Help on Problem 14.15

ntruong2H
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Help on Problem 14.15

Hi, I was wondering if someone could please help me with question 14.15, specifically part c? The question reads: Write the half-reactions and devise a galvanic cell (write a cell diagram) to study each of the following reactions:

c) $Cd(s) + 2Ni(OH)_{}3 (s) \rightarrow Cd(OH)_{2}(s) + 2Ni(OH)_{2} (s)$ the reaction in the nickel-cadmium cell.

My specific question is, in the cell diagram, where did the KOH(aq) part come from? Also, why do we need to use Ni(s) for the conducting electrode for the cathode?

The answer to this question is Cd(s)/$Cd(OH)_{2}(s)$/KOH(aq)//$Ni(OH)_{3}(s)$/$Ni(OH)_{2}(s)$/Ni(s)

Thank you!

Chem_Mod
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Re: Help on Problem 14.15

The $KOH_{(aq)}$ or KOH (aq) acts as the salt bridge between the two solutions, which allows for ion flow. The conducting electrode refers to the solid pieces of metal in solution, acting as the cathode or the anode. The previous questions only used one metal species as both cathode and anode, but in this reaction the metal with a lower potential is the anode and the metal with the higher potential is the cathode. Figure 14.4 on page 573 is a picture of the diagram.

ntruong2H
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Re: Help on Problem 14.15

Chem_Mod wrote:The $KOH_{(aq)}$ or KOH (aq) acts as the salt bridge between the two solutions, which allows for ion flow. The conducting electrode refers to the solid pieces of metal in solution, acting as the cathode or the anode. The previous questions only used one metal species as both cathode and anode, but in this reaction the metal with a lower potential is the anode and the metal with the higher potential is the cathode. Figure 14.4 on page 573 is a picture of the diagram.

Okay thank you. I understand that KOH acts as the salt bridge; however, what I am confused about is why in this cell diagram we have to include the salt bridge, but in the other problems, we did not write the salt bridge as part of the cell diagram?

Chem_Mod
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Re: Help on Problem 14.15

It would have to do with the type of reaction that is given. In the question, it is specified that this reaction is in a nickel-cadmium cell, which needs a salt bridge. The previous parts (a) was specifying a solubility equilibrium and part (b) was specifying a Bronsted neutralization reaction, both of which the species are in contact with each other and do not require a salt bridge.

ntruong2H
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Re: Help on Problem 14.15

Chem_Mod wrote:It would have to do with the type of reaction that is given. In the question, it is specified that this reaction is in a nickel-cadmium cell, which needs a salt bridge. The previous parts (a) was specifying a solubility equilibrium and part (b) was specifying a Bronsted neutralization reaction, both of which the species are in contact with each other and do not require a salt bridge.

I thought the double lines (//) were enough to represent the salt bridge? I guess I am still confused as to why in problems such as 14.13, we do not explicitly write the salt bridge. And in problems, such as 14.11, the cell diagrams do not write the salt bridge out explicitly either? Can you please explain why in this singular problem we write the salt bridge, where typically it is not written?

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