The law of mass action, rates and equilibria

The rates of chemical reactions almost always obey the law of mass action. Although the direct proportionality to concentration is sometimes modified, this law states that 

The proportionality constant is known as the rate constant for the reaction in question. For particular sorts of reaction this name may be given a rather more descriptive name, for example, the association rate constant for a reaction involving association of two molecules, the dissociation rate constant for the reverse reaction, or the isomerization rate constant for a reaction that involves isomerization of one form of a molecule to another. The rate constant is a measure of how fast a reaction takes place (for a specified concentration), or, more precisely, it indicates how frequently the reaction occurs (hence it has the units s ). At the level of single molecules the rate constant is a measure of the probability (per unit time) that the reaction will happen in the next time interval. 

It will be supposed in all that follows that rate constants are indeed constant in the sense that they change neither with time nor with reactant concentration. The value of a rate constant may, and usually will, depend 

It is important to mention in this context that in many reactions changes in ionization result in the uptake or release of protons (see sections 3.4 and 6.4). Unless the effect of proton concentration is being considered explicitly as part of the postulated reaction mechanism, it is also essential to keep pH constant during the observation period. The importance of protons, and hence of pH, was recognized by Sorensen (1912) when he introduced the concept of buffered solutions. The effects of changes in pH on the reactivity of enzymes and other proteins are discussed in sections 6.4 and 3.4. 

The use of the law of mass action to describe a simple binding reaction 

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