Nanomicrosystems: Chemistry for Nanotechnology 5
Oxidation and Reduction
Dr. Muhammed Ajmal C
&
Prof. Seunghyun Baik
School of Mechanical Engineering
Sungkyunkwan University
Contents
Oxidation reaction
Reduction reaction
Oxidation number
Adsorption (basics)
Practice problems
Oxidation and reduction reactions
Classical idea of oxidation and reduction reactions:
 Originally, the term oxidation was used to describe the addition of oxygen to an element or a compound.
While, reduction was considered as removal of oxygen from a compound. For Example,
Oxidation
Reduction
 Later, the term “oxidation” is defined as the addition of oxygen/electronegative element to a substance or
removal of hydrogen/ electropositive element from a substance. While, “reduction” reactions include
removal of oxygen/electronegative element from a substance or addition of hydrogen/electropositive
element to a substance.
Oxidation
Reduction
‘The oxidation and reduction reactions always occur simultaneously, hence, the word “redox” was coined for this class of chemical reactions’.
Oxidation and reduction reactions
Problem?
➢ In the reactions given below, identify the species undergoing oxidation and reduction:
Solution:
Oxidation and reduction reactions
Redox reactions in terms of electron transfer reactions:
 Consider the following redox reaction;
Adding half reactions
Overall reaction
Each of these steps is
called a half reaction,
which explicitly shows
involvement of electrons.
Adding half reactions
□ Half reactions that involve loss of electrons are called oxidation reactions.
□ Similarly, the half reactions that involve gain of electrons are called reduction reactions.
□ A reducing agent donates electron to each of the elements interacting with it and thus helps in reducing them.
□ While, oxidizing agents accept electrons from the other species and promotes oxidation.
Oxidation and reduction reactions
To summarize,
Problem?
➢ Write down the half reactions involved in the following redox reaction?
Na(s) + S(s) → Na2S
Solution:
First, we need to balance the chemical equation.
Therefore, half reactions are
2Na(s) + S(s) → Na2S
2Na → 2Na++ 2eS + 2e- → S2-
Oxidation and reduction reactions
Oxidation number :
 Consider the following reaction;
 Though not simple in its approach, yet we can visualize the H atom as going from a neutral (zero) state in H2 to
a positive state in H2O, the O atom goes from a zero state in O2 to a dinegative state in H2O.
 It is assumed that there is an electron transfer from H to O and consequently H2 is oxidized and O2 is reduced.
 Note that the charge transfer is only partial and is perhaps better described as an electron shift rather than a
complete loss of electron by H and gain by O.
 In order to keep track of electron shifts in chemical reactions involving formation of covalent compounds, a
more practical method of using oxidation number has been developed.
 In this method, it is always assumed that there is a complete transfer of electron from a less electronegative
atom to a more electronegative atom.
 Thus,
Oxidation and reduction reactions
Oxidation number denotes the oxidation state of an element in a compound ascertained according to a set of
rules formulated on the basis that electron in a covalent bond belongs entirely to more electronegative element.
Rules for the calculation of oxidation number:
1. In elements, in the free or the uncombined state, each atom bears an oxidation number of zero.
Evidently each atom in H2, O2, Cl2, O3, P4, S8, Na, Mg, Al has the oxidation number zero.
2. For ions composed of only one atom, the oxidation number is equal to the charge on the ion.
3. The oxidation number of oxygen in most compounds is –2. However, peroxides, superoxides, and compounds
of O with F are exceptions.
4. The oxidation number of hydrogen is +1, except when it is bonded to metals in binary compounds.
For example, in LiH, NaH, and CaH2, its oxidation number is –1.
5. In all its compounds, fluorine has an oxidation number of –1. Other halogens (Cl, Br, and I) also have an
oxidation number of –1, when they occur as halide ions in their compounds.
6. The algebraic sum of the oxidation number of all the atoms in a compound must be zero. In polyatomic ion,
the algebraic sum of all the oxidation numbers of atoms of the ion must equal the charge on the ion.
Oxidation and reduction reactions
The terms oxidation number and oxidation state are interchangeably used.
Problem?
➢ Find out the oxidation number of the Mn in the following compounds?
1. KMnO4
2. Mn2O3
Solution:
1. KMnO4
2. Mn2O3
We have, the oxidation number of KMnO4 molecule is zero.
∴ The oxidation number of Mn can be calculated by;
(+1) + Mn + [4ⅹ(-2)] = 0
∴ oxidation number of Mn in KMnO4 is +7
We have, the oxidation number of Mn2O3 molecule is zero.
∴ The oxidation number of Mn can be calculated by;
2Mn + [3ⅹ(-2)] = 0
∴ oxidation number of Mn in Mn2O3 is +3
Note that, the d-block elements exhibit variable oxidation numbers.
Oxidation and reduction reactions
Redox reactions in terms of oxidation number,
Oxidation and reduction reactions
Problem?
Solution:
Oxidation and reduction reactions
Problem?
Fe2O3(s)+ 3CO → 2Fe(s) + 3CO2
CuO(s)+ H2(g)→ Cu(s) +H2O(g)
Oxidation and reduction reactions
Competitive electron transfer reactions
Copper in aqueous solution of silver nitrate
Zinc in aqueous solution of copper nitrate
Zn>Cu>Ag
Metal reactivity series or electrochemical series
Adsorption
 What is surface chemistry?
Surface chemistry deals with phenomena that occur at the surfaces or interfaces.
Many important phenomena, noticeable amongst these being corrosion, electrode processes,
heterogeneous catalysis, dissolution and crystallization occur at interfaces.
The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.
Adsorption - the most common surface phenomena
• The surface of a solid has the tendency to attract and retain the
molecules of the phase with which it comes into contact. These
molecules remain only at the surface and do not go deeper into the
bulk.
• The accumulation of molecular species at the surface rather than
in the bulk of a solid or liquid is termed adsorption.
• The molecular species or substance, which concentrates or
accumulates at the surface is termed adsorbate and the material
on the surface of which the adsorption takes place is called
adsorbent.
Adsorption
➢ Types of adsorption: