(a) Sulfur dioxide is a pollutant in the air.
(i) State one source of sulfur dioxide in the air. [1]
(ii) Sulfur dioxide is oxidised to sulfur trioxide in the air. Oxides of nitrogen act as catalysts for this reaction. What is meant by the term catalyst? [1]
(iii) Sulfur trioxide dissolves in rainwater to form acid rain. Which one of the following pH values could be the pH of acid rain? Draw a circle around the correct answer. [1]
pH 4 pH 7 pH 9 pH 13
(iv) State one adverse effect of acid rain on buildings. [1]
(b) Sulfur dioxide melts at –73 °C and boils at –10 °C. What is the physical state of sulfur dioxide at –20 °C? Explain your answer. [2]
(c) Excess sulfuric acid reacts with ammonia to make a salt which can be used as a fertiliser. State the name of the salt formed when excess sulfuric acid reacts with ammonia. [1]
(d) The table shows some observations about the reactivity of four metals with dilute sulfuric acid.
Use the information in the table to put the four metals in order of their reactivity. Put the least reactive metal first.
▶️ Answer/Explanation
(a)(i) Ans: Burning fossil fuels or volcanic eruptions
Sulfur dioxide is primarily released from combustion of sulfur-containing fuels (coal, oil) and volcanic activity.
(a)(ii) Ans: A substance that increases reaction rate without being consumed
Catalysts like nitrogen oxides speed up the oxidation of \(SO_2\) to \(SO_3\) without undergoing permanent change.
(a)(iii) Ans: pH 4
Acid rain is weakly acidic, typically with a pH between 4 and 5 due to dissolved sulfur trioxide (\(SO_3\)) forming sulfuric acid.
(a)(iv) Ans: Corrodes limestone/marble buildings
Acid rain reacts with calcium carbonate (\(CaCO_3\)) in stone, causing erosion and structural damage.
(b) Ans: Liquid state
At –20 °C (between melting point –73 °C and boiling point –10 °C), sulfur dioxide exists as a liquid.
(c) Ans: Ammonium sulfate (\((NH_4)_2SO_4\))
Neutralization of sulfuric acid (\(H_2SO_4\)) with ammonia (\(NH_3\)) produces ammonium sulfate, a common fertiliser.
(d) Ans: Tungsten < Nickel < Iron < Magnesium
Reactivity order based on observations:
• Magnesium: Rapid reaction with bubbles.
• Iron: Slow reaction with bubbles.
• Nickel: Very slow reaction (few bubbles).
• Tungsten: No reaction.
Coal gas is made by heating coal in the absence of air. The list shows the main gases present in coal gas.
- carbon dioxide
- carbon monoxide
- ethene
- hydrogen
- methane
- nitrogen
(a)(i) Which one of these gases is an alkane?
(a)(ii) Draw the structure of a molecule of ethene. Show all of the atoms and all of the bonds.
(a)(iii) Describe how aqueous bromine can be used to tell the difference between methane and ethene.
(b) Ethene molecules react with each other to form poly(ethene).
(b)(i) What is the name given to this type of chemical reaction?
(b)(ii) Which one of the following words describes the ethene molecules in this reaction?
Draw a circle around the correct answer.
elements mixtures monomers polymers
(b)(iii) Poly(ethene) is a non-biodegradable plastic.
What is meant by the term non-biodegradable?
(b)(iv) Describe one pollution problem caused by non-biodegradable plastics.
(c) Ethanol can be made from ethene and one other reactant.
● Name the other reactant.
● State the conditions needed to make ethanol from ethene.
▶️ Answer/Explanation
(a)(i) Ans: methane
Methane (CH4) is the only alkane in the list, as alkanes are saturated hydrocarbons with single bonds.
(a)(ii) Ans: 
Ethene (C2H4) has a double bond between the two carbon atoms, with each carbon bonded to two hydrogen atoms.
(a)(iii) Ans: Bromine water remains orange with methane but decolorizes with ethene
Ethene reacts with bromine (addition reaction), turning orange bromine water colorless, while methane (saturated) does not react.
(b)(i) Ans: polymerisation / addition
Ethene monomers undergo addition polymerization to form poly(ethene), a long-chain polymer.
(b)(ii) Ans: monomers
Ethene molecules are monomers—small units that join together to form the polymer poly(ethene).
(b)(iii) Ans: Cannot be decomposed by organisms
Non-biodegradable plastics persist in the environment as microbes lack enzymes to break their strong C-C bonds.
(b)(iv) Ans: Blocks animal digestive tracts or drains
Accumulated plastics harm wildlife (e.g., choking birds) and clog drainage systems, causing environmental hazards.
(c) Ans: Reactant – steam; Conditions – high temperature and catalyst (phosphoric acid)
Ethanol is produced via hydration of ethene: C2H4 + H2O → C2H5OH, requiring 300°C and a catalyst.