I have a question about electrical potential in galvanic cells ,suppose that we have only half a cell wich means that we have only one Ox/Red couple (one metal and the other as ionic solution ...)clearly nothing will happen ,suppose that we put a wire between the metal and the ground .The ground has no voltage while the Red/OX couple has a potential does It mean that the oxidation will happen ? and the electrons will go through the ground ?If the oxidation happens with any kind of OX/RED couple ,and if we take a couple wich the deltaG of the oxidation reaction is very high (so It's very unlikely to happen all by Itself ) won't we be creating a free source of energy ? Thank you !


closed as unclear what you're asking by bobthechemist, ashu, Philipp, jonsca Sep 27 '13 at 22:23

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  • $\begingroup$ Can you specify what you mean by a "free source of energy"? It's vague as it stands. $\endgroup$ – jonsca Sep 27 '13 at 22:24

The answer to your question is difficult to explain and understand without a correct understanding of charge and the redox reaction. Another way of asking this question is: Why does current not flow if I connect two batteries using only one wire from the positive terminal of one battery to the negative terminal of the other. The common answer you will get is that you need a complete circuit, but that avoids the deeper question of why you need a circuit. To understand it a little better, you could think of a cell as two parts, one tries to pull electrons from its metal terminal and another tries to push electrons into its terminal. However, neither can do so unless the oppositely charged ions that are left behind can be exchanged between the two metals inside the battery. This means that while the cathode does have a negative charge, it will not let the electrons go unless there is a negative ion to pair up with the positive ion that gets left behind inside the cell. This negative ion comes from positive anode ions leaving the solution and attaching to the anode, but more negative ions only get created if the anode receives the electrons needed to attract the positive anode ions.

In your question, the one Ox/Red half might be the cathode. In that case it will not have a potential at all since there is nothing but other electrons to balance the charge of the positive ions created at the cathode as the electrons leave it for the wire. The ground connection acts only as a conductor of the electrons. There is no reason for an electron to leave behind a positive ion just to push other electrons through a wire to ground and recombine with a positive ion. That would require energy since the wire has some resistance. The reason the battery works is because a chemical reaction takes place that releases energy. In other words, the positive and negative ions from each half cell would rather be combined together to reach a lower energy state. This can only occur if the positive metal ions that detach from the cathode as it releases electrons can combine with negative ions in the electrolyte. However, the negative ions in the electrolyte must be replenished and that can only occur if other positive metal ions in the electrolyte are pulled out of solution by the anode, which can occur only if the anode receives electrons.

As you may know, the cells' anode and cathode are typically made of two different materials such as zinc and copper, and the electrolyte is a solution made of a salt containing positive ions of the anode metal such as copper sulfate. As the cell discharges, negative zinc ions are leaving the cathode and dissolving into the solution, while positive copper ions are leaving the solution and attaching to anode. One terminal dissolves while the other grows. As negatively charged electrons move from the zinc cathode to the copper anode via the circuit outside the battery, positively charged zinc ions now in the solution are soaking up or balancing the negative charge created in the electrolyte as the positively charged copper ions leave the solution and attach to the anode that received the extra electrons via the wire from the cathode.

The reason all of this takes place is because the process results in a lower overall energy state after it occurs. In other words, the zinc atoms would rather be combined with $\ce{SO4^2-}$ ions than with each other, and the copper ions would rather be combined with each other at the anode than with the $\ce{SO4^2-}$ in the solution. The only way this can happen is if electrons are moved from the cathode to the anode and the easiest way is via the wire and circuit outside the battery. Without the electron flow, the chemical reaction between the two half cells is inhibited. Conversely, without the chemical energy potential of the reaction between the two half cells, there is no reason for electrons to flow.


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