In the last third of the nineteenth century different scientists tried to find a connection between energy and the driving force of chemical reactions. Since reactions that go more thoroughly to completion tend to be exothermic, the French chemist Marcelin BERTHELOT (1827-1907) proposed a first principle:
"The thermal energy of a chemical reaction and the constancy of the products formed spontaneously at these reactions are a measure for the chemical affinity."
By affinity (Lat. kinship) the old chemists understood the ability of materials to react with each other; those that exhibited this tendency were considered to be "kindred". Also the English scientist Sir William THOMSON (lord KELVIN) independently developed essentially the same hypothesis, namely that endothermic reactions could not take place spontaneously (at that time no conterexamples were known; the endothermic dissolution of salts was not then considered a chemical process.)
The American chemist and physicist J. Willard GIBBS (1839-1903) of Yale university was the first to formulate a fundamental theory theory of the thermodynamics of the driving force of chemical reactions. Also the Dutch chemist van't HOFF, who worked at the Prussian academy of sciences, demonstrated experimentally in 1906 that the energy released in a reaction does not of itself determine whether or not a reaction proceeds. Van't Hoff ascertained when mixing natriumsulfate (=sodiumsulfate) with kaliumchloride (=potassiumchloride) a endotherm reaction under liquefaction. At this reaction the order in the crystall lattice changes.
In highly exothermic reactions the extent of reaction is controlled mainly by the amount of heat released, but only to a first approximation; the entropy change must also be considered. This was first grasped by the German chemist and physicist Hermann ULICH (1895-1945): An exact description of the driving force is only possible however, if the change of the order of a system, i.e. the change of the entropy before and after the reaction is considered. This was first grasped by the German chemist and physicist Hermann ULICH (1895-1945):
"Everything in our world strives in the direction
decreasing enthalpy and increasing entropy. "
(Principle of minimizing the energy and maximizing the disorder.)
The reaction enthalpy is the quantity of heat exchanged with the surroundings at constant pressure. The entropy is a measure of the disorder of a system. The Austrian physicist BOLTZMANN (1844-1906) tied the entropy with the statistical probability W of a system.
S = k * ln W (S = entropy, k = BOLTZMANN-constant)
Example: For an ideal crystal at the absolute zero point (- 273°C)there is only one possible arrangement (W=1); in this case the value for the entropy is 0.
Whether a chemical reaction is possible or not is therefore both dependent on the changes in enthalpy and entropy. The Gibbs - Helmholtz - equation expresses this connection.
d G = d H -T * d S
d = delta, difference
If you own a textbook with data for enthalpy and entropy, you can use this equation to predict whether or not a chemical reaction can proceed spontaneously. (possible =exergonic, d G has a negative value, not possible =endergonic, d G has a positive value). If not, you can see in the following examples, which results the software Gibbs-Energetik will present.
Gibbs-Energetik for Windows 3.1 and 95R combines the necessary calculations with an integrated data base which contains several hundred relevant thermodynamical values which can be changed by the user any time. Reactants and products are selected by mouse clicks into the spreadsheet as shown below:
Example: Synthesis of Ammonia from its elements
From the database in Gibbs-Energetik the reactants hydrogen and nitrogen and the product ammonia are selected:
Gibbs-Energetik sets up the chemical equation and calculates the free energy (d G), in this example at 298 K. In this case d G has a negative value, i.e. the reaction is possible under these conditions. A mouse click on the button "K" calculates the equilibrium constant at this or some other selected temperature.
Gibbs-Energetik can present all results graphically. The temperature range was chosen between 0 - 1000 K. For the synthesis of ammonia and for the synthesis of methane from the elements, you see at a glance when the reaction passes over from spontaneous to the non-spontaneous area, i.e. where the red "zero - line" cuts. The Gibbs - Helmholtz theory gives only statements whether a reaction is thermodynamically possible; It remains the art of the chemist to make it take place at a satisfactory rate.
For this example the diagram shows that the synthesis of ammonia should be accomplished at lower temperature. Nitrogen is however kinetically inert at low temperatures; below 450 K a catalyst is necessary. Suitable catalysts were first developed by Haber and Bosch at the beginning of this century. Special biocatalysts (=enzymes) containing molybdene make it possible to some plants (legumes like soybean, lenses, peas, beans) to perform this reaction at environmental temperatures.
Gibbs-Energetik makes it easy to explore new reactions. For example, you (or your students) can compare the enthalpies of non metal hydrides, the homologous series of the alkanes or alkenes and so on, all over an arbitrarily chosen temperature range.