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Gibbs Free Energy of Formation values

HI all,

I'm looking at the value for the gibbs energy of formation of water (to be -273.13kJ/mol) however when I compute the values for its enthalpy (-285.83kJ/mol) and entropy (+69.91J/mol)I dont get the same answer, just really confused here. I used the G=H-TS equation.

Also how is the gibbs free energy of hydrogen gas and oxygen gas zero when they have entropies values? enthalpy values are zero yes but what about the entropy ?
Original post by noodlemon
HI all,

I'm looking at the value for the gibbs energy of formation of water (to be -273.13kJ/mol) however when I compute the values for its enthalpy (-285.83kJ/mol) and entropy (+69.91J/mol)I dont get the same answer, just really confused here. I used the G=H-TS equation.

Also how is the gibbs free energy of hydrogen gas and oxygen gas zero when they have entropies values? enthalpy values are zero yes but what about the entropy ?


Part 1: The entropy change on forming water must be negative as you are going from 1.5 moles of gas to 1 mol of liquid. Your data is incorrect.

Absolute entropy values (at 298K): H2 =131 J K-1 ; O2= 205 J K-1; H2O = 69.9 J K-1

Hence ΔS = -163.6 J K-1

ΔG = ΔH - TΔS

ΔG = -285.8 --48.8 = -237 kJ



Part 2: Don't forget that you are talking about Gibbs free energy CHANGE of formation.

There is no entropy CHANGE in forming hydrogen gas or oxygen gas from itself!

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