Question

Why is Lewis concept more useful than Bronsted-Lowry concept?

Answer 1

According to Bronsted-Lowry concept, acid is a substance which donates hydrogen ion and base is a substance which accepts hydrogen ion.

                         $HA + Z\rightleftharpoons A^- + HZ^+$

Here HA is the Bronsted acid and Z is the Bronsted base.

Moreover only when we observe the reaction equation we can tell which species is Bronsted acid and which one is the Bronsted base.

$HCl+ H_2O\rightarrow H_3O^+ + Cl^-$ (Here $H_2O$ accepts a proton; hence Bronsted base)

$NH_3+ H_2O\rightarrow NH_4^+ +OH^-$ (Here the same $H_2O$ donates a proton; hence Bronsted acid)

According to Lewis concept acid, it is a substance which accepts a pair of electrons (electrophilic- electron attracting) and base is a substance which donates those pair of electrons in other words a nucleophile. \

Lewis bases have lone pair of electrons. Hence we can very easily identify the Lewis acids and bases.

Answer 2

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There are certain reasons for why the Bronsted-Lowry theory is more superior to the Arrhenius theory, one of them being that Arrhenius couldn't classify substances unless they were dissolved in water, his definitions of acids and bases basically rely upon these substances dissociating in aqueous solutions, this made the bronsted-owry theory more superior since he could explain and apply his model so that solutions with solvents other than water and even in reactions that occur in gas or solid phases. Also, his theory did not explain why some compounds like HCl, which contains hydrogen, dissolves in water and gives acidic solutions whereas CH4 does not, this lack of reasoning weakened his theory and made the Bronsted-Lowry theory more superior since he could explain this by stating that it was a non-polar compound. 

Also, Arrhenius did not explain why some some compounds which are bases, like NH3, don't contain OH- but are still recognized as bases and contain base-like characteristics. 





Ammonia is a base, HOWEVER, Arrhenius stated that for a substance to be a base it has to contain OH- but as seen in the sample problem hydroxide is not persent in Ammonia. Unless, Ammonia reacts with water it will not be, by Arrhenius' definition a base but by the Bronsted-Lowry theory we can now see that Ammonia in fact IS a base. The dissociation method does not work in this case.


Other advantages which make the Bronsted-Lowry theory more superior ís the fact that acids and bases can be either ions or neutral molecules ( sulfuric, nitric, and aceetic acids, and the alkali metalhydroxides) and they can be any molecule as long as they have one pair of nonbonding electrons. TheBronsted-Lowry theory also explains the role of water in acid-base reactions; the water accepts the hydrogen ions to form hydronium ions. He also proved that the relationship between acids and bases can be shown by the conjugate acid-base pairs which in turn can explain and/or show their relative strengths. The Bronsted-Lowry theory enlarges the fact that acetate, phosphate, carbonate, sulfide and the halogen ions are bases whereas the ammonium ions, the hidryonium ions and some hydrated metal cations are considered as acids. 


However, as much as the Bronsted-Lowry theory is far superior than the Arrenhius theory, this theory also has a flaw and it is that it does not explain the acid-base behaviour in the aprotic solvents like benzene and dioxane which is then a limitation which then excludes it from being the perfect theory. 


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