Remarkably, a catalytic system that uses light energy to hydrolyze water into hy

Question

Remarkably, a catalytic system that uses light energy to hydrolyze water into hydrogen H_2(g) and O(g) has been devised (Science 334, 2011: 645-648) to capture and effi­ciently store solar energy. Hydrogen is "clean burning" in that it yields only H_2O. In this problem you will contrast the combustion of 1 kg H_ (g) to that of 1 kg of n-octane. Calculate the enthalpy, entropy, and free-energy change on burning 1 kg of H_2(g) to H_2O(/) at 25 degree C and 1 bar. Calculate the enthalpy, entropy, and free-energy change on burning 1 kg of w-octane(g) to H_2O(/) and CO_2(g) at 25 degree C and 1 bar.

Explanation / Answer

the reactions

H2 + 1/2O = H2O(g)

C8H18 + 25/2O2 = 8CO2 + 9H2O

so

a) for H2:

dH = Hprod - Hreact = (H2O) - (H2 + 1/2*O2) = -241.8 - (0 + 1/2*0) = - 241.8kJ/mol

dS = Sprod - Sreact = (H2O) - (H2 + 1/2*O2) = 188.7 - (130.5+205*1/2) = -44.3 J/K

dG = Gprod - Greact = (H2O) - (H2 + 1/2*O2) = -228.4 - (0 + 0 ) = -228.4 kJ/mol

b)

for octane

dH = Hprod - Hreact = (8CO2 + 9H2O) - (octane+ 25/2*O2) =( -393.5*8 + 9*-241.8 )- (-252.1 + 1/2*0) = -5072.1 kJ/mol

dS = Sprod - Sreact = (8CO2 + 9H2O) - (octane+ 25/2*O2) = (8*213.8 + 9*188.7 ) - (361.20 +205*1/2) = 2945 J/K

dG = dH - T*dS = -5072.1 /1000 - 2945*298 = -877615.0 J/mol = -877.61 kJ/mol

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