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A bomb calorimeter can be used for accurate determination of the heat change during combustion of a fuel - AQA - A-Level Chemistry - Question 5 - 2020 - Paper 3

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A bomb calorimeter can be used for accurate determination of the heat change during combustion of a fuel. A bomb calorimeter is a container of fixed volume that wit... show full transcript

Worked Solution & Example Answer:A bomb calorimeter can be used for accurate determination of the heat change during combustion of a fuel - AQA - A-Level Chemistry - Question 5 - 2020 - Paper 3

Step 1

Calculate the heat capacity (C<sub>cal</sub>) in kJ K<sup>-1</sup>

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Answer

To calculate the heat capacity, we need the amount of hexane burned:

Amount of hexane=2.00g86.09gmol−1=0.0232mol\text{Amount of hexane} = \frac{2.00 g}{86.09 g mol^{-1}} = 0.0232 mol

Using the heat produced:

q=4154kJ for 1 mol hexaneq = 4154 kJ \text{ for 1 mol hexane}

Thus:

Ccal=qΔT=4154kJ0.0232mol⋅12.4K=16.99kJK−1C_{cal} = \frac{q}{\Delta T} = \frac{4154 kJ}{0.0232 mol \cdot 12.4 K} = 16.99 kJ K^{-1}

Step 2

Calculate the heat change, in kJ mol<sup>-1</sup>, for this combustion reaction

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Answer

For octane: We first calculate the heat change:

ΔT=12.2°C\Delta T = 12.2 °C

Now, using the heat capacity derived:

q=CcalΔTq = C_{cal} \Delta T

Substituting:

q=6.52kJK−1⋅12.2K=79.66kJq = 6.52 kJ K^{-1} \cdot 12.2 K = 79.66 kJ

Now calculating per mole of octane:

Amount of octane=2.00g114.0gmol−1=0.0175mol\text{Amount of octane} = \frac{2.00 g}{114.0 g mol^{-1}} = 0.0175 mol

Thus heat change per mole:

Heat change per mole=79.66kJ0.0175mol=4544.57kJmol−1\text{Heat change per mole} = \frac{79.66 kJ}{0.0175 mol} = 4544.57 kJ mol^{-1}

Step 3

State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change

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Answer

The heat change calculated from the bomb calorimeter experiment is not an enthalpy change because it occurs at constant volume, while enthalpy change is defined at constant pressure.

Step 4

Calculate the percentage uncertainty in this use of the thermometer

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Answer

To calculate the percentage uncertainty:

Percentage uncertainty=0.1°C12.2°C×100=0.82%\text{Percentage uncertainty} = \frac{0.1 °C}{12.2 °C} \times 100 = 0.82 \%

Step 5

Suggest one change to decrease the percentage uncertainty while using the same thermometer

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Answer

One change could be to use a larger mass of fuel, which would result in a greater temperature change, thus reducing the percentage uncertainty.

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