文档介绍：Questions and answers I
B. Inten/Extensive. Indicate whether each of the following quantities is intensive, extensive, or neither:
p: V:
n/V: T:
S: mass:
density: Cp:
(pVm): mJ:
intensive
extensive
intensive
intensive
intensive
intensive
extensive
extensive
extensive
neither
系统由相同的始态经过不同途径达到相同的末态。若途径a的, ;而途径b的。求。
解:系统热力学能变与途径无关,只与系统的始末态有关
故,
始态为25℃,200kPa的5mol某理想气体,经a,b两不同途径到达相同的末态。途径a先经绝热膨胀到-℃,100kPa,步骤的功;再恒容加热到压力为200kPa的末态,步骤的热。途径b为恒压加热过程。求途径b的及。
恒压
途径b
(1) 绝热
(2) 恒容
解:题给途径可表示为
由途径a可知,第一步为绝热过程, , 。第二步为恒容过程, 。整个过程的热力学能变,只取决于始末态,与途径无关,故
末态的体积由途径a中的(2)恒容过程求算。
整个过程的
途径b为恒压过程,故
本题也可采用下列解法。先求出题给理想气体的:
由可知
再求
,这是题给原始数据不完全配套造成的,若
,两者结果就可相同。
I. (55) Thermodynamics of perfect gases
A certain perfect gas has heat capacity CV,m = a + bT, with a = 5/2 R. and b = J K–1 mol–1. mol of this gas is carried through the following processes. In each case, calculate (in units J for energies) as many of the following as possible: q, w, △T, △P, △V, △U,△H, △S, △A, △G. Also indicate clearly why those you do not calculate cannot be evaluated.
A. The gas, initially at 279℃, is expanded isothermally and reversibly from P = atm to V = L.
B. From its final state in (A) the gas undergoes an adiabatic expansion to V = L and T = 244℃.
C. The gas is taken from T = 279℃ and P = atm to V = L and T = 244 ℃. [Hint: THINK before pute!]
D. From your results in (B), can you determine whether that process is reversible or not? Explain briefly.
(55) solution A. The gas, initially at 279℃, is expanded isothermally and reversibly from p = atm to V = L.