Assuming that arsenic-bronze was used to make bows and swords, would they be practical and offensive weapons?
It has already been made.
Although arsenical bronze occurs in the archaeological record across the globe, the earliest artifacts so far known, dating from the 5th millennium BCE, have been found on the Iranian plateau.
Among the advantages:
Whilst arsenic was most likely originally mixed with copper as a result of the ores already containing it, its use probably continued for a number of reasons. Firstly, it acts as a de-oxidiser, reacting with oxygen in the hot metal to form arsenous oxides which vaporise from the liquid metal. If a great deal of oxygen is dissolved in liquid copper, when the metal cools the copper oxide separates out at grain boundaries, and greatly reduces the ductility of the resulting object. However its use can lead to a greater risk of porous castings, owing to the solution of hydrogen in the molten metal and its subsequent loss as a bubble.
Secondly, the alloy is capable of greater work-hardening than is the case with pure copper, so that it performs better when used for cutting or chopping. An increase in work-hardening capability arises with an increasing percentage of arsenic, and the bronze can be work-hardened over a wide range of temperatures without fear of embrittlement.
Thirdly, in the correct percentages, it can contribute a silvery sheen to the article being manufactured. There is evidence of arsenical bronze daggers from the Caucasus and other artifacts from different locations having an arsenic-rich surface layer which may well have been produced deliberately by ancient craftsmen, and Mexican bells were made of copper with sufficient arsenic to colour them silver.