The following equipment was set up to measure the heat of combustion of an alkanol - HSC - SSCE Chemistry - Question 10 - 2017 - Paper 1
Question 10
The following equipment was set up to measure the heat of combustion of an alkanol.
Thermometer
Heat shield
Loose fitting lid
Conical flask containing water
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Worked Solution & Example Answer:The following equipment was set up to measure the heat of combustion of an alkanol - HSC - SSCE Chemistry - Question 10 - 2017 - Paper 1
Step 1
Black deposits were observed on the bottom of the conical flask.
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Answer
The presence of black deposits indicates incomplete combustion of the alkanol. This suggests that not all of the fuel was combusted to produce carbon dioxide and water; instead, some carbon was likely formed as soot.
Step 2
The heat of combustion measured was lower than the theoretical value.
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Answer
Lower measured heat suggests that some of the energy was lost due to heat not being fully transferred to the water (due to heat losses to the surroundings) or that not all of the alkanol was combusted, hence not all potential energy was released.
Step 3
Which equation accounts for these observations?
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Answer
The correct equation that would lead to black soot formation and a lower heat of combustion is option B:
C3H8(g)+4O2(g)→CO2(g)+4H2O(g)
This represents incomplete combustion of the propane, leading to the formation of carbon (soot) instead of only carbon dioxide.
