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From the Wikipedia entry on Electrochemical cell (

An electrochemical cell consists of two half-cells. Each half-cell consists of an electrode, and an electrolyte. The two half-cells may use the same electrolyte, or they may use different electrolytes. The chemical reactions in the cell may involve the electrolyte, the electrodes or an external substance (as in fuel cells which may use hydrogen gas as a reactant). In a full electrochemical cell, species from one half-cell lose electrons (oxidation) to their electrode while species from the other half-cell gain electrons (reduction) from their electrode.

My familiarity with the word species begins and ends with the classification of living things so I am utterly baffled as to what it means in the present context.

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The term "species" is used quite generally in physics mean something like "type of substance", anything from the subclasses of elementary particles, to (like in your case), chemicals. – user12345 Apr 8 '13 at 18:45
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Species in this context means "chemical species", i.e. a type of molecule. $\mathrm{H_2O}$ is one example of a chemical species, $\mathrm{NaCl}$ is another.

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Thanks. 1. So could the author basically have written "molecules" or "atoms" instead of "species"? Any idea why they preferred "species"? Is it to emphasize the fact that the electrolyte of one half-cell may consist of a different chemical composition than the electrolyte of the other half-cell? 2. The "species" that are losing or gaining their electrons are found in the electrolyte, right? – oyvey Apr 8 '13 at 16:06
In this case they could just have written molecules, yes. But in general it doesn't mean the same thing - the species is the type of molecule rather than the molecule itself, in the same way that the biological species "elephants" is a different thing from an actual elephant. I found the word confusing the first time I came across it as well, but it's a pretty standard term in chemistry. – Nathaniel Apr 9 '13 at 0:24

In electrochemistry a species is just something distinct that we have to put into our equations to work out what's going on. For example water is a species, but so is H$^+$ and OH$^-$. Likewise NaCl is a species, but in practise it ionises almost totally so the only species present would be Na$^+$ and Cl$^-$.

Generally a species is anything that appears on it's own in the formula for calculating concentrations. For example for water we write:

$$ K_w = \frac{[H^+][OH^-]}{[H_2O]} $$

where $K_w$ is the equilibrium constant for dissocation of water, and all three items on the right hand side are treated as seperate species (though we normally write the activity of water, [H$_2$O], as the constant value 1).

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