Question

± Gibbs Free Energy: Temperature Dependence Gibbs free energy (G) is a measure of the spontaneity of a chemical reaction. It is the chemical potential for a reaction, and is minimized at equilibrium. It is defined as G=H−TS where H is enthalpy, T is temperature, and S is entropy.

The chemical reaction that causes aluminum to corrode in air is given by

4Al+3O2→2Al2O3

in which at 298 K

ΔH∘rxn	= −3352 kJ
ΔS∘rxn	= −625.1 J/K

Part A

What is the standard Gibbs free energy for this reaction? Assume the commonly used standard reference temperature of 298 K.

Express your answer as an integer and include the appropriate units.

Part B

What is the Gibbs free energy for this reaction at 5975 K ? Assume that ΔH and ΔS do not change with temperature.

Express your answer to two decimal places and include the appropriate units.

Part C

At what temperature Teq do the forward and reverse corrosion reactions occur in equilibrium?

Express your answer as an integer and include the appropriate units.

Answer 1

A)

we know that

dGo = dHo - TdSo

so

dGo = (-3352 x 1000) - ( 298 x -625.1)

dGo = -3165.72

so

standard gibbs free energy is -3165.72 kJ

B)

now

dG = dH - TdS

given

dH and dS do not change

and

T = 5975 K

so

dG = ( -3352 x 1000) - ( 5975 x -625.1)

dG = 382.9725 kJ

so

at 5975 K

gibbs free energy for this reactioon is 382.9725 kJ

C)

we know that

at equilibrium

dG= 0

so

dH - TdS= 0

dH = TdS

T = dH / dS

so

T = -3352 x 1000 / -625.1

T = 5362.34

so

at 5362.34 K , it will be at equilibrium