※ 引述《ototokio (人生處處是考驗)》之銘言： : 當固體或液體被恆溫壓縮或膨脹時 : Gibbs free energy的變化量 : 按書上寫是代 : dG = VdP - SdT (因為恆溫 dT = 0) : 所以ΔG = V(P2 - P1) : 但是為何下列的關係式在此不適用呢? : ΔG = ΔH -TΔS : 有例題已要求先算出ΔS : 用此式代入的答案與正確答案不一樣 : 我不懂為何ΔG = ΔH -TΔS在固液體恆溫壓縮時不適用 : 請教各位 謝謝 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. -- 我的網誌： http://www.wretch.cc/blog/appleamily --

※ 發信站: 批踢踢實業坊(ptt.cc) ◆ From: 70.239.50.126 ※ 編輯: AppleAmily 來自: 70.239.50.126 (02/26 17:03)