electrolysis (copper ii sulfate) problems

edit: whoops this was sposed to go in the chemistry section ><

hey guys,

im doing an extended experimental investigation at school at the moment involving electrolysis of copper sulfate with 2 copper electrodes. i am to weigh the electrodes before and again after the process has occurred and measure change in mass

basically what i dont understand is how Voltage relates to mass changed and amps? I mean obviously a greater voltage applied from the power pack would cause a larger change in mass. however, the results i have at the moment show that although i am keeping voltage constant, the amps seems to increase and fluctuate as time progresses - what might be the cause of that? is it meant to do that, or should amps stay constant as voltage stays constant? :confused:

thanks in advance for any help you can provide (i'll provide any more info if you guys need it to understand the scope of this experimental investigation)

cheers
Jonsta

Reply 1

Dhesi

V=IR so if the voltage is constant the both the other variables could change in such a way that voltage will still remain a constant. I think however electrolysis will depend on the charge provided and charge(Q) has the equation Q=It. So I think from your experiment you will want to try and make current a constant. The mass which should theoretically be gained can be worked out as Cu2+(aq) + 2e ---> Cu(s), so for every two moles of electrons i.e. 193000 colombs you get 1 mole of copper.

If im not getting what you mean try and explain a bit more and i might get a better idea.

Mike

Reply 2

Jonsta

hello, thanks for the reply

like i said before, during my experiment as time progresses it seems that my amperage is increasing (although voltage remains constant). I have no resistors on my circuit (just voltmeter and powerpack) - so are there any other variables that could be causing the change in resistance? (e.g. temperature?) or is this fluctuation meant to occur?

from my experiment i am supposed to use a "suitable graph/data to calculate a n approximate/suitable figure of faraday's constant"

i am guessing that this suitable data is change in mass (increase/decrease) as volts (or amps?) change -
but i have no idea on how to organise equations to get a figure for the constant.

i am also unsure of how to keep current constant without adding (controlable) resistance to the experiment - perhaps i should just take the average amps and use that when calclutaing ? (or is that a very bad idea..)

sorry im asking for so much, but i guess i really dont have a solid understanding at all.

thanks

Reply 3

Intelligentsia

Courtesy of wiki:

m is the mass of the substance produced at the electrode (in grams),
Q is the total electric charge that passed through the solution (in coulombs),
q is the electron charge = 1.602 x 10^-19 coulombs per electron,
n is the valence number of the substance as an ion in solution (electrons per ion), i.e. Cu2+ = 2.
F is Faraday's constant,
M is the molar mass of the substance (in grams per mole), and
NA is Avogadro's number = 6.022 x 10^23 ions per mole.

Now you can measure m the mass produced and Q the charge delivered at given time intervals. Then if you plot a graph of m against Q
and equate it to the form y = mx, then your gradient will be equal to M/(nF) so you can find F (remember M and n are constants) .