Electrochemistry Summary (Grade 12 IEB)
INTRODUCTORY TERMINOLOGY
ELECTROCHEMISTRY is the branch of chemistry that deals with the transformation
between electrical and chemical energy.
Galvanic cell - a device that converts chemical energy to electrical energy as a result of
self-sust...
Original notes by C. M. Fairman
Modified by T. Doyle
,1. INTRODUCTORY TERMINOLOGY
ELECTROCHEMISTRY is the branch of chemistry that deals with the transformation
between electrical and chemical energy.
Galvanic cell - a device that converts chemical energy to electrical energy as a result of
self-sustaining spontaneous electrode reactions
An electrolytic cell is a device that converts electrical energy into chemical potential
energy by means of a non-spontaneous redox reaction.
Electrolysis - Chemical change, especially decomposition, produced in an electrolyte by
an electric current.
Electrolyte - a solution that conducts electricity due to the presence of free ions
An electrode is a conductor by which an electrical current passes between an electrolyte
and an external circuit.
The anode is the electrode where oxidation takes place.
The cathode is the electrode where reduction takes place.
2. REVISION OF REDOX CONCEPTS
A chemical species is an atom, a molecule or an ion.
The word redox is an abbreviation of the words reduction and oxidation.
Oxidation is defined as the loss of electrons. (OIL)
Reduction is defined as the gain of electrons. (RIG)
Redox reaction - a reaction involving the transfer of electrons
Oxidising agent is defined as a substance that accepts electrons.
Reducing agent is defined as a substance that donates electrons.
In terms of oxidation number, a redox reaction is one in which there is a simultaneous
increase and decrease of oxidation number.
Example:
A piece of zinc metal is placed in a solution of copper (II)
sulphate. The zinc is observed to gradually corrode away
and is replaced by a reddish deposit of copper. The blue
colour, imparted to the solution by the Cu2+ ions gradually
disappears.
1
, All redox reactions may be thought of as comprising two half-reactions. In one of the half-
reactions electrons are lost and in the other they are gained. For the example above, the two
half-reactions, with oxidation numbers shown, are as follows:
0 +2
Zn(s) → Zn2+(aq) + 2e−
(oxidation half-reaction: electrons lost & oxidation no. of Zn increases)
+2 0
Cu2+(aq) + 2e− → Cu(s)
(reduction half-reaction: electrons gained & oxidation no. of Cu2+ decreases)
The two half-reactions of a redox reaction are mutually dependent, meaning that the one
cannot occur without the other.
For every Zn(s) atom which enters the solution, two electrons are left behind on the zinc
surface. These two electrons are taken up by a Cu2+(aq) ion as it turns into a Cu(s) atom. The
reaction is spontaneous and occurs because zinc is a more reactive metal than copper.
The overall redox reaction is obtained by adding the two left-hand sides and the two right-
hand sides of the half-reactions as follows:
Zn(s) + Cu2+(aq) + 2e Zn2+(aq) + 2e + Cu(s)
Simplifying: Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)
Spectator ions are ions which are present in an electrolyte, but which do not undergo
change in the redox reaction (in this example the spectator ions are: SO42− ions). It is a
common practice to omit the spectator ions from the equation for the redox reaction.
However, if they were to be included, the equation above would be written as follows:
Zn(s) + CuSO4 (aq) ZnSO4 (aq) + Cu(s)
A reducing agent is any chemical species which enables reduction to take place. It is an
electron donor. Its oxidation number increases during a redox reaction.
In the example above, the Zn(s) atom is the reducing agent. (It enables the Cu2+(aq) ion to be
reduced while its oxidation number increases from 0 to +2.)
An oxidising agent is any chemical species which enables oxidation to take place. It is an
electron acceptor. Its oxidation number decreases during a redox reaction.
In the example above, the Cu2+(aq) ion is the oxidising agent. (It enables the Zn(s) atom to be
oxidised while its oxidation number decreases from +2 to 0.)
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