electrochemical series

Question

30 min of electrolysis of solution of copper sulphate produces 3.175g of Cu. how many coulombs pass through the cell and what is the current

Answer 1

Step 1: My "Shortcut" equation

Use the following equation to simplify electrolysis calculations. It will most likely never be given to you, but may be well worth the effort to memorize.

Grams Product = (I * t * M) / (n * F)

where...

I = Current in amps

t = Time in seconds

M = molar mass of the product

n = number of moles of electrons transferred in the redox reaction

F = Farday's constant = the charge on a mol of eletrons = 96,500 C/mol

Sine we want to find current, lets solve that equation for I. This gives us the following

I = (Grams product * n * F) / (t * M)

Since we are told what the product is, Cu, finding its molar mass (M), is only a matter of looking up the element on the periodic table. It is found to be 63.5. For "n", we have to set up the redox reaction as follows...

CuSO₄ –> Cu

To find the initial oxidation state of Cu, you must look to the charge of the sulfate polyatomic ion (-2). The oxidation state of Cu will be the charge that balance the molecule out to a neutral overall charge. Thus, the initial oxidation state of Cu is +2. Cu in its final oxidation state is by itself, no longer paired with the sulfate ion. Its final oxidation state is therefore 0. The change in oxidation state is from +2 to 0. This corresponds to the transfer of electrons, or more specifically the gaining of 2 electrons by Cu. The "n", number of mols of electrons transferred in this reaction is therefore 2 mols. Plugging all of this into our shortcut equation we get...

I = (3.175 * 2 * 96,500) / (1800 *63.5) = 5.36 Amps

Step 2: Finding Coulombs from Amps

Amps is equal to the number of Coulombs per second. We can therefore multiply Amps by the number of seconds that the current was applied.

5.36 Amps = 5.36 C/s * 1800 s = 9648 C

Answer 2

30 min x 60 sec/min = 1800 seconds

3.175 g Cu x 1 mol Cu/63.55 g = 0.04996 moles Cu

0.04996 moles Cu x 2 mole electrons/mol Cu = 0.09992 moles of electrons

0.09992 mol electrons x 96,500 C/mol electrons = 9642 C

1 amp = 1C/sec therefore...

9642 C/1800 sec = 5.357 amps (current)