[truf777]

can anyone explain to me how volta element works without salt bridge ?
like this animation
<a href="http://www.e-dukasi.net/mapok/mp_files/mp_259/anim/hal06.swf" target="_blank" rel="noopener noreferrer" class="bbc_link bbc_flash_disabled new_win">http://www.e-dukasi.net/mapok/mp_files/mp_259/anim/hal06.swf</a>

[Jorriss]

I think you can have any electrochemical cell without a salt bridge but if the ions diffuse at different rates you'll gain a potential difference between the two which messes it up - and a salt bridge fixes that.

That's probably not the answer but that's my first thought.

[truf777]

I think you can have any electrochemical cell without a salt bridge but if the ions diffuse at different rates you'll gain a potential difference between the two which messes it up - and a salt bridge fixes that.

That's probably not the answer but that's my first thought.

my purpose of making volta element is to make potential difference so it make electric current . so what is  messed up ?

and i need to know what zn is reacting to or cu is reacting to
and how the electron transfered from + to -

[Jorriss]

errrm, I was referring to a liquid junction potential. I'll work on the rest of your questions in the morning, I gotta sleep!

[truf777]

errrm, I was referring to a liquid junction potential. I'll work on the rest of your questions in the morning, I gotta sleep!

wow thanks a lot

[Caeldom]

Brace yourself, heh, this may be a bit long.

Basically this is a version of a Voltaic cell (I assume 'volta element' is just another name) which isn't set up in the fashion of a Daniell cell (the usual one with two half-cells and a salt bridge). It still works quite the same, but instead of two separated electrolytes, only one is used, ie. diluted sulfuric acid. However, I must point out firstly that the animation is incorrect. It has animated electrons traveling from positive copper to negative zinc, when in actual fact, the electrons should be traveling in the opposite direction. The reason lies in the process of the cell.

What happens is first the diluted sulfuric acid is ionised into positive hydrogen and negative sulfate ions. Now the zinc is more reactive and has a more negative electrode potential than copper (this is provided in the table of standard potentials), meaning zinc atoms in solution will tend to ionise and oxidise more easily than copper, ie. the atoms will leave their electrons on the zinc electrode, making it more negative. Thus, the zinc becomes the anode. These electrons need to go somewhere, so they push along the conducting wire (through the volt meter) and onto the more positive copper cathode. Again, note that this is opposite to what the animation is showing. Here the electrons can't really push anywhere else. Since the only copper cations are those already at equilibrium with their electrons, they won't react to form more copper. But remember the ionised sulfuric acid electrolyte? The positive hydrogen ions are repelled by the zinc cations around the zinc anode, but are attracted by the increase in migrating electrons at the copper cathode. They quickly reduce (by accepting an electron) to form hydrogen atoms that then react covalently with each other into hydrogen gas. As a result, bubbles will visibly form on the copper. And the sulfate anions? They are repelled by the migrating electrons at the copper cathode, and attracted to the zinc cations around the zinc anode, which react to form aqueous zinc sulfate (but won't be visible, and will remain mostly ionised anyway, due to the dilution).

To sum things up:

• Zinc, having the more negative E⁰ (electrode potential) value becomes the anode and site of oxidation (remember this with ANOX), where the sulfate anions are attracted to.
• Copper becomes the cathode and the site of reduction (remember this with REDCAT), where the hydrogen cations are attracted to, and are reduced to visible hydrogen gas.
• Copper is not formed as there are no copper cations.
• Also, as a side note, there is something called polarisation which occurs at the copper cathode, where hydrogen can build up to a point until it prevents other hydrogen ions from accepting electrons, thereby temporarily halting the electron flow, ie. no electricity.

Now to answer your question, a salt bridge is only used in a Daniell cell where the two electrodes have their own separate electrolytes. Both electrolytes start with a neutral charge, which allows the electrons to flow. As ions in each electrolyte are increased or decreased due to the redox reactions, it affects the charge of the electrolyte. If neutrality is not established, the electron flow will not continue, they will remain where they are to keep the charges neutral. A salt bridge simply allows soluble ions, that won't react with those in the electrolyte, to balance out those charges. Potassium nitrate is most often used because both ions are soluble in aqueous solutions.

In this example, if zinc and copper were instead set up in a Daniell cell, zinc and copper sulfate would be used as the respectful electrolytes. Also, no hydrogen is formed (thus solving the polarisation problem) and an overall higher voltage can be produced too. Zinc is oxidising into zinc ions at the anode, making the zinc sulfate electrolyte positive. Conversely, positive copper ions are reducing into copper at the cathode, making the electrolyte negative. What the salt bridge will do is provide nitrate anions to balance out the positive zinc sulfate electrolyte, and potassium cations to the other side. Thus neutrality is established and electrons will continue to flow!

Now I know that was a whole smorgasbord of information to take in (sorry!), but I tried to elaborate as much as possible to make everything clear. If you still have any questions, just ask