Carbon and its Compounds

Coal, charcoal and soot are black, and they are all forms of carbon. The name comes from the Latin word carbo, meaning coal or charcoal. Carbon in these forms burns easily in air, giving heat and light; when something containing carbon burns without enough air, it gives thick black smoke.

This gives a simple test. If a substance chars black when heated, or burns with black, sooty smoke, it contains carbon. Paper, wood, sugar, cloth, plastic, kerosene and a candle all pass this test.

Carbon is a non-metal. Its atomic number is 6 (each atom has 6 protons and 6 electrons) and its atomic mass is 12. It is an unusual non-metal: one of its forms, graphite, conducts electricity, and another, diamond, is the hardest natural substance known. And today we use carbon in more ways than ever, from diamond-tipped drills to graphite in batteries. Above all, carbon is the main element of organic compounds, the compounds of living things.

When firewood burns, it gives black smoke. A candle flame blackens a spoon held over it. Sugar heated in a pan turns into black sugar charcoal. Each time, carbon that was hidden in the substance shows itself.

Carbon is found in every living thing: it is the main element in their bodies. So it is called the basic building block of life. Compounds of carbon are in our food, our clothes, the wooden table, paper and pens. Carbon is found in three ways:

As the element (free carbon)	In inorganic compounds	In organic compounds
coal, soot, charcoal, diamond, graphite, graphene	carbonates (limestone, marble), bicarbonates (baking soda), carbides (calcium carbide CaC₂, silicon carbide SiC); in minerals and rocks; gases such as carbon dioxide (CO₂) and carbon monoxide (CO)	carbohydrates, proteins, fats, vitamins, urea, medicines, petrol, silk, cotton, paper, soap; hydrocarbon gases such as methane, ethane and LPG

A carbon atom has 6 electrons: 2 in the first shell and 4 in the outer shell, the valence shell (its electron configuration is 2, 4). These four are its valence electrons. To become stable, an atom needs 8 electrons in its outer shell (an octet). Losing four electrons or gaining four would take too much energy, so carbon does neither. Instead, it shares its four outer electrons with other atoms, forming four covalent bonds. So the valency of carbon is four.

Carbon can form these bonds with hydrogen, oxygen, nitrogen, other carbon atoms and many other elements. That is why chemists know of millions of carbon compounds, many more than the compounds of all the other elements put together.

a. Catenation

Most elements can’t join with many atoms of their own kind. Carbon can: carbon atoms join to each other by covalent bonds in long chains, branched chains and rings. This property is called catenation (from the Latin catena, “chain”).

Butane, C₄H₁₀ (the main gas in the LPG cylinders in your kitchen), has a chain of four carbon atoms (Figure 18.1 b); pentane has five. In polythene and PVC (polyvinyl chloride, the plastic of water pipes), the chains are thousands of carbon atoms long.

Because of catenation, the number of carbon compounds is almost endless. They are so many that they have a branch of chemistry of their own: organic chemistry, the chemistry of carbon compounds.

b. Allotropes and allotropy

Coal, charcoal, soot, diamond, graphite and graphene are all made of carbon atoms only. They look and behave completely differently because their atoms are arranged differently. The different forms of the same element are called its allotropes, and the existence of an element in more than one form is called allotropy.

Allotropes of an element have different physical properties (color, hardness, conduction), but similar chemical properties: diamond, graphite and charcoal all burn in oxygen to give the same gas, carbon dioxide.

The allotropes of carbon are of two kinds:

Crystalline allotropes	Amorphous allotropes
atoms in a regular, repeating pattern	no regular pattern
diamond, graphite, fullerene (and graphene, a single layer of graphite)	coal, charcoal, soot (lamp black) (and coke)

Diamond, graphite and fullerene

In diamond, each carbon atom uses all four of its outer electrons to bond with four other carbon atoms. The bonds point in every direction, making a strong, rigid network through the whole crystal. There are no free electrons. So diamond is:

• the hardest natural substance;
• a bad conductor of electricity (no free electrons to carry a current);
• transparent and sparkling when cut.

