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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 thermoche... 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 from the complete combustion of propane (C₃H₈), we use the enthalpy change ( ΔH) of the reaction.
For propane, the balanced equation for the complete combustion is:
The standard enthalpy change of combustion is given as ( ΔH = -2220 ext{ kJ mol}^{-1} ).
Next, we need to calculate the number of moles in 140.0 g of propane:
Number of moles of propane = ( rac{140.0 ext{ g}}{44.11 ext{ g mol}^{-1}} \approx 3.17 ext{ mol} ).
Now, using the enthalpy of combustion:
Energy released = moles × ΔH = ( 3.17 ext{ mol} imes (-2220 ext{ kJ mol}^{-1}) \approx -7035.4 ext{ kJ} ).
Thus, the energy released when 140.0 g of propane is completely combusted is approximately 7035.4 kJ.
Step 2
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, begin with the reactants (propane and oxygen) at a certain energy level. As the reaction proceeds to products (carbon dioxide and water), there will be a peak representing the activation energy before the energy level drops down to the products, indicating an exothermic reaction.
Label the y-axis as 'Energy' and the x-axis as 'Reaction Progress'. The graph should show an upward slope to the peak and then a downward slope to a lower energy level for the products.
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
The energy profile for the incomplete combustion of propane will appear similar but will have a higher energy level for the products compared to the complete combustion profile. This is because the products of incomplete combustion (such as carbon monoxide) have a higher energy than the products of complete combustion (carbon dioxide and water).
Additionally, the activation energy required for incomplete combustion may be lower than that for complete combustion, but it still results in a less exothermic reaction, as less energy is released due to the incomplete conversion of propane. In summary, the incomplete combustion profile shows higher energy products and a different activation energy.