There are very few reactions that go to completion as seems to be indicated by the equation A + B --> P. In principle every chemical reaction is reversible, which means that in the preceding reaction, P -> A + B. Many reactions are shifted very strongly toward the products, and some reactions are very difficult to reverse. If a precipitate or a gas is formed as product, then the reaction is going to be strongly shifted toward the product side. Many reactions actually are in a state of dynamic equilibrium, which means that the rate of production of product has reached a stationary value but the reaction is proceeding both toward the product(s) and toward the reactant(s). The stationary time rate of production of product may be anywhere from perhaps 1% to 99% but the reaction is still proceeding in both directions. We indicate this dynamic equilibrium by using double arrows (). Although each arrow in this nomenclature is of equal length, this does not mean that the reaction is not shifted more in one direction than the other. When a reaction is in dynamic equilibrium, but is known to be shifted more in one direction than the other, you will sometimes see people write double arrows with one shorter than the other. We can experimentally prove that reactions are in dynamic equilibrium by using radioactive tags as discussed in your text.
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