CIE iGCSE Co-Ordinated Science C11.5 Alkenes Exam Style Questions Paper 3
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C11.1 — Formulas and terminology (Part (a)(i))
• Topic C11.5 — Alkenes (Part (a)(ii) & (b)(i))
• Topic C11.7 — Polymers (Part (b)(ii))
▶️ Answer/Explanation
(a)(i) Ethene contains only carbon and hydrogen atoms.
Hydrocarbons are compounds that consist exclusively of carbon and hydrogen. Ethene has the formula \(\mathrm{C_2H_4}\), so it fits this definition.
(a)(ii) Correct displayed formula of ethene:
The structure must show a double bond between the two carbon atoms, with each carbon atom bonded to two hydrogen atoms. The double bond is essential because ethene is an unsaturated hydrocarbon.
(b)(i) from orange to colourless
Aqueous bromine (bromine water) is orange-brown. When added to an alkene such as ethene, it undergoes an addition reaction. The bromine adds across the double bond, and the orange colour disappears, forming a colourless dibromo compound.
(b)(ii) poly(ethene) ; monomers ; addition
Ethene molecules can join together in a polymerisation reaction to form poly(ethene) (also called polythene). The small ethene molecules are the monomers. Since this polymerisation involves the opening of the double bond and joining without loss of any small molecule, it is an example of addition polymerisation.
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C11.5 — Alkenes (Part (a))
• Topic C11.7 — Polymers (Part (b)(iii))
• Topic C11.3 — Fuels, combustion (Part (b)(i))
• Topic C11.3 — Fuels, cracking (Part (b)(ii))
▶️ Answer/Explanation
(a)(i) 6 atoms
Ethene contains 2 carbon atoms and 4 hydrogen atoms.
Total atoms = \(2 + 4 = 6\)
(a)(ii) \(\text{C}_2\text{H}_4\)
The molecular formula of ethene is \(\text{C}_2\text{H}_4\), showing 2 carbon atoms and 4 hydrogen atoms per molecule.
It belongs to the alkene homologous series (general formula \(\text{C}_n\text{H}_{2n}\)).
(a)(iii) Ethene contains a C=C double bond.
Unsaturated molecules contain at least one carbon-carbon double bond (\(\text{C=C}\)).
This means not all carbon atoms are bonded to the maximum possible number of hydrogen atoms — they are not fully “saturated” with hydrogen.
(a)(iv)
test: Add aqueous bromine (bromine water).
observations (positive result): The orange/brown bromine water turns colourless.
The \(\text{C=C}\) double bond reacts with bromine in an addition reaction, decolourising the solution.
(b)(i)
1. Carbon dioxide (\(\text{CO}_2\))
2. Water (\(\text{H}_2\text{O}\))
Complete combustion of any hydrocarbon in excess oxygen produces only carbon dioxide and water.
(b)(ii) Cracking
Cracking is the process by which large hydrocarbon molecules from petroleum are broken down into smaller, more useful molecules.
It involves high temperatures (thermal cracking) or catalysts (catalytic cracking) to break C–C bonds.
(b)(iii) During polymerisation, many small ethene molecules (monomers) join together to form a single long-chain molecule (polymer).
The \(\text{C=C}\) double bond in each ethene monomer opens up, allowing the carbon atoms to bond to adjacent monomers.
This addition polymerisation produces poly(ethene), a long-chain polymer with repeating \(-\text{CH}_2\text{CH}_2-\) units.