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a. Propane is used as a fuel for barbecues - VCE - SSCE Chemistry - Question 3 - 2023 - Paper 1
Question 3
a. Propane is used as a fuel for barbecues. i. Calculate the amount of energy released when 140.0 g of propane is completely combusted. ii. Write a thermochemical ... show full transcript
Worked Solution & Example Answer:a. Propane is used as a fuel for barbecues - VCE - SSCE Chemistry - Question 3 - 2023 - Paper 1
Step 1
i. Calculate the amount of energy released when 140.0 g of propane is completely combusted.
Answer
To calculate the amount of energy released during the complete combustion of propane (C₃H₈), we first need to know the enthalpy of combustion per mole. The thermochemical data indicates that the standard enthalpy change for the combustion reaction of propane is -2220 kJ mol⁻¹.
The molar mass of propane is:
Using the formula: ext{Energy Released} = rac{ ext{Mass of Propane}}{ ext{Molar Mass of Propane}} imes ext{Enthalpy Change}
We can input the values: ext{Energy Released} = rac{140.0 ext{ g}}{44.10 ext{ g/mol}} imes (-2220 ext{ kJ/mol})
Calculating this gives us an energy release of approximately - 1400 kJ.
Step 2
ii. Write a thermochemical equation for the complete combustion of propane at SLC.
Answer
The thermochemical equation for the complete combustion of propane at standard laboratory conditions (SLC) can be expressed as:
ightarrow 3 ext{CO₂(g)} + 4 ext{H₂O(l)} \\[\Delta H = -2220 ext{ kJ}$$ This equation shows that one mole of propane reacts with five moles of oxygen, resulting in three moles of carbon dioxide and four moles of water, with the release of energy.Step 3
iii. Sketch the energy profile for the complete combustion of propane on the axes provided below.
Answer
To sketch the energy profile for the complete combustion of propane, the graph should show:
- The y-axis labeled as 'Energy'
- The x-axis labeled as 'Reaction Progress'
- Starting from a high energy level representing the reactants (C₃H₈ and O₂), the graph should show a downward slope indicating energy release as the reaction progresses towards products (CO₂ and H₂O), reaching a lower energy level at equilibrium.
- Ensure to label the energy levels of both the reactants and products as well as the activation energy peak.
Step 4
iv. State how the energy profile for the incomplete combustion of propane would differ from the diagram you drew in part a.iii. Justify your answer.
Answer
In the energy profile for incomplete combustion of propane, the graph would depict a higher energy level for the products compared to the complete combustion profile, indicating that the products (e.g., CO and C) possess more energy compared to carbon dioxide and water.
The energy release would be lower, as incomplete combustion does not fully oxidize the propane, resulting in less energy being released. This reflects the incomplete oxidation of reactants, which correlates with the lower energy transition and greater energy retention in the products.