What makes a stable connection

Stability of chemical compounds

The question often arises as to how stable connections are. The atoms in compounds are held together by the chemical bond. All chemical bonds are based on the formation of the energetically most favorable state (lowest energy state). The formation of a fully occupied outer shell (noble gas configuration) is often mentioned here. In the following, the stability of connections will be discussed in more detail.

The stability of chemical compounds

Chemical compounds exist i. d. Usually from several atoms or ions, these are held together by the chemical bond (atomic bond, metal bond and ionic bond). This chemical bond is based on electrostatic interactions (metal and ionic bonds) or interactions between valence electrons of two atoms that form a common pair of electrons (atomic bond).

Now the question arises, how can one find out how stable a connection is?

The question arises as to what a stable connection is. This could be answered in such a way that the compound can be obtained as a product in a reaction and this can also be isolated. If one looks at an (exothermic) reaction, there are two “types” of stable products.

  • One is the kinetically stable product that is the first to arise in a reaction and which in itself forms an isolatable (detectable) product. This kinetically stable state is mostly only a metastable state, i.e. if enough activation energy is added, the system can reach the most energetically favorable state (thermodynamically stable state). As a rule, the activation energy is so low that practically only the thermodynamically stable state of a compound exists.
  • The thermodynamically stable product is different. A product is stable in the thermodynamic sense if the reaction under consideration (= production of the product) shows a negative change in Gibbs energy (D G <0). It could also be formulated in such a way that the change in Gibbs energy is positive for every chemical change in the thermodynamically stable compound (D G> 0)

This would clarify how stable compounds are in relation to the elements contained in the compound (-> compound breaks down into the elements, e.g. CS2 -> C + 2S). If the change in the Gibbs energy of the reaction from the elements to the compound is negative (D G <0), then it is a stable compound (compared to the elements contained in the compound). It should not be forgotten that this only applies to the reaction product if no energy is supplied to the product. For example, the thermodynamically stable compound water can be broken down into its components - hydrogen and oxygen - with the help of electrolysis (supply of electrical energy).

In addition, one can ask how stable a connection is compared to another connection:
The comparison between elements is much easier (this is also discussed here on Lernort-Mint.de), and one makes use of the electrochemical series. The reduced form of an element reacts with the oxidized form of an element when the reduced form has the more negative normal potential, e.g. Na (red. Form, - 2.71 V) + Cl 2 (ox. Form, + 1.36 V) to NaCl (for more details see: Redox process prediction).

The question of the stability of a connection in relation to other elements or connections is not so easy to answer universally. Since the question of "stability" must always be asked about stability in relation to what, e.g. to acids, to oxidizing agents, reducing agents, etc. There are different methods for the different concepts:

  • Base + acid in an acid-base reaction - aids are the acid constants pKs or the HASB principle
  • Compounds vs. oxidizing agents or reducing agents - Aids are the normal potentials or their difference D E °
Since there are so many different concepts, the question of the stability of a connection compared to others cannot be generally answered (one can thermodynamically argue that in a system (e.g. two connections) the most energetically most favorable products are always formed, energetically favorable products are e.g. Water). Calculating these systems would lead deep into physical chemistry or quantum theory, so it is better to know general reaction schemes (a few examples are listed below):
  • Metal oxides usually dissolve in water to form the corresponding base (metal hydroxide)
  • Non-metal oxides usually dissolve in water with the formation of the corresponding acid.
  • Base metals and water react to hydrogen and the corresponding base (metal hydroxide).
  • Precious metals do not react with non-oxidizing Brönsted acids (an oxidizing acid is e.g. HNO3)

Further information on Lernort-Mint.de