Electroplating

[Mao]

A couple questions:

1. Is there a "minimum concentration"?
I came across a question like this in the NEAP lecture notes, featuring a standard copper electroplating cell, and it read
"The cell operates with a steady current of 0.50 A for 20.0 minutes.
Calculate the minimum concentration of Cu2+ ions in solution, assume that the volume of the solution remains constant at 100mL."

couldn't the concentration be anything? there is a net flow of Cu2+ ions, but no net change in concentration. It is possible to work out the rate of reaction, but that will be meaningless without knowing the speed of migration of Cu2+ from anode to cathode....
or am I not thinking in the right mode at all?

2. If I were to electroplate a copper object with silver, wouldn't these two spontaneously react?
If I wish to substitute the electrons with an external source, would I need to connect the copper object to the power source first THEN dip in the solution? could this be potentially dangerous? [or the rate of reaction is so slow there is no need to worry about this?]

[bucket]

For the first one aren't you just plugging the numbers into that formula? ie. Working backwards to find z (or n(e-)) then plugging that into the half equation to find n(Cu2+) then working the concentration with the volume given?

wait, I think I misread the question, are you meant to find the concentration of the copper ions remaining AFTER electroplating has taken place? (first one)

[Mao]

For the first one aren't you just plugging the numbers into that formula? ie. Working backwards to find z (or n(e(-))) then plugging that into the half equation to find n(Cu²⁺) then working the concentration with the volume given?

This is the third time this has been suggested to me (one by suggested solutions, one by teacher)
I'm having problem with "minimum concentration"...

[shinny]

Well if theres a minimum amount (calculated by Faradays), a fixed volume (100mL), then doesn't that imply theres a minimum concentration?

EDIT: i.e.

[Mao]

disregarding the rate of migration of Cu2+ ions across the beaker from anode to cathode, there need not be a "minimum" amount.

suppose this cell (100mL) has [Cu2+] at 0.1M, and we pass 0.04 mol of electrons through it in 10 minutes. during this time, 0.02 mol of Cu would be deposited on the cathode and 0.02 mol of Cu2+ would have gone into the solution, whereas the concentration of the electrolyte really did not have much relevance.
we can half it to 0.1M and it still not have impact on how the cell operate.

so how does this "minimum concentration" concept work?

[shinny]

If it's any lower than this, the solution would simply run out of copper it seems. Perhaps I've overlooked something really obvious since I don't actually understand the principles behind this chapter as I haven't begun studying it, but using what I know of the maths side, this seems to be the answer o.O

[bucket]

thats exactly what I would have done :S I think Mao's question is more why is there a minimum concentration rather than how to get the answer... ie. why couldn't you get say, 0.010 mol of Cu and run it through the galvanic cell for the exact same time at the exact same current? It's not like theyre specifying the specific mass they want to be deposited. I think the wording was bad. :S

[shinny]

They should've explicitly stated that it was run for the entire 20 minutes productively (i.e. copper was deposited for the entire 20 minutes)...that was the assumption I made in those calculations. If there's any less than 0.031 M, then there'll basically be none left before the 20 mins is over.

[Mao]

but from my understanding, you have worked out how much Cu2+ has been consumed, and that is the minimum required concentration if the anode was not supplying it. When the anode is supplying it, the rate at which Cu2+ is going into the solution is the same at which it is being consumed.

My fav analogy is my bank account. If I use $200 per month and I earn $200 per month, my bank account stays the same, there need not to be a minimum balance of $200 or $2000 in there as the net change is $0.

there is a net movement of Cu2+ ions across the cell, but no net change in concentration.

unless, of course, I missing something deadly obvious

[Mao]

I think you are talking about electrolysis of a solution where I am talking specifically about electroplating...
no clue.

[shinny]

Yeh I actually have no idea how any of this is working; I just know the two formulas ._."" Wheres coblin when you need him.

[bucket]

What does the question say word for word? From your earlier post I assumed that this cell involved an inert electrode as opposed to a copper electrode as you said:

suppose this cell (100mL) has [Cu2+] at 0.1M

If the cell was using a copper electrode, wouldn't you use an electrolyte which wasn't composed of copper?

Furthermore, the question asks for the minimum concentration of copper (required I presume?) and this also indicates to me that a copper electrode isn't the source of copper ions...

[Mao]

Electroplating:

lets say you want to plate a steel spoon with copper. you would have a copper electrode at the anode, and the object at the cathode. the reaction at anode would be , the copper ions goes into the solution, and is reduced at the cathode:
The overall reaction is , basically copper ions go from anode to cathode.

as you can see here, there need not be any other compounds involved in this purpose.


the question shows a standard electroplating cell [copper electrode at anode, spoon at cathode, copper(II) sulfate solution]. the exact wording of the question is as presented in the first post.

[Collin Li]

Yeah, it's a fucked up question. You need to know the rate at which the copper ions flow from one electrode to another. If it was instantaneous, you could theoretically electroplate with just 1 copper ion in solution, as the copper ion would be instantly replaced from the copper anode.

[Mao]

Thank you coblin. [and everyone else ]

second question... anyone care to have a crack?