# 0.9 Electrochemistry/alchemy: molar mass of cu and turning cu into  (Page 4/4)

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## Alchemy: copper into gold

Place your pre-1982 copper penny in an evaporating dish and heat with a mixture that first turns it silver, then suddenly turns it gold when the penny is then heated on a hot plate.

Caution: Wear safety goggles and gloves, do the reaction in the fume hood with the sash down. Note step 10: special disposal

• Place approximately 5 g of zinc in an evaporating dish.
• Add enough NaOH solution to cover the zinc and fill the dish about one-third.
• Place the dish on a hot plate and heat until the solution is near boiling.
• Prepare a copper penny (pre-1982) by cleaning it thoroughly with a light abrasive (steel wool pads work well).
• Using crucible tongs or tweezers, place the cleaned penny in the mixture in the dish.
• Leave the penny in the dish for 3-4 min. You will be able to tell when the silver coating is complete.
• Remove the penny, rinse it, and blot dry with paper towels. (Do not rub.) Remove particles of zinc.
• Using crucible tongs or tweezers, place the coated penny on the hot plate. The gold color appears immediately.
• When the gold color forms, remove the coin, rinse it, and dry it with paper towels.
• Special disposal procedures: Do not discard the waste zinc in the trash container. When zinc dries, it forms a powder that may spontaneously ignite. Rinse the NaOH-zinc mixture several times with water. Then add the solid to a beaker that contains 200 mL of 1 M ${H}_{2}{\text{SO}}_{4}$ . When all of the solid dissolves, flush the zinc sulfate solution down the drain.

## Example of the calculation of molar mass:

• Graph the electric current (in amps) on the y-axis against time (in seconds) on the x-axis. The total charge that passed through the electrolysis cell is given by the area beneath this curve. If the current is constant, this area is:

$Q=\text{area}=I×t$

Calculate this charge in coulombs.

• Convert the coulombs of charge to mol electrons:

N =

The equation for the reduction half-reaction responsible for the plating at the cathode is

${\text{Cu}}^{2+}\left(\text{aq}\right)+{\text{2e}}^{-}\to \text{Cu}\left(s\right)$

Use the mol ratios of the preceding balanced equation to calculate the number of moles of Cu plated out:

$n\left(\text{Cu}\right)=\text{n/2}$

• Use the initial and final weighings of the copper mesh electrode to calculate the mass of copper plated out:

m(Cu) = m(final)–m(initial)

• Calculate the molar mass (M) of copper:

M =

## Electrochemistry pre-lab exercise

Hopefully here for the Pre-Lab

Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students’work (including your laboratory partner) or misrepresent your own data (see honor code).

Name(Print then sign): ___________________________________________________

Lab Day: ___________________Section: ________TA__________________________

This assignment must be completed individually and turned in to your TA at the beginning of lab. You will not be allowed to begin the lab until you have completed this assignment.

1. Write a balanced chemical equation for the electrolysis of molten potassium iodide (KI).

2. An electric current of 0.211 mA passes through an electrolytic cell for 2.00 min. How many moles of electrons have passed through the cell?

3. A voltage drop of 3.412 V is measured across a resistance of 10.51 ohms. How much current is flowing through the resistor?

4. Why is the anode not weighed before electrolysis begins?

## Report form: electrochemistry

Hopefully here for the Report Form

Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students’work (including your laboratory partner) or misrepresent your own data (see honor code).

Name(Print then sign): ___________________________________________________

Lab Day: ___________________Section: ________TA__________________________

Initial mass of cathode ___________________g

Final mass of cathode ___________________g

Mass of copper plated out ___________________g

 Time (min) Voltage (V) Current (A) 5 10 15 20 25 30 35 40 45 50 55 60

Show calculations for full credit!

Average Current = (A)

Total charge through cell ______________C

Number of moles ${e}^{-}$ through cell ______________mol

Molar mass of copper ______________g/mol

## Questions:

1. What is oxidized and what is reduced when electroplating copper?

2. Write the half reactions that occur at the anode and electrode.

3. Why is it important not to touch the cleaned cathode?

4. What is the purpose of ${\text{KNO}}_{3}$ in this experiment?

5. How would your results for the molar mass of copper be affected if hydrogen gas were also observed at the cathode?

6. What part of this procedure limits the accuracy of the molar mass determination?

## Alchemy - copper into gold

• Is this reaction an oxidation-reduction reaction?

•  Why did the penny turn "silver"?

• Why did it turn "gold"?

• Why did we heat the penny to turn it "gold"?

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