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Electrochemistry I: Galvanic Cells 65
In galvanic cells, an ammeter (which measures the current) or
voltmeter (which measures the potential) is present to measure the
flow of current, i.e. to measure Eocell. This will be discussed later in
Chapter 7.

REDOX REACTIONS-REVISION
AG, the Gibbs free energy change of a reaction and K, the equilibrium
constant, can be determined from galvanic cell calculations. Before
describing the various types of electrodes used in galvanic cells, know-
ledge of the fundamentals underlying oxidation-reduction reactions
(redox reactions) is required. The next section summarises the funda-
mental concept of a redox reaction. A knowledge of redox reactions is
essential in understanding the processes occurring at electrodes in
both galvanic and electrolytic cells respectively.

Redox Reactions
The oxidation number of an element is the apparent charge an atom
of that element has in an anion, cation, compound or complex. An
anion is a negatively charged species such as C1-, N03-, SO:-, etc.
A cation is a positively charged species, such as Na+, NH4+,
[Mn(0H2)6l2+, etc. (An easy way to remember this is: Anion-
negatively charged). Table 6.1 is a summary of the rules used to
determine the oxidation number of an element.
Five examples illustrating the calculation of oxidation numbers
follow:

Table 6.1 Rules for assigning oxidation numbers
The oxidation number of any atom as a free element is equal to zero, e.g.
Na, C12, Fe, S8, He, etc.
The elements of Group 1 (alkali metals, s'po) have an oxidation number
of I (sl --+ so) in compounds, i.e. Na, K, etc. The elements of Group 2
(alkaline earth metals, ?po) have an oxidation number of I1 (? -+ so) in
compounds, i.e. Be, Mg, Ca, erc. Group 1 and Group 2 elements all tend
to lose their one or two valence electrons in order to attain the stable inert
gas core configuration of [He], me], etc.
The elements of Group 17 (the halogens, s2ps), F, C1, Br, I, etc., normally
have an oxidation number of -I (s2p5 + ?p6) in compounds. All these
elements need just one more electron to attain a stable inert gas core.
Oxygen, with an electron configuration of [He]2s22p4, normally has an
oxidation number of -11, by gaining two electrons to form me]. In
peroxides (compounds with an 0-0 bond), such as H202, oxygen has an
oxidation number of - I.