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WJEC GCSE Additional Science Teacher’s Notes Relative formula mass of calcium carbonate = 40 + 12 + (3 × 16) = 100 Relative formula mass of calcium oxide = 40 + 16 = 56 So 100 g of calcium carbonate makes 56 g of calcium oxide. 1 g of calcium carbonate makes 0.56 g of calcium oxide 10 g of calcium carbonate makes 5.6 g of calcium oxide Percentage yield 5 100 89.3 % 5.6 20. The combustion of ethanol: C 2 H 5 OH (l) + 3O 2 (g) → 2CO 2 (g) + 3H 2 O (g). Bonds broken: 5 × C–H = (5 × 412) = 2060 kJ 1 × C–C = (1 × 348) = 348 kJ 1 × C–O = (1 × 358) = 358 kJ 1 × O–H = (1 × 463) = 463 kJ Total energy in = 3229 kJ Bonds made: 4 × C=O = (4 × 743) = 2972 kJ 6 × O–H = (6 × 463) = 2778 kJ Total energy out = 5750 kJ Heat of combustion = 5750 – 3229 = 2521 kJ – EXOTHERMIC 21. The neutralisation of sodium hydroxide by hydrochloric acid: HCl (aq) + NaOH (aq) → NaCl (aq) + H 2 O (l) ERRATUM Calculation Error: The neutralisation reaction of sodium hydroxide by hydrochloric acid is exothermic. 22. The reaction of lithium with water: 2Li (s) + H 2 O (l) → 2LiOH (aq) + H 2 (g) ERRATUM Calculation Error: The reaction of lithium with water is exothermic. 23. The reaction of nitrogen and oxygen gas: N 2 (g) + O 2 (g) → 2NO (g) Bonds broken: 1 × NN = 1 × 944 = 944 1 × O=O = 1 × 496 = 496 Total energy in = 1440 kJ Bonds made: 2 × NO = 2 × 627 = 1254 kJ Total energy out = 1254 kJ Heat of reaction = 1254 – 1440 = –186 kJ – ENDOTHERMIC 42
WJEC GCSE Science Teacher’s Notes 12 Organic chemistry _Alkanes and alkenes (pages 132–35)______________ PRACTICAL Making alkenes from alkanes (pages 133–34) This experiment is based on a practical from the Royal Society’s ‘Practical chemistry’ website www.practicalchemistry.org/experiments/cracking-hydrocarbons,139,EX.html. Technical notes are available from the link above. Questions 1. Look at the structural formula in Figure 12.5. Is this chemical an alkane or an alkene Give a reason for your answer. It is an alkene because it has a carbon-carbon double bond. 2. Alkenes are more reactive than alkanes. Suggest a reason for this, using your knowledge of their chemical structure. The double bond can be broken quite easily to attach to another atom. The structure of an alkane is more stable. _What’s the difference between a thermoplastic and a thermoset (pages 136–37) TASK Why use this plastic (page 137) The answers to this task will depend on the plastic item chosen by the pupil. 44
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