About science; About the mechanics of chemical reactions

This is about science produced by the California Institute of Technology in collaboration with station KPCC Pasadena California. The programs are made available to the station by national educational radio. This program is about the mechanics of chemical reactions with host Dr. Peter Letterman and his guest Dr. Aaron Cooper men. Here now is Dr. lesson of the pure sciences chemistry probably has its roots deepest in the antiquity of man. Maybe this is because we all practice chemistry every day in the ancient and noble art of cooking. We apply the basic principles of mixing different specified quantities of different substances permitting them mutually to react usually by the fundamental thermal process of heating and culminating in a final compound which is radically different from its basic ingredients. And usually well the unhappily not always an improvement. Whatever the reason chemistry has fascinated man

from earliest times from the ritual and magic of the Sorcerer from the arcane mysteries of the alchemist hoping against hope that the next bizarre combination of faintly repellent ingredients will real compound whose properties will unlock the mysteries of matter to the modern chemist probing ever deeper with more and more complex tools into the why and wherefore of the basic building blocks of matter and its modes of combination. Recently chemists have made great progress in understanding the basic principles of chemical reaction how a single molecule combines with another. What forces are involved. And work laws and principles govern these bonds and reactions we have with us today. The man has done very significant work in analyzing the molecular mechanics of chemical reactions. Dr. Cooperman was born and set upon going to Brazil where he did his

undergraduate work. He studied at Edinburgh University obtained his Ph.D. in physical chemistry at the University of Notre Dame and taught at the University of Illinois at the moment. He is professor of chemical physics at the California Institute of Technology. I wonder what are some of the basic ideas and principles of Bach chemical reaction. Well all chemical reactions and nature sensually without exception take place as the result of the collision between molecules for a molecule way to react with a molecule B it's absolutely essential that they approach each other and collide. This is true in organic reactions such as the ones involving combustion of gasoline and automobile engine biological reactions such as the ones involved

in the transformation of food into human tissue. And inorganic reactions in propulsion Jet Propulsion such as the reactions that occur and the propulsion system of rockets and then explosive chain reactions such as that that those that occur when a mixture of hydrogen and oxygen is heated up. Because I expect there in that this is a fairly old idea the idea that a pocked reaction through completion of Mona cubes. Well this idea is not to alter it's younger than the idea of the existence of molecules and that idea amazingly enough is just about one hundred fifty five years old. It was well when Delton developed the atomic theory and the early eighteen hundreds he didn't distinguish between an

atom and a molecule in those days people thought that somehow like atoms could not get together to form more complex aggregates and it took of a Gadar to show that you could have actual molecules formed of two atoms of the same kind. Obviously after the development and acceptance of the concept of molecules it was understood that chemical reactions must result from collisions between molecules. This however only occurred about 50 years after of about two and about 1860 one kind of sorrow was able to sell the molecular concept to a chemist so the idea of reaction by a collision between molecules is just about 100 years old and yet much of modern ideas have been formulated in the last 50 or so you say. That is right because essentially most chemical reactions

with which we are faced by nature are very complex chemical reactions such as the ones that I mentioned a moment ago. Such reactions usually involve in a whole sequence of different chemical steps. Molecule A react with molecules B to give some product see and see further reacts with B to give DNA and DNA further react and so on. So that in order to boil down the study to individual Chemical Staff steps a lot of effort is needed. An effort of this type of sensually was only successful starting about 50 years ago. And yet Aaron when talking about reaction these changes don't take place instantaneously I expect they won't take place in time soon in time some time spoons and some very long.

That is exactly correct. That is one of the beauties of chemical reactions when one tries to study them by measuring the speed at which they occur and the range of speeds that is found and in chemical reactions is fantastically large. Some reactions can be over in 10 to the minus 13 seconds whereas others can take 10 billion years namely the age of the universe to occur corresponding to a total range of about 30 orders of magnitude. And yet I expect that these very very long reactions all these fantastically short ones are not the sort of things which you would normally study. That is correct it's exceedingly difficult to study either very very fast ones I mentioned are very very slow.

