rjenkins@oracle.com (Robert Jenkins) (07/17/90)
One-way catalysts are (would be) a lazy form of Maxwell's
Demon. They could trap energy from random thermal vibrations and
store it in chemical bonds, which could later be burned or used to do
useful work. It may be possible to produce them with today's
technology, although designing them is tricky.
If A <=> B is a chemical reaction where, at equilibrium, there
is 1 part A to 100 B (1A:100B), then the reaction A => B occurs 100
times as often as B => A. B => A still occurs, just not that often.
Often the dominant reaction releases heat. The dominant reaction is
always said to increase entropy.
Catalysts are not supposed to change the equilibria of
reactions. If 1A:100B is the equilibrium without a catalyst, then
1A:100B is the equilibrium with a catalyst. The catalyst may speed up
A=>B and B=>A one millionfold, but it will speed them up
proportionately. (Note: could someone confirm this with actual
measurements for reactions with equilibria between 1:1 and 1:100?
I suspect that even known catalysts change equilibria, probably
turning 1:2 into 1:5, or the like.)
A one-way catalyst is a catalyst that *does* change the
equilibria of the reactions it encourages. If 1A:100B is an
equilibria, a one-way catalyst which encourages A=>B (but not B=>A)
may not be too useful, but one that encourages B=>A (but not A=>B)
would be useful, especially if B=>A absorbs heat. (If you could do
CO2+2H2O => CH4+2O2, you could cool the fridge and air conditioner without
supplying energy, and you could use the CH4 to run the stove. That
reaction may be too extreme to run at a useful rate at room temperature,
though.)
There are lots of possible designs for one-way catalysts. One
is to have an active site which only exists (or is only enabled) when
it is filled with reactants. That would imply the product of the
reaction can't bump into the active site (because it is only active
when it is already clogged with reactants), so the catalyst can't
encourage the reverse of the reaction.
A C-shaped catalyst could have an active site which is formed
when the tips touch. The molecule naturally rests with the tips separated.
When reactants bind to the tips, though, that would have to alter the
catalyst enough for its new natural shape to have the tips touching.
Once the tips are touching (they are already clogged with reactants),
you wait until random thermal agitation provides enough energy to run
the catalyzed reaction. The reaction may immediately reverse itself
(more than likely), in which case you gained and lost nothing. Or the
reaction could only form the product (1 in 100 chance, using A and B),
in which case the energy required to form the new bonds was drawn from heat.
The catalyst would no longer be bound to the reactants (it might be
bound to the product), so the catalyst's tips will separate again,
removing the active site. (As I said, tricky to design.)
Another approach is to have a { shaped molecule, reactants
bind to the outside causing it to change to }. Another approach is to
have electric potentials shift around and determine when the active
site is enabled or not. Another approach would be a system that winds
up and fires, like striking a match. It may even be useful to use energy
to reconfigure the catalyst, providing that more energy is trapped by
converting the reactants to a product. There are lots of approaches;
there are probably better ones I haven't even imagined.
- Bob Jenkins
RJENKINS@oracle.oracle.com
PS. I am not a chemist; please discuss and criticize my ideas, but
not my presentation of them.
PS. The second law of thermodynamics is not axiomatic; it is derived
from other laws of physics and chemistry. One of those laws is that
nothing can change the equilibria of reactions. So don't resort to
entropy; argue with charge, momentum, and the other axioms of physics,
from which the concept of entropy can be derived.
Disclaimer -- These thoughts and opinions are entirely my own.
[I'm not a chemist either, but I don't see why a catalyst shouldn't
be one-way. A simple static shape could have binding sites for
two reagents which formed a product which didn't fit the sites.
This would involve an expression of energy in the reaction, however,
and furthermore adding or removing either the reagents or products
from solution with the catalyst would also affect the entropy, so
I see no reason for this to affect the second law. I believe the
second law is essentially statistical, and is a fairly direct
mathematical consequence of the definitions of the various quantities
and the time-reversible nature of the underlying physics.
