Carbon and Its Compounds: Bonding, Allotropes and PYQs

The correct answer is D, propane and butane. Liquefied petroleum gas is a mixture of these two saturated hydrocarbons, which are gases at ordinary pressure but turn liquid under moderate pressure, so a large quantity of fuel fits into a small cylinder; a strong-smelling compound is added so that a leak is noticed. Option A is wrong because ethene is an unsaturated gas used to make polythene and to ripen fruit, not as cooking fuel. Option B is wrong because ethyne, also called acetylene, is burnt with oxygen in welding torches for its very hot flame, and it is not supplied in domestic cylinders. Option C is wrong because propene is a raw material of the plastics industry, chiefly for polypropylene. Remember also that compressed natural gas, used in vehicles, is mainly methane, a different fuel from LPG.

The correct answer is D, five. Hexane, C6H14, can be drawn as a straight chain of six carbons, as two chains of five carbons with a methyl branch in different positions, and as two chains of four carbons carrying branches, which gives five different structures with the same molecular formula. Option A is wrong because three is the number of isomers of pentane, C5H12, and candidates who remember the smaller member often answer with it. Option B is wrong because six is not the count for any of the first few alkanes; the next member, heptane, has nine isomers. Option C is wrong because four corresponds to no alkane in this part of the series either. The sequence worth memorising is simple: butane two, pentane three, hexane five, heptane nine.

The correct answer is B, dienes. The prefix di stands for two and the ending ene for a carbon to carbon double bond, so a diene is a hydrocarbon carrying exactly two such bonds; buta-1,3-diene, the raw material of synthetic rubber, is the standard example. Option A is wrong because terpenes are a class of natural compounds built from isoprene units and found in the oils of plants; they may contain several double bonds but the name says nothing about the number. Option C is wrong because the ending yne refers to a triple bond, so a name of that kind would describe three triple bonds and not double bonds at all. Option D is wrong for the same reason: it would mean two triple bonds. Reading the prefix for the count and the ending for the kind of bond answers the whole family of such questions.

The correct answer is B, ethyne. An alkyne contains a carbon to carbon triple bond, which needs at least two carbon atoms, so the series begins with ethyne, C2H2, also called acetylene and used with oxygen in welding. Option A is wrong because propyne, with three carbon atoms, is the second member of the series, not the first. Option C is wrong because a compound called methyne cannot exist as an alkyne at all: methane has a single carbon atom and there is no second carbon for it to form a triple bond with, which is exactly the trap the option is meant to spring. Option D is wrong because butyne has four carbon atoms and is the third member. The general formula of the series is CnH2n-2, and putting n equal to two gives C2H2, confirming ethyne as the first member.

The correct answer is A, complete combustion of fuel. When enough air reaches the burner, every carbon atom of the fuel is oxidised fully to carbon dioxide and the flame is blue and hot, giving the maximum heat for the gas consumed. Option B is wrong because ordinary air contains nitrogen and oxygen with only traces of hydrogen, and the colour of the flame has nothing to do with hydrogen in the atmosphere. Option C is wrong because hazardous combustion is not a chemical term at all and describes nothing; the danger in a burner lies in incomplete burning, which releases poisonous carbon monoxide. Option D is wrong because insufficient oxygen produces the opposite result, a yellow sooty flame in which unburnt carbon particles glow, and the remedy is to clean the air holes of the burner so that more air is drawn in.

The correct answer is B, catenation. Catenation is the self-linking of atoms of the same element, and carbon shows it more strongly than any other element because the carbon to carbon bond is short and exceptionally strong, which is why chains of many carbon atoms remain stable. Option A is wrong because tetravalency is the separate property of having four bonds available, which lets carbon hold four other atoms; the two properties together explain the huge number of carbon compounds. Option C is wrong because isomerism is the existence of different structures with the same molecular formula, such as the two forms of butane, and it is a consequence of catenation rather than its name. Option D is wrong because allotropy is the existence of an element in different physical forms, such as diamond, graphite and fullerene.

The correct answer is B. In graphite every carbon atom is bonded to only three others within a layer, so the fourth valence electron of each atom is free to move along the layer, and moving electrons constitute an electric current. Option A is wrong because pure graphite conducts by itself; impurities are not the reason, and the same conduction is seen in synthetic graphite of high purity. Option C is wrong because diamond and graphite are both made of carbon alone, which is the whole point of calling them allotropes; only the arrangement of atoms differs. Option D is wrong because diamond actually has an extremely high melting point owing to its rigid three-dimensional network, and melting point in any case does not decide conduction. In diamond each atom is bonded to four others, so no electron is left free.

The correct answer is A, a CH2 unit and fourteen units of mass. Going from methane to ethane, or from ethanol to propanol, adds one carbon atom of mass twelve and two hydrogen atoms of mass one each, which is fourteen in all, and it is this fixed step that defines a homologous series. Option B is wrong because CH3 is a group, not the difference between successive members; a compound cannot differ from another by an odd fragment of this kind and remain a stable molecule of the same series. Option C is wrong because C2H4 would be a jump of two members at once, skipping one homologue. Option D is wrong because CH4 is the whole molecule of methane rather than the step of the series. Members of such a series share the same functional group and show a gradual change in boiling point and density.

The correct answer is B, an addition reaction. Vegetable oils contain unsaturated chains with carbon to carbon double bonds; hydrogen adds across those double bonds in the presence of nickel or palladium, turning them into single bonds, so the oil becomes saturated and sets into a solid fat. Option A is wrong because a substitution reaction replaces one atom or group of a saturated compound with another, as when methane reacts with chlorine in sunlight, and nothing is replaced here. Option C is wrong because a displacement reaction is one in which a more reactive element pushes a less reactive one out of its compound, which describes reactions of metals rather than this one. Option D is wrong because neutralisation is the reaction of an acid with a base to give salt and water, and no acid or base takes part in hydrogenation.

The correct answer is C, the carboxyl group. Ethanoic acid, better known as acetic acid and familiar as vinegar in dilute form, has the formula CH3COOH, and the -COOH group at the end is what makes it acidic, since it releases a hydrogen ion in water and turns blue litmus red. Option A is wrong because the hydroxyl group marks an alcohol such as ethanol, C2H5OH, which is neutral to litmus. Option B is wrong because the aldehyde group ends in -CHO, as in ethanal, and an aldehyde is easily oxidised to the corresponding acid but is not itself acidic. Option D is wrong because the ketone group lies between two carbon atoms, as in propanone or acetone, and is likewise not acidic. A useful pair to remember is that ethanol oxidises to ethanoic acid.

The correct answer is B, buckminsterfullerene. It is a molecule of sixty carbon atoms arranged in a closed cage of pentagons and hexagons, exactly like the panels of a football, and it is named after the architect who designed domes of the same geometry. Option A is wrong because graphite is built of flat hexagonal layers stacked over one another, which makes it soft and slippery rather than cage-like. Option C is wrong because coke is the impure carbon left when coal is heated in the absence of air and is used in extracting metals; it is not a crystalline allotrope with a definite molecular formula. Option D is wrong because charcoal is likewise an amorphous form of carbon obtained from wood, valued for adsorbing gases and colouring matter rather than for any regular structure.