Answer
a) Bronsted-Lowry acid is HCl in the forward reaction and $NH_{4}$+ in the reverse reaction. The Bronsted-Lowry base is $NH_{3}$ in the forward reaction.
b) HCl is the Lewis acid and $NH_{4}$+ is the Lewis base.
Work Step by Step
a) According to Bronsted-Lowry acid-base theory, acids are proton donors while bases are proton acceptors. HCl is the Bronsted-Lowry since it donates a proton ($H$+) to $NH_{3}$ in the forward reaction to produce $NH_{4}$+ and Cl-. Since $NH_{3}$ accepts a proton, is the Bronsted-Lowry base.
b) According to Lewis theory, acids are electron-pair acceptors and bases are electron-pair donors. $NH_{3}$ is the Lewis base since the nitrogen in $NH_{3}$ has a lone-pair which it can donate or share with another atom. The H+ from HCl is able to accept the electron-pair from the nitrogen, thus HCl is the Lewis acid.
