Cu to Cu
Topics that could be studied:
1. Equation writing - balanced, ionic, and net ionic.
2. Limiting reagents
3. Kinetics
4. Enthalpy
Procedure:
1. Measure about 0.5 grams of Cu metal and add 5-6 ml concentrated nitric acid in a hood or outdoors.
2 . After the reaction is complete, add 75-100 ml of distilled water.
3. Add 30 ml of 3 M NaOH to the Cu2+ solution.
4. Gently and carefully heat the Cu(OH)2 solid (frothing may occur). Heat the beaker and contents until they are totally black.
5. Allow the black precipitate to settle and decant, wash precipitate with warm water and decant.
6. Slowly add 15 ml of 6 M sulfuric acid to the black solid and stir.
7. Add 2 g of Zn powder to the Cu2+ solution.
8. When the reaction is complete filter and wash the Cu solid with water. Rinse with acetone and allow to dry.
Safety alert:
All solutions contain nitric acid which is very corrosive to skin and eyes. Wash spills off yourself with lots of water. Neutralize spills on the lab table with baking soda. Solutions are toxic. Wash your hands before leaving the lab. If pipets are used to measure the solutions always use a pipet bulb. Never pipet by mouth. Sodium hydroxide is a strong base and is hazardous to skin and eyes. If you get any on yourself wash off with large amounts of water. Neutralize spills on the counter with vinegar. You will be using solutions with high concentrations of sulfuric acid and sodium hydroxide both of which are highly damaging to skin and eyes. Be careful when handling them. If you spill any on yourself wash off with lots of water. Wear chemical splash goggles and a chemical resistant apron.
Data Table:
mass of Cu metal ...................................... g
mass of filter paper and dried Cu .............. g
mass of filter paper....................................g
mass of dried Cu.........................................g
percent yield.............................................%
Questions:
1. Write equations and describe the reaction in step #1
2. Write the equation and describe the reaction in step #3.
3. Write the equation and describe the reaction in step #4.
4. Write the equation and describe the reaction in step #6.
5. Write the equation and describe the reaction in step #7.
6. What are you removing when you decant and wash the precipitate in step #5?
7. Label each reaction as exothermic or endothermic.
8. What is the gas evolved in step #7?
9. Write the equation for the production of this gas in step #7.
10. Give reasons to explain why your % yield was not 100 %.
11. How do you know each reaction was complete before moving on to the next step?
12. Order the reactions from slowest to fastest.
13. List reasons to explain your answers in question #12.
14. What are the competing reactions that take place during step #3 and step #7?
15. Give possible reasons for the competing reactions to have faster reaction times than the Cu reactions.
Teacher Notes:
The lab can be stopped after any step. Step 7 may require additional Zn metal to be added to remove Cu2+. If a white precipitate forms during step 7, add some warm water and stir. Mossy Zn can be used in step 7 but a longer reaction time is necessary.
Equations: (not balanced)
1. (Redox) Cu(s) + HNO3(aq) -> Cu(NO3)2(aq) + NO2(g) + H2O(l)
2. (Double replacement) Cu(NO3)2(aq) + NaOH(aq) -> Cu(OH)2(s) + NaNO3(aq)
3. (Decomposition) Cu(OH)2(s) -> CuO(s) + H2O(l)
4. (Double displacement) CuO(s) + H2SO4(aq) -> CuSO4(aq) + H2O(l)
5. (Single replacement) CuSO4(aq) + Zn(s) -> ZnSO4(aq) + Cu(s)
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