Question

Fermentation of glucose produces ethyl alcohol and carbon dioxide.

C₆H₁₂O₆ (l) ------->2C₂H₅OH (l) + 2CO₂ (g)

What is the delta H (rxn) (in Kilojoules) if 678.5mL ofCO₂ was collected over water at 12degrees Cand 764.68 mm Hg? The vapor pressure of wateris 10.52 mm Hg.

Species delta H_f,KJ/mol

C₆H₁₂O₆ (l)-1,260.0

C₂H₅OH (l) -277.7

CO₂ -393.5

So I began by subtracting the water vapor from thetotalpressure.

764.68 mm Hg - 10.52 mm Hg = 754.16 mmHg

convert to atm: 754.16/760 = 0.992atm

n = PV/RT

n = (.992)(.6785 L) / (0.0821)(285K)

n = 0.0288 moles

delta H rxn = [ 2(-277.7) + 2(-395.5)] - [-1260]

= -86.4 KJ/mole

delta H rxn = -86.4 KJ/mole x 0.0288moles

= -2.49 KJ