13. Carbon: An Important Element

Carbon (2, 4) needs four more electrons to complete its octet. Giving away or taking four electrons is very difficult, so it shares electrons with other atoms, forming four covalent bonds.

In a saturated hydrocarbon such as ethane (CH₃–CH₃), all the bonds between carbon atoms are single covalent bonds; in each, the two carbon atoms share one pair of electrons.

An unsaturated hydrocarbon has at least one double bond (ethene, H₂C=CH₂) or triple bond (ethyne, HC≡CH) between two carbon atoms.

All compounds obtained from plants and animals — carbohydrates, proteins, fats, DNA — contain carbon. That is why organic chemistry is called the chemistry of carbon compounds.

Along with carbon, hydrogen is found in most organic compounds; compounds of only carbon and hydrogen are called hydrocarbons. A few organic compounds, such as carbon tetrachloride (CCl₄), contain no hydrogen.

• Carbon and its compounds (wood, coal, charcoal, petrol, diesel, LPG, CNG, biogas) burn in air and give out a large amount of heat and light.
• Their ignition temperature is moderate, so they catch fire easily and burn steadily.
• They are easily available, easy to store and transport, and fairly cheap.
• Fuels such as methane burn completely with a blue flame, releasing much heat: CH₄ + 2O₂ → CO₂ + 2H₂O + heat.

• As carbon dioxide in the air, and as carbonates such as calcium carbonate (limestone, marble, chalk) and calamine (ZnCO₃).
• In fossil fuels — coal, petroleum and natural gas.
• In carbonaceous nutrients — carbohydrates, proteins and fats.
• In natural fibres — cotton, wool and silk; and in wood.

• In glass-cutting tools and rock-drilling machines, because it is the hardest natural substance.
• In ornaments, because of its brilliance.
• Diamond knives are used in eye surgery.
• Diamond dust is used for polishing other diamonds.
• For making windows that protect against radiation in space and artificial satellites.

1. Structure: in diamond each carbon atom is bonded to four others, forming a rigid three-dimensional structure; in graphite each carbon atom is bonded to three others, forming hexagonal layers that can slip over each other.
2. Hardness: diamond is the hardest natural substance; graphite is soft, slippery and brittle.
3. Appearance: diamond is transparent and brilliant; graphite is black and opaque.
4. Electrical conductivity: diamond is a bad conductor, as it has no free electrons; graphite is a good conductor, as it has free electrons in each layer.
5. Density: diamond is about 3.5 g/cm³; graphite is 1.9 to 2.3 g/cm³.
6. Uses: diamond — ornaments, glass cutting, drilling; graphite — pencils, lubricants, electrodes.

1. Arrangement of atoms: crystalline forms have a regular and definite arrangement of carbon atoms; non-crystalline (amorphous) forms have an irregular arrangement.
2. Shape: crystalline forms have a definite geometrical shape with sharp edges and plane surfaces; non-crystalline forms have no definite shape.
3. Melting and boiling points: crystalline forms have high melting and boiling points; these are not definite for non-crystalline forms.
4. Examples: crystalline — diamond, graphite, fullerene; non-crystalline — coal, charcoal, coke.

In graphite each carbon atom uses only three of its four valence electrons to bond with three neighbouring atoms in the same layer. The fourth electron of every atom is free, and these free electrons move continuously within the entire layer. When a potential difference is applied, they carry the electric current. Therefore graphite is a good conductor of electricity.

Ornaments need a substance that is lustrous, attractive and hard enough not to wear away. Graphite is black, dull (not brilliant), soft and slippery; its layers slide over one another and it leaves black marks on the skin and clothes. Therefore it is not used in ornaments.

Limewater is a solution of calcium hydroxide. Carbon dioxide reacts with it to form calcium carbonate, which is insoluble in water. The fine white particles of calcium carbonate stay suspended in the solution and make it look milky.

• Biogas is made from animal dung, dry leaves and wet garbage, which are renewable wastes; it disposes of these wastes usefully.
• It contains 55–60% methane, which burns completely with a clean blue flame and gives no smoke or soot.
• Burning methane releases comparatively little carbon dioxide.
• The spent slurry left after gas production is a very good manure, which reduces the use of chemical fertilisers.
• It reduces the cutting of trees for firewood.

Therefore biogas is an eco-friendly fuel.

A crystalline form has a regular and definite arrangement of atoms, a definite geometrical shape and high melting and boiling points. In diamond every carbon atom is bonded to four others in a regular three-dimensional structure; in graphite every carbon atom is bonded to three others in regular hexagonal layers; and in fullerenes (such as C₆₀) the carbon atoms are arranged in a regular hollow ball or tube like a geodesic dome. All three have a regular, definite arrangement of carbon atoms, so they are crystalline forms (allotropes) of carbon.

In marshy places, dead plants and other organic matter decompose under water, in the absence of air, by the action of anaerobic microbes. This produces methane, which bubbles out and is found at the surface of marshes. Methane was also first discovered by Volta while studying the gases of marshy land. Hence methane is called marsh gas.

Millions of years ago, plants and animals were buried under layers of soil, mud and rock. Under high pressure and temperature inside the earth, and in the absence of air, their remains slowly changed into coal, petroleum and natural gas. Petrol and diesel are obtained from petroleum. Since these fuels are formed from the remains (fossils) of ancient living things, they are called fossil fuels. They take millions of years to form, so they are non-renewable.

