Stefan Schuster wrote:
Referring to Pentcho Valev's question, I recall some textbook knowledge:>
>
In isobaric processes which proceed in systems neither exchanging any matter>
with their surroundings nor exchanging work except volume work, the change in>
enthalpy gives the exchanged H E A T (rather than work). >
>
The change in Gibbs free energy indicates, if negative, the maximum work the >
system can perform, and, if positive, the minimum work one has to perfom on >
the system to force the process to proceed. >
>
Both quantities do depend on concentrations. What is independent of >
concentrations are the molar standard enthalpy and molar standard Gibbs free >
energy. >
>
These are sufficient points to resolve the paradox, I think. >
Quite insufficient I am afraid. As you write, the enthalpy change is heat
only when the system does not exchange work except volume work, i.e. when
IT DOES NOT EXCHANGE USEFUL WORK. However in this case the system DOES
exchange useful work, and therefore
delta H = delta E + p(delta V) = Q + Wu /1/
where Q is the heat absorbed and Wu is the useful work the system receives.
Let the reaction on which work is done be the one described in my previous
message: transport of H+ against a transmembrane electrical gradient:
H+in -> H+out /2/
If the reaction were spontaneous, Wu would be zero and then
delta H = Q /3/
i.e. what we have in textbooks concerns spontaneous reactions. However the
reaction is not spontaneous - something must "push" H+ against the electrical
gradient - the energy comes from within this something, i.e. Q = 0 and
delta H = Wu /4/
As you can see, the enthalpy change is equal to the useful work by
definition - this is not my preference. Note also that when we say that
"the change in Gibbs free energy indicates the maximum work the system can
perform" we mean the same useful work done under specific conditions
(reversibility). The two concepts cannot be reconciled.
Pentcho
