Carbon and its Compounds

Carbon has four electrons in its outermost shell. Losing or gaining four electrons is energetically difficult, so carbon generally shares electrons with other atoms. Sharing electrons forms covalent bonds. In contrast, ionic bonding involves transfer of electrons, which is not the usual characteristic of most carbon compounds.

Catenation is the ability of an element to form covalent bonds with atoms of the same element, and carbon shows this property particularly strongly. Carbon–carbon bonds produce straight chains, branched chains, and rings, explaining the enormous number of carbon compounds. Isomerism means different structures with the same molecular formula; neutralisation is an acid-base reaction, and oxidation involves loss of electrons or addition of oxygen. Therefore, option B is correct.

Diamond and graphite are both made only of carbon, but their carbon atoms are arranged differently. Such different structural forms of the same element are called allotropes. Therefore, option B is correct. Isotopes have different numbers of neutrons in atoms of the same element, whereas diamond and graphite differ in atomic arrangement.

Diamond is an allotrope of carbon. Each carbon atom is covalently bonded to four other carbon atoms in a three-dimensional network, making diamond extremely hard. In contrast, graphite has layered arrangements of carbon atoms, and the layers can slide over one another, so it is soft.

An unsaturated hydrocarbon contains at least one carbon–carbon double bond \\(\mathrm{C=C}\\) or triple bond \\(\mathrm{C\equiv C}\\). Alkenes have a double bond and alkynes have a triple bond, so option A is correct. Hydrocarbons containing only carbon–carbon single bonds are saturated hydrocarbons; therefore, option B is not correct.

Ethene,
\(\mathrm{C_2H_4}\), has a carbon-carbon double bond,
\(\mathrm{C=C}\). Hydrocarbons containing a carbon-carbon double or triple bond are unsaturated; therefore, ethene is an unsaturated hydrocarbon (an alkene). In contrast, saturated hydrocarbons have only single carbon-carbon bonds.

Ethyne has the formula \(\mathrm{C_2H_2}\) and contains a carbon–carbon triple bond, \(\mathrm{C\equiv C}\). Hydrocarbons containing a carbon–carbon triple bond are called alkynes. In contrast, alkenes contain a double bond, not the triple bond present in ethyne.

All members of a homologous series have the same functional group and similar chemical properties. Their successive molecular formulas differ by a \\(\mathrm{-CH_2-}\\) unit, as in ethane \\(\mathrm{C_2H_6}\\) and propane \\(\mathrm{C_3H_8}\\). In isomerism, compounds have the same molecular formula but different structures, so it is not a homologous series.

Successive members of a homologous series differ by one –CH₂– unit in their molecular formula. The relative molecular mass of carbon is 12 and that of two hydrogen atoms is 2, so the increase is 12 + 2 = 14 units. This constant difference is a defining pattern of homologous series. Ten, two, and eighteen do not represent the mass of the added CH₂ group. Therefore, option A is correct.

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond \\(\mathrm{C=C}\\). Their IUPAC names generally end in “-ene”; for example, \\(\mathrm{C_2H_4}\\) is called ethene. In contrast, alkanes end in “-ane” and alkynes end in “-yne”.

Alcohols have a hydroxyl group (-OH) attached to a carbon atom. For example, ethanol (C₂H₅OH) contains an -OH group. The -COOH group is found in carboxylic acids, -CHO in aldehydes, and \(>C=O\) in ketones. Exam tip: an -OH group attached to carbon indicates an alcohol.

The characteristic functional group of carboxylic acids is the carboxyl group, \(-COOH\). It has a carbonyl part \(C=O\) and a hydroxyl part \(-OH\) attached to the same carbon atom. Thus, \(-OH\) alone is not the functional group of a carboxylic acid; it is characteristic of alcohols. For example, ethanoic acid \(CH_3COOH\) contains a \(-COOH\) group.

Ethanol has the molecular formula \(\mathrm{C_2H_5OH}\). It contains the hydroxyl functional group \(\mathrm{-OH}\), so it is an alcohol. Carboxylic acids contain the \(\mathrm{-COOH}\) group; for example, this group is present in ethanoic acid, not in ethanol.

The governing concept is the relationship between systematic IUPAC names and common names of carboxylic acids. Ethanoic acid has the formula CH₃COOH and is commonly known as acetic acid, so option A is correct. Methanoic acid, HCOOH, is called formic acid and is a different compound. Lactic acid and citric acid are also different organic acids with different structures and properties.

The governing concept is the composition and everyday use of carboxylic acids. Vinegar is mainly a dilute aqueous solution of ethanoic acid, CH₃COOH, which is also called acetic acid; therefore, option A is correct. Hydrochloric, sulphuric and nitric acids are strong mineral acids used in other contexts and are not the principal acid responsible for vinegar’s sour taste. The word “dilute” indicates that the acid is mixed with water.

The governing concept is the effect of alcohol on human health. Ethanol, when consumed excessively, can impair the brain and nervous system, reduce coordination and judgement, and cause serious long-term damage to organs such as the liver. It can also lead to dependence and other health problems. Thus, option A, harmful, is correct. The claims that it is always beneficial, has no effect, or only improves digestion ignore these established effects and are false.

In the liver, methanol is metabolised into toxic substances such as formaldehyde and formic acid. These can cause severe poisoning, loss of vision, and sometimes death. Therefore, option A is correct. Unlike option B, methanol is not a safe source of energy for the body.

In complete combustion, the carbon in a carbon compound reacts with sufficient oxygen to form carbon dioxide. For example, complete combustion of a hydrocarbon produces carbon dioxide and water. Carbon monoxide is generally formed during incomplete combustion when oxygen is limited.

A fuel requires a sufficient supply of oxygen for complete combustion. For example, complete combustion of a hydrocarbon fuel produces carbon dioxide and water. In contrast, less air may provide insufficient oxygen, causing incomplete combustion and forming carbon monoxide or soot.

When oxygen is insufficient, a carbon-containing fuel may not burn completely and can form carbon monoxide. This poisonous gas binds strongly with haemoglobin, reducing the transport of oxygen by blood. In contrast, complete combustion with sufficient oxygen mainly forms carbon dioxide.