Question

The chemical reactions:
CuSO4(aq) + Zn(s) → ZnSO4(aq) + Cu(s) Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) The copper on the surface quickly reacts with oxygen according to the following reaction: 2Cu(s) + O2(g) → 2CuO(s)

Experimental Procedure Dissolve
completely about 0.5 to 0.7 g of copper (II) sulphate pentahydrate in about 10 to 20 mL of deionized water. Calculate the amount of zinc powder that must be added to a copper sulfate solution so that the sulfate completely reacts. This calculated mass of zinc, plus an extra 0.01 g, is added to the copper sulfate. Stir. When the reaction appears to be complete (what signifies this?), add enough sulfuric acid to create a colourless solution. What is the purpose of this step? Suction filter the final product. Wash with about 5 mL of ethanol. Maintain suction for about 10 minutes. Determine the mass of the final product to 2 significant figures. Reagents 0. 6 g Copper (II) sulfate pentahydrate (CuSO4(s)) (FW = 249.68) Zinc powder (FW = 65.41) Sulfuric acid (1.5 M H2SO4(aq)) Ethanol

Data Mass of CuSO4 = 0.61g Mass of Zn = 0.17g Mass of empty watch glass = 48.72g Mass of watch glass and Cu = 48.91g

Questions:

1. Please show the calculation for the amount of Zn added to the reaction mixture and for the percentage of Cu recovered from the original copper sulfate sample (theoretical yield).

2. What is the purpose of adding enough sulfuric acid to create a colorless solution?

Answer 1

Resction between Zn and CuSO4 is an example of displacement reaction. When You add Zn powder in the solution of CuSO4 it replaces Cu from its sulphate solution to form ZnSO4 solution. CuSO4 is a blue coloured solution while ZnSO4 is colourless.

Hence this answers your first question regarding what signifies reaction completion: i.e. when the blue colour of the CuSO4 disappears it means reaction has completed.

Ques 2). What is the purpose of addition of enough sulphuric acid??

Ans: It is mentioned in the starting of the reaction that Cu on the surface quickly reacts with oxygen to form CuO(which is a black coloured mass). To remove this black coloured mass H2SO4 is added because CuO reacts with sulphuric acid to form copper sulphate(blue colored) solution again, which futher can react with extra zinc added in the beginning to give the final product i.e. ZnSO4( colourless).

2Cu(s) + O2(g) 2CuO(s)

CuO(s) + H2SO4(aq) CuSO4(aq) + H2O(l)

CuSO4(aq) + Zn(s) ZnSO4(aq) + Cu(s)