Carbon has four valence electrons. Losing or gaining four electrons is energetically unfavourable, so it generally completes its outer shell by sharing electrons and forming covalent bonds.

Catenation is the ability of an element to form bonds with atoms of the same element, producing chains, branched chains or rings. Carbon exhibits extensive catenation.

The valency of carbon is four. It can form four covalent bonds.

The general formula of open-chain alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms.

The carboxyl group, –COOH, is present in ethanoic acid.

Ethanoic acid is commonly known as acetic acid.

Carbon forms a large number of compounds mainly because of:

1. Tetravalency: Carbon forms four covalent bonds with carbon and other elements.
2. Catenation: Carbon atoms bond strongly with one another to form long chains, branched chains and rings.

Saturated hydrocarbons: Contain only carbon–carbon single bonds. Example: ethane, C₂H₆.

Unsaturated hydrocarbons: Contain at least one carbon–carbon double or triple bond. Examples: ethene, C₂H₄, and ethyne, C₂H₂.

A homologous series is a family of organic compounds having the same functional group and the same general formula, with successive members differing by a –CH₂– unit.

• Successive members differ in molecular mass by 14 u.
• They have similar chemical properties, while physical properties show gradual changes.

Most carbon compounds are covalent. They generally do not produce free ions or mobile charged particles in solution, so they do not conduct electricity effectively.

• Ethanol contains the –OH functional group; ethanoic acid contains the –COOH functional group.
• Ethanoic acid reacts with sodium hydrogencarbonate to release carbon dioxide; ethanol does not give this characteristic reaction.

Hard water contains calcium and magnesium ions. These ions react with soap to form insoluble salts, which appear as a white curdy deposit called scum. Soap therefore produces less lather in hard water.

A substitution reaction is one in which an atom or group of atoms in a molecule is replaced by another atom or group. For example, methane reacts with chlorine in sunlight:

A hydrogen atom in methane is replaced by chlorine.

1. A soap molecule has a hydrophobic hydrocarbon tail and a hydrophilic ionic head.
2. The tail attaches to oily dirt while the ionic head interacts with water.
3. Soap molecules form micelles around oily dirt, allowing it to remain dispersed in water and be washed away during rinsing.

Ethanoic acid reacts with sodium hydrogencarbonate to produce sodium ethanoate, water and carbon dioxide gas.

The gas is carbon dioxide. It turns limewater milky due to the formation of calcium carbonate.

Esterification is the reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst, generally concentrated sulphuric acid, to form an ester and water.

The ester formed is ethyl ethanoate, which has a characteristic sweet, fruity smell.

Addition reaction: Atoms are added across a multiple bond in an unsaturated compound.

Substitution reaction: An atom in a compound is replaced by another atom.

Ethanol is oxidised to ethanoic acid by suitable oxidising agents, such as alkaline potassium permanganate or acidified potassium dichromate.

Here, [O] represents oxygen supplied by the oxidising agent.

Soaps react with calcium and magnesium ions in hard water to form insoluble scum. Detergents generally do not form these insoluble precipitates, so they continue to clean effectively in hard water.

One limitation is that some detergents, particularly poorly biodegradable types, can persist in the environment and contribute to water pollution.

Ethanol loses a molecule of water to form ethene. This is a dehydration reaction, which is an elimination reaction.

Conditions: excess concentrated H₂SO₄ and approximately 443 K.

1. Combustion: Carbon compounds burn in oxygen to release energy.

2. Oxidation: Ethanol can be oxidised to ethanoic acid.

3. Addition: Hydrogen adds across the double bond of ethene.

4. Substitution: Chlorine replaces hydrogen in methane in sunlight.

5. Esterification: An alcohol and a carboxylic acid react to form an ester and water.

For full marks, write the correct equations and state the relevant reaction conditions where applicable.

Properties:

• Ethanoic acid is also called acetic acid.
• A 5–8% solution of acetic acid in water is commonly called vinegar.
• It is a weak acid and turns blue litmus red.

Reaction with a metal: With sodium, it produces sodium ethanoate and hydrogen.

Reaction with a base: With sodium hydroxide, it forms a salt and water.

Reaction with sodium hydrogencarbonate:

Esterification: It reacts with ethanol in the presence of concentrated sulphuric acid to form ethyl ethanoate and water.

Structure: A soap molecule contains a long hydrophobic hydrocarbon tail and a hydrophilic ionic head.

Cleansing action:

1. The hydrophobic tail attaches to grease and oily dirt.
2. The hydrophilic head interacts with water.
3. Soap molecules surround the dirt to form micelles, with tails directed towards the oily material and ionic heads towards the water.
4. The dirt becomes dispersed in water and is removed during rinsing.

Action in hard water: Calcium and magnesium ions form insoluble salts with soap. These produce scum, reduce lather and decrease cleaning efficiency.

Detergents generally avoid this problem and work better in hard water.

Carbon has four electrons in its outermost shell. Each hydrogen atom has one electron and needs one more to achieve a stable duplet.

Carbon shares one electron with each of four hydrogen atoms. This forms four single covalent bonds. Carbon completes its octet, while each hydrogen completes its duplet.

Simplified structural representation:

Each C–H bond represents a shared pair of electrons. In an electron-dot diagram, show the shared pair between carbon and each hydrogen.

Distilled water lacks the significant concentrations of calcium and magnesium ions found in hard water, so less soap is consumed in forming insoluble salts and more lather develops.

The deposit is scum, formed when soap reacts with calcium or magnesium ions to produce insoluble salts.

Generally, no. Detergents do not form the same insoluble calcium and magnesium salts as ordinary soaps, so they can produce lather and clean effectively in hard water.

Liquid A is ethanol. It contains the alcohol functional group, –OH.

Ethanol reacts with sodium to produce sodium ethoxide and hydrogen gas.

Liquid B, ethanoic acid, gives brisk effervescence because it reacts with sodium hydrogencarbonate to release carbon dioxide gas. Ethanol does not give this characteristic reaction.