From owner-biophysics@net.bio.net Thu Jan 08 22:00:00 1998 Path: biosci!daresbury!not-for-mail From: Pentcho Valev Newsgroups: bionet.biophysics Subject: Chemical work (reply to Erik Van Nimwegen) Date: 9 Jan 1998 09:34:04 -0000 Lines: 59 Sender: lpddist@mserv1.dl.ac.uk Distribution: bionet Message-ID: <694quc$mlj@mserv1.dl.ac.uk> Original-To: biophys@dl.ac.uk Erik wrote:>>>>>>>>>> I've been trying to follow this discussion a bit and I'm getting kind of confused about what the problem is. Ok, first, as far as I know, a reversible process is a process for which the entropy of the system remains fixed, i.e. dS = 0.<<<<<<<<<<<< Only for an isolated system. For a normal closed system dS is not zero. One can easily imagine this in the particular case in which a solute reaction releases a gas, e.g. A + B <-> C + D where D is a gas. Then we can carry out the reaction reversibly in both directions with the help of a piston. The reaction can be either endo- or exothermic, and, in addition, some work is done as the piston is pushed. The sum of the endothermic heat dQend and the work done dW gives dS: dQabsorbed = dQend + dW = TdS /1/ In fact, this is a true REVERSIBLE process - when I started the discussion I intended to prove in the end that delta G work is done only in such processes - it is statistical work which comes from a simultaneous conversion of the thermal energy of a great number of particles. However in chemical systems there is a different and independent kind of work - when in the elementary chemical act a molecule (e.g. ATP) transfers some of its internal energy to another molecule. So the solution is simple - when, in a thought experiment, a TRUE REVERSIBLE process is possible (e.g. for galvanic and osmotic systems) we can expect delta G work to be done or approached. When such a process is impossible (as in all cases in which ATP does work), delta G work is also impossible - then only molecular work is done which obeys a different quantitative description. Of course, there are complications - in the chemiosmotic process for instance the two types of work are interconvertible. My dream was to constructively discuss this as well, but at the moment I am still trying to prove that I am not just ignorant. >>>>>>>>>>>>>>>>>>> So, to me it seems that: Yes, thermodynamics is fine. Yes, the maximal work thermodynamics predicts a process can deliver can in general not be obtained by a real system because we don't know how to set up the system such that it remains arbitrarily close to equilibrium. Does that solve the problem? <<<<<<<<<<<<<<<<<<<<< To some extent. If the reversible process is possible and definable, delta G is the maximum work and a small deviation from reversibility decreases slightly the work. (This is strictly analogous to what is valid for an ideal gas system). If the reversible process is not definable at all (as in the cases in which ATP does work), we should just see "what transfers energy to what". If the process is isothermal, this energy transfer cannot be heat exchange - then it is WORK DONE. I would agree that thermodynamics is fine, but we should not ignore any REAL physical event just because thermodynamics does not adequately describe it. I still believe that a quantitative theory describing energy transfer at the molecular level is much needed. Best regards, Pentcho .