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Topic: Workings of a Daniel Cell  (Read 5530 times)

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BravoTwo

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Workings of a Daniel Cell
« on: March 21, 2006, 09:35:36 PM »
Hello

I am reviewing the basics of high school chemistry, using "Chemistry for Dummies".
The book contains a description of the workings of a Daniel Cell, composed of the following:

-A piece of zinc metal in a solution of zinc sulfate (anode)
-A piece of copper metal in a solution of copper (II) sulfate (cathode)
-A wire between the anode and the cathode
-A salt bridge, filled with a concentrated salt solution, between the two solutions

I do not understand why a salt bridge is necessary, in addition to the wire between the two electrodes, for electrons to flow.

More specifically, since copper ions  accept electrons from zinc metal, and copper and zinc are conductors, why can't electrons just move from the anode to the cathode, through the wire, as if "pulled" by copper ions in the copper (II) sulfate ? Why do we need the circuit to be closed by a bridge?

Could somebody explain what's missing in my "reasoning"?
Thank you very much for your help.
 ???

tonyliruhan

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Re:Workings of a Daniel Cell
« Reply #1 on: March 21, 2006, 10:07:31 PM »
the salt bridge is a part of the current circle.
the electrons will flow when there is a completed circle

BravoTwo

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Re:Workings of a Daniel Cell
« Reply #2 on: March 22, 2006, 04:13:40 AM »
Thank you for your reply. I think my question was not clearly stated.

So here it is:

Why do electrons need a complete circle to flow?

Let's imagine there is no salt bridge. In that case, what would prevent electrons from moving from the anode to the cathode, through the wire, when copper ions in the copper (II) sulfate "pull" them?

It may be obvious, but I just don't get it. Thank you for your help.

« Last Edit: March 22, 2006, 04:15:05 AM by BravoTwo »

Offline xiankai

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Re:Workings of a Daniel Cell
« Reply #3 on: March 22, 2006, 04:20:29 AM »
maybe this picture answers your question?

« Last Edit: March 22, 2006, 04:21:07 AM by xiankai »
one learns best by teaching

BravoTwo

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Re:Workings of a Daniel Cell
« Reply #4 on: March 22, 2006, 06:11:45 AM »
Thank you for the picture. This is roughly the same picture as is in my textbook.

So, suppose we remove the bridge. We have now a species that gives electrons, a species that accepts electrons, and a metallic wire between them. What would prevent electrons from going, through the wire, from one species to the other?



Thank you. ???

tonyliruhan

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Re:Workings of a Daniel Cell
« Reply #5 on: March 22, 2006, 06:40:17 AM »
I know where your problem is
suppose the salt bridge is removed
as you said, the electhons flow to the specie which
except electrons, so it is negative charged.in the same words,
the opposite side is positive charged.
maybe you know that the current flows from high protential to low protential, and maybe you know the electrons flow in the opposite direction of the current.so the electron is supposed to move back and  the situation you discribed can not exist

anyway, your question is more likely to be a physical problem

BravoTwo

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Re:Workings of a Daniel Cell
« Reply #6 on: March 22, 2006, 07:19:22 AM »
 :)?In other words, I need to have a deeper understanding of what a current is..
For now, I will just blindly accept that we need "a closed circuit for the current to flow". I'll check later in a physics textbook.
Thank you everyone for taking the time to reply.
Have a good day. ;D
« Last Edit: March 22, 2006, 07:20:04 AM by BravoTwo »

Chomp

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Re:Workings of a Daniel Cell
« Reply #7 on: April 01, 2006, 05:37:21 PM »
If there was no salt bridge, the electrons would initially enter the copper cathode and Cu(+2) cations would react with the electrons to form solid Cu.  HOWEVER, there is a counter-ion in solution, i.e. the SO4(-2) ion.  If there was no salt bridge, there would be a build-up of negative charge in the solution.  Similarly, at the anode, a build-up of positive charge would result, as Zn(+2) ions are released into solution.  This would effectively remove the electrochemical gradient.  The salt bridge serves to balance the charge of the solution, effectively "completing the circuit".

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