浙江大学物理化学(甲)第二章.ppt
1,热力学第一定律,First law of thermodynamics,Chapter 3 The First Law: the machinery,2,3.1 State functions and path functions,State functions,Path functions,Exact differentials,Inexact differentials,状态函数法,3,3.2 Changes in internal energy,Definition:,Internal pressure,4,3.3 The relation between Cv and Cp,1. The relation between Cv and Cp,(1)定性分析:,恒容加热,不做功,吸收的热量转化为系统的 热力学能(内能),所以,每升高一度,恒压过程所需热比恒容过程大:,CpCv,5,(2) 定量计算:,恒压下两边除以dT,得:,6,Definition:,Internal pressure,Expansion coefficient,Therefore:,7,(3) 讨论:,时,Cp=CV,水在277.15K时, Cp=CV,理想气体,8,实验过程:如右图所示。,实验结果: 系统(气体)与环境(水浴) 间没有能量交换,即Q=0; (2) 气体向真空膨胀,W=0 。,Joule在1843年做了实验:,实验现象:没有观察到温度变化 (dT=0),2.The Joule experiment, U and H of perfect gas,9,热力学分析:对于定量的封闭系统, dT=0,dU=0(实验结果),显然, dV0(dV0),物理意义:恒温下改变体积,气体的热力学能不变。,10,同理:, dT=0,dU=0(实验结果), dp0,物理意义:恒温下改变压力,气体的热力学能不变。,U=f(T),所以理想气体,单纯pVT变化时,不管恒容与否,理想气体热力学能只是温度的函数,11,对于焓,恒温条件:,所以理想气体,单纯pVT变化时,不管恒压与否,12,理想气体的热力学能和焓仅是温度的函数,而与p、V无关。, Cp、Cv也仅是温度的函数。,小结:对于理想气体,13,对于理想气体:,理想气体的Cp与Cv之差,14,液体、固体:,同理,实际上, 仅有实际气体或压力变化很大时的液体需要校正!,15,The Joule experiment:,Internal energy and enthalpy of a perfect gas only dependent on temperature!,16,Example 1:,A sample consisting of 1 mol Ar is expanded isothermally at 273 K from 22.4 L to 44.8 L (a) reversibly, (b) against a constant external pressure equal to the final pressure of the gas, and (c) freely.For the three processes calculate .,17,Answer:,解题步骤:,(1)确定始终态,画出状态图(框图或p-V图),Expanded isothermally reversibly against a constant external pressure freely,(2)判断问题类型和过程特征,Internal energy and enthalpy of a perfect gas only dependent on temperature!,(3)选择可用的工具(理论、公式等),判断先算“谁”。常先找出“等于零”的函数变化。,(4)计算、检验。,18,(b),(a),(c),19,Example 2:,A sample of 1 mol H2O(g) is condensed isothermally and reversibly to liquid water at 373.15 K. The enthalpy of vaporization of water at 373.15 K is 40.656 kJ mol-1. Find for this process. (Assuming that H2O(g) is a perfect gas).,20,Answer:,Condensed isothermally and reversibly Phase transition,Enthalpy of vaporization: 40.656 kJ mol-1 H2O(g) is a perfect gas,21,22,3.4 Throttling expansion (节流膨胀) and the Joule-Thomson effect,焦耳实验:1843年,自由膨胀,理想气体 U=f(T) H=f(T),焦耳汤姆生实验:1852年,节流膨胀,实际气体 U=f(T,V) H=f(T,p),实验过程(下页图):,23,Joule-Thomson expansion,24,热力学分析:,在左侧压力p1下气体膨胀功为:,气体通过小孔膨胀,功为:,实验事实: p1p2,V1V2,T1T2,(绝热),(第一定律),25,即,节流过程是个等焓过程 (isenthalpic process)。,移项,实验结果: H1=H2,实验结论: H=f(T,p) U=f(T,V),26,Joule-Thomson coefficient(焦耳-汤姆生系数),0 致冷,0 致热,=0 温度不变,思考:,27,Isothermal Joule-Thomson coefficient :,28,判断下列气体节流膨胀后温度怎样变化?,温度升高,