How cathode is positively charged in voltaic cell?

[DrDu]

Yes, if you have a salt bridge, both cations and anions will contribute to the current. The precise extent of current carried by anions and cations depends on their respective mobility.

 

[gracy]

Yes, if you have a salt bridge, both cations and anions will contribute to the current. The precise extent of current carried by anions and cations depends on their respective mobility.

You mean as we have studied protons do not produce current but electrons do,is it not true for ions?I mean do cations also produce current?What's the direction of the current produced by ions?In the direction of anions flow or cations flow?

 

[DrDu]

Who sais that protons don't carry current? In solution, protons have the highest mobility of all charge carriers. Anions moving in the same direction as electrons will prduce a current of the same sign, while cations moving in the same direction as electrons will produce a current in the other direction. In a salt bridge, anions and cations move in different directions (anions moving to the + pole, cations to the - pole), hence the sign of the current carried by them is the same.

 

[gracy]

Who sais that protons don't carry current?

In our class we have been told that it was a misconception that flow of proton produces current it was later learned but convention was not changed so still current direction is indicated by direction of proton (opposite of direction of flow of electrons).

 

[DrDu]

At the time the direction of current and the + and - were fixed, people had no idea about the existence of electrons and protons. So nobody ever claimed that the current in a wire were due to protons. Anyhow, that the current in a wire is carried by electrons does not mean that cations can't move (and can be the main charge carriers) in other media.

 

[gracy]

.

Who sais that protons don't carry current?

Here is a link
http://www.enotes.com/homework-help/why-direction-current-opposite-flow-electrons-349665

 

[gracy]

Anyhow, that the current in a wire is carried by electrons does not mean that cations can't move (and can be the main charge carriers) in other media.

I got your point.

 

[gracy]

In solution, protons have the highest mobility of all charge carriers.

Is current due to positive charges only possible in solutions?If not,what are the other examples of current because of positive charges?

 

[Borek]

what are the other examples of current because of positive charges?

Electron holes in semiconductors.

 

[DrDu]

Here is a link
http://www.enotes.com/homework-help/why-direction-current-opposite-flow-electrons-349665

So what? I suppose the respondenr had a metallic conductor in mind.

 

[gracy]

I have one more doubt .Sometimes it is said that cations of inert electrolytes of salt bridge go to cathode half cell and anions of inert electrolyte travel into anode half cell to balance the charge imbalance produced in them.Is it right?

 

[DrDu]

Sodium doped alumina. Glas, e.g. in ph electrodes.

 

[DrDu]

I have one more doubt .Sometimes it is said that cations of inert electrolytes of salt bridge go to cathode half cell and anions of inert electrolyte travel into anode half cell to balance the charge imbalance produced in them.Is it right?

Depends on whether you are talking about electrolysis or a galvanic cell.

 

[gracy]

Sometimes it is said that cations of inert electrolytes of salt bridge go to cathode half cell and anions of inert electrolyte travel into anode half cell to balance the charge imbalance produced in them.Is it right?

This condition is seen in electrolytic cell,right?But why electrolytic cell needs salt bridge?

 

[DrDu]

Yes, this is right both for electrolytic and Galvanic cells. You are right that not all electrolytic cells need a salt bridge or thelike. The copper rafination I mentioned earlier is a counter example. On the other hand, if you take two half cells, e.g. Cu/Cu2+ and Zn/Zn2+ with a salt bridge, you can operate it both as a Galvanic and an electrolytic cell. With the now generally accepted convention of anode and cathode, cations will allways wander to the cathode, anions to the anode.

 

[gracy]

I want to know which of these is correct
Cations and anions from oxidation and reduction half cell respectively move through the salt bridge into reduction an oxidation half cell and inert electrolyte present in salt bridge is just to provide conducting medium
OR
The inert electrolyte present in salt bridge dissociates into cations and anions and travel into the reduction and oxidation half cell respectively.For eg.KNO3 will dissociate int K+ and NO3- ions ,K+will go in reduction half cell to balance excessive negative charge and NO3- would go in oxidation half cell to balance excessive positive charge .

 

[DrDu]

The second option is correct, although ions from the half cells will also enter into the salt bridge.

 

[gracy]

ions from the half cells will also enter into the salt bridge.

Both of the processes will occur simultaneously ,the one i have mentioned in first option is to complete the circuit, and the second one to maintain neutrality ,right?

 

[gracy]

The last doubt about salt bridge is
In my textbook it is written that salt bridge prevents the mechanical mixing of the solution.what it means?

