Exam-useful questions from this chapter, grouped by marks, each with a full model answer. Try writing the answer yourself first, then check it against this one.
1 1-Mark Questions
Q. What is a covalent bond?
Answer: A covalent bond is a bond formed by the sharing of a pair of electrons between two atoms, so that both attain a completely filled outermost shell.
Q. What is the atomic number and valency of carbon?
Answer: Carbon has atomic number 6 and a valency of four (it has four valence electrons).
Q. Name the hardest substance known and state how many carbon atoms it bonds each atom to.
Answer: Diamond is the hardest substance known; each carbon atom in it is bonded to four other carbon atoms.
Q. Define catenation.
Answer: Catenation is carbon’s unique ability to form bonds with other carbon atoms, giving rise to long chains, branched chains or rings.
Q. What is the common name of ethanoic acid, and what is its 5-8% water solution called?
Answer: Ethanoic acid is commonly called acetic acid; a 5-8% solution of it in water is called vinegar.
Q. Why is ethanoic acid also called glacial acetic acid?
Answer: Pure ethanoic acid has a melting point of 290 K, so it often freezes into ice-like crystals during winter in cold climates, which is why it is called glacial acetic acid.
Q. What is a catalyst?
Answer: A catalyst is a substance that causes a reaction to occur, or to proceed at a different rate, without the reaction itself changing the catalyst.
Q. Name two oxidising agents mentioned in the chapter.
Answer: Alkaline potassium permanganate and acidified potassium dichromate.
Q. What are soap molecules chemically?
Answer: Soap molecules are sodium or potassium salts of long-chain carboxylic acids.
Q. What is a micelle?
Answer: A micelle is the structure formed when soap molecules cluster in water, with their hydrocarbon (hydrophobic) tails pointing toward an oil droplet and their ionic (hydrophilic) ends facing outward into the water.
2 2-Mark Questions
Q. Why do carbon compounds generally have low melting and boiling points?
Answer: The covalent bond within a carbon compound’s molecules is strong, but the force of attraction between different molecules is weak, so relatively little energy is needed to separate the molecules from each other, giving low melting and boiling points.
Q. State two ways diamond and graphite differ in their physical properties, and explain why, despite this, they are said to have the same chemical properties.
Answer: Diamond is extremely hard and does not conduct electricity, while graphite is soft, slippery and conducts electricity. They have the same chemical properties because both are made of nothing but carbon atoms; the difference is only in how those carbon atoms are bonded to each other.
Q. What is a homologous series? Name the functional group and give one example of a homologous series.
Answer: A homologous series is a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain, with successive members differing by a -CH₂- unit. Example: the alcohol series with the -OH functional group, such as methanol, ethanol, propanol and butanol.
Q. Distinguish between a saturated and an unsaturated carbon compound, with one example each.
Answer: A saturated compound has only single bonds between its carbon atoms and is normally not very reactive, e.g. ethane (C₂H₆). An unsaturated compound has a double or triple bond between carbon atoms and is more reactive, e.g. ethene (C₂H₄).
Q. What are structural isomers? Explain using butane as the example.
Answer: Structural isomers are compounds with the same molecular formula but different structures. Butane’s four carbon atoms (C₄H₁₀) can form either a straight-chain skeleton or a branched skeleton; both share the formula C₄H₁₀ but are different compounds, so they are structural isomers.
Q. Write the equation for the reaction between ethanol and sodium, and name the compound formed alongside hydrogen gas.
Answer: 2Na + 2CH3CH2OH → 2CH3CH2O-Na+ + H₂. The compound formed alongside hydrogen gas is sodium ethoxide.
Q. How is ethanol converted to ethene? Name the reagent, the temperature, and what that reagent does in this reaction.
Answer: Ethanol is heated at 443 K with excess concentrated sulphuric acid, which removes water from ethanol (CH₃CH₂OH → CH₂=CH₂ + H₂O). Concentrated sulphuric acid acts here as a dehydrating agent.