In graphite, each carbon atom bonds with only three others, making flat sheets of hexagons, like chicken wire. The fourth electron of every atom is free to move along the sheet. The sheets lie on top of each other, held only by weak forces. So graphite is:

• a good conductor of electricity (and heat), thanks to its free electrons;
• soft and slippery: the sheets slide over each other easily. That is why pencil lead leaves a mark (sheets rub off onto the paper), and why graphite powder is used as a dry lubricant in locks and machines.

Fullerene (C₆₀, “buckminsterfullerene”) is a hollow ball of 60 carbon atoms, arranged in 20 hexagons and 12 pentagons, exactly like the patches of a football.

1. Carbon is found as a solid. Diamond, graphite and fullerene are its crystalline allotropes; coal, charcoal and soot are amorphous.
2. Carbon does not dissolve in water (nor in most other liquids).
3. On strong heating, carbon doesn’t melt at ordinary pressure: at about 3,600 °C it turns directly from solid to gas. (This is called sublimation.)
4. Most forms of carbon are black, but diamond is colorless and transparent.
5. Most forms are dull, but diamond sparkles and graphite has a dull gray shine.
6. Most forms are bad conductors of heat and electricity, but graphite is a good conductor. (Diamond, though it doesn’t conduct electricity, conducts heat extremely well.)
7. The density of carbon depends on the allotrope: from about 1.5 g/cm³ (amorphous forms such as charcoal) and 2.2 g/cm³ (graphite) to 3.5 g/cm³ (diamond).

1. Combustion

Carbon burns in oxygen. What it makes depends on how much oxygen there is.

With plenty of oxygen (complete combustion), carbon burns to carbon dioxide:

$$\text{C} + \text{O}_2 \longrightarrow \text{CO}_2 + \text{heat}$$

With too little oxygen (incomplete combustion), it gives carbon monoxide:

$$2\text{C} + \text{O}_2 \longrightarrow 2\text{CO} + \text{heat}$$

With even less air, some carbon doesn’t burn at all and goes up as black smoke: soot.

Carbon monoxide is a deadly gas. It has no color and no smell, so you can’t tell it is there. When you breathe it in, it sticks to the hemoglobin in your blood about 200 times more strongly than oxygen does, so your blood can no longer carry oxygen. First comes a headache and dizziness, then sleepiness, and then death.

2. Reaction with metals

Heated strongly with some reactive metals, carbon forms compounds called carbides.

$$\text{reactive metal} + \text{carbon} \xrightarrow{\text{heat}} \text{metal carbide}$$

Reaction	Carbide formed
4Al + 3C → Al₄C₃	aluminium carbide
3Fe + C → Fe₃C	iron carbide (cementite)
2Li + 2C → Li₂C₂	lithium carbide
Ca + 2C → CaC₂	calcium carbide

Iron carbide is what makes steel harder than pure iron. Calcium carbide gives off a gas called acetylene when it gets wet; it is sometimes misused, dangerously, to ripen mangoes and bananas in a hurry (the carbide often carries arsenic and phosphorus compounds as impurities).

3. Reducing property

Removing oxygen from a compound is called reduction, and a substance that removes oxygen is a reducing agent. Carbon is a good reducing agent. Heated with the oxides of less reactive metals, it pulls the oxygen away and leaves the metal behind:

$$\text{metal oxide} + \text{carbon} \xrightarrow{\text{heat}} \text{metal} + \text{carbon monoxide}$$

Reaction	Metal obtained
Fe₂O₃ + 3C → 2Fe + 3CO	iron
ZnO + C → Zn + CO	zinc
PbO + C → Pb + CO	lead

This is how iron has been made for 3,000 years: iron ore is heated with charcoal (today, with coke, a form of carbon made from coal) in a furnace.

Carbon also reacts with some other oxides at very high temperatures (about 2,000 °C, in an electric furnace) to form carbides and carbon monoxide:

$$\text{SiO}_2 + 3\text{C} \xrightarrow{\text{heat}} \text{SiC} + 2\text{CO}$$
$$\text{CaO} + 3\text{C} \xrightarrow{\text{heat}} \text{CaC}_2 + \text{CO}$$

Silicon carbide (SiC), made from sand and carbon, is nearly as hard as diamond: it is the grit on sandpaper and grinding wheels.

