Is HBr a strong acid?

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.