From owner-biophysics@net.bio.net Sat Jan 10 22:00:00 1998 Path: biosci!agate!howland.erols.net!wnfeed!204.127.130.5!worldnet.att.net!news.u.washington.edu!fujimoto From: fujimoto@u.washington.edu (Bryant Fujimoto) Newsgroups: bionet.biophysics Subject: Re: Chemical work (reply to Erik Van Nimwegen) Date: 11 Jan 1998 09:47:28 GMT Organization: University of Washington Lines: 76 Distribution: bionet Message-ID: <69a4fg$n3q$1@nntp3.u.washington.edu> References: <694quc$mlj@mserv1.dl.ac.uk> NNTP-Posting-Host: homer11.u.washington.edu X-Trace: nntp3.u.washington.edu 884512048 23674 (None) 140.142.64.4 X-Complaints-To: help@cac.washington.edu NNTP-Posting-User: fujimoto Pentcho Valev writes: >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/ Pentcho, Where did you get this equation? Assuming you define your system correctly, dQabsorbed = TdS for a reversible reaction. However, I see no reason why they should necessarily equal dQend + dW. >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), Pentcho, What ever gave you the idea that it wasn't definable? Carrying out such a reaction would be a problem, but that doesn't make it undefinable. >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. Pentcho, If the thing you want is energy transfer, thermodynamics already has it. Its called (Delta U). Regards, Bryant .