DP Chemistry (last assessment 2024)

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Question 18M.3.sl.TZ2.12

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Date May 2018 Marks available [Maximum mark: 3] Reference code 18M.3.sl.TZ2.12
Level sl Paper 3 Time zone TZ2
Command term Calculate, State, Suggest Question number 12 Adapted from N/A
12.
[Maximum mark: 3]
18M.3.sl.TZ2.12

The process of converting heat to electricity is limited by its thermal (Carnot) efficiency.

Thermal efficiency = temp. of steam at source (K)  −  temp. heat sink (K) temp. of steam at source (K) × 100

(a)

Calculate the thermal efficiency of a steam turbine supplied with steam at 540°C and using a river as the choice of sink at 23 °C.

[1]

Markscheme

« 813 K − 296 K 813 K  × 100» = 64 «%»

[1 mark]

(b)

Power plants generating electricity by burning coal to boil water operate at approximately 35% efficiency.

State what this means and suggest why it is lower than the thermal efficiency.

[2]

Markscheme

35% of chemical/potential energy available in coal is transformed to electricity/electrical energy

 

not all chemical energy from burning fuel transferred into heating water

OR

energy dispersed elsewhere/energy lost due to friction of moving parts

OR

heat loss to the surroundings

 

Accept “stored energy” for “potential energy”.

[2 marks]

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