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Chemistry 231

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Chemistry 231

Chapter 3 – Enthalpy and Internal Energy: The Importance of State Functions

Let’s examine the pressure of any system as a function of T and V

- For a fluid system or a solid
- Isothermal Compressibility

- For solids and fluid systems
- The coefficient of thermal expansion

Assume that we will write pressure as a function of T and V

In general, we write U as a function of T and V

Examine the second partial derivative

O

O

O

O

C

C

F

F

50

120

100

40

30

80

20

60

10

40

0

20

10

0

20

20

30

40

40

60

50

A

T1, Vm,1, P1

B

Stirrer

Thermal insulation

Valve

is known as the Joule coefficient, J.

The partial derivative

The change in the internal energy under isothermal conditions is related to the Joule Coefficient

- In general

For an ideal gas

In general, we write H as a function of T and P

Define the constant pressure heat capacity, CP

Examine the second partial derivative

Thermal insulation

O

O

O

O

C

O

C

O

O

O

F

F

C

C

F

F

50

120

50

120

100

40

100

40

30

80

30

80

20

20

60

60

10

10

40

40

0

0

20

20

10

10

0

0

20

20

20

20

30

30

40

40

40

40

60

50

60

50

T2, P2, Vm,2

T1, P1, Vm,1

Porous Plug

is known as the Joule-Thomson coefficient, JT.

The partial derivative

The change in the enthalpy under constant pressure conditions is related to the Joule-Thomson Coefficient