Calculation of absolute electrode potential and salt bridge qns

[sgstudent]

>According to http://www.chemguide.co.uk/physical/redoxeqia/introduction.html each electrode has an absolute electrode potential which is the difference between the positiveness and negativeness. So if we had a Zn plate in water and it has a negative charge of -5 on the metal. Would the absolute electrode potential be -5-5=-10?

>So when we connect that zinc electrode to another electrode say copper. Copper so has it's own absolute electrode potential say -1-1=-2. So would the voltage recorded be -10-(-2)=-8V? Or would the absolute electrode potential be like -5-0=-5 since the solution in a cell is electrically neutral and so the potential differece recorded be -5-(-1)=-4V? So do we just take the difference in charge between the metal and the overall charge of the electrolyte to get the absolute electrode potential and the difference of the two absolute electrode potential is the voltmeter reading or am I wrong here?

>Then now if we didn't have a salt bridge, how does the presence of more cations cause an opposite potential difference? The way I see it is like this: looking at the zinc electrode the negative charge on the zinc metal would remain constant but the solution outside becomes more positive. So initially it's absolute electrode potential was -5-0=-5 but soon it's -5-(1)=-6V and so on with the (1) continuing to increase. But even if keeps increasing there wouldn't be an opposite potential difference. So why would the reaction stop after a while?

 

[Borek]

You are mixing charges and voltages in a dangerous way.

Do you know what is the definition of the potential difference?

 

[sgstudent]

You are mixing charges and voltages in a dangerous way.

Do you know what is the definition of the potential difference?

Hi Borek,

Is potential difference the difference in the electrode potential at two different points? So I thought in a cell each electrode has a potential difference which is the difference in the electrode potential of the electrode and the solution which I thought would just decrease even if there are excess ions in the solution? Like because the metal is negative and after each metal atom oxidizes the negative charge remains constant but having more cations in solution the difference in potential becomes more negative? But at the other electrode a similar process occurs so overall potential difference of the two half cells still remains the same?

Which part of my understanding is wrong here?
Thanks for the help Borek :)

 

[Borek]

Is potential difference the difference in the electrode potential at two different points?

What is potential definition then?

 

[sgstudent]

What is potential definition then?

It is the electric potential energy which is a scalar quantity. Hmm is this where I'm wrong when I assigned them a negative or positive charge? If so would it be like this: The initial charge before any build up of positive charges is just 5-0=5. Then as the positive charge builds up then we go to 5-5=0 so there is no more there is no more potential difference at all and so the circuit would stop?

And if this is true then when we just put a piece of zinc in water after the equilibrium has been reached would the potential difference of that zinc and the solution be also 0? But then why would there be any of the forward reaction when the zinc was just initially put in the water?

thanks again Borek! :)

 

[sgstudent]

What is potential definition then?

Oh wait never mind my previous post I just checked and electrical potential may be a salt but it still van have a negative value like height as the reference point is zero. Electrical potential is the electrical potential energy/Coulomb charge. So if we consider the same cell as just now initially it's -5-0=-5 and when there is too much charge it's -5-5=-10? But at the other side if initially it's -2-0=-2 then after a while it's -2-(-5)=3. So initially the potential difference between the two electrode/solution would be -5-(-2)=--3V? But finally it would be -10-(-3)=-7V?

 

[Borek]

You are juggling terms and numbers, but IMHO you still failed to mention the proper definition.

All numbers you list have no units, so it is impossible to tell what you are adding to what. What you write suggests you are summing charges, but miraculously the final result is in volts.

 

[sgstudent]

You are juggling terms and numbers, but IMHO you still failed to mention the proper definition.

All numbers you list have no units, so it is impossible to tell what you are adding to what. What you write suggests you are summing charges, but miraculously the final result is in volts.

Oh yeah I shouldn't associate the charge with the voltage. So potential difference is the difference in the electric potential between two points. So how would we use that to explain why there would be an opposite potential if the solution becomes charged?

 

[Borek]

At the risk of repeating myself - what is the definition of the potential? What does the potential measure?

 

[sgstudent]

At the risk of repeating myself - what is the definition of the potential? What does the potential measure?

It is the amount ofelectric potential energy that a unitary point charge would have when located at that point. But the electrical potential at a point is the electric potential energy per unit Coulomb.

 

[Borek]

OK. I was hoping for a slightly different definition, one that involves work done when moving the electric charge. It makes it - at least in my opinion - easier to see what is happening in the solution and on the electrode surfaces, where the charge resides.

 

[sgstudent]

OK. I was hoping for a slightly different definition, one that involves work done when moving the electric charge. It makes it - at least in my opinion - easier to see what is happening in the solution and on the electrode surfaces, where the charge resides.

That other definition would be the work done per unit charge to move a charged particle from a reference point to a designated point in a static electric field. But I don't quite know how to apply this to the electrodes and solution.

 

[Borek]

Now, the electrodes are charged - but each is charged differently, so there is an electric field between them. To move a charge from one electrode to another you need to do some work against the electric field - and this is where the potential difference comes from.

 

[sgstudent]

Now, the electrodes are charged - but each is charged differently, so there is an electric field between them. To move a charge from one electrode to another you need to do some work against the electric field - and this is where the potential difference comes from.

So the anode and cathode are both negatively charged but the anode has more electrons than the cathode so the electrons would flow from the anode to the cathode. But as the charge in each individual solution increases the anode would become more neutral as positive charges accumulate on it cancelling out the negative on the anode. But the cathode would get more negative as more negative anions build up in the solution so would this cause the opposite potential difference that was mentioned?