chandni and ankur , the eq you talkin abt is used whenever a rxn is at equili or may attain , the basic definition of gibbs free energy is given by
G = H - TS
at contant temperature , change in enthalpy is zero , and i hope you can calculate TS at constant temp
You see all those people are wrong, it's not at all like this .I too got the same doubt when i was studying the Nerst equation.Basically what we have to consider in the logarithm is the reaction quotient & the reaction quotient is not defined as the ratio what we have defined in chemical equilibrium i.e its not the ratio of product of active concentrations ......... but it is the ratio of ACTIVITIES which we see later in our life if we are to do some research in quantum mechanics. So you need not bother much about it .But to say what we write in euilibrium is wrong is not right. As we deal with fairly low concentrations we have activities= concentration, this is the reason why pH isn't -ve.Here aloso we take the -ve logarithm of the ACTIVITY of the H+ ion and we know that activity cant exceed 1.Hence pH ain't -ve.So to say ln{p1/p2} is a real mess,but in our level we are justified to use it but do prefer using concentration, as the ideal gas equation is not valid in our normal chemical systems .You are a smart fellow you should be knowing this.If you say that this is thermodynamics you are mistaken even in thermo dynamics we use activities and not pressures.
MANOJ


Your reply is a very sensible one except for the fact that you are being too general by quoting quantum mechanics and stuff.
Can you reword yor reply sticking just to the points?
It would be easier to comprehend.
And please write in paragraphs.