Rates and Reaction Pathways The How Fast Question

The chemical reactions frequently used as classroom demonstrations are usually fast. The color change, bubbles of gas, or explosion happens right away as visible proof that a reaction has occurred. Many reactions are also naturally slow, however. At everyday conditions of temperature and pressure, for example, the conversion of carbon monoxide to carbon dioxide is slow. 

It is sometimes unfortunate that this reaction isn t fast. Breathing too high a concentration of carbon monoxide is fatal. 

Out of curiosity and also for practical reasons, it is interesting to discover what makes reactions fast or slow. Ideally, to do this a chemist would like to watch the pathway of each atom from its position in the reactants to its position in the products. 

What is the connection between the rate of a process and its pathway Suppose that there are several hundred books in a storeroom on the first floor and that you have been hired to move them to the new third-floor library. Depending on the conditions, there are a variety of possible pathways. One pathway might require the following steps (1) put 10 books (the maximum number you can lift) into a carton, (2) carry the carton up one flight of stairs to the second floor, (3) rest a bit, (4) carry the carton up another flight of stairs to the third floor, (5) empty the carton, and (6) return for another load. Another pathway might involve a different series of steps (1) fill four cartons with books, (2) pile the cartons onto a dolly, (3) push the dolly onto the elevator, (4) ride to the third floor, (5) push the dolly off the elevator, (6) empty the four cartons, and (7) return for another load. The second pathway would probably be faster than the first. To compare them quantitatively, the rate for each pathway could be measured in books moved per hour. 

The details of chemical reaction pathways can be extremely complex, and they are very hard to study. With the aid of computers, sophisticated electronics, and clever new techniques, however, advances in this area of observation are being announced regularly. 

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