Why does an electrochemical cell work?

Question

My textbook says that an electrochemical cell works because of difference in reduction potential of two metals which cause one to lose electrons and other one to accept them. If a salt bridge is not used then the solutions of the two metals become charged and the reaction proceeds till the difference in reduction potential is cancelled by the difference in potential due to charges. All this theory seems to work well on paper, but it is never really explained why this reaction occurs.

What I mean is suppose you have a copper and a silver half cell. They are connected. The copper part does not know whether it is connected by a silver half cell or sodium have cell and I see no apparent of this information being communicated between the cells, so how does it know whether to get dissolved in the solution or precipitate out of the solution?

The book says that due to the difference in reduction potential, a potential gradient is set up which causes the charge to flow. I agree with this completely yet I am not able to grasp why this physically happens. What force causes to electrons to move from one electrode to the other? Especially when there is no salt bridge, the electrons move from positive electrode to negative electrode, which is completely opposite to anything I know.

I think that there must be some difference in local phenomena in the solution which spontaneously causes electrons to flow without any information being shared between the two electrodes. eg: At first I thought maybe copper having lesser reduction potential than silver would dissolve and precipitate out faster (basically a fast(?) equilibrium) which is completely independent of the other electrode and this difference speed would somehow cause the electrons to flow. I rejected this idea after a while but I still think difference in some sort of local phenomena in a half cell which is completely independent of the other half cell would drive the reaction forward.

Any thoughts?

Answer 1

Maybe you would understand better with an image, or if you admit that all metals "are wanting" to loose electrons. And in order to do that, they must find another atom which is weak enough or "humble" enough to accept to work "against its will". In other words, some atoms are more "willing" than others. It is like a fight, with a "strong" and a "weak" atom. In your copper-silver cell, copper is more assertive : it imposes its electrons to the "weak" silver atom. Silver cannot emit its own electrons. It is even obliged to accept electrons from copper and have these electrons react with its proper ions. producing more silver atoms. Some atoms are stronger than copper. Zinc for example, opposed to copper, imposed its electrons to copper. This order of "strength" is experimental, and cannot be derived by calculations.

Now to speak more scientifically, you may replace the present notion of strength by the reduction potential. The more the reduction potential is negative, the more the metal has a tendency to loose its electrons.

Anyway, a salt bridge is absolutely necessary for the cell to work. If there is no salt bridge, it is not a cell, and it produces no chemical reaction, and no electric currant.

• Complement, required by Manit. If you want a local phenomena to explain the cell, you may consider the following development. You may admit that copper "wants" to create more $\ce{Cu^{2+}}$ ions in solution. These ions are positively charged. So the copper compartment must attract negative ions to compensate the new positive charges appearing around the copper plate. As a consequence, the new $\ce{Cu^{2+}}$ ions attract negative ions from where they are available, namely in the other compartment. And as $\ce{Ag}$ is not able to create $\ce{Ag+}$ ions with the same force, Silver plate must admit that it looses its negative nitrate ions to send them to the copper compartment. And as a consequence, it must admit that the positive ions $\ce{Ag+}$ should also "disappear" from the compartment. And the only way to make this charge disappear from the compartment is to accept electrons, so that the $\ce{Ag+}$ ion is transformed into metallic silver. OK ?

Answer 2

An important thing is not to limit yourself to the idea of electrochemical cell with metals, being dissolved or deposited.

More general image is the general redox half-reactions:

$\ce{<oxidized form> + n e- <=> <reduced form>}$

The oxidized form may, or may not be a metallic ion like $\ce{Cu^2+}$ or $\ce{Ag+}$.
The reduced form may, or may not be a metal of the electrode, like $\ce{Cu(s)}$ or $\ce{Ag(s)}$.

E.g. one of industrial energy storage types uses cells with this aquaeous vanadium system with inert electrodes:

$$\begin{align} \ce{V^2+(aq) &<=>[discharging(anode)][charging(cathode)] V^3+(aq) + e-}\\ \\ \ce{VO2+(aq) + e- + 2 H+(aq) &<=>[discharging(cathode)][charging(anode)] VO^2+(aq) + H2O} \end{align}$$

There are ongoing reactions in both directions at electrodes, even if not connected to any circuit. If an electrode happens to have the equilibrium potential, both reactions have the same rate with the zero net electron production.

If an electrode has a lower than the equilibrium potential, the reduction reaction consuming electrons is faster and the electrode potential raises towards the equilibrium one. Unless the potential is externally forced, e.g. at alectrolysis. Similarly, if an electrode has a higher than the equilibrium potential, the oxidation reaction producing electrons is faster and the electrode potential falls down towards the equilibrium one.

Electronic chips are known to be using electronic charge pumps to internally generate needed voltage levels. Electrode systems can be analogically considered as chemically powered charge pumps.

If 2 electrodes are connected by a galvanic circuit, the current disbalances their potentials and "the chemical charge pumps" start feeding and collecting the electrons to/from the electrodes. If the chemical processes are fast, the cell is a hard voltage source with low internal resistance ( like car acid lead batteries ). If they are slow, the cell is a soft source with high internal resistance.