An עצם is more generic. For example: עצם בלתי מזוהה = unidentified flying object.
A חפץ is more like a thing/singular for stuff. Did you pack your things? = ארזת את החפצים שלך?
חפץ Means "desires" in the bible . So as a noun it probably means desirables
עצם Means "bone" or "essence" in the bIble
I think you are confusing HEFETZ with HAFETZ. Hefetz is an object, while Hafetz is desires.