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Product at anode of the electrolysis of MgBr2

  • #1
1. Homework Statement
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Why is the product at anode Br2 for MgBr2? Instead of O2?

Homework Equations




The Attempt at a Solution


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The EΘ value for the oxidation of OH- is -0.40 V,
And that of Br- is -1.07V
And so, shouldn't oxidation of OH- be easier? And hence, O2 is formed at the anode?
 

Answers and Replies

  • #2
Borek
Mentor
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If memory serves me well, the explanation lies in the kinetics. In general yes, oxygen should be formed. In practice oxygen evolution is quite slow, so you need an overpotential to speed it up, which moves you into the voltages where Br2 is produced. It is the same with brine solutions, where you can easily smell chlorine despite the fact oxygen should evolve first.
 
  • #3
If memory serves me well, the explanation lies in the kinetics. In general yes, oxygen should be formed. In practice oxygen evolution is quite slow, so you need an overpotential to speed it up, which moves you into the voltages where Br2 is produced. It is the same with brine solutions, where you can easily smell chlorine despite the fact oxygen should evolve first.
Woah I see.. How am suppose to know this during the exam ?
Anyways, thanks. Are there any other exceptions other than brine and MgBr2?
 
  • #4
Borek
Mentor
28,364
2,757
I believe oxygen in general evolves slowly, so whenever it is one of possible products, other things can be produced as well at higher potentials.

Note: I am not entirely sure about this explanation, take it with a grain of salt.
 
  • #5
I believe oxygen in general evolves slowly, so whenever it is one of possible products, other things can be produced as well at higher potentials.

Note: I am not entirely sure about this explanation, take it with a grain of salt.
Alright, thanks!
 

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