The purpose of this experiment is to test the Law of Conservation of Mass by determining the percent recovery of copper from a sequence of chemical reactions involving copper in several chemical forms. The sequence is:
Cu(s) → Cu2+(aq) → Cu(OH)2(s) → CuO(s) → Cu2+(aq) → Cu(s)
Step 1 – Weigh about 0.35 g of copper into a 250 mL beaker. Describe the color and form of the sample. Record the mass of the sample. WORKING IN THE HOOD, Add 3-5 mL of concentrated HNO3 slowly to dissolve the copper. Warm gently with a Bunsen burner if all of the copper does not dissolve. Then slowly add 10 mL of deionized H2O. Note the color of the solution.
Step 2 – Return to your desk. Add 6 M NaOH(aq) very slowly, with stirring, until the solution turns red litmus paper blue. (Use a stirring rod to test a drop of solution). Do not confuse the color of the precipitate with the color of the litmus paper. Note any color changes; a solid will form (called a precipitate). Describe the solid and the “supernatant.”
Step 3 – Add deionized water to make a total volume of about 100 mL. Heat the solution gently and slowly over a low flame (no yellow/luminous flame) until the solid completely changes color. Note the color change. DO NOT BOIL THE SOLUTION. Heating gently prevents the solution from bumping. Allow the solution to cool. Prepare a filter by folding a piece of filter paper or a fluted filter paper to fit your funnel. Filter the mixture and discard the filtrate. Use your wash bottle (filled with deionized water) to transfer the last traces of solid onto the filter. Heat about 40 mL of deionized water; wash the residue 3 times with generous portions of this hot water. Allow each portion to drain through the filter. Keep the solid, with the filter paper, on a watch glass in your drawer, until you are ready for step 4.
Step 4 – Record the color of the residue and the color of the filtrate. Dissolve the solid copper oxide by adding about 10 mL of 3 M H2SO4 to the residue on the filter paper from step 3. Let the solution drain into a 250 mL beaker. If any solid remains in the filter, pour the solution through the filter again. Repeat this step until the solid is completely dissolved. Note the color change. Wash the empty filter paper with cold deionized water; add the washings to the acid solution and save the solution for step 5.
Step 5 – WORKING IN THE HOOD, Add about 2 g of zinc metal to the acidic copper solution. If any blue color remains after the zinc has dissolved, add another gram of zinc. Dissolve any excess zinc with a small amount (about 10 mL) of 3 M H2SO4. Allow the precipitated copper to settle; decant the supernatant liquid from the solid; wash the solid 3 times with 20 mL portions of deionized water, stirring and allowing the solid to settle. Some liquid remains with the solid by this procedure. Wash your evaporating dish and dry it for ten minutes in the oven. (Write your name on the bottom.) Cool and weigh it. Transfer the copper into the cool, dried evaporating dish, then using a stream of water from the wash bottle, wash the acid off the copper. Pour off as much of that waste water as you can without disturbing the solid. Repeat three times. Next clean the water off the copper by adding a small portion of acetone or ethanol and then decanting off that solvent. Do this three times and let the dish sit until all of the solvent odor is gone and the solid is dry. Allow the dish and copper to dry and then weigh them. As long as the dish is cool to the touch on the bottom, the solvent is still evaporating – WAIT LONGER for it to dry. Calculate the weight of the copper by difference. Describe any differences in the copper’s properties after the sequence of reactions. Turn in the copper sample, in a labeled vial, to your instructor.
Pre-Laboratory Questions: (Answer these in your lab notebook, you do not have to write the questions, but it’s a good idea to do so.)
1. Find in the procedure and describe:
- A test for a basic solution.
- A test to decide whether enough zinc has been added.
2. Does observing a color change always indicate that a chemical change has occurred? Explain.
3. The equation for the re-conversion of copper into copper metal is as follows:
Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq)
- Calculate the theoretical mass of zinc needed to carry out the reaction for a 0.35 g copper sample.
- Compare the theoretical mass of zinc with the amount we actually use. Explain any differences.
Report Pages and Questions: Copy the report pages into your laboratory notebook and record your data directly in the lab notebook. You do not have to answer the questions in the lab notebook. That can be answered directly on the printed pages.