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Re: Electolysis of water

electrolytic cell .... bubbles are coming out at some constant
temperature. I suspect that the second law can be used to determine
the ideal possible efficiency

The second law has little to do with it.
You should be able to come arbitrarily close
to reversibility, i.e. 100% efficiency.

The second law has everything to do with it! Please consult
a good thermal physics book, like Kittel and Kroemer, or a
mediocre one, like Schroeder. Without the second law,
there would be no such concept as reversibility. As for
efficiency, please define this term! At standard temp
and pressure, electrolysis of one mole of water results
in a net energy increase of 282 kJ for the system. The
system also performs 4 kJ of mechanical work as it expands
against atmospheric pressure, so its enthalpy increases
by 286 kJ. But the minimum electrical work that you must
provide is only 237 kJ. That's because the system's
entropy is increasing during this process, by 163 J/K,
and therefore it can absorb up to 49 kJ of heat from the
constant-T environment (Q = T dS = (298 K)(163 J/K) = 49 kJ).
To see how these numbers change with temperature and
pressure, you need to use partial-derivative relations
like (dG/dT)_P = -S.

Cheers,

Dan

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.