October 31, 2024, 07:51:02 PM
Forum Rules: Read This Before Posting


Topic: Electrolysis - Which metal will deposit first  (Read 4141 times)

0 Members and 1 Guest are viewing this topic.

Offline sinthreck

  • Regular Member
  • ***
  • Posts: 43
  • Mole Snacks: +1/-0
Electrolysis - Which metal will deposit first
« on: November 09, 2010, 11:34:24 PM »
Quote
An electrolysis cell contains a mixture of the following salts in solution in the cell around the cathode:
0.2 M CdCl 2, 1.0 x 10 -5 M CoCl 2, 5 M FeCl 2, and 1.0 x 10 -5 M NiCl 2.

Given the following standard reduction potentials, in which order are the metals deposited at the cathode as the voltage of the electrolysis cell is slowly increased?

Ni2+ ( aq) + 2e - → Ni ( s)      E°= -0.275 V
Co2+ ( aq) + 2e - → Co ( s)    E°= -0.280 V
Cd2+ ( aq) + 2e - → Cd ( s)    E°= -0.400 V
Fe2+ ( aq) + 2e - → Fe ( s)     E°= -0.440 V
ANS: Fe, Ni, Co, Cd

My understanding is that the reaction which is most easily reduced will plate out first, however, I suppose it also depends on concentration.

My next thought is to use the nerst equation but what concentration do I put for the anode?

Offline FreeTheBee

  • Regular Member
  • ***
  • Posts: 87
  • Mole Snacks: +12/-1
Re: Electrolysis - Which metal will deposit first
« Reply #1 on: November 10, 2010, 01:53:15 AM »
If the reaction on the anode is the same in all cases, do you really need to know any details about it to make a comparison?

Offline sinthreck

  • Regular Member
  • ***
  • Posts: 43
  • Mole Snacks: +1/-0
Re: Electrolysis - Which metal will deposit first
« Reply #2 on: November 10, 2010, 04:05:39 AM »
thanks for that.

Only thing is, I got a slightly different answer to their's

Fe, Cd, Ni, Co

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27820
  • Mole Snacks: +1808/-412
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Electrolysis - Which metal will deposit first
« Reply #3 on: November 10, 2010, 04:45:57 AM »
Cd & Ni order is highly temperature dependent, it can be either Fe, Cd, Ni, Co or Fe, Ni, Cd, Co (that is assuming we know concentrations with a high precision). But in general you are right and they seem to be wrong.

Or at least you are wrong in a good company ;)
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline FreeTheBee

  • Regular Member
  • ***
  • Posts: 87
  • Mole Snacks: +12/-1
Re: Electrolysis - Which metal will deposit first
« Reply #4 on: November 10, 2010, 05:08:25 AM »
If I use the numbers you wrote I get the same order as you. According to my tables the standard potential for nickel should 0.257V though. But with that number I don't get the "correct" order either.
The numbers are so close that in practice they will have little predictive power, but I guess that is not so relevant for the exercise  :)


Offline sinthreck

  • Regular Member
  • ***
  • Posts: 43
  • Mole Snacks: +1/-0
Re: Electrolysis - Which metal will deposit first
« Reply #5 on: November 10, 2010, 07:16:55 PM »
thanks guys!

Exam in 4 hrs time.

50% of entire course in only 2hrs!

Sponsored Links