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Summary Redox reactions
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- Introduction to chemistry
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- University Of South Africa (Unisa)
Summary of redox reactions
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redox reactions 235Unit 7
redox reactions
Where there is oxidation, there is always reduction –
Chemistry is essentially a study of redox systems.
After studying this unit you will be
able to Chemistry deals with varieties of matter and change
of one kind of matter into the other. Transformation of
• identify redox reactions as a class
of reactions in which oxidation matter from one kind into another occurs through the
and reduction reactions occur various types of reactions. One important category of such
simultaneously; reactions is Redox Reactions. A number of phenomena,
• define the terms oxidation, both physical as well as biological, are concerned with
reduction, oxidant (oxidising redox reactions. These reactions find extensive use in
agent) and reductant (reducing pharmaceutical, biological, industrial, metallurgical and
agent); agricultural areas. The importance of these reactions is
• explain mechanism of redox apparent from the fact that burning of different types of
reactions by electron transfer fuels for obtaining energy for domestic, transport and
process;
other commercial purposes, electrochemical processes
• use the concept of oxidation for extraction of highly reactive metals and non-metals,
number to identify oxidant and manufacturing of chemical compounds like caustic
reductant in a reaction;
soda, operation of dry and wet batteries and corrosion of
• classify redox reaction into metals fall within the purview of redox processes. Of late,
combination (synthesis),
decomposition, displacement
environmental issues like Hydrogen Economy (use of
and disproportionation liquid hydrogen as fuel) and development of ‘Ozone Hole’
reactions; have started figuring under redox phenomenon.
• suggest a comparative order
7.1 CLASSICAL IDEA OF REDOX REACTIONS –
among various reductants and
oxidants; OXIDATION AND REDUCTION REACTIONS
• balance chemical equations Originally, the term oxidation was used to describe the
using (i) oxidation number addition of oxygen to an element or a compound. Because
(ii) half reaction method; of the presence of dioxygen in the atmosphere (~20%),
• lear n the concept of redox many elements combine with it and this is the principal
reactions in terms of electrode reason why they commonly occur on the earth in the
processes. form of their oxides. The following reactions represent
oxidation processes according to the limited definition of
oxidation:
2 Mg (s) + O2 (g) → 2 MgO (s) (7.1)
S (s) + O2 (g) → SO2 (g) (7.2)
Rationalised 2023-24
Unit 7.indd 235 10/10/2022 1
, 236 chemistry
In reactions (7.1) and (7.2), the elements broadened these days to include removal
magnesium and sulphur are oxidised on of oxygen/electronegative element from
account of addition of oxygen to them. a substance or addition of hydrogen/
Similarly, methane is oxidised owing to the electropositive element to a substance.
addition of oxygen to it. According to the definition given above,
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) (7.3) the following are the examples of reduction
processes:
A careful examination of reaction (7.3) in
which hydrogen has been replaced by oxygen 2 HgO (s) 2 Hg (l) + O2 (g) (7.8)
prompted chemists to reinterpret oxidation (removal of oxygen from mercuric oxide )
in terms of removal of hydrogen from it and, 2 FeCl3 (aq) + H2 (g) →2 FeCl2 (aq) + 2 HCl(aq)
therefore, the scope of term oxidation was (7.9)
broadened to include the removal of hydrogen
from a substance. The following illustration is (removal of electronegative element, chlorine
another reaction where removal of hydrogen from ferric chloride)
can also be cited as an oxidation reaction. CH2 = CH2 (g) + H2 (g) → H3C – CH3 (g) (7.10)
(addition of hydrogen)
2 H2S(g) + O2 (g) → 2 S (s) + 2 H2O (l) (7.4)
2HgCl2 (aq) + SnCl2 (aq) → Hg2Cl2 (s)+SnCl4 (aq)
As knowledge of chemists grew, it was
natural to extend the term oxidation for (7.11)
reactions similar to (7.1 to 7.4), which do (addition of mercury to mercuric chloride)
not involve oxygen but other electronegative In reaction (7.11) simultaneous oxidation
elements. The oxidation of magnesium with of stannous chloride to stannic chloride is
fluorine, chlorine and sulphur etc. occurs also occurring because of the addition of
according to the following reactions : electronegative element chlorine to it. It was
soon realised that oxidation and reduction
Mg (s) + F2 (g) → MgF2 (s) (7.5)
always occur simultaneously (as will be
Mg (s) + Cl2 (g) → MgCl2 (s) (7.6) apparent by re-examining all the equations
given above), hence, the word “redox” was
Mg (s) + S (s) → MgS (s) (7.7) coined for this class of chemical reactions.
