Problem about Nitrate electrodeposition

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Discussion Overview

The discussion revolves around a homework problem concerning the electrodeposition of nickel from an aqueous sulfate solution, specifically focusing on the pH range required to avoid hydrogen gas evolution and nickel hydroxide precipitation. Participants explore the solubility product (Ksp) and its implications for the problem.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant attempts to determine the pH range for electrodeposition by calculating the equilibrium potential and suggests a pH of 4.22 to avoid hydrogen gas evolution.
  • Another participant challenges the formula for Ksp provided by the first participant, asserting it is incorrect conceptually, not just mathematically.
  • Subsequent posts continue to debate the correct formulation of Ksp, with various participants proposing different expressions and corrections.
  • One participant suggests a formula involving the dissociation of nickel hydroxide and the relationship between hydroxide ions and nickel ions.
  • Several participants express confusion and seek clarification on the correct approach to calculating the concentrations involved in the Ksp expression.
  • A later reply indicates that once the correct formula for Ksp is established, solving for hydroxide concentration and converting it to pH should be straightforward.
  • One participant reflects on their initial confusion regarding the problem's presentation and acknowledges a learning moment regarding the definition of the solubility product.

Areas of Agreement / Disagreement

Participants do not reach consensus on the correct formulation of the Ksp or the approach to solving the problem. Multiple competing views and corrections are presented throughout the discussion.

Contextual Notes

There are unresolved issues regarding the assumptions made in the Ksp calculations and the definitions used, which may affect the conclusions drawn from the discussion.

cuigm371
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Homework Statement


Hey guys,
I am preparing a technical exam and am facing an exam problem below:

Ni is electrodeposited from an aqueous sulphate solution that has a composition that fluctuates from 1 to 0.01M due to changes in the feed composition of the process.
a) what is the pH range that this electrodeposition can be conducted in order to avoid hydrogen gas evolution and nickel hydroxide precipitation? Given:
Ni 2+ + 2e- <-> Ni , E0 = -0.25V
Ni 2+ + 2H2O <-> Ni(OH)2 (s) + 2H+ log K = -12.73

Homework Equations



Ksp= [Ni(OH)2] * [H+]^2/[Ni+]

The Attempt at a Solution



a. pH range: take Eh-PH of H2O with respect to H2/H+ where:
[H2/H+] = -0.0592 pH = E0 = -0.25 V
so pH = 0.25/0.0592 = 4.22 to avoid hydrogen gas evolution

For Ni(OH)2 precipitation:
Ksp= [Ni(OH)2] * [H+]^2/[Ni+]^2 = 10^(-12.73)

this is where I was stuck- how to calculate the [Ni(OH)2] and the [Ni+]? using 1M and 0.01M respectively?
 
Last edited:
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Your formula for Ksp is wrong.
 
Borek said:
Your formula for Ksp is wrong.

Thank you for pointing out. the formula should've been:

Ksp= [Ni(OH)2] * [H+]^2/[Ni+] = 10^(-12.73)

I still am not clear what to do next. Could you please advise?
 
No, this is still not a correct formula for solubility product. It is not a matter of a wrong exponent, it is conceptually wrong (and that's what stops you from solving the problem).
 
Ni(OH)2 <-> Ni 2+ + 2 (OH)-
and Ksp = [Ni2+] [OH-]^2/[Ni(OH)2]

this seems more reasonable. Correct?
 
No, you are making the same error again and again. Please check with your textbook.
 
Borek said:
No, you are making the same error again and again. Please check with your textbook.

I believe it is right as below. THANK YOU VERY MUCH!

H+ + OH- = H2O Kw
Ni(OH)2= Ni2+ + 2OH- Ka

so, for Ni2+ + 2H2O = Ni(OH)2 + 2H+
K = Kw^2/Ka
 
Borek said:
Nope, it is not a solubility product.

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch18/ksp.php

Refer to the textbook page attached.

Ni(OH)2 = Ni2+ + 2(OH)-
Ks = [Ni]2+ x (OH)-^2 = 10^(-12.73)
Ni+ ranges from 1 to 0.01M, so OH- can be calculated accordingly. Is this right now?

Problem_Ni.JPG
 

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  • #10
Yes, when you have a correct formula for Ksp it is as trivial as solving for OH- and converting it to pH.
 
  • #11
Borek said:
Yes, when you have a correct formula for Ksp it is as trivial as solving for OH- and converting it to pH.

Thank you very much indeed!
looking back I think I was confused initially by the way it was given,
Ni 2+ + 2H2O <-> Ni(OH)2 (s) + 2H+

and forgot to go to the definition of the solubility product. this is a good lesson learned.
 

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