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Students were asked to perform a first-hand investigation to determine the molar heat of combustion of ethanol - HSC - SSCE Chemistry - Question 17 - 2001 - Paper 1
Question 17
Students were asked to perform a first-hand investigation to determine the molar heat of combustion of ethanol. The following extract is from the practical report o... show full transcript
Worked Solution & Example Answer:Students were asked to perform a first-hand investigation to determine the molar heat of combustion of ethanol - HSC - SSCE Chemistry - Question 17 - 2001 - Paper 1
Step 1
After completing the calculations correctly, the student found that the answer did not agree with the value found in data books. Suggest ONE reason for this.
Answer
One possible reason for the discrepancy between the calculated value and the literature value could be heat loss to the surroundings during the experiment. In calorimetry, it is crucial to minimize heat exchange with the environment, as this can lead to inaccurate results.
Step 2
Propose TWO adjustments that could be made to the apparatus or experimental method to improve the accuracy of the results.
Answer
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Use a lid on the calorimeter to reduce heat loss to the environment. This would help to ensure that more heat is retained within the system and contributes to the heating of the water, leading to a more accurate measurement of temperature change.
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Place insulating material around the calorimeter to further minimize heat exchange between the calorimeter and the surroundings. This would prevent any heat from escaping or entering, thus leading to a more precise determination of the heat of combustion.
Step 3
Calculate the molar heat of combustion of ethanol, using the student’s data.
Answer
First, calculate the mass of ethanol burned:
Mass of ethanol burned = Initial mass of burner - Final mass of burner
Mass of ethanol burned = 221.4 g - 219.1 g = 2.3 g
Next, calculate the heat absorbed by the water:
Heat absorbed (q) = mass of water × specific heat capacity of water × change in temperature
Given that the specific heat capacity of water is approximately 4.18 J/g°C, calculate the temperature change:
Change in temperature = Final temperature - Initial temperature
Change in temperature = 59.0°C - 19.0°C = 40.0°C
Now, substituting into the equation for q:
q = 250.0 g × 4.18 J/g°C × 40.0°C
q = 41800 J
To find the molar heat of combustion, divide the total heat absorbed by the moles of ethanol burned:
Molecular weight of ethanol (C2H5OH) = 46.07 g/mol
Moles of ethanol burned = Mass of ethanol burned / Molar mass
Moles of ethanol burned = 2.3 g / 46.07 g/mol ≈ 0.0499 mol
Finally, calculate the molar heat of combustion:
Molar heat of combustion = Total heat absorbed / Moles of ethanol burned
Molar heat of combustion = 41800 J / 0.0499 mol ≈ 838000 J/mol or 838 kJ/mol.