Chemistry stinks.

by mezz • 13 Apr 2010, 22:34 • Journals
Ok Ladies ... im desperate enough to ask scientific stuff on crossfire:

Here is the problem:


How much time does it take to refine a 0,1cm layer of copper (Cu) on a cathode (sheet) electrolytically?


Known facts:

current density: I = 0,1A / mm²
density Cu : 8,92 g / cm³
M Cu: 63.55 g / mol


I would understand it if I could relate the current density with the surface of sheet ...

Any ideas?
Comments
9
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varit 13 Apr 2010, 23:39
i hate chemistry thats all!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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mezz 13 Apr 2010, 23:41
I have to graduate in Chemistry ... in 9 days :D
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varit 13 Apr 2010, 23:44
urgh.. ;) gl mate
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huNthi 13 Apr 2010, 23:39
add it up and ur with the answer.


Your sincerely, huNti the master of world
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RazZaH 13 Apr 2010, 23:47
hard enough in german

but in english O_o
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Nemesis- 13 Apr 2010, 23:59
You need to know the basic equation for refining copper:

Cu3+ + e- --> Cu2+

Firstly, you find the number of moles of copper per volume (plug M Cu into density Cu). Multiply by 1 mm^2 / 0.1 A to get mol Cu / A*area

Uhh... my brain just stopped working there. From here, just try to manipulate the Current :P I = dQ/dt so you can find how many Coulombs of electrons 0.1 A is. Use the above equation to see that the stoichiometry implies 1 mol of electrons purifies 1 mol of Copper.

Hope that helps, if not I'll try to follow through the steps again :P
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mezz 14 Apr 2010, 00:06
First, thanks alot ;) But my brain stops working at the same position ... I have the answer so close in front of my mind but im totally braindear for today -.-
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Nemesis- 14 Apr 2010, 00:15
Was gonna write up the full solution, but I think you should refer to Loctarar's comment below. That's the main connection imo

EDIT:

Here's the full calculation if you want to see it ^^

I(t) = i * S
V = Sd = 0.1S

Moles of copper = 8.92/63.55 = 0.14036 mol/cm^3

By Stoichiometry, the ratio of moles Cu to moles e- is 1:1

Total moles of copper = 0.14036*V = 0.014036*S mols = Total moles of electrons

Using Faraday’s constant, F = 9.6845 x 10^4 C/mol (the charge of one mole of electrons) you multiply this with 0.014036*S to get the total charge equal to 1359*S Coulombs

By definition of current, I = Q/t
I = 1359*S/t = 10 A/cm^2 (converting from mm to cm to conform with the above calculations) * S

Luckily for you, the S cancels out so you just get t = 135.9s or approximately 2.27 minutes.
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loctarar 14 Apr 2010, 00:08
The total current is the current density (let's call it It) * the surface:
It = I * S.
The volume of the sheet is:
V = S*d (d is the thickness 0.1cm)
The mass is now density * V = density * S * d. From here you get the mol count.

I don't know how the equations are but my guess is this:
water + some electrons -> some ions that get neutralized by the copper atoms.

You can find the total amount of electrons using the definition of It = Q/t and then match in the equation with the number of copper mols.
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