Lesson 13: Carbon: An Important Element

Rule 1: Facts about carbon — Carbon: symbol C, atomic number 6, atomic mass 12, electron configuration 2, 4, valency 4; a nonmetallic element. The name comes from the Latin carbo = coal. It is about 0.27% of the earth's crust and about 0.03% of the atmosphere (as CO₂). It occurs free as diamond and graphite and combined in CO₂, carbonates, fossil fuels, carbohydrates, proteins, fats and fibres like cotton, wool and silk.

• Easy : Carbon has 2, 4 electrons, so its valency is 4.
• Medium : If the crust of 1000 kg of rock had 0.27% carbon, it would contain 1000 × 0.27 ÷ 100 = 2.7 kg of carbon.
• Exam-type : Sugar, cotton, wool and silk come from living things, so they are organic; marble (CaCO₃) comes from minerals and is called inorganic, even though it holds carbon.

Rule 2: Organic chemistry and allotropy — Compounds from plants and animals are organic, and those from minerals are inorganic. All organic compounds contain carbon, including DNA and RNA. Wöhler made urea from the inorganic ammonium cyanate, so organic chemistry is the chemistry of carbon compounds. In allotropy the forms have the same chemical properties but different physical properties (carbon, sulphur, phosphorus). Crystalline allotropes of carbon: diamond, graphite, fullerene.

• Easy : Diamond, graphite and fullerene are crystalline allotropes of carbon.
• Medium : Diamond burns to give only CO₂, which shows it is pure carbon.
• Exam-type : Sulphur has allotropes too, so allotropy is not only for carbon.

Spot the difference

In the figure : In the figure diamond and graphite are both pure carbon but the atoms are arranged differently.

Rule 1: Diamond — Each carbon is bonded to four others in a 3-D structure. It is the hardest natural substance with density 3.5 g/cm³ and melting point 3500 °C. It burns in oxygen at 800 °C to give only CO₂, is insoluble and unaffected by acids and bases, and is a bad conductor since it has no free electrons. Uses: glass cutting, rock drilling, ornaments, eye-surgery knives, polishing, radiation-protection windows. Found at Golconda and Panna.

• Easy : Diamond is the hardest natural substance.
• Medium : 10 cm³ of diamond has mass 10 × 3.5 = 35 g.
• Exam-type : Diamond does not conduct electricity because all four valence electrons of each carbon are held in bonds, leaving none free.

Rule 2: Graphite — Each carbon is bonded to three others in hexagonal layers. One layer is graphene. The layers slip over each other, so graphite is black, soft, brittle and slippery. Free electrons make it a good conductor. Density 1.9 to 2.3 g/cm³. Uses: pencil lead (mixed with clay), lubricant, electrodes, paints and polish, arc lamps.

• Easy : A pencil mark is made of graphite layers that slip off onto the paper.
• Medium : Graphite is a good conductor, unlike diamond, because of its free electrons.
• Exam-type : Graphite is used in electrodes and as a lubricant, because it conducts and its layers slide.

Rule 3: Fullerene — Rarely found, in soot and interstellar space. C₆₀ is buckminsterfullerene, named after the architect Richard Buckminster Fuller. Kroto, Curl and Smalley got the 1996 Nobel prize. Buckyballs and buckytubes have 30 to 900 carbon atoms. Soluble in organic solvents; used as insulators and catalyst in water purification; becomes a superconductor at a certain temperature.

• Easy : C₆₀ is a ball made of 60 carbon atoms.
• Medium : Fullerene was discovered in 1985 and its discoverers got the Nobel prize in 1996.
• Exam-type : 3 C₆₀ balls hold 3 × 60 = 180 carbon atoms.

Spot the difference

In the figure : In the figure the four types of coal are shown.

Rule 1: Coal, coke and charcoal — Coal is a fossil fuel with carbon, hydrogen and oxygen (and N, P, S). Four types by carbon content: peat (less than 60%, the first step), lignite (60 to 70%), bituminous coal (70 to 90%) and anthracite (about 95%, hard, purest). Charcoal comes from bones and horns, or from wood burnt in limited air. Coke is the pure coal left after coal gas is removed; it is a fuel and a reducing agent and is used to make water gas and producer gas.