Q. What is esterification? Write the equation for ethanoic acid reacting with ethanol.
Answer: Esterification is the reaction of a carboxylic acid with an alcohol, using an acid catalyst, to form an ester and water. CH₃COOH + CH₃CH₂OH → CH₃COOC₂H₅ + H₂O.
Q. Why does soap not lather well in hard water?
Answer: Hard water contains calcium and magnesium salts. These react with soap to form an insoluble, curdy precipitate (scum) instead of foam, so more soap is needed before any lather forms.
Q. How are detergents different from soaps in the way they behave with hard water?
Answer: Detergents are usually sodium salts of sulphonic acids, or ammonium salts with chloride or bromide ions. Their charged ends do not form an insoluble precipitate with the calcium and magnesium ions of hard water, so detergents remain effective and foam well even in hard water, unlike soap.
3 4-Mark Questions
Q. Explain why carbon forms such a large number of compounds, covering both properties the chapter names.
Answer: Two properties of carbon explain this. (1) Catenation: carbon has a unique ability to bond with other carbon atoms, forming long chains, branched chains or rings, and the carbon-carbon bond is strong and stable, so these chains remain stable however long they are; no other element shows this to the same extent (silicon manages only 7-8 atoms before its hydrogen chains become very reactive). (2) Tetravalency: carbon has a valency of four, so each atom can bond to four other atoms, whether more carbon atoms or atoms of other elements such as oxygen, hydrogen, nitrogen, sulphur or chlorine. Carbon’s small size lets its nucleus hold the shared electron pairs strongly, so these bonds are also strong and stable.
Q. Describe the structures of diamond and graphite, and explain how their structures lead to their different properties.
Answer: In diamond, each carbon atom is bonded to four other carbon atoms, building a rigid three-dimensional network; this is why diamond is extremely hard and does not conduct electricity (all four valence electrons are used in bonding, leaving none free). In graphite, each carbon atom is bonded to only three other carbon atoms in the same plane, one of these three bonds being a double bond so that carbon’s valency is fully satisfied, and this forms hexagonal sheets that are stacked in layers; graphite is soft and slippery because the layers are held together only weakly and can slide over each other, and it conducts electricity, which is unusual for a non-metal.
Q. Explain the naming of carbon compounds using the chapter’s own 4-step method, illustrating each step with propane as the base chain.
Answer: Step 1: count the carbon atoms; three carbons give the base name propane. Step 2: a functional group present is shown with a prefix or suffix, e.g. -one for a ketone. Step 3: if that suffix starts with a vowel, the chain name’s final “e” is dropped first: propan(e) – e + one = propanone. Step 4: if the chain is unsaturated, the final “ane” is replaced by “ene” (double bond, propene) or “yne” (triple bond, propyne) instead.
Q. Write all three reactions of ethanoic acid described in the chapter, as balanced equations, and name what each produces.
Answer: (1) Esterification: CH₃COOH + CH₃CH₂OH → CH₃COOC₂H₅ + H₂O, producing a sweet-smelling ester and water. (2) Reaction with a base: NaOH + CH₃COOH → CH₃COONa + H₂O, producing sodium ethanoate (sodium acetate) and water. (3) Reaction with carbonates/hydrogencarbonates: 2CH3COOH + Na₂CO₃ → 2CH3COONa + H₂O + CO₂, and CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂, producing sodium acetate, water and carbon dioxide gas.
Q. Explain the mechanism of the cleaning action of soap, in steps.
Answer: Soap molecules have an ionic (hydrophilic) end that mixes with water, and a hydrocarbon (hydrophobic) tail that mixes with oil. Around a droplet of oily dirt, many soap molecules arrange themselves with their hydrocarbon tails buried in the oil and their ionic ends facing outward into the water; this cluster is called a micelle. Because their outward-facing ionic ends repel each other, the micelles stay suspended in the water as a colloid rather than clumping and separating out. Agitation (rubbing, shaking, or a washing machine’s motion) then physically pulls the whole micelle, dirt included, away from the fabric and into the water, so it can be rinsed away.
