[skp524]

By1st law of thermodynamics :dU=dq+dw
Isothermal process: dU=0 , dT=0
By definition : H=U+PV=U+nRT (ideal gas, PV=nRT)

Therefore, dH= dU+ nRdT = 0 + 0 =0
Am i right?


Also, i want to ask whether delta G= -RT lnK  K refers to Kp or Kc?

[tamim83]

By1st law of thermodynamics :dU=dq+dw
Isothermal process: dU=0 , dT=0
By definition : H=U+PV=U+nRT (ideal gas, PV=nRT)

Therefore, dH= dU+ nRdT = 0 + 0 =0
Am i right?

Yes, you are. 

Also, i want to ask whether delta G= -RT lnK  K refers to Kp or Kc?

I believe it can be used for either. 

[Borek]

Also, i want to ask whether delta G= -RT lnK  K refers to Kp or Kc?

I believe it can be used for either. 

When there are gases involved and there is a volume change, Kc doesn't equal Kp, so it won't work.

[tamim83]

When there are gases involved and there is a volume change, Kc doesn't equal Kp, so it won't work.

Hmm, so which constant does the delta G = -RTlnk equation refer to?  When I went back through my textbooks, it didn't say.  Does it just depend on the reaction?

[Borek]

To be honest I don't know. My bet is that Kp is the correct one, and Kc is only an approximation. Note that when dealing with equilibrium where both gas and solutes are present we mix both concentrations and pressures in reaction quotient. However, as we deal mostly with solutions we are using Kc and treat it as a correct one, when it is only an approximation. But I can be wrong.

[Jorriss]

From my textbook, it's formally derived for Kp standard and from there, it's just used whenever necessary in any form, which, is odd come to think of it. For sure, it's for Kp standard, I've seen the derivation. But we also use it for solubility products, hetergeneous equilibrium constants, etc.