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The acidity concept – new perspective
The acidity concept – new perspective
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The acidity concept – new perspective
08/07/2008 13:25:09 »
Background and actual explanation
Even the substances with acid and alkaline character are known from ancient times, first theories regarding these phenomena are proposed later, after XVII century.
Boyle was first who proposed this classification in acids and basis having as background their properties:
Acids present sour taste, are corrosive to metals and become less acidic when mixed with bases.
Bases feel slippery and become less basic when mixed with acids.
Lavoisier was trying to explain the comportment and composition of acids; according to him acids are compound which contain oxygen. The idea turned out to be wrong as was demonstrated by Davy’s experiments, who supposed that hydrogen was the cornerstone component of an acid. Liebig proposed that an acid is a hydrogen-containing substance in which the hydrogen could be replaced by a metal.
Arrhenius proposed the first argued theory of acid and basis based on electrolytic dissociation. According to this, depending on the chemical structure, certain substances remain ionized in solution all the time.
An acid is any substance which delivers hydrogen ion (H+) to the solution and a base is any substance which delivers hydroxide ion (OH¯) to the solution.
The theory limit the acid-base character only to hydrogen or hydroxyl containing substances and does not take into consideration the importance and influence of solvent used.
Brønsted and Lowry published, in 1923, a new theory about acids and bases behavior, where an acid is a proton (hydrogen ion) donor, comparatively with a base which is a proton (hydrogen ion) acceptor.
The theory is used even in our days, with small amendments (a proton is not freely donate, it is necessary an energy to be removed from acid), because it is considered to describe well the comportment of acid and basis in protic solvents. The theory fails to explain the acid base behavior in case of aprotic solvents.
Lewis overpasses this problem and in his proposed theory and acid is an electron acceptor, by comparison with a base which is electron donor. In a modern approach, "Lewis acids" are electron-deficient substances and "Lewis bases" are electron-rich substances.
Why are the actual theories defective?
1. The acidity strength problem
If the acidity strength of hydrogen halides is analyzed, it is observed that the acidity increase from HF to HI according to following series:
HF, HCl, HBr, HI
Taken into consideration the electronegativity of elements, when water is used as solvent, the actual interpretations appear contradictory.
As is observed in tab1., the polarity of OH bond is intermediate between HF and HCl bound.
Arrhenius electrolytic dissociation theory supposes that water can dissolve many compounds by separating them into their individual ions.
If the concept of solvent polarity is the reason for electrolytic dissociation, according to up presented data, the comportment of hydrogen halides should be:
• Water dissociate in a stronger manner HI, less HBr and lesser HCl;
• When HF is added to water, due to its stronger polarity, water should be dissociated in ions according to following reaction:
The explanation is very simple: flourine is the most electronegative element and of course flourine gains a proton from water. It is absurd in actual explanation that oxygen takes a proton from a HF.
• In actual scientific texts the influence of acid over solvent is not considered. This is an absurd thing. For example, in case of a 50% concentration of HCl solution, there should be a closed number of chloride ions and hydroxyl ions. This is because chlorine and oxygen present closed electronegativity and consequently not only water convert HCl in ions but also HCl convert water in ions
In proposed theory when hydrogen halides are dissolved in water, depending on the strength of H-X bond, a greater or smaller number of radicals are formed in solution according to reaction:
where X = F, Cl, Br, I
The water molecule has no force to separate a covalent molecule into their individual ions. In the same time, not only the hydrogen halide liberates radicals, depending on the concentration of solute, the water (solvent) liberate radicals, too.
In this case it is very easy to explain the increasing of acidity when H-X series is analyzed. With increasing number of shells in halogen atom, it is more accessible to break homolitical a HI bound instead of a HF bound.
The acid character, in the proposed theory, is given by the number of free hydrogen atoms (radicals) existent at a certain moment in solution.
2. Acidity structure problem
Let’s consider some simple reactions between a metal and an oxoacid with formation of hydrogen. For the simplicity of example a monovalent metal will be used, but the conclusions are the same in case of other less reactive metals like Ca, Zn, etc.
