Problem: 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.Calculate the heat evolved on combustion of 6 kg of each substance.

FREE Expert Solution

We are asked to calculate the heat evolved on combustion of 6 kg of each substance.


ΔH°rxn=ΔH°f, prod-ΔH°f, react


C3H4(g) + 4 O2(g) → 3 CO2(g) + 2 H2O(g).

ΔH°rxn=(3 mol CO2)-393.5 kJ1 mol CO2+(2 mol H2O)(-241.8 kJ1 mol H2O)           -(1 mol C3H4)+185.4 kJ1 mol C3H4+0

ΔH˚rxn = –1849.5 kJ/mol


Calculate for 6 kg C3H4

Molar mass C3H4 =40.07 g/mol

6 kg × 103  g 1 kg ×1849.5 kJmol×1 mol40.07 g 

= -2.77 x 105 kJ 


The heat evolved for 6 kg of C3H4 is -2.77 x 105 kJ 



C3H6(g) + 9/2 O2(g) → 3 CO2(g) + 3 H2O(g).

ΔH°rxn=(3 mol CO2)-393.5 kJ1 mol CO2+(3 mol H2O)(-241.8 kJ1 mol H2O)           -(1 mol C3H6)+20.4 kJ1 mol C3H6+0

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Problem Details

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.

Calculate the heat evolved on combustion of 6 kg of each substance.