Daily Exam Challenge: Chemistry (Higher)
Mark Scheme — 20 marks total
Higher TierMarking Instructions
- • Accept equivalent answers unless the mark scheme states otherwise.
- • M marks are for method — award even if the final answer is wrong.
- • A marks are for accuracy — only award if the method mark has been earned.
- • B marks are independent and can be awarded without other marks.
- • Where a range of answers is acceptable, this is indicated in the mark scheme.
A student carries out a titration to find the concentration of a solution of sulfuric acid (H₂SO₄). 25.0 cm³ of 0.200 mol/dm³ NaOH is neutralised by 12.5 cm³ of H₂SO₄. 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O (a) Calculate the number of moles of NaOH used. (b) Calculate the number of moles of H₂SO₄ that reacted. (c) Calculate the concentration of the H₂SO₄ in mol/dm³. (d) Calculate the concentration of the H₂SO₄ in g/dm³.
Acceptable Answer
(a) Moles NaOH = 0.200 × 25.0/1000 = 0.005 mol (b) Ratio 2:1, so moles H₂SO₄ = 0.005/2 = 0.0025 mol (c) Concentration = 0.0025/(12.5/1000) = 0.0025/0.0125 = 0.200 mol/dm³ (d) Mr of H₂SO₄ = 2+32+64 = 98. Concentration = 0.200 × 98 = 19.6 g/dm³
Explain why the melting point of sodium chloride (801°C) is much higher than that of silicon dioxide which has a similar structure? Actually, silicon dioxide melts at 1713°C. Explain why SiO₂ has a higher melting point than NaCl, even though both have giant structures.
Acceptable Answer
Both NaCl and SiO₂ have giant structures requiring lots of energy to break. NaCl has ionic bonds — electrostatic attraction between Na⁺ and Cl⁻ ions. SiO₂ has a giant covalent structure where each silicon atom is covalently bonded to 4 oxygen atoms in a tetrahedral arrangement. Covalent bonds in SiO₂ are stronger than the ionic bonds in NaCl because covalent bonds involve shared electrons held tightly between two nuclei. More energy is needed to break the many strong covalent bonds throughout the SiO₂ structure, so its melting point is higher.
Iron can be extracted from iron oxide using carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂ (a) Identify what is oxidised and what is reduced. Explain your answer. (b) 500 tonnes of iron ore contains 70% Fe₂O₃ by mass. Calculate the maximum mass of iron that could be extracted.
Acceptable Answer
(a) Carbon monoxide (CO) is oxidised — it gains oxygen to become CO₂. Iron oxide (Fe₂O₃) is reduced — it loses oxygen. In terms of electrons: Fe³⁺ gains electrons to become Fe (reduction). Carbon in CO loses electrons going from C²⁺ to C⁴⁺ in CO₂ (oxidation). (b) Mass of Fe₂O₃ = 500 × 0.70 = 350 tonnes. Mr Fe₂O₃ = 160, Mr 2Fe = 112. Mass of Fe = (112/160) × 350 = 245 tonnes.