Na + HClO4 = NaClO4 +1/2 H2
2 Na +H2SO4 = Na2SO4 +H2
3 Na +H3Po4 = Na3PO4 +3/2 H2
According to actual interpretation acids are dissociated in ions in solution and the reactions are simple change between metal and hydrogen with electron transfer.
There is a procedure of attributing formal charge to elements and according to this the entire process can be described as follows:
It is considered that Na loose an electron, and is transformed in cation which bind to the existing anion (perchlorate) and the proton gain an electron from sodium, followed by a combination between two hydrogen atoms and molecule generation. As is observed the perchlorate anion remains unchanged as structure and as charge according to actual quantum chemistry.
The structure and length of bounds in case of perchloric acid is presented in fig. 1. As is observed hydrogen is bound to an oxygen atom and this oxygen is bound to chlorine atom by a different length bound in comparison with the other three Cl-O bounds. Using actual quantum mechanic terminology, an oxygen atom is bound to Cl by a sigma bound, and the other three oxygen atoms form double bound with chlorine.
Figure 1. Perchloric acid molecule details
The structure and length of perchlorate anion shows another reality. All bound and angle in perchlorate anion are equals as presented in fig. 2
Figure 2. Perchlorate anion details
How can be interpreted in frame of actual quantum chemistry the change in the length of Cl-O bound during this process?
According to quantum chemistry when perchloric acid is dissolved in water, there is a dissociation process and the proton is removed from molecule. The electrons of H-O bound remain at oxygen and the supplementary electron jump from oxygen to oxygen in order to give the well known resonance formula. Probably the electron makes this jump through a quantum tunnel effect or a spatio-temporal distortion, because it is impossible to pass via chloride atom.
The same similar discussion can be made in case of other oxoacids ( sulfhuric, phosphoric, etc.) reactions.
In proposed mechanism the anion participate active at reaction process.
First difference between actual and proposed theory regards the ionization phenomena. During ,,so called” acid ionization, in proposed theory there is not a separation of a covalent molecule into their individual ions. In this process, which must be renamed because is not ionization, only a transformation in radicals is counted.
Consequently in case of a perchloric acid solution, the perchlorate radical in water has quite the same structure like in perchloric acid. It is possible to have a small modification of angles or Cl-O lengths in order to compensate the homolitic cleavage of O-H bound. For a simpler representation, the following structures are represented without taking into consideration the geometry of double bound (as was presented the magnetic moments of the pi type bound are perpendicular on the line which pass through the nuclei).
The global reaction can be formulated between metal and acid is:
If the reaction is analyzed in detail, it is observed the anion primordial importance for reaction.
As was already presented with 7 electrons on the last shell, when all these electrons are decoupled, chlorine can form 7 covalent bound due to the compensation of these magnetic moments with other 7 magnetic moments from other atoms participating at bounds formation. The structure of perchloric radical presents an uncompensated electron magnetic moment at one oxygen atom.
In the reaction condition the stabilization of these radical is made due an electron transfer from metal (Na) to chlorine which arrive at 8 electrons on the last shell. In proposed theory must be highlighted that an 8 electrons structure is not a sine qua non condition for a compound stability. Most important is the stability of spatial arrangement of atoms in a spatial template.
Chlorine with the electron gained from metal form a new covalent bound with unpaired electron from oxygen through a compensation of their magnetic moments.
The hydrogen radical existent in solution combines and forms hydrogen molecules.
In proposed theory the formal charge and other accepted concepts of actual chemistry are ruled out, being without any reasonable support.
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Last Edit: 08/07/2008 20:21:59 by sorincosofret
Neilep Level Member
Re: The acidity concept – new perspective
Reply #1 on:
08/07/2008 20:04:36 »
For Christ's sake stop wittering on about radicals.
Radical reactions are well documented, they show up in ESR experiments, they are frequently photochemical. They are markedly affected by the presence of radical scavengers such as iodine or oxygen.
None of these properties applies to the dissociation of HCl in water. It's not, never will be, and never has been a radical reaction.
While I'm at it "water should be dissociated in ions according to following reaction:"
That might be your interpretation but I don't think you will find many who agree.
is not the same as
Last Edit: 08/07/2008 20:13:33 by Bored chemist
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