LIMITATIONS: THE THEORIES (of Arrhenius & Bronsted-Lowry)


The theory of Arrhenius has a lot of limitations; in many cases his theory does not state true. Therefore his theory is in today´s society mostly spoken about and referred to in order to relate it with the more specifically detailed theories (in order to see the evolvement of the theories on acids and bases). Some limitations of his theory will be discussed:

Even though Arrhenius´s theory states true for many acids and bases, we must begin by criticizing it since the theory does not clarify why Ammonia (NH3) might act as a base; because it is proven that NH3 cannot produce any hydroxide ions (OH-)  – so it cannot be a base!  When it has neutralized, it only works as a base.
Here is an example of a neutralization reaction of ammonia with hydrochloric acid (HCl) where ammonia acts as a base:
(All aqueous) NH3  + HCl   --> NH4Cl             
We can clearly see that there is no OH- ion involved in this neutralization reaction. SO; in order for the issue to be overcome we can state that: “Hydroxide ions (OH-) and ammonium (NH4+) is formed when ammonia (NH3) reacts the water (H2O) which it is a solution in”. We can prove this by this “Reverse reaction”:
NH3(aq) + H2O(l) -->
 NH4+(aq) + OH-(aq)
Studies show that the reverse reaction of NH3 with H2O just reacts (at equilibrium) to an extremely small amount (less than 3%) and therefore only produces a small amount of hydroxide ions (OH-). Since there is actually OH- present, a lot of people state that Arrhenius’s theory covers the “ammonia problem” and that it is only described as a weak base in water.  Additionally, the requirement for hydroxide as the base made Arrhenius suggest the formula NH4OH as the formula for ammonia in water. This led to the confusion that NH4OH is the actual base instead of NH3.

Then, it is also proven by this formula, that the EXACT SAME product is produced when Ammonia and Hydrogen Chloride Gas react as the product they produced in aqueous solution even though the Gas form is involved in this case.
NH3(g) + HCl(g)  -->
As we can see and understand, this reaction is not actually an Arrhenius Acid-Base reaction. There are no hydroxide ions present. But it is still producing the same product as it did when it was in aqueous solution.

Another problem with Arrhenius´s theory is that odd base and acid types like for instance `ternary and polyprotic acids´ are not analyzable and explainable by Arrhenius theory. 

As a final point, Arrhenius theory does not take the solvent into thought. It is proven that an acid should to be an acid in whichever solvent, but as it is proved; this is not the case. For instance, HCl is an acid in aqueous solution, but if HCl is dissolved in benzene (C6H6), there will be no dissociation and it would therefore not be considered as an ´Arrhenius acid´. 

Improvements from theory of Arrhenius:
If we now look at the difference between the two theories - it can easily be said that the Bronsted-Lowry theory has just added some specific details to it as mentioned earlier. So if we look at the example of the reaction between the Ammonia (NH3) and Hydrogen Chloride Gas (HCl) it is not an Arrhenius acid-base reaction because there is no OH- present, however, with Bronsted-Lowry´s definition of acids and bases, it is actually an acid-base reaction.
NH3(g) + HCl(g)  -->
The H+ (proton) from the HCl is accepted by the NH3. An ionic bond between the NH4+ and the Cl- is then formed in order to form ammonium chloride. The need of OH- ions is not necessary in the Brønsted-Lowry theory. As long as a compound can accept a proton, it is a base, and as a result, ammonia is certainly a Brønsted-Lowry base. The existence of NH4OH is no longer necessary to explain why ammonia is actually a base, and no longer are there any misunderstandings about whether NH4OH or NH3 is the actual base.

Another improvement is that the Brønsted-Lowry theory explains the idea of conjugate pair relationships and amphotheric substances: some substances, for instance water, can actually accept and donate protons. These substances are therefore so called: “amphiprotic”:
H2O + H+ -->
H2O + NH3 -->
 NH4+ + OH-
Then if we look at for instance this equation:
HNO3 + OH-
à NO3- --> H2O.
Then, here the HNO3 donates an H+ ion to the OH- to make it into H2O. The HNO3 therefore becomes NO3- because it has lost a hydrogen ion.
The base pairs are OH- and H20, and HNO3 and NO3, because they are the ones which have been changed to each other.

The limitations:
This theory works for all protic solvents (acetic acid, water, liquid ammonia, etc.), not only for water as Arrhenius’ theory. But it does not explain the acid-base behavior in aprotic solvents such as benzene and dioxane.

Also Bronsted-Lowry´s theory cannot explain the reactions between acidic oxides for instance like CO2, SO2, SO3 etc.  and also the basic oxides like CaO, BaO, MgO etc. which also take place even in the absence of the solvent, for instance show in this reaction:
CaO + SO3 --> CaSO4           (There is no proton transfer in this example)

Finally, substances like for instance: BF3, AlCl3 and many more do not have hydrogen present and therefore they cannot donate a proton but are known to act as acid.