Important Questions and Answers: Copper

1. What is the effect of heat on copper sulphate pentahydrate ?

Ans: When it is heated to 100oC, it lose two more molecules of water. Heating upto the temperature of 250oC, blue vitriol loses all of its water molecule and further heating on more than 700oC decomposes to cuprous oxide and oxygen.

$$\ce{CuO.5H2O->[air]CuSO4.3H2O->[{100^{\circ}C}]CuSO4.H2O->[{230^{\circ}C}]CuSO4->[{700^{\circ}C}]->SO3 ^ + CuO}$$

2. Name any two important ores of copper.

Ans: The two important ores of copper are:

  1. Copper pyrite
  2. Copper glance

3. What happens when NH3 is passed over red hot CuO ?

Ans: When NH3 is passed over red hot CuO, metallic copper is obtained.

$$\ce{3CuO + 2NH3->3Cu + 3H2O + N2}$$

4. Cu++ is more stable than Cu+. Why ?

Ans: An ion carrying higher charge has greater lattice and hydration energy. Since, Cu++ has more charge, it is more stable than Cu+.

5. Cupric salts are blue in color but cuprous salts are colorless. Why ?

Ans: When the electron of cupric salts jump from one d-level to another d-level, they absorb and emit light of visible spectra. This light is blue in color. So, cupric salts are blue in color. In cuprous salts, there is no possibility of excitation of the electrons. Hence, no color is shown by cuprous salts.

6. Name any two ores of copper with molecular formula.

Ans: The two important ores of copper are:

Copper pyrite [CuFeS2]

Malachite [CuCO3.Cu(OH)2]

7. Copper becomes green when exposed to air for long time. Why ?

Ans: Copper remains unaffected by dry air at ordinary temperature, but when it is exposed to air in presence of carbon dioxide, is changes to green basic carbonate.

$$\ce{2Cu + CO2 + O2 + H2O->$\underset{\text{Basic carbonate of copper}}{\ce{ CuCO3.Cu(OH)2}}$}$$

8. What are blister copper and copper matte?

Ans: The copper metal which contains bubble of SO2 is called blister copper. It is 98% pure.

The sulphide of copper and iron formed at  the lower layer of blast furnace during smelting is  called copper matte. It is 50% pure.

9. What happens when:

a. Copper is dropped into conc. HNO3 in a test tube.

Ans: When copper is dropped into conc.HNO3 in a test tube, a brown gas of NO2 is obtained.

$$\ce{Cu + 4HNO3->[Cu(NO3)2] + 2NO2 ^ + H2O}$$

b. Ammonia gas is passed in copper sulphate solution.

Ans: When CuSO4 solution is treated with ammonium hydroxide, bluish white precipitate is formed. When excess of ammonium hydroxide is added, deep blue coloration of tetra-amine copper (II) sulphate is produced. The product is also called Schwitzer’s reagent.

$$\ce{CuSO4 + 2NH4OH ->Cu(OH)2 + $\underset{\text{Bluish white precipitate}}{\ce{(NH4)SO4}}$}$$

$$\ce{Cu(OH)2 + (NH4)2SO4 + 2NH4OH->$\underset{\text{Blue ppt.}}{\ce{[Cu(NH3)4]SO4}}$ + 4H2O}$$



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