What is a positive enthalpy change
| free enthalpy G|
The first two reaction-driving forces, enthalpy and entropy, say a lot about the direction of a reaction. With regard to the entropy, however, there is a problem, because in a closed system one has to take into account not only the entropy change within the system, but also those of the environment. It would be far easier if one could make statements about the direction of the reaction just by looking at the changes in the system. For this purpose, a new quantity is defined, the free enthalpy G. It is a combination of the quantities enthalpy, entropy and absolute temperature:
Since DH and DS are changes in equations of state, DG is also defined as a measure for the change in a new equation of state.
The free enthalpy says the following about reactions:
At constant temperature and constant pressure, a reaction can only take place spontaneously if DG is less than zero:
The sign of the free enthalpy shows us whether a reaction can take place spontaneously or not in a closed system at constant temperature and constant pressure. Their numerical value indicates how far the initial state is from equilibrium.
Whether a reaction can take place in a closed system is now discussed by considering the individual terms of the free enthalpy equation:
If DH is negative and has a high value, which is the case with strongly exothermic reactions, then DG is also negative when the entropy increases. This means that a reaction can take place although the order increases. An example of this is the reaction between oxygen and hydrogen to form water:
Here, 2 parts by volume of hydrogen react with 1 part by volume of oxygen to form 2 parts by volume of water:
So there are 3 parts 2 formed. As a result, the entropy drops. In addition, water is a gas that condenses into a liquid very easily, which also reduces the entropy. Due to the strongly exothermic reaction, however, the enthalpy change has such a large negative value that the positive entropy values still make the equation negative. The change in the free enthalpy G is consequently negative and thus the reaction can take place without any work having to be added.
The entropy part of the equation has a decisive influence when the enthalpy has a small value or when it is positive (= endothermic).
For example, the following reaction is strongly endothermic:
Carbon dioxide breaks down into carbon monoxide and oxygen.
This reaction takes place even at moderately high temperatures, even though it is endothermic. The drive here is due to entropy. Two gas molecules result in three, which increases the total number by 50% and thus leads to more disorder (= increase in entropy):
Just like when a salt is dissolved in water, where the enthalpy is endothermic and the entropy increases sharply. (see entropy).
The free enthalpy is also called the Gibbs function (after John Willard Gibbs, who formulated it in 1886).
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