CIE iGCSE Co-Ordinated Science C11.5 Alkenes Exam Style Questions Paper 3
Question
(a) Fig. 8.1 shows three hydrocarbon molecules X, Y and Z.
Hydrocarbon molecule X is unsaturated.
(i) Explain why molecule X is unsaturated.
▶️Answer/Explanation
Molecule X is unsaturated because it contains a carbon-carbon double bond (C=C). Unsaturated hydrocarbons have at least one double or triple bond between carbon atoms, which allows them to undergo addition reactions.
(ii) State the name of molecule X.
▶️Answer/Explanation
The name of molecule X is ethene.
(iii) An aqueous solution of element P is used to test if hydrocarbons X and Y are saturated or unsaturated. State the name of element P.
▶️Answer/Explanation
The name of element P is bromine.
(iv) Describe what, if anything, is observed when an aqueous solution of element P is mixed with hydrocarbon molecule X and with hydrocarbon molecule Y.
▶️Answer/Explanation
When an aqueous solution of bromine is mixed with hydrocarbon molecule X (ethene), the bromine solution will decolorize or become colorless, indicating that an addition reaction has occurred. When mixed with hydrocarbon molecule Y (a saturated hydrocarbon), the bromine solution will remain orange as no reaction occurs.
(b)(i) When hydrocarbon molecule Y is completely combusted in oxygen, carbon dioxide and water are made. Balance the symbol equation for this reaction.
\[ \text{CH}_4 + \ldots \text{O}_2 \longrightarrow \text{CO}_2 + \ldots \text{H}_2\text{O} \]
▶️Answer/Explanation
The balanced equation is:
\[ \text{CH}_4 + 2\text{O}_2 \longrightarrow \text{CO}_2 + 2\text{H}_2\text{O} \]
(ii) This reaction is an exothermic reaction. State what is meant by an exothermic reaction.
▶️Answer/Explanation
An exothermic reaction is a chemical reaction that releases energy in the form of heat to the surroundings.
(c) Ethanol has the formula \( \text{C}_2\text{H}_5\text{OH} \).
(i) Explain why ethanol is not a hydrocarbon.
▶️Answer/Explanation
Ethanol is not a hydrocarbon because it contains an oxygen atom in its structure (C2H5OH). Hydrocarbons consist only of hydrogen and carbon atoms.
(ii) State two methods of producing ethanol.
▶️Answer/Explanation
Two methods of producing ethanol are:
- Fermentation of sugars by yeast.
- Reaction of ethene with steam in the presence of a catalyst.