Question

Calculate the enthalpy, entropy, gibbs energy, and internal energy of combustion of cyclohexane.

Answer 1

Cyclohexane = C6H12

C6H12 + O2 --> CO2 + H2O

Balance

C6H12 + 9O2 --> 6CO2 + 6H2O

Enthalpy of Combstion:

H = Hprod- Hreact = (6HCO2 + 6HH2O) - (HC6H12 + 9*HO2) =  (6*−393.51+ 6*−241.826) - (–156.4 + 9*0) = -3655.616 kJ/mol

S = Sprod- Sreact = (6SCO2 + 6SH2O) - (SC6H12 + 9*SO2) = (6*213.6 + 6*188.7 ) - (204 + 9*205.0 ) = 364.8 J/molK

G = H-TS

G = -3655.616 kJ/mol - 298*(364.8 J/molK)/1000 J/kJ = -3764.31 kJ/mol

INTERNAL ENERGY

H = PV + U

U = H-PV

Pressure is constant and P = 1 atm

V = there is a change in volume ( 1l + 9g --> 6g+6g) the chang ein gas moles is 12-9 = 3

Therefore

dV = Vf-Vi = 12Vi - Vi = 3Vi

at STP, there are 22.4 liter per mol

the change of moles is 3, therefore 22.4*3= 67.2 Liters

PV = 1atm*246.4L = 101325Pa*0.0672m3= 6809.04 J or 6.8 kJ

U = H-PV = -3655.616 -6.8= -3662.416 kJ/mol