iGCSE Chemistry (0620) Theory (Core):11.5 Alkenes: Exam Style Questions Paper 3- New Syllabus
Question
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 11.1 — Formulae, functional groups and terminology (Parts a, b, d, f)
• Topic 3.2 — Relative masses of atoms and molecules (Part c)
• Topic 11.3 — Fuels: Petroleum fractions (Part e)
• Topic 11.5 — Alkenes: Cracking and addition polymerisation (Parts g, h)
▶️ Answer/Explanation
(a) $\mathrm{C}_{10}\mathrm{H}_{16}\mathrm{O}_2$
By counting the atoms in the displayed formula, there are 10 carbon atoms, 16 hydrogen atoms, and 2 oxygen atoms.
(b) C=C circled
An unsaturated compound contains one or more carbon-carbon double bonds. Circling any $\mathrm{C=C}$ double bond in the displayed formula correctly identifies the molecule as unsaturated.
(c)
| type of atom | number of atoms | relative atomic mass |
|---|---|---|
| hydrogen | 14 | $14 \times 1 = 14$ |
| oxygen | 3 | $3 \times 16 = 48$ |
relative molecular mass $= 132 + 14 + 48 = 194$
The relative molecular mass ($M_r$) is the sum of the relative atomic masses of all atoms present in one molecule of the compound.
(d) (compound) which contains carbon and hydrogen only
A hydrocarbon is an organic compound consisting exclusively of hydrogen and carbon atoms. Any compound containing other elements (e.g., oxygen) is not a hydrocarbon.
(e)(i) gasoline / petrol
Fractions at the top of the fractionating column consist of small, light molecules with weak intermolecular forces, resulting in the lowest boiling points.
(e)(ii) lubricating oil
Fractions collected near the bottom of the column contain large, heavy molecules that become entangled, leading to high viscosity (thickness).
(e)(iii) kerosene / paraffin
The kerosene/paraffin fraction is specifically used as aviation fuel for jet engines.
(f)(i) $\mathrm{C}_7\mathrm{H}_{16}$
Alkanes follow the general formula $\mathrm{C}_n\mathrm{H}_{2n+2}$. For heptane with 7 carbon atoms ($n=7$), the number of hydrogen atoms is $2(7)+2=16$.
(f)(ii) any value $-55$ to $-89$ ($^\circ$C) inclusive
Table 4.2 shows that melting points increase with chain length. Heptane ($\mathrm{C}_7$) melts at $-90^\circ$C and nonane ($\mathrm{C}_9$) at $-54^\circ$C. The melting point for octane ($\mathrm{C}_8$) is expected to lie between these two values.
(g)(i) cracking
Cracking is the process of breaking long-chain, less useful alkanes into smaller, more useful alkanes and alkenes using heat and a catalyst. It is an endothermic reaction.
(g)(ii)
The displayed formula shows a double bond between two carbon atoms, each of which is also singly bonded to two hydrogen atoms. The molecular formula is $\mathrm{C}_2\mathrm{H}_4$.
(h)(i) addition
Addition polymerisation occurs when monomers containing a carbon-carbon double bond (like ethene) join together, with the double bond opening up, without the loss of any atoms or molecules.
(h)(ii) Any two from:
- fills landfill
- accumulates in oceans
- forms toxic / acidic gases on burning
Plastics are non-biodegradable, persisting in landfills for centuries. They accumulate in marine environments, entangling wildlife and entering the food chain. Incineration can release harmful and toxic gases.