For burning, a fuel needs oxygen. Carbon dioxide does not burn and does not support burning, and it is heavier than air. When it is sprayed on a fire, it forms a layer around the burning material, cuts off the supply of oxygen and puts out the fire.

In a soda-acid type extinguisher, sodium bicarbonate and dilute sulphuric acid are kept separately; on pressing the knob, the capsule breaks, they react and CO₂ is released forcefully:

In modern extinguishers, liquid or solid CO₂ is filled under pressure; when the pressure is released it becomes gas and rushes out through the hose. CO₂ extinguishers do not cause corrosion and do not conduct electricity, so they are used for fires in electrical and electronic equipment. They are useful only for small fires.

• To make aerated (soda) drinks.
• Solid CO₂ (dry ice) is used in cold storage and to keep milk products and frozen foods cool during transport, and to create a mist effect in dramas and movies.
• In fire extinguishers.
• Liquid CO₂ is used to remove caffeine from coffee, and as a solvent in eco-friendly dry cleaning.
• Plants use CO₂ from the air for photosynthesis.
• To make washing soda and baking soda.

Coal is a fossil fuel containing carbon, hydrogen and oxygen, with some nitrogen, phosphorus and sulphur. It is formed in stages, and there are four types:

1. Peat: the first stage in the formation of coal; contains a high proportion of water and less than 60% carbon, so it gives little heat.
2. Lignite: formed from peat by increased pressure and temperature inside the earth; contains 60–70% carbon.
3. Bituminous coal: the third stage; contains 70–90% carbon.
4. Anthracite: the purest form of coal; hard, and contains about 95% carbon.

Uses of coal: as a fuel in factories and homes; in thermal power plants to generate electricity; to obtain coke, coal gas and coal tar. Charcoal is used to purify water and organic materials.

Apparatus: pencil lead (graphite), a cell, a small bulb, connecting wires.

Procedure: remove the lead from a pencil. Connect the cell, the bulb and the pencil lead in series with wires, as shown, and close the circuit.

Observation: the bulb glows.

Inference: the circuit is complete, so electric current is flowing through the graphite. Graphite has free electrons in each of its layers, which carry the current. Hence graphite is a good conductor of electricity.

• Basic facts: symbol C, atomic number 6, atomic mass 12, electronic configuration 2, 4, valency 4; it is a non-metal.
• Allotropy: carbon occurs in many forms with the same chemical properties but different physical properties — crystalline (diamond, graphite, fullerene) and non-crystalline (coal, charcoal, coke).
• Covalent bonding: carbon completes its octet by sharing electrons and forms four covalent bonds.
• Catenation: carbon atoms join with one another by single, double or triple bonds to form long chains and rings. So carbon forms a very large number of compounds (hydrocarbons and other organic compounds).
• Solubility: carbon (coal powder) does not dissolve in water, kerosene or cooking oil; it is insoluble in common solvents.
• Reaction with oxygen: carbon burns in air to form carbon dioxide, giving out heat: C + O₂ → CO₂. The gas turns moist blue litmus faintly red, showing it is acidic.

Carbon occurs in the free state (diamond, graphite) and in the combined state (carbon dioxide, carbonates, fossil fuels, nutrients, fibres). Its free forms, the allotropes, are classified as follows:

First prepare CO₂ by adding dilute hydrochloric acid to pieces of marble or limestone (CaCO₃) and collect it in several gas jars by upward displacement of air:

Then perform the following tests, using a separate gas jar for each:

1. Colour: observe the gas — it is colourless.
2. Odour: waft the gas gently towards the nose — it has no smell.
3. Burning candle: lower a burning candle into the jar — the candle goes out. CO₂ does not burn or support burning.
4. Limewater: pour limewater into the jar and shake — it turns milky: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
5. Water: shake some water in the jar — CO₂ dissolves and forms carbonic acid: CO₂ + H₂O → H₂CO₃.
6. Universal indicator: add a few drops and shake — the colour shows a slightly acidic solution (pH a little below 7).
7. Litmus paper: drop moist blue and red litmus papers into the jar — blue litmus turns faintly red, red litmus does not change. CO₂ is acidic.
8. Density: since the gas is collected by upward displacement of air and can be poured from one jar into another like a liquid, it is heavier than air.

Model: use a large plastic container (or a big bottle) as the underground digester tank. Fix a funnel with a pipe near the bottom as the inlet, and a side outlet pipe for the spent slurry. Seal the top with a lid carrying a thin pipe with a tap for the gas outlet, attached to a balloon or tube. Fill it partly with a mixture of cow dung and water (1 : 1), close it airtight, and keep it in a warm place. After some days the balloon slowly inflates with gas.

Process for the presentation: the dung-and-water slurry enters the digester through the inlet chamber. In the absence of air, anaerobic microbes decompose it in two stages:

1. Production of acids: microbes break down complex biodegradable organic compounds into organic acids.
2. Production of methane: methanogenic bacteria act on these acids to form methane: CH₃COOH → CH₄ + CO₂.

The gas, containing 55–60% methane and the rest mostly carbon dioxide, collects at the top and is taken out through the gas pipe for cooking, lighting or producing electricity. The spent slurry flows out through the outlet chamber and is used as good manure.