[Technicalhuman]

Hi I have some questions regarding half cell potentials. Here is a link http://www.chemguide.co.uk/physical/redoxeqia/introduction.html

So the link stated that the some magnesium would turn into a magnesium ion leaving behind the 2 electrons in the piece of metal. But how can this be so? If the magnesium simply turns into Mg2+ and goes into the solution, the solution would have an overall positive charge. How can this be so? And how can the magnesium metal have a overall negative charge?

Also, they went on to explain hydrogen alone with the sulfuric acid solution. Again I don't get how can more H₂ turn into ions as it would result in the solution to be charged.

Is there something missing in these two reactions such as another half equation to balance the charges? In the text it doesn't see likely though.

Thanks in advance for the help.

[Corribus]

Half reactions are bookkeeping devices to keep track of oxidation and reduction components of a system.  The oxidation counterpart, however, does not happen independently of the reduction component.  In a real electrochemical cell, there is usually a salt bridge between the anode side and the cathode side that balances the production of cations (or anions) in the respective cell solutions.  As for whether electrons built up in an electrode - no, not generally.  In a complete cell, the electrons that are retained in one electrode due to electrode oxidation flow to the other electrode via a wire, which is what produces current.  At the other electrode, these electrons perform more chemistry (reduction of cations in solution), which completes the circuit.  When the cell is depleted, equilibrium is reached and the cell no longer has enough potential energy to generate current flow (here's where you Gibb energy equations from the other thread come into play).  At that point the cell is dead.  However it can be recharged by applying an electric potential in the other direction, which causes electrons to flow in reverse.  This is only spontaneous with the application of the external potential.   Once a new non-equilibrium situation is produced, the external potential is removed and the flow of current in the battery is spontaneous again.  In principle this process can be repeated indefinitely, but certain irreversible reactions at the electrode surfaces reduce the efficiency of the battery after each recharge, which means that batteries do have a limit to the number of times they can be recharged.

[Technicalhuman]

Half reactions are bookkeeping devices to keep track of oxidation and reduction components of a system.  The oxidation counterpart, however, does not happen independently of the reduction component.  In a real electrochemical cell, there is usually a salt bridge between the anode side and the cathode side that balances the production of cations (or anions) in the respective cell solutions.  As for whether electrons built up in an electrode - no, not generally.  In a complete cell, the electrons that are retained in one electrode due to electrode oxidation flow to the other electrode via a wire, which is what produces current.  At the other electrode, these electrons perform more chemistry (reduction of cations in solution), which completes the circuit.  When the cell is depleted, equilibrium is reached and the cell no longer has enough potential energy to generate current flow (here's where you Gibb energy equations from the other thread come into play).  At that point the cell is dead.  However it can be recharged by applying an electric potential in the other direction, which causes electrons to flow in reverse.  This is only spontaneous with the application of the external potential.   Once a new non-equilibrium situation is produced, the external potential is removed and the flow of current in the battery is spontaneous again.  In principle this process can be repeated indefinitely, but certain irreversible reactions at the electrode surfaces reduce the efficiency of the battery after each recharge, which means that batteries do have a limit to the number of times they can be recharged.

Hi Corribus thanks for the great post.

So if I were to just put a piece of metal in a solution with its ions, there would be an equilibirum between the metal and the solution Mⁿ⁺ +ne^-  M so given this equilibrium, definitely some of the M would turn into ions. So the metal would gain a negative charge while the solution would gain a positive charge.

When I read on my book, they mentioned the reason why a salt bridge is needed is that the solution would slowly get charged causing an opposite potential difference to be set up preventing the redox reaction from occurring.

What does this mean? Why would having more of the charged ions in the solution result in an opposite potential difference?

Thanks so much for the help given.

[Corribus]

As you build up positive (or negative) charge in a battery cell, it would become harder and harder to add additional positive (or negative) ions into the solution.  This would severely limit the ability of a battery to produce current.  The salt bridge provides a way to neutralize excess charge in the cell, so that more cations (or anions) can be produced by the electrochemical reaction.

Remember, a battery is really just a single oxidation-reduction reaction.  What makes it special is that the oxidation and reduction components are spatially separated by rather than squished together in a single reaction pot.  The funneling of the transferred electrons from one part to the other over a wire is what produces current. 

[Technicalhuman]

As you build up positive (or negative) charge in a battery cell, it would become harder and harder to add additional positive (or negative) ions into the solution.  This would severely limit the ability of a battery to produce current.  The salt bridge provides a way to neutralize excess charge in the cell, so that more cations (or anions) can be produced by the electrochemical reaction.

Remember, a battery is really just a single oxidation-reduction reaction.  What makes it special is that the oxidation and reduction components are spatially separated by rather than squished together in a single reaction pot.  The funneling of the transferred electrons from one part to the other over a wire is what produces current.

Hi Corribus thanks for another awesome post.

Actually, why would having more ions in the solution make it harder to add more of the ions in it?

Thanks so much for the help.

[Corribus]

The simplest answer is that like charges repel each other.  If you didn't have a salt bridge, unbalanced minus charges would build up on one of the battery and unbalanced plus charges would build up on the other.  This would effectively create a voltage difference between the anode and cathode and would tend to resist the flow of additional negative charge (electrons) toward the end with built up minus charges.  In essence, you'd be creating a large driving force for electrons to flow in the reverse direction.