Question

Acidity of $B{F_3}$ can be explained on the basis of which of the following concepts?
(A) Arrhenius concept
(B) Bronsted Lowry concept
(C) Lewis concept
(D) Bronsted Lowry as well as Lewis concept.

Answer 1

The Arrhenius concept and the Bronsted Lowry concept of acidity have one thing in common which is that both these concepts define acids as substances capable of donating hydronium ions on dissolution in water. An acid must contain hydrogen according to both these concepts.

Complete answer:
Acids and bases can be distinguished based on their physical as well as chemical properties. Acids can be identified using indicators, pH scale or with the help of the theories that categorize different substances as acids or bases.
The Arrhenius theory and the Bronsted Lowry theory fail to provide a logical explanation for the acidic character shown by boron trifluoride $B{F_3}$ molecule as it does not contain hydrogen atoms in its formula that can be released as proton on dissolution.
The electron deficiency of boron trifluoride is responsible for its acidic behavior. The Lewis theory of acids defines acidic substances to be compounds or ions that have low electron density and look for lone pair suppliers.
$\Rightarrow$ Therefore boron trifluoride $B{F_3}$ is an acid according to Lewis acid theory.
Option (C) is correct.

Note:
Boron compounds are electron deficient because boron can only form three covalent bonds which results in six electrons around it and it therefore lacks a pair of electrons to acquire the stable noble gas configuration. Lewis bases like ammonia that contain a lone pair form adducts with boron trifluoride.