Question

Which of the following are Lewis acids?

A. BCl3 and AlCl3

B. PH3 and BCl3

C. AlCl3 and SiCl4

D. PH3 and SiCl4

BCl3BCl3

Answer 1

Hint: We should know the Lewis acid-base theory to determine the answer. The Lewis acid-base theory is based upon the electron pairs. The species having electrons less than octet requirement is an electron-deficient species and species having electrons more than octet requirement is an electron excess species.

Complete step by step solution:
According to Lewis's acid-base pair theory, acids are the substance that accepts a nonbonding electron pair and the bases are the substance that donates a non-bonding electron pair. In an acid-base reaction, acids accept electron pairs and bases donate electron pairs.

Generally, every atom forms bonds to complete its octet. The species having electrons less than the octet requirement can accept electron pairs and the species having electrons more than the octet requirement can donate electron pairs.

In a compound if the octet of central metal is not complete then it is an electron-deficient species, so it will take an electron pair thus, it will work as acid.
So, we will find the molecule whose octet is not complete.
The Lewis structure of all given molecule is as follows:

The boron in , is correct.

Note: The electron-deficient species or having incomplete octet work as acid. The species having lone pairs work as a base. Lewis's theory has some limitations such as it does not explain the relative strength of acids and bases and it does not explain the behavior of HCl. According to Arrhenius, the acids are proton donors and the base are hydroxyl acceptors. So, HCl is an acid. According to Bronsted, the acids are proton donors and bases are proton acceptors.