• Easy : Anthracite has the most carbon (about 95%).
• Medium : In order of increasing carbon: peat, lignite, bituminous coal, anthracite.
• Exam-type : A sample of coal has 80% carbon, which lies in the 70 to 90% range, so it is bituminous coal.

Rule 2: Hydrocarbons — Hydrocarbons contain only carbon and hydrogen. Carbon shares electrons to form four covalent bonds (methane CH₄). Covalent compounds have low melting and boiling points, do not dissolve in water, dissolve in organic solvents and conduct poorly. Saturated hydrocarbons have only single bonds (ethane C₂H₆, propane C₃H₈). Unsaturated ones have a double or triple bond (ethene C₂H₄, ethyne C₂H₂, propene, propyne).

• Easy : Methane CH₄: 1 carbon, 4 hydrogen, four single bonds.
• Medium : Ethane C₂H₆ has only single bonds, so it is saturated; ethene C₂H₄ has a double bond, so it is unsaturated.
• Exam-type : Methane is 12 ÷ 16 × 100 = 75% carbon by mass (C = 12, H = 1, mass 16).

Spot the difference

In the figure : In the figure CO₂ turns limewater milky.

Rule 1: Occurrence and preparation — CO₂ is about 0.03% of air and about 4% of exhaled air. It is present in chalk, marble and limestone and is released on burning wood and coal. Prepared by CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. It turns limewater milky (CaCO₃ forms). With NaOH it gives sodium carbonate (washing soda), and with sodium carbonate solution it gives sodium bicarbonate (baking soda).

• Easy : Blow through a straw into limewater: it turns milky because exhaled air has CO₂.
• Medium : CO₂ mass = 12 + 2 × 16 = 44. In 44 g of CO₂ there are 12 g of carbon.
• Exam-type : 22 g of CO₂ is half of 44 g, so it holds 12 ÷ 2 = 6 g of carbon. Balanced: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ has Ca 1, C 1, O 3, H 2, Cl 2 on both sides.

Rule 2: Uses and fire extinguishers — Uses: aerated drinks, solid CO₂ (dry ice) for cold storage and mist effects, fire extinguishers, removing caffeine from coffee, dry cleaning, and photosynthesis. A fire extinguisher releases CO₂ when sulphuric acid meets sodium bicarbonate. CO₂ does not corrode or conduct electricity, so it suits electrical fires, but only small fires.

• Easy : Dry ice makes white mist at a stage show (handle it only with tongs).
• Medium : NaHCO₃ + H₂SO₄ gives CO₂ in a fire extinguisher.
• Exam-type : A small fire in an electric panel can be put out with a CO₂ extinguisher, since CO₂ does not conduct electricity. Water must not be used.

Spot the difference

In the figure : In the figure biogas comes from the anaerobic breakdown of waste.

Rule 1: Methane — Methane (mass 16) is 87% of natural gas. It forms by anaerobic decomposition and is found in biogas, coal mines and marshes (marsh gas). It is colourless, with melting point −182.5 °C and boiling point −161.5 °C; liquid methane is lighter than water, and the gas dissolves only sparingly in water. It burns with a bluish flame: CH₄ + 2O₂ → CO₂ + 2H₂O + heat. It reacts with chlorine in light (chlorination). It is an industrial and domestic fuel. Volta discovered it (1776 to 1778).

• Easy : Methane burns with a bluish flame.
• Medium : CH₄ + 2O₂ → CO₂ + 2H₂O: left 16 + 64 = 80 g; right 44 + 36 = 80 g. Mass is conserved.
• Exam-type : Methane's boiling point is −161.5 °C, so on a normal day it is a gas; in Kelvin it is 273 − 161.5 = 111.5 K.

Rule 2: Biogas — Biogas has 55 to 60% methane and the rest mostly CO₂. It is made by anaerobic microbes from dung, dry leaves and wet garbage in two stages: first organic acids form, then methanogenic bacteria make methane. The manure left over is a useful by-product.

• Easy : A biogas plant turns dung and kitchen waste into fuel and manure.
• Medium : 100 L of biogas with 60% methane has 100 × 60 ÷ 100 = 60 L of methane.
• Exam-type : 40 L of biogas with 55% methane has 40 × 55 ÷ 100 = 22 L of methane.

Spot the difference