why would acyl halides give higher yields of amides compared to carboxylic acids or esters on reaction with NH3?

Answer 1

The first step that takes place in the amide formation reaction, is that the :NH3 molecule (the semicolon represents the lone pair) donates its lone pair to the carbon atom (which is connected to the oxygen atom), forming a bond.

The more electrophilic (wanting an electron/having a low electron density) the carboxyl/carbonyl carbon is, the easier this will be.

Now, in acyl halides, the halide ion connected to the carbon atom is electron withdrawing, and it hence increases the tendency of the carbon atom to accept a lone pair (makes it more electrophilic). Since halide atoms do this better than oxygen atoms in carboxylic acids or esters, the reaction is easier in acyl halides and gives a higher yield.