Yes a few of us live long enough to study the time scales of 10 billion years of that one. That's right those one can find something out about by looking at planets and inferring what they must have been and their formative stages and what they are not. But the usual laboratory type chemical reactions is studied when when a chemist mix mixes reagents and separates products. O have a typical times of the order of minutes to hours and the number of molecules of the new substance that he prepares when the study is a chemical reaction is of the order of magnitude of a. Billion billion molecules per hour per minute so that in measure the numbers of molecules these numbers are faint dastardly large so that even although we can we can conduct experiments which take place in a time scale a matter of minutes which we can handle. We still are

dealing with as you say a fantastically large number of molecules and I suppose it's out of the question to discuss what each particular molecule guised in this this huge sample of a billion billion molecules that you're talking about. Indeed you would have been correct if you had asked me that question about 10 years ago or 12 years ago. However with modern developments it is now actually possible to attempt to find out both experimentally and theoretically what does happen with individual molecules. In other words you mean that one can actually identify molecule and find out what speed it's going and how its reaction depends upon the characteristics of the moment. Yes and indeed one can in the ordinary chemical

experiments in the laboratory. That is not the case there. During a chemical reaction different molecules have different speeds and collide with other molecules with a very wide range of molecular speeds. Some of these collisions produce chemical reactions and others do not. And what we measure when we measure the overall rate of a chemical reaction is an overall average of the results of all these kinds of collisions and to the average this very complicated. An average process. It was essentially impossible until as I mentioned about a dozen years ago but nowadays techniques haven't even been developed that permits one to ask. Very interesting questions about chemical reactions such as How does the probability of a chemical reaction taking place during a

molecular collision depend on the relative speed of the two colliding molecules. About the direction of impact. Where does the energy liberated in the chemical reaction go does it go to make the two products of the reaction speed away from each other. Or does it make polyatomic products vibrate and rotate like mad. These questions were although always very interesting not really asked in the past because where there was no real hope of getting close to an answer. When you talk about probability do you imply that when two men accuse meet sometimes they will do one thing and other times they will do something quite different. Yes that is indeed the case when two molecules encounter one another. If they are just in the right orientation the chemical reaction might take place. If they

are encounter each other with with their axes making a slightly different angle with each other then they might just bounce off each other and the chemical reaction can take place. Instead some of their speed might be transfer into vibration of the bonds that make up these molecules so that to say that a chemical collision to say that a collision occurs between a molecule does not necessarily imply a chemical reaction. There is a certain probability that this reaction might occur in other words to get some idea of how the reaction will occur or what it will result in. We have to know in rather precise detail exactly how these particular individual atoms are behaving how on earth can we go about finding out how particular atoms are behaving when in a general sample as you point out we get a whole mass of atoms all of them having different characteristics and it seems like a

group of people. How do we answer these questions. Well to answer such questions experimentally. One has to start out putting one's house in order. For example I pointed out that. All sorts of speeds molecular speeds were represented in the reacting system. One way of getting you type information is by making the reagents have a single speed or a single energy. And there are tricky experimental techniques which permits one to do this. One is with the help of the photochemistry. Let us consider for example the action of light on some molecule. As we know the action of light is responsible for some very well-known chemical

reactions. The formation of smog is started by a chemical reaction. The beautiful blonde that gets a beautiful suntan is has had her skin undergoing a chemical reaction. When we take pictures chemical reactions also due to the. Action of light are taking place with the photographic emulsion because I think that we should bear in mind Aaron is that most of us are familiar with the fact that when you heat things things happen but also when you shine light on things I launch a number of chemical processes that Karen as you pointed out these occur every day things such as photographic smog or suntan doesn't matter in what sort of way you like these things does sunlight or light from a lamp. Do they have different effects on what goes on when you effect them by light.

