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Home  / GENERAL CHEMISTRY Textbook / Chapter 11. Physical and chemical properties of substances / Chemical properties of substances

Chemical properties of substances

Until recently, the periodic law was generally formulated as follows:

"Properties of elements are in periodic dependence on the charges of  their nuclei".   

As experimental examples confirming this law, data were given on the FIEs, affinity, valence of elements, and identity of the chemical and physical properties of the elements from one group in the periodic table. This experimental data was not logically connected, but served as an illustration of the periodic law.   

According to our theory, the chemical and physical properties of elements are defined by the FIEs of these elements and by the number of electrons in the outermost shells.  Elements with close FIE values and the same number of electrons in the outermost shell should have almost identical chemical properties.  

Experimental data on elements' FIEs and the number of electrons in the outermost shell change periodically, which leads to periodic changes in the chemical and physical properties of the elements. The periodic system of elements is the main experimental proof of the correctness of our theory of chemical bonding.   

Our theory explains the property changes even when the number of electrons in the outermost layer is the same, while the FIEs change significantly.

For example, all the inert gases have 8 electrons in the shell while their FIEs decrease:  He (24.58eV), Ne (21.56 eV), Ar (15.76 eV), Kr (14 eV), Xe (12.1 eV), Rn (10.7 eV) which accounts for the differences in their chemical properties. 

Now let's see, with the help of some concrete examples, just how our theory of chemical bonding together with the theory of Elementary Interactions (TEI) explain the chemical properties of substances.  

The main chemical properties of substances are their capacity to react.  According to TEI, substance A-B usually reacts with radicals, ions, and conences. As far as radical reactions are concerned, as already mentioned, they proceed along the following scheme:  


1
2 3
A:B + M˙ A:B ... M˙ ⇆  A˙... B:M  →  A˙ + B:M
The reaction rate is defined by stage 3 with a constant rate: 

 W3 = k CA .... B : M e-E/RT 

here — C.... : M  is the concentration of A....: M;  E is the energy necessary to break bond A....B in compound A....: M;  and  W3 is the reaction rate. 

The concentration of A....: M is defined by the radical concentration in the system (M) and by the degree of displacement of equilibrium 2 towards compound A....: M... The concentration of radicals in an M type system is defined by the bonding energy and the concentration of the substance M-M. Concentration M exponentially depends on the bonding energy, i.e., K =  e-E/RT where E is the bonding energy of  M-M.   

The equilibrium constant of isomerization stage 2 is proportional to the difference in the bonding energies of B:M and A:B, i.e., the greater the bonding energy in B:M, as compared with that of A:B, the more of compound A....B:M there will be in the reaction mixture.  Here we have not considered the importance of the Van der Waals bonds (VWB) in both compounds, since these are weaker by more than one order of magnitude, as compared to covalent bonds. Thus, we have: 

      W3  =  k3  exp (EBM - EAB) / RT 

here EBM  and  EAB  are respectively the bonding energies in compounds BM and AB.   

According to this reasoning, the most active radical reactions are molecules, where the M-M bond is weak, and the B-M bond is strong. 

In agreement with our theory of chemical bonding, a weaker bond is observed in diatomic molecules with the atoms' FIEs below 500 kJ/mol and over 1,400 kJ/mol (for homoatomic bonds); the energy of heteroatomic bonds increases proportionally to the difference between the FIEs of the atoms, which form these bonds.  We can expect some active interaction of the molecules formed out of atoms having FIEs below 500 kJ/mol with those formed from atoms with FIEs over 1,400 kJ/mol. 

In keeping with the FIE table, such groups of substances include alkaline metals (Li2, K2, Na2, etc.) and halogens (F2, Cl2, Br2, I2).  The interactions of these substance groups proceed, under normal conditions, with an explosion due to the chain radical mechanism.  

It is evident from our theory that radicals should form out of the molecules of alkaline metals since the covalent bonding energy in these molecules is smaller than that in the halogen molecules.

The correlation of interaction rates is defined by the bonding energy in the metals' molecules and by the difference of energies M-h and h-h where h is a halogen atom. 

