文档介绍：不可能把热从低温物体传到高温物体,而不引起其它变化
物理化学电子教案第3章(中)(添)
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热力学第二定律的本质和熵的统计意义
气体混合过程的不可逆性
将N2和O2放在一盒内隔板的两边,抽去隔板, N2和O2自动混合,直至平衡。
这是混乱度增加的过程,也是熵增加的过程,是自发的过程,其逆过程决不会自动发生。
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Concentrating on the system
Entropy is the basic concept for discussing the direction of natural changes. Nevertheless, in order to use it we have to investigate both the system and its surroundings. We have seen that it is always very simple to calculate the entropy change in the surroundings, and now we shall show that it is possible to devise a method of taking the surroundings into account automatically.
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This focuses attention on the system,
and simplifies thermodynamic discussions.
Consider a system in thermal equilibrium with its surroundings(so that ),
then the Clausius inequality reads
The importance of this inequality is that
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it expresses the criterion
for natural,spontaneous change solely in terms of the properties of the system.
From now on the superscript denoting the system will be dropped, and everything we do will relate to the system.
The last equation can be developed in two ways.
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First take the case where the heat
is lost from a constant volume system.
Then dQv can be identified with dU if there is no non-pV work involved. Putting this into
dS - dU/T ≧0, or 0 ≧ dU- T dS
dU≤TdS (const. V, no non-pV work)
When the heat is lost from the system under conditions of constant pressure and when no non-pV work is involved,
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the only modification is to identify
dQp with dH, the change of enthalpy of the system.
Then dS – dH /T ≧ 0, or 0 ≧ dH - T dS
dH≤ TdS (const. p, no non-pV work)
These expressions can be simplified even more by introducing two new thermodynamic functions.
They are defined as follows:
Helmholtz function: A = U – T S
Gibbs function: G = H – T S
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How G and A simplify the expressions
Consider what happens to them when the state of a system changes at constant tempe