV) oxide, carbonates such as chalk, limestone, marble, sea shells etc. and in mineral oil (petroleum)
  • Impure forms of carbon are made of graphite lattice.
  • Carbon in pure form exhibits two allotropic forms, diamond and Graphite.
  • Existence of an element in more than one form without change of state is known as allotropy



ALLOTROPES OF CARBON

Allotropes only differ in physical properties but have similar chemical properties eg when equal masses of  the pure forms of carbon(graphite and diamond) are burned in excess air they yield equal quantities of carbon (IV) oxide

Amorphous carbon

These are impure forms of carbon and are made up of small crystals of graphite. It is black opaque which conducts both heat and electricity.

Wood charcoal.

A source of energy

Formed when wood is heated in absence or presence of limited air (a process known as destructive distillation of wood)

It is black porous solid and capable of absorbing gases up to 100 times its volume, therefore it used in gas masks.

It is also used as pigment in shoe /boot polish.

Coke

Formed in retorts when coal is burned in absence of air

It does not burn easily and thus used in blast furnaces, ovens and boiler’s

Also used in extraction of Iron, Zinc and Lead.

Lampblack

Formed when most hydrocarbons such as petroleum, turpentine, kerosene and natural gas burn in a limited supply of air.

  • Used in making printing ink and black paint.
  • Added to rubber tires to make them tough.

CRYSTALLINE FORMS OF CARBON: DIAMOND AND GRAPHITE

Diamond

Colourless, transparent crystalline substance with high density 3.15g/cm3

Hardest naturally occurring substance.

Produced under extreme conditions of temperature and pressure

All carbon atoms are bonded to four others using strong covalent bonds in a tetrahedral manner forming a giant covalent structure.

This explains why diamond is very hard.

It does not conduct since it does not have free electrons.

DIAMOND    Uses

  • Jewelry due to its high lustre when cut and polished.
  • Glass cutting and drilling of rocks because of its hardness.




Graphite

GRAPHITE

Structure of graphite:

In graphite, the carbon atoms are arranged in flat planes of hexagonal rings stacked on one another.

Each carbon atom is attached to three others on the same plane. Thus, only three out of four valence electrons are used in carbon-carbon bonding. The fourth valence electron is not used in bonding and is delocalised in the structure hence diamond is a good conductor of heat and electricity. The week van der Waal forces allows the layers to slide over each other making graphite soft, slippery and useful as a lubricant.

A lot of energy is required to break the covalent bonds within the layers hence diamond has high melting and boiling point.

Graphite is a soft, slippery, grayish-black substance. It has a metallic luster and is opaque to light.

Uses of graphite:

The important uses of graphite are as follows.

  1. The major use of graphite is in making pencils “leads” of different hardness, by mixing it with different proportions of clay. This is because the layers can slide over each other.
  2. Due to its slippery nature and high melting point, graphite is used as a dry lubricant in machine parts involving high temperature where oil cannot be used.
  3. The presence of delocalised electrons makes graphite a good conductor of electricity and it is used to make electrodes.
  4. Graphite has the ability to absorb fast-moving neutrons, thus, it is used in nuclear reactors to control the speed of the nuclear fission reaction.

Synthetic forms

Fullerenes

 

Fullerenes
  • They are molecules composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube.
  • Spherical fullerenes are sometimes called buckyballs, the C60 variant is often compared to a typical white and black soccer football.
  • Cylindrical fullerenes are called buckytubes.
  • Fullerenes are similar in structure to graphite, which is composed of a sheet of linked hexagonal rings, but they contain pentagonal (or sometimes heptagonal) rings that prevent the sheet from being planar.

 

Evidence that diamond and graphite are allotropes of carbon is:

When equal amounts of pure allotropes are heated separately in pure oxygen they yield equal amounts of carbon (IV) oxide.

Chemical properties of carbon

  1. Reaction with oxygen

All forms of carbon burn freely in oxygen/air to produce carbon (IV) oxide

C (s)    +   O2 (g) → CO2 (g)

However in limited supply of oxygen carbon (II) oxide and carbon (IV) oxide are formed.

3C (s)    +   O2 (g) → 2CO (g)     +   CO2 (g)




Carbon readily removes combined oxygen from oxides of metals below it in the reactivity series

Carbon is oxidized to carbon (IV) oxide and metal oxide reduced to metal.

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

 

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

2ZnO (s)        +     C (s) → 2Zn (s)      +     CO2 (g)

 

2Fe2O3 (s)      +   3C (s) → 4Fe (s)      +     CO2 (g)

This property of carbon is utilized in extraction of iron, zinc and lead by reducing their oxides.

NB: All oxides of metals below it in reactivity series can be reduced by it

Reactivity series

Potassium

Sodium

Calcium

Magnesium

Aluminium

Carbon

Zinc

Iron

Tin

Lead

Hydrogen

Copper

Silver

Gold

Platinum



Carbon slowly reduces hot conc sulphuric (VI) acid to sulphur (IV) oxide.

C (s)         +      2H2SO4 (l) → CO2 (g)    +    2SO2 (g)    +   2H2O (l)

It also reduces concentrated nitric (V) acid

C (s)         +     4HNO3 (l) → 4NO2 (g)    +    CO2 (g)    +   2H2O (l)

NB: They react with carbon because they are strong oxidizing agents

 Reaction with steam

When steam is blown through red hot charcoal, a mixture of carbon (II) oxide and hydrogen gas is obtained.

The mixture is referred to as water gas.

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

This mixture is used as a fuel since it gives nonpoisonous substances when burnt

You can now move to

carbon (IV) oxide

carbon (II) oxide

Solvay process