J. R. S. PLEASE HELP

Question

The Standard enthalpies of formation of gaseous propyne (C3H4), propylene (C3H6), and propane (C3H8), were experimentally determined to be +136.1 kJ, +36.1 kJ, and -136.1 kJ respectively.

A complete, balanced equation for the complete combustion of each substance to yield carbon dioxide gas and water vapor is shown below:

𝐶3𝐻4(𝑔)+4𝑂2(𝑔)→3𝐶𝑂2(𝑔)+2𝐻2𝑂(𝑣)C3​H4​(g)+4O2​(g)​3CO2​(g)+2H2​O(v)

2𝐶3𝐻6(𝑔)+9𝑂2(𝑔)→6𝐶𝑂2(𝑔)+6𝐻2𝑂(𝑣)2C3​H6​(g)+9O2​(g)​6CO2​(g)+6H2​O(v)

𝐶3𝐻8(𝑔)+5𝑂2(𝑔)→3𝐶𝑂2(𝑔)+4𝐻2𝑂(𝑣)C3​H8​(g)+5O2​(g)​3CO2​(g)+4H2​O(v)

1. calculate the heat of combustion per mole of C3H4.
2. calculate the heat of combustion per mole of C3H6.
3. calculate the heat of combustion per mole of C3H8.
4. Calculate the heat evolved upon combustion of one kilogram of C3H4.
5. Calculate the heat evolved upon combustion of one kilogram of C3H6.
6. Calculate the heat evolved upon combustion of one kilogram of C3H8.

Answer 1

The approach and calculations are the same for all three. Doing the first one (C₃H₄) should hopefully be sufficient to help you do the other two on your own. If not, you can re-submit the question for the C₃H₆ and C₃H₈.

C₃H₄(g) + 4O₂(g) ==> 3CO₂(g) + 2H₂O(g)

(1). To calculate the heat of combustion per mole C₃H₄(g), we need the ∆Hf of all reactants and products. Then, we use the equation: ∆H_rxn = ∑∆Hf_products - ∑∆Hf_reactants

∆Hf C₃H₄ = +136.1 kJ/mol (given)

∆Hf O₂(g) = 0

∆Hf CO₂(g) = -394 kJ/mol (looked up in a table)

∆Hf H₂O(g) =-286 kJ/mol (looked up in a table)

∆H_rxn = ∑(3 * -394 + 2 * -286) - (136.1 + 0)

∆H_rxn = (-1182 + -572) - 136.1

∆H_rxn = -1754 - 136.1

∆H_rxn = -1890.1 kJ/mol = heat of combustion per mole C₃H₄

(4). To calculate the heat evolved upon combustion of 1 kg of C₃H₄, we first convert 1 kg to moles using the molar mass of C₃H₄ (40.06 g/mol), and then use the calculated ∆H_rxn as follows:

convert 1 kg C₃H₄ to moles: 1 kg x 1000 g / kg x 1 mole / 40.06 g = 24.96 mols C₃H₄

24.96 mols C₃H₄ x -1890.1 kJ/mol = -47,179 kJ of heat evolved from 1 kg of C₃H₄