Concentration cell

In battery technology, a concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same composition differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst equation.[1] A concentration cell produces a small voltage as it attempts to reach chemical equilibrium, which occurs when the concentration of reactant in both half-cells are equal. Because an order of magnitude concentration difference produces less than 60 millivolts at room temperature, concentration cells are not typically used for energy storage.

A concentration cell generates electricity from the reduction in the thermodynamic free energy of the electrochemical system as the difference in the chemical concentrations in the two half-cells is reduced. The same reaction occurs in the half-cells but in opposite directions, increasing the lower and decreasing the higher concentration. The energy is generated from thermal energy that the cell absorbs as heat, as the electricity flows. This generation of electricity from ambient thermal energy, without a temperature gradient, is possible because the convergence of the chemical concentrations in the two half-cells increases entropy, and this increase more than compensates for the entropy decrease when heat is converted into electrical energy.

Concentration cell methods of chemical analysis compare a solution of known concentration with an unknown, determining the concentration of the unknown via the Nernst Equation or comparison tables against a group of standards.

Concentration cell corrosion occurs when two or more areas of a metal surface are in contact with different concentrations of the same solution. There are two general types of concentration cells.

Concentration cells can be electrode concentration cells or electrolyte concentration cells.

Electrolyte Concentration cell - In this type of cell electrodes in both the half cells are made up of same substances and electrolyte is the solution of same substances but with different concentration.

Electrode Concentration cell - In this type of cell two electrodes of same substance but with different concentration are dipped in same solution.

Metal ion concentration cells

In the presence of water, a high concentration of metal ions will exist under faying surfaces and a low concentration of metal ions will exist adjacent to the crevice created by the faying surfaces. An electrical potential will exist between the two points. The area of the metal in contact with the high concentration of metal ions will be cathodic and will be protected, and the area of metal in contact with the low metal ion concentration will be anodic and corroded.

Oxygen concentration cells

Water in contact with the metal surface will normally contain dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse uniformly into the solution, thereby creating a difference in oxygen concentration between two points. Corrosion will occur at the area of low-oxygen concentration, which are anodic.

Active-passive cells

If a metal is protected against corrosion by a tightly adhering passive film (usually an oxide) and salt deposits on the surface in the presence of water, the active metal beneath the film will be exposed to corrosive attack in areas where the passive film is broken. An electrical potential will develop between the large area of the passive film (cathode) and the small area of the exposed active metal (anode). Rapid pitting of the active metal will result.

gollark: There are lots of *imaginable* and *claimed* gods, so I'm saying "gods".
gollark: So basically, the "god must exist because the universe is complex" thing ignores the fact that it... isn't really... and that gods would be pretty complex too, and does not answer any questions usefully because it just pushes off the question of why things exist to why *god* exists.
gollark: To randomly interject very late, I don't agree with your reasoning here. As far as physicists can tell, while pretty complex and hard for humans to understand, relative to some other things the universe runs on simple rules - you can probably describe the way it works in maybe a book's worth of material assuming quite a lot of mathematical background. Which is less than you might need for, say, a particularly complex modern computer system. You know what else is quite complex? Gods. They are generally portrayed as acting fairly similarly to humans (humans like modelling other things as basically-humans and writing human-centric stories), and even apart from that are clearly meant to be intelligent agents of some kind. Both of those are complicated - the human genome is something like 6GB, a good deal of which probably codes for brain things. As for other intelligent things, despite having tons of data once trained, modern machine learning things are admittedly not very complex to *describe*, but nobody knows what an architecture for general intelligence would look like.
gollark: https://media.discordapp.net/attachments/348702212110680064/896356765267025940/FB_IMG_1633757163544.jpg
gollark: https://isotropic.org/papers/chicken.pdf

See also

References
  1. Almost any textbook on physical chemistry, e.g. by I. N. Levine or P. W. Atkins, and also many general chemistry texts.
  •  This article incorporates public domain material from the National Aeronautics and Space Administration document: "Concentration Cell Corrosion".
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