Chemical Thermodynamics of Tin - Volume 12 - OECD Nuclear ...

32
II Standards, conventions and contents of the tables
gation of errors. In practice the parameters affected by the change between these two
standard state pressures are the Gibbs energy and entropy changes of all processes that
involve gaseous species. Consequently, changes occur also in the Gibbs energies of
formation of species that consist of elements whose reference state is gaseous (H, O, F,
Cl, N, and the noble gases). No other thermodynamic quantities are affected significantly.
A large part of the following discussion has been taken from the NBS tables of
chemical thermodynamic properties [1982WAG/EVA], see also Freeman [1984FRE].
The following expressions define the effect of pressure on the properties of all
substances:
⎛∂H
⎞ ⎛∂V
⎞
⎜ ⎟ = V − T⎜ ⎟ =
⎝ ∂p
⎠
T
T ⎝∂
⎠p
V(1 −αT)
(II.39)
2
⎛∂C
p ⎞ ⎛∂
V ⎞
⎜ ⎟ = −T
⎜ 2 ⎟
⎝ ∂p
⎠ ∂T
T ⎝ ⎠p
(II.40)
⎛∂S
⎞
⎛∂V
⎞
⎜ ⎟ = −V
α = −⎜ ⎟
⎝∂p⎠
∂T
T
⎝ ⎠p
(II.41)
⎛∂G
⎞
⎜ ⎟
⎝ ∂p
⎠T
= V
(II.42)
where α ≡
1 ⎛∂V
⎞
⎜ ⎟
V ⎝∂T
⎠p
(II.43)
For ideal gases, V = RT / p and α = R / pV = 1 /T. The conversion equations
listed below (Eqs. (II.44) to (II.51)) apply to the small pressure change from 1 atm to 1
bar (0.1 MPa). The quantities that refer to the old standard state pressure of 1 atm are
assigned the superscript (atm) , and those that refer to the new standard state pressure of
1 bar are assigned the superscript (bar) .
For all substances the changes in the enthalpy of formation and heat capacity
are much smaller than the experimental accuracy and can be disregarded. This is exactly
true for ideal gases.
(bar)
(atm)
ΔfH ( T) − ΔfH ( T) = 0
(II.44)
(bar)
(atm)
C ( T) − C ( T) = 0
(II.45)
For gaseous substances, the entropy difference is:
p
(atm)
(bar)
(atm)
S T − S T R ⎜ R
(bar) ⎟
p
⎛ p ⎞
( ) ( ) = ln = ln1.01325
⎝ p ⎠
= 0.1094 J·K –1·mol –1 (II.46)
This is exactly true for ideal gases, as follows from Eq. (II.41) with α = R / pV .
The entropy change of a reaction or process is thus dependent on the number of moles
of gases involved:
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012