Physical Chemistry - Electrochemistry - 3

Before reading this post, please read the first two posts here :
Physical Chemistry - Electrochemistry - 1

Physical Chemistry - Electrochemistry - 2

ELECTROCHEMICAL SERIES
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It is series where elements are arranged in increasing order of Standard Reduction potential (or decreasing order of Standard Oxidation potential) as compared to that of S.H.E. It is experimentally determined.

Some General Characteristics of this series are:
$\bullet$ Elements near the top of the series are highly electropositive. They lose electrons readily to form cations.
$\bullet$ Elements near the bottom of the series are highly electronegative. They gain electrons readily to form anions.

$\bullet$ Note that, the Standard Reduction Potential of Hydrogen is zero.

Keeping these points in mind, we can draw some general conclusions, such as,
$\bullet$ If metal A is situated above metal B in the electrochemical series, then A can displace B from its salt. The reason for this is that, since $E_{red}^o (A) < E_{red}^o (B)$, implying that $E_{ox}^o (A) > E_{ox}^o (B)$, thus suggesting that A has more tendency to form cations than B.

$\bullet$ Metals situated above Hydrogen, are easily rusted. This is because these metals will have a higher tendency to oxidize (tendency to form cations). And since the definition of rusting is oxidation of metal, these metals will rust easily.

Before we can learn how to use this series to calculate the EMF of cells, we need to understand how to represent a cell as an equation. We cannot write long sentences to represent a cell everytime. It is time-consuming, and also unnecessary.

NOTATION FOR REPRESENTING A GALVANIC CELL
First of all, a Galvanic cell works through a redox reaction. One metal is oxidized, while another metal's cation is reduced. I think it will be best to approach this idea through an example.

For eg, consider the cell which uses the following reaction :
$Zn(s) + Cu^{+2}(aq) \to Zn^{+2}(aq) + Cu(s)$

$\bullet$ The interfaces, across which a P.D exists, are shown by a vertical line ( | ). In this case, the two half cells are represented as,
$Zn|Zn^{+2}$ and $Cu^{+2}|Cu$
These indicate that a P.D. exists at the $Zn$ and $Zn^{+2}$ ions interface, and similarly at the $Cu$ and $Cu^{+2}$ interface.

$\bullet$ Sometimes, a comma is observed in the formation of the half cells. For example,
$Zn,Zn(Cl)_2|Cl^{-}$
These indicate that $Zn$ and $Zn(Cl)_2$ together constitute the electrode.

$\bullet$ The contact between two solutions by means of a Salt Bridge (will be explained later) is indicated by double vertical line (||) between them. For example,
$Cu^{+2}||Zn^{+2}$

$\bullet$ The anode half-cell (oxidation half-cell) is always written on the left hand side, and the cathode half-cell (reduction half-cell) on the right, with the respective metal electrodes on the outside extremes. For example,
$Zn|Zn^{+2}||Cu^{+2}|Cu$

$\bullet$ The concentration of solution, pressure of gases and physical state of solids and liquids involved are indicated in the cell formation. For example,

$Pt,H_2(0.9atm)|H^{+}(0.1M)||Cu^{+2}(0.1M)|Cu$

In the next post, we shall learn how to calculate the standard EMF of any cell, with the help of our new tool : Electrochemical Series.