--JoSH]landman@hanami.eng.sun.com (Howard A. Landman x61391) (07/22/90)
In article <Jul.16.22.11.36.1990.26770@athos.rutgers.edu> rjenkins@oracle.com (Robert Jenkins) writes: > One-way catalysts are (would be) a lazy form of Maxwell's >Demon. They could trap energy from random thermal vibrations and >store it in chemical bonds, which could later be burned or used to do >useful work. It may be possible to produce them with today's >technology, although designing them is tricky. The reason this is not possible is that microscopic physics is reversible: for every physically possible motion, the reverse motion (reversing the direction of time) is also possible. That means that any reaction that can run forward can also run backward. If you can make *ANY* reaction run entropically uphill, then it is trivial to design a perpetual motion machine powered by letting it run downhill again. So, you are claiming that molecular perpetual motion engines are feasible to build. See the good article on Maxwell's Demon in Sci Am a year or so back. > If A <=> B is a chemical reaction where, at equilibrium, there >is 1 part A to 100 B (1A:100B), then the reaction A => B occurs 100 >times as often as B => A. B => A still occurs, just not that often. You're talking about two different sets of conditions here. At equilibrium, BY DEFINITION, forward and reverse reactions occur at the same rate (otherwise the concentrations would be changing). If the above equilibrium holds, then AT EQUAL CONCENTRATIONS OF A AND B (a different set of conditions!) the reaction A => B would be occurring 100 times as often as B => A. > Catalysts are not supposed to change the equilibria of >reactions. If 1A:100B is the equilibrium without a catalyst, then >1A:100B is the equilibrium with a catalyst. The catalyst may speed up >A=>B and B=>A one millionfold, but it will speed them up >proportionately. Right. > A one-way catalyst is a catalyst that *does* change the >equilibria of the reactions it encourages. >If you could do >CO2+2H2O => CH4+2O2, you could cool the fridge and air conditioner without >supplying energy, and you could use the CH4 to run the stove. More perpetual motion machines here. These don't exist. > There are lots of possible designs for one-way catalysts. One >is to have an active site which only exists (or is only enabled) when >it is filled with reactants. Not possible because self contradictory. If that's hard to understand ... >That would imply the product of the >reaction can't bump into the active site (because it is only active >when it is already clogged with reactants), so the catalyst can't >encourage the reverse of the reaction. Which violates microscopic reversibility, because we know the product can LEAVE the active site. >PS. The second law of thermodynamics is not axiomatic; it is derived >from other laws of physics and chemistry. One of those laws is that >nothing can change the equilibria of reactions. So don't resort to >entropy; argue with charge, momentum, and the other axioms of physics, >from which the concept of entropy can be derived. Sorry, but you can't dismiss entropy that easily. It is fundamentally a STATISTICAL phenomenon, and so derivable via pure math from the underlying physical laws. It makes no such assumption about equilibria. >[I'm not a chemist either, but I don't see why a catalyst shouldn't > be one-way. A simple static shape could have binding sites for > two reagents which formed a product which didn't fit the sites. So then the reaction could never happen, because when it did you'd have a product which "didn't fit the site" fitting the site! QED. > I believe the > second law is essentially statistical, and is a fairly direct > mathematical consequence of the definitions of the various quantities > and the time-reversible nature of the underlying physics. > --JoSH] You got THAT right! Honestly, what's wrong with the way nature solves these problems in living organisms? If you want a reaction to proceed that otherwise wouldn't, you couple it with something that has a greater tendency to proceed, and then recycle the coupling byproduct: A + ATP <=> B + ADP + Pi + heat energy + ADP + Pi <=> ATP + heat -------------------------------- NET: energy + A <=> B + heat The overall reaction tends to proceed because of the heat released, even if A => B is energetically unfavorable. In the above equations I've shown Adenosine Triphosphate <=> Adenosine Diphosphate plus inorganic Phosphate, but there are numerous other examples of this sort of coupling (using NADH, FAD, ...). The total amount of heat released is equal to the energy input plus(minus) any heat released(absorbed) by the A => B reaction. The advantage of this approach is that you only need ONE high-energy species in your mixture (ATP) and can use it to drive many different reactions; this is something like using money instead of having to barter for everything. Also the amount of ATP required is small (since it is reused) which makes the task of purification easier. Finally, the distribution of the energy needed to convert A=>B is accomplished by diffusion, a simple reliable process needing no infrastructure or supervision, and (at cellular scales) not even very slow. -- Howard A. Landman landman@eng.sun.com -or- sun!landman [Let's straighten out some terminology here, and at the same time explain the entropy business in a way that is parhaps easier for computer types to think about. First, if you take "one-way catalyst" to mean something which is microscopically irreversible and reduces entropy, Howard is right and there is no such thing. On the other hand, if you mean something which "uses energy", i.e. increases entropy like any macroscopic engine (of course energy is not destroyed but simply moved around) then such things are not only possible but are indeed ubiquitous. The mechanism I mentioned is indeed possible and is just what one finds in a catalytic hand warmer or the catalytic converter in your car. Why, if the catalyst is microscopically reversible, doesn't it then take the CO2 and H2O and give us oxygen, carbon, and hydrogen? Microscopic reversibility means that for every microstate leading into a particular molecular reaction there is one that leads back, namely the exact reverse path. Why aren't there the same number of molecules going each way? The answer is that when energy is expressed in a reaction the product (hotter) side of the reaction has so many more microstates available that the set leading back is a very tiny subset. It's as if you crowded 100 people into a small room in the Pentagon, opened the door, and told them to wander at random. It's as easy for someone to walk in as out but after an hour you'd be very lucky ever to see even one person in the original room again. This is why endothermic reactions are relatively rare: They do exist; a common examle is the solution of sugar in water. The added entropy, i.e. number of microstates, expands along the positional axes so much as to overcome the slight shrinkage in the energetic dimensions. But normally the heat from an exothermic reaction will cause the product side to have a greater entropy. Let me finish this up with a flat statement: There is no reason whatsoever to believe that nanomachines, biocatalysts, or any other such molecular mechanism could break the second law of thermodynamics. None. Any such violation requires either ending or branching microstates and currently held physics doesn't allow this. --JoSH]