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 withstands the change in pressure during the reaction. The fuel is mixed with pure oxygen in the calorimeter, ignited and the temperature change is recorded. The total heat capacity ($C_{cal}$) of the calorimeter is calculated using a fuel for which the heat change is known. In an experiment to calculate $C_{cal}$, 2.00 g of hexane (M$_{r}$ = 86.05) is ignited. A temperature change ($ riangle T$) of 12.4 °C is recorded. Under the conditions of the experiment, 1.00 mol of hexane releases 4154 kJ of energy when combusted. 1. Calculate the heat capacity ($C_{cal}$) in kJ K$^{-1}$ [3 marks] 2. When the experiment is repeated with 2.00 g of octane (M = 114.0) the temperature change recorded is 12.2 °C. Calculate the heat change, in kJ mol$^{-1}$ for this combustion reaction. [2 marks] 3. State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change. [1 mark] 4. The thermometer used to measure the temperature change of 12.2 °C in Question 05.2 has an uncertainty of ± 0.1 °C in each reading. Calculate the percentage uncertainty in this use of the thermometer. [2 marks] 5. Suggest one change to this experiment that decreases the percentage uncertainty while using the same thermometer.

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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

Question 5

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 with... 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

1. Calculate the heat capacity ($C_{cal}$) in kJ K$^{-1}$

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Answer

To determine the heat capacity of the calorimeter, use the formula:
$C_{cal} = \frac{q}{\Delta T}$
Here, the heat released ( $q$ ) is 4154 kJ for 1.00 mol, and the temperature change ( $\Delta T$ ) is 12.4 °C.
Since 2.00 g of hexane corresponds to:
$\text{Amount of hexane} = \frac{2.00 \, g}{86.05 \, g \, mol^{-1}} = 0.0232 \, mol$
Thus, the heat released for 2.00 g is:
$q = 0.0232 \, mol \times 4154 \, kJ = 96.5 \, kJ$
Substituting into the heat capacity equation:
$C_{cal} = \frac{96.5 \, kJ}{12.4 \, K} = 7.77 \, kJ \, K^{-1}$ .

Step 2

2. Calculate the heat change, in kJ mol$^{-1}$ for this combustion reaction.

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Answer

For the second part, when burning 2.00 g of octane:

Calculate the amount of octane:
$\text{Amount of octane} = \frac{2.00 \, g}{114.0 \, g \, mol^{-1}} = 0.0175 \, mol$
The temperature change is 12.2 °C.
Using the previously determined heat capacity:
$q = C_{cal} \times \Delta T = 7.77 \, kJ \, K^{-1} \times 12.2 \, K = 94.7 \, kJ$
The heat change per mole of octane:
$\text{Heat change per mole} = \frac{q}{\text{moles of octane}} = \frac{94.7 \, kJ}{0.0175 \, mol} = 5420 \, kJ \, mol^{-1}$ .

Step 3

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 the reaction occurs at constant volume rather than constant pressure.
Enthalpy pertains to heat changes at constant pressure.

Step 4

4. Calculate the percentage uncertainty in this use of the thermometer.

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Answer

To calculate the percentage uncertainty, use the formula:
$\text{Percentage uncertainty} = \left( \frac{\text{Uncertainty}}{\text{Measurement}} \right) \times 100$
Here, the uncertainty in measurement is ±0.1 °C, and the measurement is 12.2 °C:
$\text{Percentage uncertainty} = \left( \frac{0.1}{12.2} \right) \times 100 \approx 0.82\%$ .

Step 5

5. Suggest one change to this experiment that decreases the percentage uncertainty while using the same thermometer.

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Answer

One effective modification could be to increase the mass of the fuel used in the experiment.
A larger mass would produce a more significant temperature change, minimizing the percentage uncertainty of the measurement.

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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

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

1. Calculate the heat capacity ($C_{cal}$) in kJ K$^{-1}$

2. Calculate the heat change, in kJ mol$^{-1}$ for this combustion reaction.

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

4. Calculate the percentage uncertainty in this use of the thermometer.

5. Suggest one change to this experiment that decreases the percentage uncertainty while using the same thermometer.

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