※ 引述《ototokio (人生处处是考验)》之铭言： : 当固体或液体被恒温压缩或膨胀时 : Gibbs free energy的变化量 : 按书上写是代 : dG = VdP - SdT (因为恒温 dT = 0) : 所以ΔG = V(P2 - P1) : 但是为何下列的关系式在此不适用呢? : ΔG = ΔH -TΔS : 有例题已要求先算出ΔS : 用此式代入的答案与正确答案不一样 : 我不懂为何ΔG = ΔH -TΔS在固液体恒温压缩时不适用 : 请教各位 谢谢 I guess it's coz V is not a constant when the system is under compression or expansion so you can't really use the equation V(P2-P1) because that way you won't be able to know what V you should use. In most cases when solving for ΔG, one would calculate ΔH first by taking the integral of Cp with respect to dT. And since the ΔS is already known, ΔG = ΔH -TΔS can be used to obtain the value of ΔG. However, this is based on the assumption that the pressure change is insignificant and the dependency of ΔH with respect to pressure is neglegible. However, in the cases of compression/expansion of gases, the above assumption is not a proper one to make because, since the process is isothermal as stated in the problem, there will be no change in enthalpy and the major effect of the process will not be considered. In this case, one should calculate ΔH by using dH = VdP and taking the integral. -- 我的网志： http://www.wretch.cc/blog/appleamily --

※ 发信站: 批踢踢实业坊(ptt.cc) ◆ From: 70.239.50.126 ※ 编辑: AppleAmily 来自: 70.239.50.126 (02/26 17:03)