EMF and Spontaneity

[Wakesc]

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While looking at a problem I got that the EMF came out to be positive. Their explanation says that since it is positive that indicates that the cell is electrolytic and spontaneous to the left(reverse). I understand the electrolytic aspect but I'm not sure about the reaction being spontaneous to the left. since a positive EMF is spontaneous to the right does that necessarily mean that if its negative it is spontaneous to the left? since electrolytic cells aren't spontaneous how could the reaction be spontaneous in any direction?

 

[TwoPaddles]

While looking at a problem I got that the EMF came out to be positive. Their explanation says that since it is positive that indicates that the cell is electrolytic and spontaneous to the left(reverse). I understand the electrolytic aspect but I'm not sure about the reaction being spontaneous to the left. since a positive EMF is spontaneous to the right does that necessarily mean that if its negative it is spontaneous to the left? since electrolytic cells aren't spontaneous how could the reaction be spontaneous in any direction?

Who is "their" and what right or left are you talking about? In electrolytic cells, you introduce a voltage that's higher than the original voltage, so the electron flows in the opposite direction, making it nonspontaneous. If you don't introduce the "higher voltage" it's already spontaneous (as in galvanic cells).

 

[Wakesc]

Who is "their" and what right or left are you talking about? In electrolytic cells, you introduce a voltage that's higher than the original voltage, so the electron flows in the opposite direction, making it nonspontaneous. If you don't introduce the "higher voltage" it's already spontaneous (as in galvanic cells).

Oh sorry, its out of an old Kaplan book. The right or left has to do with the reactants/products of a redox reaction.

Sm^3+ + Rh + 6Cl- --> [RhCl6]^3- + Sm

So its saying that the reaction would spontaneously Oxidize Sm and Reduce [RhCl6]^3-. Which I guess I understand since a positive EMF would make the left side spontaneously react to produce the right. Therefore, is the opposite true for a negative EMF? If no voltage is introduced it spontaneously goes left? If a voltage was applied it would then go right.... right 🙂? Thanks for your input.

 

[Rabolisk]

All reactions are either in equilibrium or spontaneous in at least one direction. An electrolytic cell is nonspontaneous in the direction as written, but it is spontaneous in the other direction.

 

[PeterPesto]

Oh sorry, its out of an old Kaplan book. The right or left has to do with the reactants/products of a redox reaction.

Sm^3+ + Rh + 6Cl- --> [RhCl6]^3- + Sm

So its saying that the reaction would spontaneously Oxidize Sm and Reduce [RhCl6]^3-. Which I guess I understand since a positive EMF would make the left side spontaneously react to produce the right. Therefore, is the opposite true for a negative EMF? If no voltage is introduced it spontaneously goes left? If a voltage was applied it would then go right.... right 🙂? Thanks for your input.

If Sm is oxidized and Rh is reduced, the reaction proceeds spontaneously from right to left and therefore has a positive emf. The reaction, as written from left to right, would not occur spontaneously. The emf of both reactions are the same magnitude but opposite signs.