The heat of combustion of ethanol is 1367 kJ mol⁻¹. In a first-hand investigation to determine the heat of combustion of ethanol, the experimental value determined differed from the theoretical value. (a) Identify a reason for this difference. (b) Calculate the theoretical mass of ethanol required to heat 200 mL of water from 21.0°C to 45.0°C.

SSCE HSC Chemistry Polymers: The heat of combustion of ethano

The heat of combustion of ethanol is 1367 kJ mol⁻¹ - HSC - SSCE Chemistry - Question 17 - 2005 - Paper 1

Question 17

The heat of combustion of ethanol is 1367 kJ mol⁻¹. In a first-hand investigation to determine the heat of combustion of ethanol, the experimental value determined d... show full transcript

Worked Solution & Example Answer:The heat of combustion of ethanol is 1367 kJ mol⁻¹ - HSC - SSCE Chemistry - Question 17 - 2005 - Paper 1

Step 1

Identify a reason for this difference.

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Answer

The difference between the experimental and theoretical values may be attributed to heat loss to the surroundings during the combustion process. In a laboratory setting, not all the heat generated by the combustion of ethanol is absorbed by the water; some may be dissipated into the environment, leading to a lower measured value.

Step 2

Calculate the theoretical mass of ethanol required to heat 200 mL of water from 21.0°C to 45.0°C.

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Answer

To calculate the mass of ethanol required, we can use the formula:

$q = mcΔT$

Where:

$q$ is the heat absorbed by water
$m$ is the mass of water
$c$ is the specific heat capacity of water (4.18 kJ/kg°C)
$ΔT$ is the change in temperature.

Calculate the change in temperature: $ΔT = 45.0°C - 21.0°C = 24.0°C$
Calculate the heat absorbed by the water:
- The mass of 200 mL of water is 0.2 kg. $q = (0.2 ext{ kg}) imes (4.18 ext{ kJ/kg°C}) imes (24.0°C) = 20.096 ext{ kJ}$
Knowing the heat of combustion of ethanol is 1367 kJ/mol, we find the moles of ethanol required:
- From $q = n imes ext{heat of combustion}$ : $n = rac{q}{ ext{heat of combustion}} = rac{20.096 ext{ kJ}}{1367 ext{ kJ/mol}} \approx 0.0147 ext{ mol}$
Finally, we convert moles of ethanol to mass. The molar mass of ethanol (C₂H₅OH) is about 46.07 g/mol: $ext{mass} = n imes ext{molar mass} = 0.0147 ext{ mol} imes 46.07 ext{ g/mol} ≈ 0.678 ext{ g}$

Thus, the theoretical mass of ethanol required is approximately 0.678 g.

The heat of combustion of ethanol is 1367 kJ mol⁻¹ - HSC - SSCE Chemistry - Question 17 - 2005 - Paper 1

Worked Solution & Example Answer:The heat of combustion of ethanol is 1367 kJ mol⁻¹ - HSC - SSCE Chemistry - Question 17 - 2005 - Paper 1

Identify a reason for this difference.

Calculate the theoretical mass of ethanol required to heat 200 mL of water from 21.0°C to 45.0°C.

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