I am bringing in a little chemistry to brilliant community and I am hoping to hear from at least one of you. This is about Thermodynamics. It is said dS=q/T only for reversible reactions but while deriving the equation for Gibbs free energy we put dS = q/T for change in entropy of surroundings though we use the formula to find the spontaneity of reactions even irreversible. Derivation of G = H - TS

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The second law of thermodynamics tells us that the entropy of system is

dS = dq/T

for a reversible process. Note that this definition only applies to a reversible process, but that is consistent because the entropy of the system will always be calculated along a reversible path. In general, if we consider the system and the surroundings the second law states that the entropy of both (i.e. the universe) tends toward a maximum. If a change is reversible the heat exchanged between the system and surroundings dqrev will have a magnitude such that dqrev/T exactly is equal to the entropy change in the system dS. For an irreversible process the heat exchanged between the system and surroundings, dqirr will be less than TdS such that

dS > dqirr/T

The above explanation can be found here.

Also, remember that when calculating spontaneity of a reaction that S, G and H are STATE FUNCTIONS and therefore independent of path--so it doesn't matter whether q or qrev is used--the values of G, S and H will be the same; what will change is q--the value of the heat itself. – N. E. · 3 years, 7 months ago

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