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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 to calculate the enthalpy change
Answer
To calculate the enthalpy change, we first identify the bonds broken and formed in the reaction. The bonds broken in methane (C–H and O=O) and those formed in carbon dioxide (C=O) and water (O–H) must be taken into account. From the data booklet:
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Breaking:
- C–H bonds: 4 (there are 4 C–H bonds in CH₄, each with an enthalpy of 412 kJ/mol)
- O=O bonds: 2 (each with an enthalpy of 498 kJ/mol)
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Forming:
- C=O bonds: 2 (each with an enthalpy of 743 kJ/mol)
- O–H bonds: 4 (each with an enthalpy of 463 kJ/mol)
Steps:
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Calculate total energy for breaking bonds:
- Total breaking = (4 × 412) + (2 × 498) = 2644 kJ/mol
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Calculate total energy for forming bonds:
- Total forming = (2 × 743) + (4 × 463) = 3338 kJ/mol
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Finally, calculate the enthalpy change:
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 given compound, whereas mean bond enthalpy is the average energy needed to break a type of bond in various compounds. This average can vary due to different bonding environments and molecular structures.
Step 3
Calculate the mass, in g, of carbon dioxide
Answer
To find the mass of CO₂ produced, we first need to determine the moles of methane combusted:
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The volume of 200 cm³ of methane is converted to liters:
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Using ideal gas law, at standard conditions:
- 1 mole of gas occupies 24 L, so:
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From the balanced chemical equation, 1 mole of CH₄ produces 1 mole of CO₂. Thus, moles of CO₂ = 0.00833 mol.
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Now, we calculate the mass of CO₂ produced:
- Molar mass of CO₂ = 12 + (16 × 2) = 44 g/mol
Step 4
State the measurements required to calculate the mass of heptane burned
Answer
To calculate the mass of heptane burned, we require:
- The initial and final temperature of water to determine the temperature change.
- The volume of water used in the calorimeter experiment.
- The specific heat capacity of water (usually 4.18 J/g°C) to calculate the heat absorbed by the water.
Step 5
Calculate the enthalpy of combustion for heptane
Answer
The heat absorbed by the water can be calculated using:
where:
- is the mass of water (400 g),
- is the specific heat capacity of water (4.18 J/g°C),
- is the temperature change (Final - Initial = 49°C - 26°C).
Calculating: ( \Delta T = 49 - 26 = 23 \text{°C} ) ( q = 400 \cdot 4.18 \cdot 23 = 38440 J = 38.44 kJ )
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To find the enthalpy of combustion per mole:
- Moles of heptane = mass burned / molar mass = 1.1 g / 100 g/mol = 0.011 moles.
- Enthalpy change:
Step 6
Suggest why the experimental value is different from the theoretical value
Answer
The experimental value may be lower than the theoretical value due to several factors:
- Heat losses to the surroundings, which are not accounted for in the calculations.
- Inefficiencies in the calorimeter, such as incomplete combustion of heptane.
- Not all the released heat energy may be transferred to the water due to evaporation or heat absorbed by the container.