enthalpy and entropy

[americanpierg]

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What is the change in enthalpy and entropy for an adiabatic compression?

A. both entropy and enthalpy increases
B. entropy increases and enthalpy decreases
C. entropy increases and enthalpy stays the same
D. entropy stays the same and enthalpy increases
E. both entropy and enthalpy stays the same

 

[Tommy43087]

If its an irreversible adiabtic process there is an increase in entropy and i would think that since there is no heat exchange that the enthalpy wouldnt change?(but i also know that Enthalpy is usually measured at a constant presssure). So i guess i would go with "C" But i dont know thats just my 2cents

And im not sure if your posting this to inquire, or to simply quiz people?

 

[americanpierg]

If its an irreversible adiabtic process there is an increase in entropy and i would think that since there is no heat exchange that the enthalpy wouldnt change?(but i also know that Enthalpy is usually measured at a constant presssure). So i guess i would go with "C" But i dont know thats just my 2cents

And im not sure if your posting this to inquire, or to simply quiz people?

hmm no answer, looked it up myself


Answer was D

no heat exchange, so q = 0, entropy is q/T

enthalpy increases b/c enthalpy = U + pv, pv is constant, which leaves U, which is q + w, and its compression, so w has to increase, and since q stays the same, U goes up and enthalpy goes up

 

[brightsmiles9]

I don't really get that, can anyone explain it?

 

[UCB05]

An adiabatic process is one in which pressure and/or volume of a system changes, no heat exchange occurs. Since entropy is a function of heat, entropy does not change. However, the gas was compressed, meaning work was done on the system, and since no heat exchange occurs between the system and its surroundings, the system itself has to have gained energy. Hence an increase in enthalpy