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Author Topic: Electric Potential  (Read 5587 times)

Offline Wally

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Electric Potential
« on: 23/11/2007 03:55:33 »
Hi passionated Scientists:

Perhaps you have used a redox reaction in the laboratory to analize some sustance, so I wanna ask you

Can you explain me whats "potential" in an electrochemical cell? what is its meaning?

what´s the meaning of ΔΕ? I know it could be a diffcult theme, but, would you help me?


 

Offline lightarrow

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« Reply #1 on: 23/11/2007 13:23:27 »
Hi passionated Scientists:

Perhaps you have used a redox reaction in the laboratory to analize some sustance, so I wanna ask you

Can you explain me whats "potential" in an electrochemical cell? what is its meaning?

what´s the meaning of ΔΕ? I know it could be a diffcult theme, but, would you help me?
The meaning is that it's a "potential"! :)
You have your electrode, you have an hydrogen standard electrode, you take a voltmeter, one point on your electrode and the other on the hydrogen electrode and you read the voltage = potential. Where's the problem?

You always measure voltage differences, but when you refer to the standard hydrogen electrode, you call that difference "E"; in general cases you call it "ΔE".
« Last Edit: 23/11/2007 13:25:48 by lightarrow »
 

lyner

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Electric Potential
« Reply #2 on: 26/11/2007 19:01:24 »
That's a chemist's answer. I'm surprised at you lightarrow.
 

Offline lightarrow

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« Reply #3 on: 26/11/2007 21:25:47 »
That's a chemist's answer. I'm surprised at you lightarrow.
If you know it's a chemist's answer, you are a chemist too... :)
I studied physics at university (4 years, didn't get degree) but chemistry has always been my hobby.
 

lyner

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Electric Potential
« Reply #4 on: 27/11/2007 14:02:52 »
There are several 'open' statements in your answer that you could help me with.
How do you get  hold of a Hydrogen Electrode?
What input resistance can your voltmeter have before your measurement goes wrong?
(The first is a genuine question.)

What is potential at a point? It's the energy needed to bring one coulomb of charge to that point. The Potential difference between two points is the energy needed to bring one coulomb of charge from one point to the other. (Energy may be put in or taken out, depending on the sign of the charge / direction of the field, so the potential may be + or -).
Is that answer any use to a Chemist?
There seem to be a huge leap between connecting wires up to an electric circuit and measuring  chemical potentials.
« Last Edit: 28/11/2007 13:38:22 by sophiecentaur »
 

Offline lightarrow

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« Reply #5 on: 28/11/2007 19:00:14 »
There are several 'open' statements in your answer that you could help me with.
How do you get  hold of a Hydrogen Electrode?
Google "reference electrodes sell". The first in the list:
http://www.radiometer-analytical.com/pdf/meterlab/Electrode_Guide_en.pdf

Quote
What input resistance can your voltmeter have before your measurement goes wrong?
Don't know.

Quote
(The first is a genuine question.)
So the second is not... :)

Quote
What is potential at a point? It's the energy needed to bring one coulomb of charge to that point. The Potential difference between two points is the energy needed to bring one coulomb of charge from one point to the other. (Energy may be put in or taken out, depending on the sign of the charge / direction of the field, so the potential may be + or -).
Is that answer any use to a Chemist?
There seem to be a huge leap between connecting wires up to an electric circuit and measuring  chemical potentials.
For some reasons, two of which are:
1. It's impossible, technically, to measure the electric potential of a semi-cell.
2. If you use a voltmeter to measure the potential difference between a metal electrod inside a solution with ions, and the solution, you authomatically introduce, at least, a contact potential between the metal electrode and the metal tip of the voltmeter, and probably (unless your voltmeter has platinum tips and the solution is not chemically active on it) another unknown potential between the voltmeter tip and the solution. For this reason in those measurements it's used a reference electrode.
« Last Edit: 28/11/2007 19:03:08 by lightarrow »
 

Offline Bored chemist

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Electric Potential
« Reply #6 on: 28/11/2007 19:52:35 »
"What input resistance can your voltmeter have before your measurement goes wrong?"
The unhelpful answer is that the resistance needs to be infinite.
There are a couple of ways round this. The traditional aproach is to use a potentiometer where (in principle) no current flows from the cell.
A more sensible answer is to use a high resistance voltmeter, say 10M, measure the voltage then put a 10M resistor in parallel with the meter and measure the voltage again.
The second voltage will be a little lower than the first From the difference you can measure the effective source resistance and from that you can calculate the voltage with no load (to a good aproximation it's the initial voltage + the difference between the 2 measured voltages).

BTW, since this thread is in "Chemistry", did anyone expect a biologist's answer?
 

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Electric Potential
« Reply #6 on: 28/11/2007 19:52:35 »

 

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