4 6-Mark Questions
Q. Ethanol and ethanoic acid are both common carbon compounds with very different properties. Compare them on: (a) their effect on litmus, (b) their reaction with sodium, (c) their reaction with sodium hydrogencarbonate, (d) their melting points and what these lead to, (e) one reaction each that is unique to the compound, (f) one everyday use of each.
Answer: (a) Ethanol is neutral to litmus; ethanoic acid turns blue litmus red, being acidic. (b) Both react with sodium to give hydrogen gas: ethanol gives sodium ethoxide, ethanoic acid gives sodium ethanoate. (c) Ethanol does not react with sodium hydrogencarbonate; ethanoic acid reacts with it to give brisk fizzing of carbon dioxide gas, which is the standard test to identify a carboxylic acid. (d) Ethanol’s melting point is 156 K, so it stays liquid at ordinary temperatures; ethanoic acid’s melting point is 290 K, so pure ethanoic acid freezes in a cold winter, earning it the name glacial acetic acid. (e) Ethanol alone undergoes dehydration to ethene at 443 K with concentrated sulphuric acid; ethanoic acid alone undergoes esterification with an alcohol to form a sweet-smelling ester. (f) Ethanol is used as a solvent in medicines such as tincture iodine and cough syrups; dilute ethanoic acid (as vinegar) is used as a preservative in pickles.
Q. Describe, with balanced equations, the four reactions of carbon compounds covered in Section 4.3 of the chapter: combustion, oxidation, addition and substitution, giving one example equation for each.
Answer: Combustion: carbon and its compounds burn in oxygen releasing heat and light, e.g. CH₄ + 2O2 → CO₂ + 2H2O + heat and light. Saturated hydrocarbons burn with a clean flame; unsaturated ones, or any hydrocarbon burnt with too little air, give a yellow, sooty flame. Oxidation: an oxidising agent such as alkaline potassium permanganate adds oxygen to a substance, e.g. it oxidises ethanol to ethanoic acid. Addition: an unsaturated hydrocarbon adds hydrogen in the presence of a nickel or palladium catalyst to become saturated, e.g. the industrial hydrogenation of vegetable oils. Substitution: in sunlight, chlorine replaces the hydrogen atoms of a saturated hydrocarbon one by one, e.g. CH₄ + Cl₂ → CH₃Cl + HCl.
5 Case Studies
(A) 1 mark What gas was evolved when sodium reacted with ethanol?
Answer: Hydrogen gas.
(B) 1 mark Name the other product formed in this reaction, besides hydrogen.
Answer: Sodium ethoxide (CH₃CH₂O-Na+).
(C) 2 marks What role does concentrated sulphuric acid play when ethanol is heated with it at 443 K?
Answer: It acts as a dehydrating agent, removing a molecule of water from ethanol to form ethene.
(D) 2 marks Why did the product of the second demonstration decolourise bromine water? Name the product.
Answer: The product is ethene (CH₂=CH₂), an unsaturated hydrocarbon with a double bond; unsaturated compounds react with and decolourise bromine water, while saturated compounds do not.
(A) 1 mark Which class of compound does liquid X most likely belong to?
Answer: A carboxylic acid (such as ethanoic acid), since it is acidic to litmus and reacts with NaHCO₃ to release CO₂.
(B) 1 mark Which class of compound does liquid Y most likely belong to?
Answer: An alcohol (such as ethanol), since it is neutral to litmus but still reacts with sodium to give hydrogen gas.
(C) 2 marks What is liquid Z most likely to be, and what structure would have formed in its cloudy mixture?
Answer: Liquid Z is most likely a soap solution; the cloudy, stable mixture forms because the soap molecules surround the oil droplets as micelles, keeping the oil suspended in the water instead of separating out.
(D) 2 marks Explain why liquid X reacts with sodium hydrogencarbonate but liquid Y does not.
Answer: Liquid X is a carboxylic acid, and carboxylic acids react with carbonates and hydrogencarbonates to release carbon dioxide gas along with a salt and water. Liquid Y is an alcohol, and alcohols do not react with sodium hydrogencarbonate at all, which is exactly why this reaction is used to distinguish the two experimentally.