Why is it reasonable to assume that each of the reactions of copper go to completion?
A little vague. Perhaps you are talking about the copper cycle lab. We can see that the reactions go to completion because there is a distinct change between reactants and products. Cu and nitric acid. There is no copper metal left. Instead we get a blue solution, Cu(NO3)2. Cu(NO3)2 and NaOH. There is the formation of a blue precipitate, Cu(OH)2. Cu(OH)2 decomposes. There is no blue solid remaining. Instead it has turned black, forming CuO. CuO and H2SO4. There is no black solid, but instead a blue solution of CuSO4. CuSO4 and Zn. The blue in the solution disappears indicating that there is no copper ion left in solution. Instead we see copper metal, Cu.
