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Derive the thermodynamic identity for G (equation 5.23), and from it the three partial derivative relations 5.24.

Short Answer

Expert verified

The expression for change in G is (μdN-SdT+VdP) and the relations are

S=-∂G∂TP,N,V=∂G∂PT,Nandμ=∂G∂NT,P.

Step by step solution

01

Explanation

Write the expression for Gibbs free energy.

G=U-TS+PV

Here, G is Gibbs free energy, T is the absolute temperature, S is the entropy, P is the pressure and V is the volume.

Write the expression for the infinitesimal change in G.

dG=dU-TdS-SdT+PdV+VdP……..(1)

Write the expression for the infinitesimal change in U.

dU=TdS-PdV+μdN

Substitute (TdS-PdV+μdN)for dU in expression (1).

dG=μdN-SdT+VdP……..(2)

Rearrange expression (2) for constant P and constant N.

dG=-SdT

Rearrange the above expression.

S=-∂G∂TP,N
02

Calculation

Rearrange expression (2) for constant T and constant N.

dG=VdP

Rearrange the above expression.

V=∂G∂PT,N

Rearrange expression (2) for constant T and constant P.

dG=μdN

Rearrange the above expression.

μ=∂G∂NT,P

Thus, the expression for change in G is (μdN-SdT+VdP) and the relations are

S=-∂G∂TP,N,V=∂G∂PT,Nandμ=∂G∂NT,P.

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Most popular questions from this chapter

Plot the Van der Waals isotherm for T/Tc = 0.95, working in terms of reduced variables. Perform the Maxwell construction (either graphically or numerically) to obtain the vapor pressure. Then plot the Gibbs free energy (in units of NkTc) as a function of pressure for this same temperature and check that this graph predicts the same value for the vapor pressure.

Consider the production of ammonia from nitrogen and hydrogen,

N2+3H2→2NH3

at 298 K and 1 bar. From the values of ΔH and S tabulated at the back of this book, compute ΔG for this reaction and check that it is consistent with the value given in the table.

Consider a fuel cell that uses methane ("natural gas") as fuel. The reaction is

CH4+2O2⟶2H2O+CO2

(a) Use the data at the back of this book to determine the values of ΔHand ΔGfor this reaction, for one mole of methane. Assume that the reaction takes place at room temperature and atmospheric pressure.

(b) Assuming ideal performance, how much electrical work can you get out of the cell, for each mole of methane fuel?

(c) How much waste heat is produced, for each mole of methane fuel?

(d) The steps of this reaction are

at-electrode:CH4+2H2O→CO2+8H++8e-at-electrode:2O2+8H++8e-→4H2O

What is the voltage of the cell?

Use the Clausius-Clapeyron relation to derive equation 5.90 directly from Raoult's law. Be sure to explain the logic carefully.

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At temp 298K and pressure 1 bar.

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