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Q-5
Spontaneous Cell Reactions
Any two half-cells can be combined to form an electrochemical cell. Electrons given off in one half-cell
must equal those gained by the other. For the cell reaction to occur spontaneously, E
cell
must be positive.
When half-cell reactions are written as reduction processes, and are ordered in a displacement series, so
that the better reducing agent is at the top (electrode potential is more negative), an equation for a
spontaneous reaction will always result by taking a half-cell reaction and adding to it any oxidation half-cell
reaction above it. An oxidation reaction is simply the reverse of a reduction reaction. If the direction of
the reaction is reversed then the sign on the half-cell potential must also be reversed. In a voltaic cell the
half-cell with the more positive reduction potential will be the reduction (cathode) half-cell and the other
half-cell will be the oxidation (anode) half-cell. The cell potential will be the difference of the two standard
reduction potentials:
E
cell
= E
red, cathode
- E
red, anode
(7)
II. Relation of the Halogen Substitution Series to the Metal Substitution Series
Halogens:
nonmetallic elements of group 17 of the Periodic Table. They exist as nonpolar
diatomic molecules, X2. (For example chlorine, Cl2; bromine, Br2; iodine, I2.)
Halides:
negatively charged monatomic ions of group 17 elements of the periodic table.
(For example: chloride, Cl
-
; bromide, Br
-
; iodide, I
-
).
There is a rule of solubility of polar/charged and nonpolar substances in solvents that states “Like dissolves
like”. This rule can be understood as follows:
a)
Polar/charged species are very soluble in polar solvents. (The most common polar solvent is
water.) Therefore halides (Cl
-
, Br
-
, I
-
) are very soluble in water and practically insoluble in
nonpolar solvents. (The nonpolar solvent used in this experiment is dichloromethane, CH2Cl2.)
b)
Nonpolar species are slightly soluble or insoluble in water and are soluble in nonpolar solvents.
For this reason halogens (Cl2, Br2, I2) are very soluble in dichloromethane and only slightly
soluble in water.
