Electroplating with Copper: Correct Reaction at Negative Electrode?

[Hevonen]

Question is: "A metallic object is electroplated with copper using a solution of copper(II) sulfate. Which statementis correct?"
A. The positive electrode increases in mass.
B. The concentration of Cu2+ ions in the solution decreases.
C. Reduction occurs at the positive electrode.
D. The reaction occurring at the negative electrode is Cu2+ + 2e− → Cu.

The right answer is D but I would consider that B is also correct. However, B is not correct but do you know why?

Thank you in advance!

 

[Kushal]

well, for electroplating, copper is used as the anode and the metallic object is used as the cathode. in such a cell, the copper anode is oxidised into Cu2+ and it is that Cu2+ that gets discharged at the cathode (negative electrode) and not the Cu2+ from the copper sulphate solution. therefore, the concentration of Cu2+ in the solution remains constant.

it is more energetically favorable for the Copper anode to get oxidised to Cu2+ which then get reduced at the cathode. but i don't why it is more favorable. can anyone explain?!

 

[symbolipoint]

A few comments from http://en.wikipedia.org/wiki/Electroplating will help you.

Apparantly for electroplating metals, reduction occurs at the cathode. This will help you in deciding among choices A, C, D. Your feeling about B is understandable, but I believe you are correct in choosing B. If the plating solution contains Cu(II) as a supply of the metal to form the plating, then as plating occurs (copper is reduced), the concentration of this ion in solution must decrease.

 

[Hevonen]

Does the word elecroplating involves that the anode is the metal used to plate the other metal?

 

[GCT]

The copper in the solution is reduced at the cathode to form the sheet of copper while the copper at the anode is oxidized proportionally to supply Cu2+ into the solution and thus the amount of Cu2+ in the solution isn't changed. Also if I remember correctly electroplating is an electrolysis process and thus the process need not to be favorable in terms of free Gibbs.

 

[Hevonen]

I will rephrase my question. How can one know from the question that the anode is made of copper? Electroplating would happen even if the anode was made of, let say, nickel (Ni). In that case, Ni oxidises on anode and the concentration of Cu2+ decreases as the reduction of Cu2+ is on the cathode. Hence, the answer B should be equally right. Please, inform me if there is some rule or standard that I am overlooking.

 

[symbolipoint]

I will rephrase my question. How can one know from the question that the anode is made of copper? Electroplating would happen even if the anode was made of, let say, nickel (Ni). In that case, Ni oxidises on anode and the concentration of Cu2+ decreases as the reduction of Cu2+ is on the cathode. Hence, the answer B should be equally right. Please, inform me if there is some rule or standard that I am overlooking.

You have the correct idea. Choice B depends on the anode being something other than copper metal solid. You must specifically be given that information in the problem description. (for next statement, textbook needs to be checked): Also, oxidation occurs at the anode, so choice D is wrong.

 

[symbolipoint]

More thorough thought: Look at choice D again. The cathode is supplying negative charge which attracts and reduces Cu(II) to Cu metal solid. This negative electrode is negative because it is the source of the negative charge. Choice D then makes sense. The cathode is where reduction occurs, the negative electrode; the anode is where oxidation occurs, the positive electrode.

So, for copper plating process, choice D is for certain. Choice B depends on being given that the anode be copper. You are only telling us that copper(II) is in solution, not that the anode is of copper.