 

[Bystander]

textbook it is written that salt bridge prevents the mechanical mixing of the solution

Very little, really. All a salt bridge accomplishes is that it slows the mixing of different solutions in two half cells.

Couple general remarks: electrochemical/electrolytic cells are used for batteries, either disposable or rechargeable, as sources of electric power; for electrolytic refining/production of metals or other chemicals (Cu, Al, Cl, Na, others); or, they are used for laboratory measurements of free energy, and analytical applications. I'm not aware that any textbook explicitly states that there are these three general areas of applications.

Salt bridges are used in laboratory applications, that is for cells that do no electrical work, nor have any electrical work done on them. One might be interested in comparing the concentrations of two different solutions of ferrous sulfate, for example, and it is necessary to prevent the solutions from mixing over the time frame of the measurement. Mixing is prevented by use of the salt bridge which allows for ionic current from a solution in one half cell (Fe/Fe⁺²) at one concentration to the second half cell (Fe/Fe⁺²) at a different concentration without the two solutions mixing or diffusing into one another and reducing the potential difference between the two half cells to zero before the measurement can be accomplished. Check the index in the textbook for "concentration cells."

 

[gracy]

Is my 48th post,correct?

 

[gracy]

Salt bridges are used in laboratory applications, that is for cells that do no electrical work, nor have any electrical work done on them. One might be interested in comparing the concentrations of two different solutions of ferrous sulfate, for example, and it is necessary to prevent the solutions from mixing over the time frame of the measurement. Mixing is prevented by use of the salt bridge which allows for ionic current from a solution in one half cell (Fe/Fe+2) at one concentration to the second half cell (Fe/Fe+2) at a different concentration without the two solutions mixing or diffusing into one another and reducing the potential difference between the two half cells to zero before the measurement can be accomplished. Check the index in the textbook for "concentration cells."

But according to the video I have seen on concentration cell ,electrons are transferred not the ions.This is the video i am talking about

 

[gracy]

Mixing is prevented by use of the salt bridge which allows for ionic current from a solution in one half cell (Fe/Fe+2) at one concentration to the second half cell (Fe/Fe+2) at a different concentration without the two solutions mixing or diffusing into one another

What is mixing of solutions then?I thought that if ions of one half cell travel to second half cell ,it is said that solutions are mixed.

 

[Bystander]

,electrons are transferred not the ions.

This is in the external circuit. The part of the circuit where chemistry is happening carries current in one, or both, directions by ion transport.

Both of the processes will occur simultaneously ,the one i have mentioned in first option is to complete the circuit, and the second one to maintain neutrality ,right?

Yes.

I thought that if ions of one half cell travel to second half cell ,it is said that solutions are mixed.

Any transfer of solute, ions, solvent, from one half cell to the other is "mixing." The idea behind the salt bridge is that the ion transfer is insignificant over the course of a measurement due to difficulty of diffusion through the salt bridge, or that the actual ion transport carrying the current involves the ions of the salt bridge, and that the salt be chosen to minimally interfere with the chemistry being measured/studied.

 

[gracy]

The idea behind the salt bridge is that the ion transfer is insignificant over the course of a measurement due to difficulty of diffusion through the salt bridge,

Only sufficient enough to complete the circuit?

 

[Bystander]

sufficient enough to complete the circuit?

Yes.

 

[gracy]

that the actual ion transport carrying the current involves the ions of the salt bridge,

Ions of salt bridge also produce current?I thought they are only to maintain neutrality .

 

[Bystander]

Ions of salt bridge also produce carry current?

For every cation reduced at the cathode, to maintain neutrality, cation(s) must move through the salt bridge (carrying charge/current).

 

[gracy]

For every cation reduced at the cathode, to maintain neutrality, cation(s) must move through the salt bridge (carrying charge/current).

But at the same time For every anion oxidized at the anode, to maintain neutrality, anions must move through the salt bridge (carrying charge/current)Won't it cancel the current produced by cations of salt bridge?

 

[Bystander]

Won't it cancel the current produced by cations of salt bridge?

The two contributions are NOT necessarily equal. If you look at current through a Cu/Cu⁺²//Cu⁺²/Cu cell for refining/producing electrolytic copper, almost all the current carried through the cell is by Cu⁺² ions. There is very little transport by SO₄^-2 ions which more or less maintain the electrical neutrality by playing "hot potato" or passing Cu⁺² ions from one to another.