Gases in the Atmosphere
6 marks · NormalThis question is about the nature of oxides and the greenhouse effect.
Magnesium and sulfur are burned separately in oxygen. State the type of oxide each element forms (acidic or basic) and explain your choice for each.
Carbon dioxide is released by burning fossil fuels and contributes to the greenhouse effect.
Describe how an increase in atmospheric carbon dioxide can affect global temperatures, according to the greenhouse effect.
Calculate the mass of CO₂ formed when 5.0 g of calcium carbonate are completely decomposed by heating. (Relative atomic masses: Ca = 40, C = 12, O = 16)
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(a) — · must show Magnesium = basic oxide; Sulfur = acidic oxide; Both with correct explanations about metal/non-metal and reaction with acids or alkalis
(b(i)) Increased CO₂ in the atmosphere traps more infrared radiation, causing average global temperatures to rise · must show CO₂ absorbs IR radiation, Energy re-emitted, warms atmosphere, Temperature increase
(b(ii)) 2.2 g · must show mole calculation for CaCO₃, correct use of equation: CaCO₃ → CaO + CO₂, finding mass of CO₂ produced
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Marking points
B1 (a) · 1 mark · Magnesium forms a basic oxide; sulfur forms an acidic oxide.
B2 (a) · 1 mark · Explanations relate to metallic character: metal oxides are basic (react with acids), non-metal oxides are acidic (react with bases/alkalis).
B3 (b(i)) · 1 mark · States that CO₂ absorbs IR radiation from the Earth's surface.
B4 (b(i)) · 1 mark · States/how this leads to increased atmospheric temperature (greenhouse effect described).
M1 (b(ii)) · 1 mark · Correct use of formula mass and mole calculation for CaCO₃ and CO₂. (follow-through)
A1 (b(ii)) · 1 mark · Correctly calculates mass of CO₂ evolved as 2.2 g (allow rounding to 2.2). (follow-through)
Worked solution
a) Magnesium is a metal. When it burns in oxygen, it forms magnesium oxide (MgO), which is a basic oxide because it reacts with acids to form salts. Sulfur is a non-metal. It forms sulfur dioxide (SO2), a non-metal oxide, which is acidic as it reacts with alkalis to form salts.
b)(i) Carbon dioxide is a greenhouse gas. An increase in atmospheric CO₂ leads to more absorption of infrared radiation emitted by the Earth’s surface. The CO₂ molecules re-emit this energy in all directions, causing the atmosphere to warm up and average global temperatures to increase.
b)(ii) Calculate the molar mass of CaCO₃: 40 + 12 + (16×3) = 100 g/mol. Moles of CaCO₃ = 5.0 g ÷ 100 g/mol = 0.05 mol. The decomposition reaction is: CaCO₃ → CaO + CO₂. Moles of CO₂ produced = 0.05 mol. Molar mass of CO₂ = 12 + (16×2) = 44 g/mol. Mass of CO₂ produced = 0.05 mol × 44 g/mol = 2.2 g.