4. Reaction with steam

When steam is passed over red-hot carbon (coke) at about 1,000 °C, it forms carbon monoxide and hydrogen:

$$\text{C} + \text{H}_2\text{O} \xrightarrow{1000\,^\circ\text{C}} \text{CO} + \text{H}_2$$

This mixture of carbon monoxide and hydrogen is called water gas. Both gases burn, so water gas is used as a fuel in industry, and as a source of hydrogen for making ammonia and fertilizers (Unit 19).

Some compounds around us contain carbon and some don’t. Chemists sort compounds into two groups.

Organic compounds

Methane, ethane, methanol, urea, insulin, proteins, carbohydrates and oils are organic compounds. Organic compounds are the hydrocarbons (compounds of carbon and hydrogen only) and the compounds made from them. In almost every organic compound, carbon is bonded to hydrogen, and the atoms are joined by covalent bonds. Besides hydrogen and other carbon atoms, carbon in organic compounds is often bonded to oxygen, nitrogen, the halogens (such as chlorine), sulphur and phosphorus; a few organic compounds even contain metals (chlorophyll contains magnesium).

Carbon dioxide, carbon monoxide, carbonates, bicarbonates and carbides contain carbon, but they are counted as inorganic compounds: they behave like minerals.

Most organic compounds don’t dissolve in water: ghee, wax, petrol and kerosene float on water and don’t mix with it. Their molecules can’t form hydrogen bonds (a special attraction) with water molecules. Organic compounds that can form hydrogen bonds with water, such as sugar and alcohol, do dissolve. Most organic compounds dissolve in organic solvents, liquids such as ether, carbon tetrachloride, benzene, petrol and kerosene.

Inorganic compounds

Inorganic compounds are all the compounds that are not organic. Carbon is not needed in them: they are made from all sorts of elements. Most inorganic compounds are held together by electrovalent (ionic) bonds; some have covalent bonds, and some have both. Inorganic acids, bases and salts are inorganic compounds, and so are the oxides of carbon, carbonates, bicarbonates and carbides.

Most inorganic compounds dissolve in water, because in water they split up (dissociate) into ions: sodium chloride (NaCl), hydrochloric acid (HCl) and sodium hydroxide (NaOH), for example. But not all: silver chloride (AgCl), barium sulphate (BaSO₄) and calcium carbonate (CaCO₃, limestone) hardly dissolve in water at all.

Differences between organic and inorganic compounds

Basis	Organic compounds	Inorganic compounds
physical state	found as solids, liquids and gases	mostly solids; a few liquids (water, hydrochloric acid) and gases (CO₂, H₂S)
chemical bond	covalent bonds	mostly electrovalent (ionic) bonds; sometimes covalent
ionization	mostly don’t split into ions	many split into ions in water
solubility	most don’t dissolve in water but dissolve in organic solvents	most dissolve in water but not in organic solvents
melting and boiling points	low	high
combustibility	most burn easily (ghee, oil, petrol, LPG)	most don’t burn (salt, limestone)

From the moment you wake up, you are surrounded by organic compounds:

1. Food: dal, rice, vegetables, fish, meat, milk, curd and fruits contain carbohydrates, proteins, fats and vitamins, all organic compounds.
2. Clothes: cotton, silk, wool and jute are natural organic materials; nylon, polyester and acrylic are organic compounds made in factories.
3. Medicines: most medicines, from paracetamol to antibiotics, are organic compounds.
4. Energy: petrol, diesel, kerosene, LPG and spirit are organic fuels, used in vehicles, kitchens and lamps.
5. Farming: crops such as rice, dal, vegetables and fruits are made of carbohydrates, proteins and vitamins; and much of what farmers use, such as many insecticides, weedkillers and animal feed, is organic too.
6. Cosmetics: soaps, shampoos, creams, lipsticks and perfumes are mostly organic compounds.

Add to these plastics, paper, rubber, paints, dyes and the ink in your pen.

1. Choose the correct answer

2. Differentiate between

3. Give reasons

4. Answer the following questions