The combustion of ethane, C2H6, produces carbon dioxide and liquid water at 25°C. — Thermodynamics Chemistry Question
Problem Context
The combustion of ethane, C2H6, produces carbon dioxide and liquid water at 25°C.
Write an equation for this reaction.
Model Answer
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l) (note: dividing all coefficients by 2 was given full credit)
Given that ∆H°comb for ethane under these conditions is -1560.5 kJ/mol ethane, calculate
i. ∆Hf° for ethane.
Model Answer
∆Hrxn = 4∆Hf(CO2) + 6∆Hf(H2O) – 2∆Hf(C2H6)
so –3121 = 4(–393.5) + 6(–285.8) – 2∆Hf(C2H6) and ∆Hf(C2H6) = –83.9 kJ·mol-1
ii. the bond energy of the C=O bond.
Model Answer
∆Hrxn = 2BEC–C + 12BEC–H + 7BEO=O – 8BEC=O – 12BEH–O
so –3121 = 2(347) + 12(413) + 7(495) – 12(464) – 8BEC=O and BEC=O = 6668 / 8 = 833 kJ.mol-1
Given ∆G° = –1467.5 kJ/mol, Calculate ∆S° for this reaction in J.mol-1.K-1.
Model Answer
∆Go = ∆Ho – T∆So so –1467.5 = –1560.5 – 298 ∆So
solving for ∆So yields
∆So = (1560.5 – 1467.5) / –298 = –312 J·K-1
Compared with combustion to form liquid water at 25 °C, how would combustion to form H2O(g) affect each of the following;
i. ∆H°combustion
Model Answer
The measured ∆Hcombustion is less negative because the heat of vaporization was not released.
ii. ∆S°combustion
Model Answer
The ∆Scombustion is more positive (less negative) because H2O(g) has greater entropy than H2O(l)
iii. ∆G°combustion
Model Answer
The ∆Gcombustion is less negative due to the combination of these two effects.