TPR GC Diagnostic: Electrochem Confusion

[justadream]

Question in picture:
http://postimage.org/image/3kgax3ewf/

The answer explains that you flip the value for E0 for oxidation which I understand. My questions is: How do you know which chemical is being oxidized and reduced?

I thought that the lower (more negative) the E0, the better the material is as a reducing agent (and hence the other material will be reduced). In this problem, Al has a highly negative E0 so I thought that Al would be the material being oxidized while the other would be the material being reduced.

Apparently in the correct answer choice involving Al and Ag, it is Ag that is being oxidized - not Al.

Can someone explain this?

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[Postictal Raiden]

Question in picture:
http://postimage.org/image/3kgax3ewf/

The answer explains that you flip the value for E0 for oxidation which I understand. My questions is: How do you know which chemical is being oxidized and reduced?

I thought that the lower (more negative) the E0, the better the material is as a reducing agent (and hence the other material will be reduced). In this problem, Al has a highly negative E0 so I thought that Al would be the material being oxidized while the other would be the material being reduced.

Apparently in the correct answer choice involving Al and Ag, it is Ag that is being oxidized - not Al.

Can someone explain this?

The more positive the overall voltage of the reaction the more spontaneous it is, and inversely, the more negative it is the more nonspontaneous it is.

Unlike galvanic cells, electrolytic cells provide the a voltage to drive a nonspontaneous reaction. Meaning, electrolytic cells deal with reactions that have an over all negative potential.

Based on what I calculated, the E value for each answer choice is as follows:

A. +2.48
B. -2.76
C. +1.48
D. -1.20

To rephrase the question, it is asking for the least favorable reaction to occur in an electrolytic cell. Since we know that electrolytic cells deal with nonspontaneous redox reactions (rxns with negative E values), we look for a reaction that has a the highest positive value (the most spontaneous one). Answer A is the most positive and the most spontaneous, so the reaction is the least favorable to occur in an electrolytic cell. Answer B on the other hand, is the most favorable redox reaction to occur in an electrolytic cell because it has the most negative E value.

 

[justadream]

Thank you for the reply.

But how did you calculate the values you got for each choice?

For example for A, you did -(E of Al) + (E of Ag)
But when in B, you did (E of Al) + -(E of Br2)

How did you decide to flip the value of Al for A but not for B? In both cases, the E0 of Al started out less than the other component.

 

[Postictal Raiden]

Thank you for the reply.

But how did you calculate the values you got for each choice?

For example for A, you did -(E of Al) + (E of Ag)
But when in B, you did (E of Al) + -(E of Br2)

How did you decide to flip the value of Al for A but not for B? In both cases, the E0 of Al started out less than the other component.

To have Al3+ the reaction must be ran backward giving an opposite E value. On the other hand, to have Ag, the reaction must run forward, so you keep the same value.