Chemical equilibrium

The chemical equilibrium is a state in which the overall reaction appears dormant, so no changes are visible. The externally observable reaction rate is zero. Nevertheless, the chemical reactions ( "go" - and " back " reaction) continue to run from, and indeed the same speed in both directions. It is therefore not a static balance as viewed externally appear, but a dynamic equilibrium, further reactions take place in the. The Gibbs free energy (G ) served a minimum and the equilibrium constant (K ) is defined by the law of mass action.

Of equilibrium

In the steady state the ratio of the product of the concentrations of the products and the product of the concentrations of the starting materials is constant. The concentration of the reactants at equilibrium is called equilibrium concentration.

The value of the equilibrium constant is a function of temperature and characteristic of each reaction. It is the case of homogeneous reactions in solutions but also depends on the solvent in which the reaction takes place.

Although return reaction to proceed resistant, ie reactants into products and these are in turn converted into reactants, the concentrations of reactants and products do not change in balance. This is because in equilibrium the rate of forward and reverse reaction is exactly the same size, that is, per unit of time as much reactant to product reacts as product is consumed by the reactant.

The equilibrium state is not to be confused with a dormant chemical reaction. Even with this, the concentration of reactants and products change virtually nonexistent, but the chemical reaction is slowed down further.

The law of mass action

→ Main article: Law of mass action


The rate of a chemical reaction is directly proportional to the activity of the reactants:

The higher the activity of the starting materials, the faster the reaction proceeds.

In the course of an equilibrium reaction, the activity of the starting materials will decrease the time. This also reduces the speed of the forward reaction. Simultaneously, the activity of the products is constantly increasing. Thus, the rate of the reverse reaction increases. Finally, both reaction rates equal, in the same time intervals as much product as the starting material is formed, i.e., that equilibrium is reached.

In the equation of the equilibrium arrow is used to describe:

The forward reaction rate of the chemical or the chemical reaction is thereby reverse:

It is the rate constant for the forward reaction Khin Krück and the rate constant of the reverse reaction.

At equilibrium, the speeds of the forward and the reverse reaction are equal:

It follows for the equilibrium or equilibrium constant:

Equilibrium position

The position of equilibrium - and thus the equilibrium constant - is determined by the reaction conditions, temperature, pressure and molar concentration:

  • Is the equilibrium constant of very large, the equilibrium is practically completely on the side of the products.
  • If the equilibrium constant is very small, the equilibrium lies almost completely on the side of the reactants.

The equilibrium constant tells us something about which side of the chemical equation, the equilibrium is: An increase in the equilibrium constant K is a shift in the equilibrium to the product side, a decrease of K implies a shift of the equilibrium to the reactant side.

Explanation: Have almost all reactants react to products, it is said " the balance lies with the products."

Influence of a catalyst

A catalyst accelerates or decelerates return reaction in the same manner. It does not change so that the equilibrium concentrations of the reactants and products, but the effect that the state of equilibrium faster. The function of a catalyst based on the opening of a new reaction path, which runs on other elementary reactions than the uncatalyzed reaction. At these elementary reactions, the catalyst is indeed personally involved, but he leaves himself unchanged the process ( chemical).

Disturbance of the equilibrium - Le Chatelier's principle

If a chemical equilibrium is disturbed, then that reaction is accelerated from, which makes this disorder reversed. This is called therefore also the "principle of least constraint " ( Le Chatelier's principle ): The " coercion", which is imposed on the balance by the disturbance is compensated by the accelerated reaction.

Interference are:

  • Concentration changes or changes in quantities ( by adding or removing one of the substances involved in the balance )
  • Supply or removal of heat or temperature changes
  • Change in the pressure
  • Change in the volume of gas reactions


The greater the difference in the free enthalpy () between reactants and products, the more the equilibrium lies on the side with the lower free energy. (Note: see standard condition )


See a free enthalpy

For redox reactions is considered:


For an electrochemical redox reaction results in the free enthalpy of the converted amount of substance n mol, the Faraday constant F and the potential difference. Energy is delivered as long until the electrochemical equilibrium is reached:

Entropy in reactions

Whether a reaction of the reactants towards the products passes ( and how far ), that depends on whether it increases the entropy. For example, this is already the case when a gaseous product can spread over a larger area. But it is not only the change in entropy of the reacting components. In the course of a reaction and heat ( enthalpy ) is usually exchanged with the environment and this causes there is also a change in entropy: Dividing the equation

By the absolute temperature T, we obtain a relationship between three quantities having the dimension of entropy ( J / K):

This refers merely a step along the turnover variable. is the amount specified in the entropy of the environment that has absorbed or released heat of reaction. The fact that a negative to a net increase in the total entropy corresponds to, is on the sign, which depend on the reacting system: Is this eg heat, then becomes negative and the expected contribution to is also negative. ( But in the area takes the entropy by the same positive amount to. ) If also the change in entropy of the reacting system is positive, obtained by the minus sign in the equation for a negative contribution. The reaction proceeds so long, so long G decreases or is negative. At a minimum, the Gibbs-Energie/Freien enthalpy G then are back and reverse reactions at equilibrium.


  • Calcite precipitation
  • Ammonia synthesis (Haber -Bosch process )
  • Chlorine production by the Deacon process
  • Synthesis of Esters