华南理工大学：《物理化学》课程教学课件（PPT讲稿）第三章 热力学第二定律（中）

物理化学电子教案第3章(中)(添) 热学能巴 不可能把热从低温 物体传到高温物体 而不引起其它变化 The Secons as of hiearmmodsymatmics 4上一内容下一内容令回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 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）

3.7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。 4上一内容下一内容◇回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 3.7 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

3.8 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 ver 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 automaticall 4上一内容下一内容◇回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 3.8 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

This focuses attention on the system and simplifies thermodynamic discussions Consider a system in thermal equilibrium with its surroundings(so that T SyS T surT then the clausius inequality reads 0O Sys SVs ≥0 T The importance of this inequality is that 4上一内容下一内容令回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 This focuses attention on the system, and simplifies thermodynamic discussions. Consider a system in thermal equilibrium with its surroundings(so that ), sys surr T = T then the Clausius inequality reads  0  − T Q dS sys sys The importance of this inequality is that

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 4上一内容下一内容◇回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 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

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 eq gives dS-dUT全0.0r0全dU-TdS 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 )V ork is involved 4上一内容下一内容◇回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 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 eq.gives 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

the only modification is to identify dQp with dh, the change of enthalpy of the system Then ds-dH/T≡0.or0≡dH-TdS dhs 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-TS Gibbs function G=H-TS 4上一内容下一内容◇回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 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

How G and a simplify the expressions Consider what happens to them when the state of a system changes at constant temperature da=dU -Tds(const. t) dG=dH-Tas(const T) Now introduce Tas>du (const. V) Tds>dH(const. P) daso (const. T,V, no non-pV work) dG<o (const T, P, no non-pV work) 4上一内容下一内容令回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 How G and A simplify the expressions Consider what happens to them when the state of a system changes at constant temperature: dA = dU –TdS (const.T) dG = dH – TdS (const.T) Now introduce TdS≥dU (const. V) TdS≥dH (const. P) dA≤0 (const. T,V,no non-pV work) dG≤0 (const. T,P,no non-pV work)

These inequalities are the most important conclusions from thermodynamics in chemistry. Some remarks on the helmholtz function When the changes in a system are constrained to occur under conditions of constant temperature and volume eq above determines whether they can occur spontaneousl 4上一内容下一内容◇回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 These inequalities are the most important conclusions from thermodynamics in chemistry. Some remarks on the Helmholtz function. When the changes in a system are constrained to occur under conditions of constant temperature and volume eq. above determines whether they can occur spontaneously

A change corresponding to a decrease in a can occur spontaneously Systems tend to move naturally towards states of lower A and the criterion of equilibrium is(dATV=. (n,W=0) We shall now demonstrate the connection between △ A and w max 4上一内容下一内容◇回主目录 返回 2021/2/21

上一内容 下一内容 回主目录 返回 2021/2/21 A change corresponding to a decrease in A can occur spontaneously. Systems tend to move naturally towards states of lower A,and the criterion of equilibrium is (dA)T,V =0. ( n,W`=0) We shall now demonstrate the connection between △A and Wmax