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The combustion reactions of methane and heptane can be studied in different ways - Scottish Highers Chemistry - Question 5 - 2019
Question 5
The combustion reactions of methane and heptane can be studied in different ways. (a) The combustion of methane produces carbon dioxide and water vapour when carrie... show full transcript
Worked Solution & Example Answer:The combustion reactions of methane and heptane can be studied in different ways - Scottish Highers Chemistry - Question 5 - 2019
Step 1
Using bond enthalpies and mean bond enthalpies from the data booklet, calculate the enthalpy change, in kJ mol⁻¹, for this reaction.
Answer
To calculate the enthalpy change, we consider the bond enthalpies:
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Identify Bonds in Reactants and Products:
- Reactants: Methane (CH₄) has bonds: 4 C-H bonds.
- Oxygen (O₂) has 2 O=O bonds for 2 moles, contributing 2
O=Obonds. - Products: Carbon dioxide (CO₂) has 2 C=O bonds, and water (H₂O) has 2 O-H bonds for 2 moles, contributing 4 O-H bonds.
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Calculate the total energy for bond breaking:
- Energy required =
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Calculate the total energy for bond formation:
- Energy released =
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Calculate enthalpy change:
- Enthalpy change (ΔH) = Energy required - Energy released = 2644 - 3338 = -694 kJ mol⁻¹.
Step 2
Explain the difference between bond enthalpy and mean bond enthalpy.
Answer
Bond enthalpy refers to the energy required to break a specific bond in a molecule, while mean bond enthalpy is the average energy required to break a bond of a specific type across different compounds. Bond enthalpy can vary based on the molecular environment, whereas mean bond enthalpy provides a general value based on typical bond breaking in similar bonds.
Step 3
Calculate the mass, in g, of carbon dioxide produced by combustion of 200 cm³ methane in excess oxygen.
Answer
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Calculate moles of methane:
- Using the molar volume of 24 liters for 1 mole, we find the number of moles for 200 cm³:
ext{Moles of CH₄} = rac{200}{24000} = 0.00833 ext{ moles}
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Determine mass of CO₂ produced:
- From the reaction, 1 mole of CH₄ produces 1 mole of CO₂, thus:
- Molar mass of CO₂ = 44 g/mol.
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Calculate the mass of CO₂:
Step 4
Step 5
Calculate the enthalpy of combustion, in kJ mol⁻¹, for heptane from the experimental results given.
Answer
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Calculate the heat absorbed by the water:
- Using the formula for heat transfer,
Where:
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m = mass of water = 400 g
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c = specific heat capacity of water = 4.18 J/g°C
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ΔT = Final temperature - Initial temperature = 49°C - 26°C = 23°C
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Thus,
or 38.44 kJ
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Calculate the moles of heptane burned: Using the mass of heptane burned (1.1 g) and molar mass (100 g/mol):
ext{Moles of heptane} = rac{1.1}{100} = 0.011 ext{ moles}
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Calculate enthalpy of combustion: Enthalpy change per mole = -q / moles of heptane burned:
ext{Enthalpy} = - rac{38.44 kJ}{0.011} = -3494 kJ/mol.
Step 6
The theoretical value for the enthalpy of combustion of heptane is significantly higher than the experimental value. Suggest why the experimental value is different from the theoretical value.
Answer
The experimental value may be lower due to various factors:
- Loss of heat/energy to the surroundings during the experiment.
- Incomplete combustion of heptane, leading to less heat produced than expected.
- Heat absorbed by the calorimeter or container itself, which is not accounted for in the calculations.
- Heat loss due to evaporation or other inefficiencies in the setup.