0.9 Electrochemistry/alchemy: molar mass of cu and turning cu into  (Page 3/4)

Through exhaustive experimentation, the charge of a single electron has been determined to be $1\text{.}\text{602}\times {\text{10}}^{-\text{19}}$ coulombs (C). (The coulomb charge unit–defined as useful for much larger charged objects–is inconvenient for expressing such a small charge, so other electrical charge units are commonly used.) One mole of electrons has a total charge calculated to be 96,485 C; this quality is defined as faraday (F):

  $\text{1 F}={\text{96,485 C/mol e}}^{-}$

 Electric currents (l) are measured in amperes (A), amps for short, and defined in terms

I = Q/t

1 A = 1 C/s

 For example, a constant current of .600 A (milliamperes) over a period of $2\text{.}\text{00}\times {\text{10}}^2$ seconds represents

$Q=I\times 5=0\text{.}\text{600 A}\times \text{200s}=0\text{.}\text{600 C/s}\times \text{200s}=\text{120 C}$

a movement of 120 coulombs. The number of moles of electrons (n) transported during the time interval is

$=1\text{.}\text{24}\times {\text{10}}^{-2}{\text{mol e}}^{-1}$

Time intervals measured in minutes and hours must be converted to seconds in such calculations. Another useful equation in Electrochemistry is Ohm's Law, where:V = I R where V is the potential difference in volts, I is the current in amperes and R is a constant, measured in ohms, called the resistance.

Experimental procedure

CAUTION WEAR EYE PROTECTION!

CAUTION The 6 M nitric acid used in the next step will burn and stain the skin as well as damage clothing. In case of skin or clothing contact, wash the area immediately with large amounts of water.

• Obtain a piece of copper foil (about $\text{2 cm}\times \text{8 cm}$ ). Holding the foil with tweezers or tongs, dip it into 6 M nitric acid several times until its surface is bright and shiny. Do not allow tweezers or tongs to touch the acid solution. Rinse the foil in de-ionized water and set it aside. This is the anode. Set the nitric acid aside to use in the electroplating exercise.
• Obtain a piece of copper mesh (about $\text{5 cm}\times \text{8 cm}$ ) and remove any loose pieces of copper. Clean and rinse it as in step 1. Place the copper mesh on a watch glass in the drying oven. Be careful not to touch the cleaned surfaces. This is the cathode.
• Add 350 mL 1.0 M ${\text{KNO}}_3$ solution to a 400 mL beaker.

 CAUTION The copper sulfate used in the next step is toxic. Avoid skin contact

• To this solution, add about 5 mL of 1 M $H_2{\text{SO}}_4$ and 10 g of ${\text{CuSO}}_4{\text{5H}}_{2}O$ . Stir until the copper sulfate pentahydrate is fully dissolved.
• Assemble the apparatus shown in Figure 1, but leave the copper mesh electrode in the oven. Add a magnetic stirring bar to the beaker. If necessary, add additional 1.0 M ${\text{KNO}}_3$ to bring solution level in the beaker within 2 cm of the rim. You will either measure the electric directly with an ammeter in series with the electrolytic cell or you will measure the current indirectly by measuring the voltage across a resistor of known value (about 10 ohms).
• Remove the copper mesh electrode from the oven, let it cool, and determine its mass to the nearest milligram.
• Attach the copper mesh electrode to the negative terminal of your power supply using an alligator clip. Turn on the magnetic stirrer.
• Turn on the low voltage power supply and adjust the current until about 140 mA are flowing through the cell. Record the time and current.
• Record the time and current every five minutes for an hour.
• After the last reading, gently remove the cathode from the solution while yet attached to the power supply. After the copper mesh has cleared the solution, remove the wire and turn off the power supply.
• Gently dip-rinse the copper mesh electrode several times in a beaker of deionized water, and place it on a watch glass in the drying oven.
• When dry, remove the electrode from the oven and let it cool. Reweigh the mesh electrode.
• Remove the magnetic stirring bar from your beaker and dispose of the solution in the sink.