Law of mass action
The law of mass action (or " MWG " ) describes the ratio of the activities of the products and reactants of a (chemical) reaction in chemical equilibrium. The ratio is constant. It is applicable to any reaction that is reversible and has already reached its steady state. The resulting solid has a constant, specific for the reaction under consideration value only on the external conditions (e.g., temperature) can be affected under the given conditions. The thermodynamic explanation for this is that there is always a lowest energy and thus best condition. In the chemical and thermodynamic equilibrium so that the reaction does not come to a standstill: both the way as the reverse reaction are balanced, that is, they run the same speed.
Exact formulation
The general formulation is:
These are:
The equilibrium constant K indicates the position of the equilibrium, ie, describes how many product molecules are, how many reactant molecules.
Instead of having the activity of the mass action law is commonly associated with the concentration (in solution), situated to the partial pressure ( reactions in the gas phase) or the mole fraction, which generally changes the numerical value of K. The law of mass action can also be expressed by a combination of these variables (pressure, concentration ... ). To distinguish one adds in the index of K to specify the size, was calculated using those K ( Kc concentration, Kp for the partial pressure, Kx for the mole fraction ), were added. The various Ks can be converted into each other by simple relations. For reactions in dilute solution the concentration normally used. For more concentrated solutions, however, the activity coefficient can differ significantly from 1, so that this approximation should be used with caution. The law of mass action, for example, for the reaction:
Formulated as follows:
Here, c (A ), c ( B) c (C ), c ( D), the molar equilibrium concentrations of the reactants or products. They are also often referred to as [ A], [B ], [C ] and [D ] noted. In the exponent, there are the stoichiometric coefficients, ie the number of particles of this species, which are needed for a formula sales.
An exact derivation of the law which is independent of the pathway takes place in thermodynamics with the aid of the chemical potential.
The law of mass action was first formulated in 1864 by the Norwegian chemists Cato Maximilian Guldberg and Peter scale, but it is listed under Guldberg 1867. They had the law of mass action even from the so -called " active mass " derived ( an archaic term for the activity ) instead of the concentration.
Understanding
Here are some points to watch:
- The MWG is done for each partial reaction. Often it appears in the sum, as there is a response from only one reaction step, but is actually made up of many individual steps with more species than those that appear in the equation, together. This must also be considered (eg all chain reactions).
- The MWG describes only the thermodynamically most favorable state. Factors such as high activation energies may result in that the actual state of equilibrium is not reached ( diamond is under normal conditions only a metastable form of carbon. However, the activation energy for migration to graphite is so high that the reaction in general not take place ).
- All activities used are equilibrium activities whose determination is often difficult.
- At a given reaction, the equilibrium constant K of the temperature and pressure dependent. The pressure dependence is very weak in the condensed phase and is often neglected.
The MWG in semiconductor electronics
The law of mass action states that in semiconductors in thermal equilibrium, the product of the charge carrier densities of valence and conduction band is constant.
With the intrinsic charge carrier density and the density of the free electrons and holes in the thermal equilibrium. The law of mass action applies in intrinsic, ie undoped, as well as in doped semiconductors.