Yes indeed they do. The chemical effects of light depend very strongly on their color. Let us consider an interesting example for the point of view to my talk that we're having now. Let's consider the action of light on a very simple chemical molecule hydrogen Iodide. It is made up of molecules which have just two atoms each one is a hydrogen atom and the one is an AI Edina and these two atoms are bound by a chemical bond. Now if one shines light on this molecule which is absorbed by it. The light or the energy absorbed absorbed by the molecule can undergo a chemical decomposition into hydrogen atoms and iodine atoms. Now the

minimum energy needed to produce such a dissociation is the equal to the strength of the chemical bond between the two atoms but if we shine light which is more energetic than that there is this excess energy above the minimum needed to break the bond will find itself in the hydrogen and iodine atoms and make them speed apart from each other. And since the hydrogen atoms are one hundred and twenty seven times lighter than the iodine ones they will. Receive almost all of this excess energy just like when one shoots a bullet out of a cannon. Most of the energy goes with the bullet the cannon recoils a little and that absorbs some of the energy but but a relatively small fraction. In other words and one can use light as an energy source to actually

accelerate atoms. Is this a similar sort of thing to what physicists do in what they talk about is the nuclear processes. In some ways it is and in others it is not. If one changes the color of the light with which one decomposes the hydrogen iodide and uses light of higher and higher energy one produces by photo dissociation hydrogen atoms of higher and higher energies. And in this sense this is a true atomic accelerator permitting one to produce accelerated hydrogen atoms. Accelerators that physicists use are really ion accelerators. They don't take electrically neutral atoms they take a charge to atoms ions which are either bare nuclei or or atoms which have had some of their electrons removed and they grab hold

of the net charge of these ions. With the help of the electric and magnetic fields and accelerate them in that way so that there really ion accelerators that are involved. Not new to atom accelerators as we use I see. And this is a fascinating idea that one can use the energy supplied by light to cause chemical reactions and to cause different chemical compounds to break up. The other interesting thing is the fact that I expect there must be a minimum amount of energy that you require for any particular chemical compound to cause it to break up and that of course is one of your major fields. Could you talk a little bit about this idea of the minimum energy required to cause a chemical reaction. Yes. Let us consider one of the simplest chemical reactions in nature the reaction between a dual Tira matter and a

hydrogen molecule and I do to remap them is just like a hydrogen atom The only difference is that its nuclear is instead of being made up of a proton alone is made up of Deuteronomy which consists of a proton and a neutron So other than it being twice as heavy as a duty or a map as as a hydrogen atom. A deal to a mountain is just chemically the same as a hydrogen atom. Now let's consider the reaction between a duty or man to man a hydrogen molecule to form a deuterium hydride molecule made up of one dough to a man to man one hydrogen atom. And this reaction liberates another hydrogen atom. It has been known for quite some time that for this reaction to occur the relative speed of the two reagents matters and hydrogen molecules must be above a certain minimum. But there was no good direct way of determining this minimum.

We have been able to do this by producing deuterium atoms of known and controllable energy by the atomic accelerator just mentioned in which we dissociate not hydrogen iodide but its rated counterpart tyramine diet. And by changing the color of the light towards light lower and lower energy we produce in this way deuterium atoms of lower and lower energy until no deuterium hydride is formed any more and under these conditions we have a measurement of the minimum energy required for this particular reaction. Now you have been working on this subject for a number of years. How long have you been studying this and your work must of given rise to some very interesting and maybe

illuminating results. Well it is interesting how the ideas for for this type of experiment come about in principle the techniques available for an experiment of this type to be performed have been available for about 15 years. However it has only been much more recently than that and the reason is that number one there was no theoretical reason for wanting to answer such a question because the theory of chemical reactions was in a very primitive stage in the last 5 years or so. Theoretical tools have developed to the point of making possible to also answer similar type questions from first principles and this spurred

motivation towards doing or finding experiments that could do the same thing. And secondly it's a matter of having all of the right equipment at the right time in the right place. Ten or fifteen years ago one would have had to build all of this equipment specially and one man rarely had the know how of all of the experimental techniques involved. Nowadays some of this equipment is purchasable commercially and other parts of this equipment is available elsewhere for example here at Cal Tech. We have done the necessary chemical analysis of the products of the reaction using a very very special mass spectrometer which happened to be available in the department of geology belonging to Professor Sam Epstein who built it for an entirely different reason but was glad to let us use it. Well had we not had that instrument

