OxidatiOn & ReductiOn
Introduction to oxidation and reduction
Some common oxidising agents and reducing agents (EXT)
Standard electrode potentials (AHL)
TOK Are oxidation numbers real?
I remember contemplating on the nature of reality
back in Chapters 2 and 4, with regard to electrons and
hybridization respectively. The implication always
seems to be that reality is in some way desirable.
Maybe my bank account might be a useful analogy
to oxidation numbers? At the end of every month
there is a number assigned to it, but as the month
progresses this number gets smaller and the numbers
assigned to the local supermarket, the petrol station,
the government etc. all go up. Hardly any of it gets
turned into bank notes for me to carry around and
even then is a bank note any more real than the
number printed on my bank statement. Isn’t the fact
that this piece of paper is worth $20 just another,
very convenient, shared fiction? Along as we are
all happy to share the belief that the figures on my
bank statement and the number on my bank notes
mean something then real or not these numbers, like
oxidation numbers, are very useful and certainly a
lot easier than pushing around wheelbarrows full of
gold, or wearing a necklace of sea-shells with holes
drilled in them. The whole thing is perhaps closely
related to the philosophy of Charles Sanders Peirce,
who developed the concept of “pragmatic truth” that in which it was convenient for society to believe; in other words truth is more an attribute of a society
than an attribute of the physical world. Are oxidation
numbers useful? Certainly they are capable of giving
us a definitive answer as to whether in a chemical
change, a particular atom is oxidised, reduced, or
neither, but is the change any more than “electron
accounting” that helps us balance equations? F going
to F– is just a change of –1 whereas N going to N3– is a change of –3, but the former is far more energetically
9.1 intROductiOn tO OxidatiOn & ReductiOn
9.1.1 Define oxidation and reduction in terms
of electron loss and gain.
9.1.2 Deduce the oxidation number of an
element in a compound.
9.1.3 State the names of compounds using
9.1.4 Deduce whether an element undergoes
oxidation or reduction in reactions using
© IBO 2007
Oxidation and reduction are most commonly defined in
terms of the loss and gain of electrons. Oxidation is the
loss of electrons, so if an iron(II) ion (Fe2+) is converted to an iron(III) ion (Fe3+), then the iron(II) ion has lost an electron and so has been oxidised.
Fe (aq) + e
Conversely reduction is the gain of electrons, so if
hydrogen ions (H+) are converted to hydrogen gas (H2)
the hydrogen ion has gained electrons and is therefore
2 H+ (aq) + 2 e–
Note that in these equations the charge as well as the
numbers of atoms must balance.
A useful mnemonic (memory aid) for this is:
LEO (the lion) goes GER
Loss of Electrons is Oxidation;
Gain of Electrons is Reduction.
(Oxidation Is Loss of electrons, Reduction Is Gain of electrons). Consider the reaction between zinc and iodine:
Zn (s) + I2 (aq)
Zn (aq) + 2 I (aq)
In this the zinc atom loses two electrons to form the zinc
ion and so is oxidised. The iodine molecule gains two
electrons to form iodide ions and so is reduced. This is
most clearly shown by splitting the overall equation into
two ‘half equations’, i.e.
Zn2+ (aq) + 2 e–
I2 (aq) + 2 e
2 I– (aq)
The full equation can be produced by combining the
appropriate half equations in such a way that the electrons
There are occasions...