(a) Sulfur dioxide is a pollutant in the air.
(i) State one source of sulfur dioxide in the __________
(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?
(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
(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.
(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.
(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 / volcanoes
Sulfur dioxide is primarily produced from combustion of sulfur-containing fuels in power plants and vehicles.
(a)(ii) Ans: substance which speeds up a reaction without being consumed
Catalysts provide an alternative reaction pathway with lower activation energy, increasing reaction rate.
(a)(iii) Ans: pH 4
Acid rain has pH below 5.6 due to dissolution of acidic gases like SO3 in rainwater.
(a)(iv) Ans: corrodes limestone/marble buildings
Acid rain reacts with calcium carbonate in building materials, causing erosion and structural damage.
(b) Ans: liquid
At -20°C (between melting point -73°C and boiling point -10°C), SO2 exists as a liquid.
(c) Ans: ammonium sulfate
2NH3 + H2SO4 → (NH4)2SO4; this salt provides nitrogen for plant growth.
(d) Ans: tungsten < nickel < iron < magnesium
Reactivity order based on observations: Mg reacts vigorously (most reactive), W shows no reaction (least reactive).
(a) Name the main ore of iron. [1]
(b) In a blast furnace used for the extraction of iron, carbon reacts with oxygen from the air to form carbon monoxide. Complete the chemical equation for this reaction. [2]
$$ ….. \rm{C} + \ldots \rightarrow 2\mathrm{CO} $$
(c) In the hotter parts of the furnace, carbon reacts with the iron(III) oxide present in the iron ore.
$$ 3\mathrm{C} + \mathrm{Fe}_2\mathrm{O}_3 \rightarrow 3\mathrm{CO} + 2\mathrm{Fe} $$
How does this equation show that carbon is oxidised? [1]
(d) Limestone is added to the blast furnace. The limestone is converted into calcium oxide and carbon dioxide. The reaction is endothermic.
$$ \mathrm{CaCO}_3 \stackrel{\text{heat}}{\longrightarrow} \mathrm{CaO} + \mathrm{CO}_2 $$
(i) What type of chemical reaction is this? [1]
(ii) What type of oxide is calcium oxide? Give a reason for your answer. [2]
(e) Iron is a metal. Give three physical properties that are characteristic of metals. [3]
(f) The structure of a compound of iron is shown.
Deduce the molecular formula of this compound to show the number of iron, carbon and oxygen atoms. [1]
▶️ Answer/Explanation
(a) Hematite (Fe2O3) is the main ore of iron. [1]
(b) $$ \underline{2}\mathrm{C} + \underline{\mathrm{O}_2} \rightarrow 2\mathrm{CO} $$ [1 mark for each blank]
(c) Carbon is oxidised because it gains oxygen to form CO (oxidation state increases from 0 to +2). [1]
(d)(i) Thermal decomposition (a compound breaks down into simpler substances when heated). [1]
(d)(ii) Basic oxide (1) because it reacts with acids to form salts (calcium is a metal, and metal oxides are basic). [1]
(e) Three characteristic properties of metals: [3]
1. Good electrical conductivity
2. Malleability (can be hammered into sheets)
3. High melting point (or ductility/lustrous appearance)
(f) From the structure: Fe2C9O9 (counts as 2 Fe, 9 C, and 9 O atoms). [1]