Question

Be sure to answer all parts.

Calculate the heats of combustion for the following reactions. Use the standard enthalpies of formation.

(a) C₂H₄(g) + 3O₂(g) →2CO₂(g) + 2H₂O(l)   

__________kJ/mol

(b) 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)

__________kJ/mol

Answer 1

Standard enthalpy change in kJ/mol as follows:
Ethane C2H4= +52.47, Hydrogen sulfide H2S is - 52.47 , Sulfur dioxide SO2 is - 296.84,
Water liquid H2O is - 285.83,Carbon dioxide CO2 is - 393.51 and for Oxygen O2 is zero.

A]

ΔrH° = C₂H₄(g) + 3O₂(g) →2CO₂(g) + 2H₂O(l)

ΔrH° = 2·ΔfH°(CO2(g)) + 2 ·ΔfH°(H2O(l)) - ΔfH°(C2H4(g)) - 3 ·ΔfH°(O2(l))

= 2·(-393.52kJ/mol) + 2 ·( -285.83kJ/mol) - (52.47kJ/mol) - 3 ·(0kJ/mol)

ΔrH° = 2( - 393.51) + 2( - 285.83) - ( + 52.47) - 0
ΔrH° = - 1411.15 kJ/mol
Reaction is exothermic

B]

2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)

ΔrH° = 2H2O + 2SO2 - 2H2S - 3O2
ΔrH° = 2( - 285.83) + 2( - 296.84) - 2( - 20.63) - 0
ΔrH° = - 1124.08 kJ/mol
Reaction is exothermic