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a. Propane is used as a fuel for barbecues - VCE - SSCE Chemistry - Question 3 - 2023 - Paper 1

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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 thermochemic... 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.

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Answer

To calculate the energy released from the combustion of propane (C₃H₈), we start with the balanced chemical equation:

C3H8(g)+5O2(g)→3CO2(g)+4H2O(l)\text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(l)

The standard enthalpy change (ΔH) for the combustion of propane is approximately -2220 kJ/mol. We can determine how many moles of propane are in 140.0 g:

Molar mass of C3H8=3(12.01)+8(1.008)=44.10 g/mol\text{Molar mass of C}_3\text{H}_8 = 3(12.01) + 8(1.008) = 44.10 \text{ g/mol}

Now, we can calculate the moles of propane:

n=140.0 g44.10 g/mol≈3.17 moln = \frac{140.0 \text{ g}}{44.10 \text{ g/mol}} \approx 3.17 \text{ mol}

The energy released can then be calculated using:

Energy=n×ΔH=3.17 mol×(−2220 kJ/mol)≈−7040kJ \text{Energy} = n \times \Delta H = 3.17 \text{ mol} \times (-2220 \text{ kJ/mol}) \approx -7040 kJ

Step 2

ii. Write a thermochemical equation for the complete combustion of propane at SLC.

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The thermochemical equation for the complete combustion of propane at standard laboratory conditions (SLC) is:

C3H8(g)+5O2(g)→3CO2(g)+4H2O(l)ΔH=−2220 kJ\text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(l) \quad \Delta H = -2220 \text{ kJ}

Step 3

iii. Sketch the energy profile for the complete combustion of propane.

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The energy profile for the complete combustion of propane should be sketched to show:

  1. Reactants at a certain energy level (initial).
  2. An activation energy (Ea) peak, representing the energy required to initiate the reaction.
  3. Products at a lower energy level than the reactants, indicating that the reaction is exothermic.

Label the axes clearly:

  • Y-axis: Energy
  • X-axis: Reaction Progress

The graph should show the overall decrease in energy from reactants to 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.

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Answer

In the incomplete combustion of propane, the energy profile would differ in the following ways:

  1. Higher products energy level: The products of incomplete combustion (such as carbon monoxide) would have a higher energy level than those of complete combustion, but still lower than the reactants.
  2. Less energy released: The overall energy change (ΔH) would be less negative (or more positive) compared to the complete combustion, indicating less energy is released due to the incomplete oxidation of reactants.

This is because the incomplete combustion does not fully utilize all the energy-producing components of propane, resulting in a distinct energy profile with increased energy levels for the products.

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