Week 3, Lesson 1
Chapter 5 – Analysing Oxidants &
Reductants
Redox Reactions
• Redox reactions involve the transfer of
electrons.
• Two half equations occur during a redox
reaction.
• One reactant loses electrons – this is
oxidation.
• One reactant gains electrons – this is
reduction.
• These two processes occur simultaneously.
Redox Reactions cont…
• Substances that cause oxidation are called
oxidants, or oxidising agents.
• While, substances that cause reduction are
called reductants, or reducing agents.
Oxidation Numbers
• To determine whether a reaction is a redox
reaction, oxidation numbers are assigned to
elements involved in the reaction.
• These numbers are determined by applying a
set of rules that treat molecular substances as
if they were composed of ions.
• Oxidation numbers have no physical meaning.
• Oxidation numbers are also known as
oxidation states.
Oxidation Number Rules
• Free elements have an oxidation number of 0.
• In ionic compounds composed of simple ions, the oxidation number
is the same as the charge on that ion.
• Some elements have oxidation numbers that are regarded as fixed,
except in a few exceptional circumstances:
– Oxygen usually takes -2 in compounds, except in peroxides it is -1.
– Hydrogen takes +1 in compounds, except in metal hydrides if is -1.
• The sum of the oxidation number in a neutral compound is zero.
• In a polyatomic ion the sum of the oxidation numbers is equal to
the charge on that ion.
• The most electronegative element in a compound has the negative
oxidation number.
Variable Oxidation Number
• In all their compounds, the group I metals only
have an oxidation number of +1 and the group
II metals only have an oxidation number of +2.
• The oxidation numbers of transition metals
vary depending on the compound.
• The oxidation numbers of non-metallic
elements may also vary from compound to
compound.
Changing Oxidation Numbers
• Once oxidation numbers are assigned to the
elements involved in a chemical reaction, we
can look for a change in these numbers during
a reaction.
• An increased oxidation number means the
element has been oxidised.
• A decreased oxidation number means the
element has been reduced.
Writing Half Equations
• Half equations that involve atoms or simple ions
can be written with little difficulty, as you just
need to add electrons to the appropriate side.
• Half equations involving polyatomic ions is a little
more difficult.
• The process is as follows:
1. Balance all elements except hydrogen and oxygen
in the half equation.
2. Balance the oxygen atoms by adding water.
3. Balance the hydrogen atoms by adding H+ ions.
4. Balance the charge by adding electrons.
Volumetric Analysis
• This process can also be used to find the
concentration of solutions of substances in a
redox reaction.
• Instead of reacting an acid with a base, a
redox titration involves reaction of an oxidant
with a reductant.
• To find the concentration, the same process is
followed as in an acid-base reaction.