Teacher's notes and answers to questions in the book - Hodder Plus ...

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