Question

PART C: Copper

1. Take two wads of copper turnings of approximately equal size, and roll each into a marble-sized ball.

2. Set up two large test tubes with 5 mL of dilute sulfuric acid in each.

3. Place the first ball in the first test tube containing dilute sulfuric acid.

4. Use tongs to hold the second copper ball in the outer cone of a Bunsen burner flame for 2 minutes until the substance turns a black color.

5. Allow the heated ball to cool; then place it into the second large test tube of acid. Warm gently with a Bunsen burner until something noticeable happens to the liquid (do not evaporate the liquid away – if you see a lot of vapor, stop and notify your instructor).

6. Allow the contents to stand for about 5 minutes.

7. Observe the colors of the copper balls and of the liquids in both test tubes. Record observations.

8. Dispose the contents in these test tubes into the waste container for Part C.

Question

1. Compare the original copper metal with the material obtained after heating the copper.

2. Compare the color of the liquid obtained after putting the unheated copper in the sulfuric acid to the color of the liquid obtained after putting the heated copper (black material) in the sulfuric acid. In other words, compare the color of the liquids in the two test tubes.

Answer 1

1)

• Original copper (Cu) metal is yellow-orange in color.
• On heating Cu metal in air it reacts with atmospheric Oxygen available giving black colored compound, CuO

Cu + 1/2 O₂ CuO (black)

2)

• When unheated Cu is added to dilute sulfuric acid (dil. H₂SO₄) no reaction takes place because the reduction potential of copper is more than hydrogen, and the solution remains colorless,

Cu²⁺ + 2 e^- Cu (reduction potential=0.34V)

2 H₃O⁺ + 2 e- H₂ + 2H₂O (reduction potential=0.00V)

Hence, Cu will not be oxidised by H⁺ .

• When heated Cu (CuO, black and insoluble) is added to dil. H₂SO₄ and heated gently there is a simple acid-base reaction that occurs giving CuSO₄ which on hydration gives blue color to the solution.

CuO (base) + dil. H₂SO₄ (acid) CuSO₄ + H₂O