Incorporating the reactions (7.5 to Problem 7.1
7.7) within the fold of oxidation reactions
In the reactions given below, identify
encouraged chemists to consider not only
the species undergoing oxidation and
the removal of hydrogen as oxidation, but
reduction:
also the removal of electropositive elements
as oxidation. Thus the reaction : (i) H2S (g) + Cl2 (g) → 2 HCl (g) + S (s)
2K4 [Fe(CN)6](aq) + H2O2 (aq) →2K3[Fe(CN)6](aq) (ii) 3Fe3O4 (s) + 8 Al (s) → 9 Fe (s)
+ 2 KOH (aq) + 4Al2O3 (s)
is interpreted as oxidation due to the removal (iii) 2 Na (s) + H2 (g) → 2 NaH (s)
of electropositive element potassium from
Solution
potassium ferrocyanide before it changes to
potassium ferricyanide. To summarise, the (i) H 2 S is oxidised because a more
term “oxidation” is defined as the addition electronegative element, chlorine is added
of oxygen/electronegative element to to hydrogen (or a more electropositive
a substance or removal of hydrogen/ element, hydrogen has been removed
electropositive element from a substance. from S). Chlorine is reduced due to
addition of hydrogen to it.
In the beginning, reduction was considered
as removal of oxygen from a compound. (ii) Aluminium is oxidised because
However, the term reduction has been oxygen is added to it. Ferrous ferric oxide
Rationalised 2023-24
Unit 7.indd 236 10/10/2022 1
, redox reactions 237
For convenience, each of the above
(Fe3O4) is reduced because oxygen has
processes can be considered as two separate
been removed from it.
steps, one involving the loss of electrons
(iii) With the careful application of the and the other the gain of electrons. As an
concept of electronegativity only we illustration, we may further elaborate one of
may infer that sodium is oxidised and these, say, the formation of sodium chloride.
hydrogen is reduced.
2 Na(s) → 2 Na+(g) + 2e–
Reaction (iii) chosen here prompts us to
think in terms of another way to define Cl2(g) + 2e– → 2 Cl–(g)
redox reactions. Each of the above steps is called a half
reaction, which explicitly shows involvement
7.2 REDOX REACTIONS IN TERMS OF of electrons. Sum of the half reactions gives
ELECTRON TRANSFER REACTIONS the overall reaction :
We have already learnt that the reactions
2 Na(s) + Cl2 (g) → 2 Na+ Cl– (s) or 2 NaCl (s)
2Na(s) + Cl2(g) → 2NaCl (s) (7.12)
Reactions 7.12 to 7.14 suggest that half
4Na(s) + O2(g) → 2Na2O(s) (7.13) reactions that involve loss of electrons are
2Na(s) + S(s) → Na2S(s) (7.14) called oxidation reactions. Similarly, the
are redox reactions because in each of these half reactions that involve gain of electrons
reactions sodium is oxidised due to the addition are called reduction reactions. It may not
of either oxygen or more electronegative be out of context to mention here that the
element to sodium. Simultaneously, chlorine, new way of defining oxidation and reduction
oxygen and sulphur are reduced because to has been achieved only by establishing a
each of these, the electropositive element correlation between the behaviour of species
sodium has been added. From our knowledge as per the classical idea and their interplay
of chemical bonding we also know that sodium in electron-transfer change. In reactions (7.12
chloride, sodium oxide and sodium sulphide to 7.14) sodium, which is oxidised, acts as
are ionic compounds and perhaps better a reducing agent because it donates electron
written as Na+Cl– (s), (Na+)2O2–(s), and (Na+)2 to each of the elements interacting with it and
S2–(s). Development of charges on the species thus helps in reducing them. Chlorine, oxygen
produced suggests us to rewrite the reactions and sulphur are reduced and act as oxidising
(7.12 to 7.14) in the following manner : agents because these accept electrons from
sodium. To summarise, we may mention that
Oxidation : Loss of electron(s) by any species.
Reduction : Gain of electron(s) by any species.
Oxidising agent : Acceptor of electron(s).
Reducing agent : Donor of electron(s).
Problem 7.2 Justify that the reaction:
2 Na(s) + H2(g) → 2 NaH (s) is a redox
change.
Solution
Since in the above reaction the compound
formed is an ionic compound, which may
also be represented as Na+H– (s), this
suggests that one half reaction in this
process is :
2 Na (s) → 2 Na+(g) + 2e–
Rationalised 2023-24
Unit 7.indd 237 10/10/2022 1
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