we would have had to build one ourselves. Having had it we were able to carry out the experiments in a couple of years. So like many famous results one is able to arrive at the onto as one is looking for by interaction with different people who are building equipment which are quite unrelated to what you are talking about or producing ideas which seem quite unrelated and yet all these things can be combined into the whole to produce ideas and solutions where none existed before. Then there's one thing that I would like to pursue. You were talking a little earlier a botch producing a B or A a rheumy of molecules all having exactly the same velocity. This seems to me a a beautiful idea. I'd like to talk about how you do it. Yes this is one of the other very powerful experimental techniques developed in recent years to

study chemical reactions just as one can produce a beam of light by having a lamp and in front of it a pair of cardboard walls with two pin holes so that only light coming through the direction of the two pin holes can get through. One can do exactly the same thing by us for molecules instead of light by substituting the source of light the lamp by a source of molecules namely a vessel containing a gas which has a small opening in it. Then if one strongly pumps vacuum pumps in the space between that container and the first wall containing the pinhole and the space between the first wall and the second wall containing the second pinhole one can produce beyond the second wall beam of molecules that having left

the gas container went through the first wall pinhole then through the second wall pinhole in and finally into. A third region of space where one can use these molecules and if one gate these pinholes such that when one opens the first one and then us a short time later the second one only the molecules that have the right speed to traverse the distance between the pinhole and the and the time interval between opening the first and opening the second will be able to go through. The molecules that move faster reach the second pinhole while it's closed to get there too early in other words molecules will come to move to slowly get there after the second pinhole have been opened opened and closed and still can't make it through.

So in this way one can produce the last selection by adequate gating techniques and so we can produce then a beam of molecules all having a very closely controlled velocity and. The interaction of these beams with other beams and then study the reaction products by the mass spectrometer. But then up to now you've talked entirely about experiment. How does the theoretical side the scratchings on paper fit in with this work. What you are doing once more until a few years ago one would say that although one can write down the basic equations that describe what happens one wouldn't know how to solve them because the solutions require high speed digital computers. Essentially one can say that in a molecule the motion of the electrons doesn't be ordinary mechanics at a base quantum mechanics which

is quite different from our daily experience. But if instead of looking at the motion of the electrons within a molecule one looks at the motion of an MRI of molecules in space as they approach one another. Then one can say that to a very good first approximation not completely accurately but a first approximation and this motion of molecules towards one another seems to be a Newtonian mechanics the same ordinary classical everyday mechanics that describes the motion of baseballs of planets around the sun and of lunar probes. In a chemical reaction one can first of all ask what is the nature of the forces that molecules exert upon one another. And to answer that question one can one has to solve the Quantum mechanics of the electrons which provide the so-called chemical loo. And this can be done nowadays with

modern Kemah with modern computers. And after that once one knows the forces one has to ask how do the these forces affect the motion of molecules as they approach each other and produce a chemical reaction and this involves the exact same type of calculations done by. People involved in the rocket or loon are pro-business and calculating the trajectory of a probe between the Earth and the moon. The main difference is that the forces are different and that in chemical reactions the calculation has to be repeated tens of thousands of times to represent the situation of different types of molecular collisions. However this can be done and for the first time in the history of chemical of chemical dynamics it has been possible to calculate and to measure the reaction probabilities and compare the two with one another so the

theoretician sitting in his office can calculate exactly what the experimenter in his test tube in the room next door is in time to expect. Thank you Aaron. This was about science with host Dr. Peter Dyson and his guest Dr. Aaron Cooper men. Join us again for our next program when host Dr. Albert Hibbs will lead a discussion about hydro magnetics about science is produced by the California Institute of Technology in cooperation with station KPP C. Pasadena California. The programs are made available to the station by national educational radio. This is the national educational radio network.