Answers to questions on Energy sources

2. (a) Amount of carbon in 1000 g of anthracite = ((95/100) x 1000) ÷ 12.01 = 79.1 mol
Amount of hydrogen in 1000 g of anthracite = ((3/100) x 1000) ÷ 1.01 = 29.9 mol
Heat evolved from combusting carbon = 79.1 x 394 kJ = 31165.4 kJ
Heat evolved from combusting hydrogen (H₂) = (29.9/2) x 286 = 4275.7 kJ
Heat evolved from combusting 1 kg of anthracite = 31165.4 + 4275.7 = 35441 kJ
Specific energy = 35400 kJ kg^-1 (to 3 significant digits)

(b) Anthracite has a much higher specific energy than wood so weight for weight produces more than twice as much energy. However anthracite (coal) is a fossil fuel and is more dangerous to mine than growing and felling wood. Anthracite cannot be replaced and contributes to global warming by releasing carbon dioxide into the atmosphere. Wood is a renewable fuel and does not significantly alter the total amount of carbon dioxide in the atmosphere.

3. (a) Amount of energy emitted by 60 W light bulb = 800 x 1/683 = 1.17 W (J s^-1)
Efficiency of energy transfer = (1.17/188) x 100 = 0.62%

(b) Any five from:
Mining and processing of coal
Transportation of coal to power station
Heat transfer from combustion of coal to produce steam
Generation of mechanical energy to drive turbine
Transfer of mechanical energy to electrical energy in generator
Transfer of electricity through grid to the home
Transfer of electrical energy to light energy in light bulb.

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