Bonding energy M-M in row Li-Na-K-Rb decreases, while the bonding energy of the M-h compounds increases.  Thus, we can assume that the reaction rate of both the molecules and the radicals with halogens, will increase in this row. 

Bonding energy h-h in row F2-Cl2-Br, according to our theory and the experiment, proceeds via the maximum: Cl2 has the maximal bonding energy.

Bonding energy M-h decreases in the row F-Cl-Br-I because the difference in the FIEs decreases insignificantly in row Li-Na-K, while the halogens' FIEs change significantly. For instance, F2 should react with the atoms of alkaline metals more vigorously than any other halogens; Cl2 is much less active than F2.  

One of the TEI conclusions is that the molecules of a substance interact as active species (radicals, ions, conences). 

Previously (before the elaboration of our theory of chemical bonding and the theory of elementary interactions) the reactions of alkali metals with oxides (including halogens) were regarded as oxidation-reduction reactions of the M + h → M - h type. And it was absolutely incomprehensible how these reactions could proceed from the viewpoint of energetics, and how ionic molecules can exist in general.  

A comparison of only the first and second ionization energies with the data on the affinity of the elements to the electron have shown that even the first ionization energies of the elements of the first group have a 2-fold greater affinity energy for fluorine (F) — the most electronegative element. That is, an interaction of the Na + Cl →Na+ + Cl- type is endothermic to about 140 kJ/mol. The energy gain at the expense of the formation of bond Na+ - Cl-comprises a value smaller than 160 kJ/mol.  That is, the bonding energy in molecule NaCl is, according to this calculation, about 20 kJ/mol, i.e., it breaks up at 100° C. 

For form radicals of sodium (Na˙) and chlorine (Cl˙) out of molecules, it is necessary to expend energy equal     ½(EbCl2) + ½(EbNa2); (where Eb is the bonding energy in molecules Cl2 and Na2 relatively). That is, it is necessary to use additionally 256 kJ/mol to break the bonds, and therefore, reaction: Na2+ Cl2 → 2NaCl is endothermic, while reaction 2NaCl → Na2 + Cl2 is exothermic, i.e., proceeding in the reverse direction. Thus, molecule NaCl is thermally unstable.

According to experimental results, reaction Na2 + Cl2 →2NaCl proceeds with an explosion thereby proving that it is exothermic. Thus, the experiment contradicts the theory qualitatively and not quantitatively, which proves the illogicality of the theory of ionic bonding, the TAC, and the TST. 

In the framework of our chemical bonding theory, the calculated energy necessary to break the NaCl molecule into two radicals, is equal to 439 kJ/mol while the experimentally defined result was 455 kJ/mol.  In the course of the reaction, one endothermic stage Na2 ⇆ 2Na which needs an energy of 36 kJ/mol in order to proceed, guarantees a possibility of a versatile procedure of two stages

1 2
Na˙ + Cl2 Na˙ ...Cl Na : Cl...Cl˙, 
which are exothermic.  

At Stage 2 an energy is discharged that is equal to the difference between the enthalpy (1/2 of bonding energy) and the bonds in molecules Na : Cl and Cl :Cl which amount to about 90 kJ/mol. This is sufficient energy for breaking a VWB of Cl...Cl in molecule Na : Cl...Cl˙ which has a value of less than 20 kJ/mol. This means that reaction: Na : Cl...Cl˙ → Na : Cl + Cl˙ is chemically active and can proceed at a great speed. The continuation of the following chain 

1 2 3
Cl˙ + Na2 Cl˙...Na : Na Cl : Na:..Na Cl : Na + Na˙ 

is also a highly exothermic chemically activated route.  

At Stage 2 bond Na : Na breaks and a new bond Na : Cl forms with a discharge of energy that is equal to the difference of the enthalpy energies of these two bonds. That is, about 160 kJ/mol — which is more than enough to break a VWB Cl...Cl in molecule Na : Cl...Cl.  That is, the initiation stage of the chain is the exothermic stage: 

      Na2 → Na˙ + Na˙ 

According to the conditions of the experiment, the topmost Na atoms are radicals, which are the main sources for the initiation of the chain. 

Chapter 11. Physical and chemical properties of substances  >>  
Chemical properties of substances  
Theory of Metallic Bonding   >> 
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