Why do the electrons in a galvanic cell not flow through the half cells themselves, but through an external conductor from one electrode to the other? Isn’t the salt solution in the half cells also electrically conductive? Usually, a galvanic cell is represented in textbooks by two containers connected by an ion bridge (salt bridge, porous wall, or the like). The two containers, the half cells, contain an aqueous solution of a certain metal salt and have a suitable electrode. The half-reactions do their job, a potential difference is created between the two electrodes, the principle of a battery. I can follow the electrochemistry behind it fairly well, except for one thing: The electrons run from electrode to electrode, for example via an external conductor. The ions in the half cells flow through the solution via the salt bridge. Why don’t the electrons also flow via the liquid and salt bridge, which are also electrically conductive? Why then do the electrons choose the path of the external conductor? What creates that internal resistance that prevents the electrons from flowing through the solution?
Answer
Hi Roel,
To fix the thought I refer to Fig. 1, which depicts a typical galvanic cell. The electrodes, in this case a strip of copper and silver metal, serve as a landing stage for the electrons.
At the copper electrode the reaction takes place:
Cu(v) –> Cu2+ + 2 e–
Because both electrons are attracted to the silver electrode, the copper electrode becomes positively charged. The electrons flow through the external wiring to the silver electrode. There the reaction happens:
2 Ag+ + 2 e– –> 2 Ag(v)
(The number of electrons given off and taken up must of course be the same!). Due to the presence of those extra electrons, the silver electrode becomes negatively charged.
The electrons cannot travel through the solution, because a “naked” electron is a very energetic particle, and would immediately undergo interactions with the molecules present. This is comparable to the damage that some types of radioactive radiation can cause, such as ionistasis and unwanted chemical reactions.
In a metal, on the other hand, the valence electrons are only weakly bonded, and can therefore move under the influence of a limited force, being the potential difference between the two electrodes.
In the solution, the ions present are electrostatically attracted to the electrodes: the copper electrode is positively charged, and will therefore attract ALL negative ions (anions). In our example that is sulfate (SO42-) and the positive copper ions (Cu2+) push off. The sulfate ions are attracted to the electrode, but will not react there. The potential difference is not big enough for that. The silver electrode is negatively charged and will therefore attract all positive ions (cations) and repel the negative ones. In our example, the Ag+ that is attracted, and NO3– that is discarded.
In the external wiring the charge carriers are the electrons, in the solution the charge carriers are all ions present. This is clarified in Fig. 2.
Answered by
Dr. Etienne Jooken
Chemistry
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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