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The standard enthalpies of formation of gaseous propyne (C3H4), propylene (C3H6), and propane (C3H8) are +185.4, +20.4, and -103.8 kJ/mol, respectively.
(a) Calculate the heat evolved per mole on combustion of each substance to yield carbon dioxide and water.
(b) Calculate the heat evolved on combustion of 1 kg of each substance.
(c) Which is the most efficient fuel in terms of heat evolved?
The strings:
S7P6A66 (grouping/interaction - chemical).
The math:
Pj Problem of Interest is of type grouping/interaction (chemical).
(ai) Balanced chemical equation of propyne combustion:
C3H4(g) + 4O2(g) -------> 3CO2(g) + 2H2O(g)
ΔH0reaction = ΣnΔH0f(products) - ΣmΔH0f(reactants)
Where ΔH0 = standard enthalpy change at standard pressure (1 atm) and temperature 250C (298 K).
ΔHf = enthalpy of formation
n is coefficients of products and m is coefficients of reactants.
Given Enthalpies Of Formation:
ΔH0f of C3H4 = +185.4 kJ/mol
ΔH0f of O2 = 0 (since this is the most stable state of oxygen)
ΔH0f of CO2(g) = -393.5 kJ
ΔH0f of H2O(g) = -241.8 kJ
From Equation for standard enthalpy of reaction:
ΔH0reaction = -3(393.5) -2(241.8) - (185.4) =-1180.5-483.6-185.4 = -1849.5 kJ/mol.
(aii) Balanced chemical equation of propylene combustion
2C3H6(g) + 9O2(g) -------> 6CO2(g) + 6H2O(g)
Simplified equation
C3H6(g) + (9/2)O2(g) -------> 3CO2(g) + 3H2O(g)
From Equation for standard enthalpy of reaction:
ΔH0reaction = -3(393.5) -3(241.8) - (20.4) = -1180.5-725.4-20.4 = -1926.3 kJ/mol.
(aiii) Balanced chemical equation of propane combustion
C3H4(g) + 5O2(g) -------> 3CO2(g) + 4H2O(g)
From Equation for standard enthalpy of reaction:
ΔH0reaction = -3(393.5) -4(241.8) - (-103.8) = -1180.5-967.2+103.8 = -2043.9 kJ/mol
(bi) Divide by molar mass of each substance
Molar mass of C3H4(g) = 36 + 4 = 40 g = 0.040 kg
So, for C3H4(g), heat evolved per kg = -1849.5/0.040 = -4.62 x 104 kJ/kg.
(bii)
Molar mass of C3H6(g) = 36 + 6 = 42 g = 0.042 kg
So, for C3H4(g), heat evolved per kg = -1926/0.042 = -4.59 x 104 kJ/kg.
(biii)
Molar mass of C3H8(g) = 36 + 8 = 44 g = 0.044 kg
So, for C3H8(g), heat evolved per kg = -2043.9/0.044 = -4.65 x 104 kJ/kg.
(c) Propyne, propylene and propane release approximately the same amount of heat per unit mass, however, heat released by propane is marginally the highest.
The point . is a mathematical abstraction. It has negligible size and a great sense of position. Consequently, it is front and center in abstract existential reasoning.
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