iGCSE Chemistry (0620) Theory (Core):3.3 The mole and the Avogadro constant: Exam Style Questions Paper 3- New Syllabus
Question
This question is about organic chemistry.
(i) State which physical property of hydrocarbons allows them to be separated by fractional distillation.
(ii) Describe how petroleum is separated into fractions by fractional distillation.
(iii) Name the process that breaks down long chain hydrocarbon molecules to form short chain hydrocarbon molecules.
(iv) Some of the fractions obtained from petroleum are shown.
Draw a line from each fraction to its use.
(b) Ethanoic acid is an organic molecule.
(i) Draw the displayed formula of a molecule of ethanoic acid.
(ii) Ethanoic acid reacts with sodium carbonate. Name the salt produced in this reaction.
(c) Ethane is burned in excess oxygen.
Name the two products of this process.
(d) Ethanol is another organic molecule.
State two ways to manufacture ethanol.
(e) Fig. 3.1 shows the displayed formula of an organic molecule.
(i) Deduce the molecular formula of the molecule in Fig. 3.1.
(ii) Explain why the molecule in Fig. 3.1 is unsaturated.
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 11.3 — Fuels (Parts (a)(i), (a)(ii), (a)(iii), (a)(iv))
• Topic 11.7 — Carboxylic acids (Parts (b)(i), (b)(ii))
• Topic 11.4 — Alkanes (Part (c))
• Topic 11.6 — Alcohols (Part (d))
• Topic 3.3 — The mole and the Avogadro constant (Part (e)(i))
• Topic 11.1 — Formulae, functional groups and terminology (Part (e)(ii))
▶️ Answer/Explanation
(a)(i)
For the correct answer:
different boiling points
Fractional distillation is a physical separation technique. It works because each hydrocarbon molecule in the petroleum mixture has a different chain length and molecular mass, which gives it a unique boiling point. The process separates them based on these differences.
(a)(ii)
For the correct answer:
1. petroleum is heated / vapourised / evaporated
2. fractionating column has a temperature gradient / column is hot at bottom and cold at top
3. fractions condense (when they get to their boiling points)
4. fractions are piped off as liquids
The hot vaporised crude oil enters the fractionating column. The long-chain hydrocarbons with high boiling points condense immediately at the hot bottom, while the short-chain hydrocarbons with low boiling points rise as vapours to the much cooler top before condensing. This allows the continuous collection of different fractions at different levels.
(a)(iii)
For the correct answer:
cracking
Cracking is a thermal decomposition reaction. It uses high temperature and a catalyst to break the strong C-C bonds in large, less useful alkane molecules, converting them into smaller, more useful molecules like short-chain alkanes (for petrol) and alkenes (for making polymers).
(a)(iv)
For the correct answer:
bitumen → making roads
lubricating oil → waxes and polishes
naphtha → chemical feedstock
Each fraction is a complex mixture of hydrocarbons with a specific range of boiling points. Bitumen is the thick, viscous residue, perfect for road surfacing. Lubricating oil is used for lubricants, waxes, and polishes. Naphtha is a vital chemical feedstock used to produce plastics, solvents, and other chemicals.
(b)(i)
For the correct answer:
A displayed formula showing all atoms and bonds: a $\mathrm{CH_3}$ group bonded to a $\mathrm{C}$ atom, which is double-bonded to an $\mathrm{O}$ atom and single-bonded to an $\mathrm{OH}$ group.
The molecular formula is $\mathrm{CH_3COOH}$. The functional group is the carboxyl group, -$\mathrm{COOH}$. The displayed formula is a 2D drawing that explicitly shows every single atom and every single covalent bond in the molecule. To gain full credit, the bond between the O and H in the hydroxyl group must be drawn.
(b)(ii)
For the correct answer:
sodium ethanoate
The reaction is: ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide. A salt is formed when the hydrogen ion of the acid is replaced by a metal ion. The metal here is sodium ($\mathrm{Na}^+$), and the acid anion is ethanoate ($\mathrm{CH_3COO}^-$). The salt is therefore sodium ethanoate ($\mathrm{CH_3COONa}$).
(c)
For the correct answer:
carbon dioxide and water
Ethane is an alkane with the formula $\mathrm{C_2H_6}$. The complete combustion of any hydrocarbon in excess oxygen always produces the same two products: carbon dioxide ($\mathrm{CO_2}$) from the carbon, and water ($\mathrm{H_2O}$) from the hydrogen. The balanced equation for this exothermic reaction is $2\mathrm{C_2H_6} + 7\mathrm{O_2} \rightarrow 4\mathrm{CO_2} + 6\mathrm{H_2O}$.
(d)
For the correct answer:
1. Fermentation (of aqueous glucose using yeast)
2. (Catalytic addition of) steam to ethene
Fermentation is a biological process using yeast to anaerobically convert sugars (e.g., glucose) into ethanol and carbon dioxide at $25$-$35^{\circ}\mathrm{C}$. The second, industrial method is an addition reaction: ethene ($\mathrm{C_2H_4}$) is reacted with steam ($\mathrm{H_2O}$) under high pressure ($6000\mathrm{kPa}$) and a moderate temperature ($300^{\circ}\mathrm{C}$) in the presence of a phosphoric acid catalyst.
(e)(i)
For the correct answer:
$\mathrm{C_7H_{12}O_4}$
The empirical formula is the simplest whole-number ratio of the atoms in a compound. To find it, divide the subscripts in the molecular formula by their highest common factor. For $\mathrm{C_7H_{12}O_4}$, the numbers 7, 12, and 4 share no common factor other than 1. Therefore, the empirical formula is identical to the molecular formula: $\mathrm{C_7H_{12}O_4}$.
(e)(ii)
For the correct answer:
unsaturated
An unsaturated compound is defined as an organic molecule that contains one or more carbon-carbon double bonds ($\mathrm{C=C}$) or triple bonds. The presence of a $\mathrm{C=C}$ double bond means the molecule does not have the maximum possible number of hydrogen atoms bonded to the carbon chain. This unsaturation allows for characteristic reactions like the decolorisation of bromine water.