Some reactions, such as the burning of a hydrocarbon in air to form carbon dioxide and water, are highly exothermic, and the products of the reaction are widely dispersed. Such reactions proceed essentially in one direction only. However, most reactions do not proceed in only one direction. If two compounds, A and B, can react together to form X and Y then X and Y can react to form A and B. The reactions can be written as:

  • 1) A + B ^ X + Y
  • 2) X + Y ^ A + B

Starting with only A and B in the solution, at first only reaction (1) will occur, forming X and Y However, as X and Y accumulate, so they will undergo reaction (2), forming A and B. Similarly, starting with X and Y at first only reaction (2) will occur, forming A and B. As A and B accumulate, so they will undergo reaction (1), forming X and Y

In both cases, the final result will be a solution containing A, B, X, and Y. The relative amounts of [A+B] and [X+Y] will be the same regardless of whether the starting compounds (substrates) were A and B or X and Y. At this stage the rate of reaction (1) forming X and Y and reaction (2) forming A and B, will be equal. This is equilibrium, and the reaction can be written as:

If there is a large difference in energy level between [A+B] and [X+Y] — i.e. if the reaction is exothermic in one direction (and therefore endothermic in the other direction) — then the position of the equilibrium will reflect this. If reaction (1) above is exothermic, then at equilibrium there will be very little A and B remaining — most will have been converted to X and Y. Conversely, if reaction (1) is endothermic, then relatively little of the substrates will be converted to X and Y at equilibrium.

At equilibrium the ratio of [A + B]/[X+Y] is a constant for any given reaction, depending on the temperature. This means that a constant addition of substrates will disturb the equilibrium and increase the amount of product formed. Constant removal of products will similarly disturb the equilibrium and increase the rate at which substrate is removed.

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