Willard Gibbs defined a function known as Gibbs energy (G), to calculate the changes in entropy and enthalpy values. The maximum work done is the amount of energy produced, given by the decrease in the thermodynamic property called Gibbs free energy.
Gibbs Free energy formula is given below.
ΔG=Gibbs free energy
ΔH = change in enthalpy
ΔS=change in entropy
T=temperature
Gibbs free energy has extensive property and a function with a single value.
Example 1
Determine the standard Gibbs free energy change for the creation of methane from carbon and hydrogen at 298 K. Given ΔH = -74.9KJ/mol and ΔS = -80.7J/K.mol
Solution
change in enthalpy,ΔH = -74.9 KJ/mol
change in entropy,ΔS = -80.7J/Kmol
ΔS = -0.0807 KJ/Kmol (1KJ=1000J)
We have the equation
ΔG = ΔH – T ΔS
Substitute the above values in this equation
ΔG =( -74.9 ) – (298 ) (-0.0807)
ΔG = -50.9KJ/mole
Example 2
Determine the standard free energy change for the following reaction at 25oC.
N2 + 3H2 —> 2NH3
Given ΔH and ΔS are -91.8KJ and -198.0J/K
Solution
We have an equation
ΔG = ΔH – T ΔS
change in enthalpy,ΔH = -91.8 KJ/mol
change in entropy,ΔS = -198.0J/Kmol
ΔS = -0.198 KJ/Kmol (1KJ=1000J)
Substitute the above values in this equation
ΔG = (-91.8 )– (298 ) (-0.1980)
ΔG = -32.8KJ
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