Is one who intended to do good (such as going to the mosque for prayer) then committed a sin on the way punished and rewarded?

Allah rewards a good intention with a good deed. If the intention is acted upon, then it is rewarded tenfold. Sins committed on the way to the mosque do not invalidate the good deeds a Muslim intended or performed, but they may affect the level of devotion in prayer, thereby diminishing its reward. Some sins may also reduce or nullify good deeds.