In the Bronsted-Lowry definition, an acid is anything which donates #H^+# ions in an aqueous solution.
#HBr# would be considered an acid, because it does dissociate in aqueous solutions in the following dissociation reaction:
#HBr(g) rightleftharpoons H^+(aq) + Br^(-)(aq)#
Now, a strong acid is an acid which almost completely dissociates in aqueous solutions.
#HBr# fulfils this, because it very readily dissociates—here's why.
#HBr# is a hydrohalic acid, or an acid with the formula #HX# where #X# is a halogen.
In all hydrohalic acids except for #HF#, the bond between #H# and #X# is very weak because halogens tend to:
- have a high number of energy levels, increasing the size of the halogen. This decreases attraction between #H# and #X#.
- Be very electronegative, causing the #H-X# bond to be very polar.
These two factors cause hydrogen to be very easily "snapped off" of #HX#, causing it to dissociate into #H^+# and #X^-#.
This is also true for #HBr#—because bromine is so electronegative and large, hydrogen will very easily dissociate from #HBr#, causing #HBr# to readily dissociate into #H^+# and #Br^-#.
Dissociation happens so readily that we can say #HBr# almost completely dissociates, fulfilling the criteria for a strong acid.
