Chemical reactions can be irreversible, like the combustion of hydrogen gas, or reversible, like the ionization of acetic acid. Equilibrium constants can be calculated for reversible reactions, which depend on system conditions like temperature. Irreversible reactions also follow mathematical relationships, but the equilibrium constant is either infinity or 0, making the information useless. Removing a product from the system can make a reversible reaction irreversible.
Some chemical actions proceed irreversibly in one direction. An example is the combustion of hydrogen gas (H) in oxygen (O) to produce water, as shown in the formula 2 H2 + O2 => 2 H2O. The opposite reaction, 2 H2O => 2 H2 + O2 does not happen under these conditions, no matter how much time passes. There are reversible reactions, as the chemist Claude-Louis Berthollet discovered in 1803. Reversible reactions proceed in one direction until the reverse reactions become the favored ones, resulting in equilibrium and making it possible to calculate equilibrium constants.
These equilibrium constants have been derived from mathematical relationships revealed over time through the efforts of many scientists. These reports use the ratios of the concentrations of the dissolved species in the reaction system. A simple example is the ionization of acetic acid. Another is the reversible breakdown of dinitrogen tetroxide gas. In these, as in all examples, the equilibrium constants depend on system conditions such as temperature.
Acetic acid dissociates into a positive hydrogen ion plus a negative acetate ion. What makes the reaction reversible is that these ions can and do recombine into acid molecules. Other acetic acid molecules then dissociate to replace those that have recombined. The result is equilibrium, which leads to a mathematical expression. The concentrations of ions and acids refer to the equilibrium constant by the expression K = (H+)(Ac-)/(HAc). Logically, the equilibrium constant for the reverse reaction is the inverse of this K, because acid concentration becomes the numerator and ion concentrations become the denominator.
For dinitrogen tetroxide, which contains nitrogen (N) and oxygen, the chemical reaction is written N2O4 ⇆ 2 NO2. Any change in the proportion of these two species in a closed system depends on the change in system pressure; for every molecule of tetroxide that decomposes, two molecules of nitrogen dioxide are formed, increasing the pressure. This requires energy and, beyond a certain point, unfavorable splitting. The equation is K = (NO2)(NO2)/(N2O4). As with acetic acid, the equilibrium constant for the reverse reaction, as with all equilibrium constants for all reverse reactions, is the inverse of this K.
Irreversible reactions obey the same mathematical relationships as reversible reactions. In such cases, however, the denominator becomes 0 or infinity, whether the forward reaction or the reverse reaction is examined. This suggests an equilibrium constant having an opposite value, either infinity or 0. Such information is useless. Also interesting is the possibility of completing a reaction, making it irreversible by removing one of the products from the system, for example through a semi-permeable membrane which retains the reactants.
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