Fundamentals of chemical equilibrium
In the following graphic, we have first drawn a simple model example for equilibrium reactions – the apple war: At the beginning of the experiment there are 9 apples on both sides of the field. Both parties, i.e. the grandfather and his grandson, now throw the apples on the other’s property. After some time, a state of equilibrium is reached. However, since grandchildren and grandfathers do not stop throwing and the balance does not change, this is referred to as a so-called dynamic balance. What happens here can be described as an equilibrium reaction, even if the apple war is of course a very abstract representation.
Fig. 1: Model experiment ‘The Apple War’ on the chemical equilibrium
law of mass action
What applies here in the abstract model ‘the apple war’ also applies to chemical equilibrium reactions. This provides a general formula for this Maggregation law. The law of mass action makes statements about the position of the equilibrium of a chemical reaction.
To illustrate, consider the following general reaction as an example:
The formula for the law of mass action is then:
If you want to learn more about calculations using the law of mass action, check out our detailed summary.
entropy
An entropy value can be uniquely assigned to each equilibrium state of thermodynamics. In the entropy it is one extensive state variable of thermodynamics. In the so-called TS diagram one can record and visualize irreversible chemical reactions. The entropy S is an extensive state variable. Dividing the entropy S by its mass gives the specific entropy s. If you want to learn more about entropy, feel free to check out our in-depth article on entropy on .
enthalpy
The enthalpy is also a measure in the field of thermodynamics. The heat content of a thing is determined by the enthalpy. The symbol for the enthalpy is the H (heat content) and the associated unit Joule.
The general formula of enthalpy is:
- Enthalpy H
- internal energy E
- pressure P
- Volume V
free enthalpy
the free enthalpy is a measure of the driving force of thermodynamic processes. It is first determined by the ‘normal’ enthalpy as described above and then by the Gibbs-Helmholtz equation calculated. The free enthalpy indicates the change in the so-called Gibbs energy. It represents a special form of enthalpy.
Theorem by Hess
Of the Theorem by Hess is used to calculate changes in enthalpy. According to the law of conservation of energy, the reaction path has no influence on the energy that is absorbed or released during a reaction. Hess’s theorem is also called Hess’ heat theorem and reads as follows:
Haber Bosch method
That Haber-Bosch process is a useful tool for the synthesis of ammonia, which is often used in large-scale industries. The Haber-Bosch process was named after the two German chemists Fritz Haber and Carl Bosch. Ammonia is mostly used to make explosives or other nitrogenous chemicals.
In the following graphic you can see exactly how the Haber-Bosch process works in practice:
principle of least constraint
Henry Le Chatelier and Ferdinand Braun formulated this in 1884-1888 principle of least constraint, also called Le Chatelier’s principle. It says:
«When a chemical system in equilibrium is subjected to a compulsion, it responds in such a way that the effect of the compulsion becomes minimal.»
In simple terms, this means that when an outside force is applied to a system in equilibrium, a new equilibrium will then be established which evades the outside compulsion. A compulsion can be, for example, a change in temperature, pressure or concentration of the amount of substance.
Here is an example:
- The compulsion to increase the temperature is avoided with heat consumption
The most important things about equilibrium reactions at a glance!
- Equilibrium reactions = reactions in which the equilibrium remains unchanged over time
- a simple example to illustrate equilibrium reactions is provided by the model experiment ‘the apple war’
- the general formula for chemical equilibrium was formulated using the law of mass action
- Important units of measurement: enthalpy with Hess’ theorem, the free enthalpy, and the entropy
- The so-called Haber Bosch process is an important tool in the synthesis of ammonia
- The principle of least constraint states that chemical systems switch to external constraints with a modified chemical equilibrium
Made!
Now you should be an expert in the field of equilibrium reactions or chemical equilibrium. You can find in-depth exercises and learning materials on this and many other school topics on . Have fun with your studying!