Oxidation Reduction – Redox Reactions

Oxidation:

Old Concept:

  • It is a process in which addition of oxygen takes place.

2Mg + O2  →      2MgO

  • It is a process in which addition of electronegative radical takes place.

2FeCl2 +  Cl2 →  2FeCl3

  • It is a process in which removal of hydrogen takes place.

H2S + 2 [Cl] →  S + 2HCl

  • It is a process in which removal of electropositive radical takes place.

2KI + H2O2 → I2 + 2KOH



Moden Concept:

  • According to the electronic concept, a reaction in which loss of electrons from an atom or an ion takes place is called oxidation.

Consider reaction, 2Mg + O2 →  2MgO

  • In this reaction, the valency of magnesium is increased’ from zero (in the atomic state) to + 2 (in MgO).

i.e.  Mg0 →  Mg2+ + 2 e

  • In this reaction, magnesium is losing electrons. And hence oxidation of magnesium takes place.

Reduction:

Old Concept:

  • It is a process in which addition of hydrogen takes place.

Cl2 + H2  →  2HCl

  • It is a process in which addition of electropositive radical takes place.

2HgCl2 +  SnCl2 → Hg2Cl2 + SnCl4 .

  • It is a process in which removal of oxygen takes place.

CuO + 2 [H] → Cu + 2H2O

  • It is a process in which removal of electronegative radical takes place.

FeCl3 + H → FeCl2 + HCl

Modern Concept:

  • According to the electronic concept, a reaction in which gain of electrons by an atom or an ion takes place is called reduction.

Consider reaction, 2HgCl2 +  SnCl2 → Hg2Cl2 + SnCl4

  • In this reaction , valency of mercury is decreased’ from +2  (in HgCl2) to +1 (in Hg2Cl2).

i.e.   Hg2+ +   e  →   Hg +

  • In this reaction mercury is gaining electron. And hence reduction of mercury takes place.


Redox Reaction:

  • In any of a chemical reaction if one of the reactants is oxidized, other is surely reduced, i.e. oxidation and reduction always take place simultaneously.
  • Example – 1:  in the reaction,  2Mg + O2 →  2MgO,  Mg is oxidized to MgO (addition of oxygen, i.e. increase in positive valency of Mg i.e. loss of electrons by Mg), whereas oxygen is reduced to MgO (addition of positive radical, i.e. increase in negative valency of oxygen, i.e. gain of electrons by oxygen)
  • Example – 2:  in the reaction,   2HgCl2 +  SnCl2 → Hg2Cl2 + SnCl4, HgClis reduced to Hg2Cl2 whereas SnCl2 is oxidised to SnCl4.
  • Thus oxidation and reduction take place simultaneously.  Therefore, all such reactions are called as reduction-oxidation reactions or redox reactions.  In all such reactions, one of the reactants loses the electrons (oxidized) while other gains those electrons (reduced)
  • However, it should be remembered that all the chemical reactions are not redox reactions.  There are several other types of reactions also.

NaCl + AgNO3 → AgCl + NaNO3

  • In such reactions none of’ the reactants is oxidized or reduced; simply the exchange of cation or anion takes place.

Oxidation Reduction 01

 Oxidizing Agent (Oxidant):

  • The substance which excepts electrons and makes the other substance to lose electrons is called oxidizing agent or oxidant.
  • e.g.  Consider reaction,  2Mg + O2 →  2MgO, In this reaction, oxygen is making magnesium to lose electrons and hence in this reaction oxygen is the oxidizing agent.
  • e.g.  Consider reaction, 2K + Cl2   2KCl, In this reaction, chlorine is making potassium to lose an electron and hence in this reaction chlorine is the oxidizing agent.

Characteristics of Oxidizing Agent:

  • The substance which excepts electrons and makes the other substance to lose electrons is called oxidizing agent or oxidant.
  • In a reaction, the oxidizing agent oxidizes the other substance but is itself reduced.
  • Oxygen, or a substance capable of giving oxygen, is always a good oxidizing agent.
  • According to electron concept, an oxidizing agent is that which is capable of de-electronating the other substance.
  • An oxidizing agent is an electron acceptor and during the redox reaction, it is electronated.
  • Fluorine (F) has a maximum tendency to accept electrons hence it is the strongest oxidizing agent.


Examples of Common oxidizing Agents:

  • Oxygen (O or O2), Ozone (O3), Hydrogen peroxide (H2O2), Sulphuric acid (H2SO4), Nitric acid (HNO3), Perchloric acid (HClO4), Potassium chlorate (KClO3), Acidified potassium dichromate (K2 Cr2 O7 + H2SO4), Acidified potassium permanganate (KMnO4 + H2SO4), Alkaline potassium permanganate (KMnO4 + KOH)

Reducing Agent (Reductant):

  • The substance which loses electrons and makes the other substance to gain electrons is called reducing agent or reductant.
  • e.g. Consider reaction, 2Mg + O2 →  2MgO, In this reaction magnesium is making oxygen to gain electrons and hence in this reaction magnesium is reducing agent.
  • e.g.  Consider reaction, 2K + Cl2   2KCl, In this reaction potassium is making chlorine to gain an electron and hence in this reaction potassium is reducing agent.

Characteristics of Reducing Agent:

  • The substance which loses electrons and makes the other substance to gain electrons is called reducing agent or reductant.
  • In a reaction, the reducing agent reduces the other substance but is itself oxidised.
  • Hydrogen, or a substance capable of giving hydrogen, is always a good reducing agent.
  • According to electron concept, a reducing agent is that which is capable of electronating the other substance.
  • A reducing agent is an electron donor and during the redox reaction, it is de-electronated.
  • Sodium (Na) has a maximum tendency to donate electron hence it is the strongest reducing agent.

Examples of Common Reducing Agents:

  • Hydrogen (H or H2), Hydrogen iodide (HI), Hydrogen sulphide (H2S), Lithium aluminium hydride (LiAI H4), Sodium borohydride (NaB H4), Sulphur dioxide (SO2), Carbon (C), Ozone (O3), Hydrogen peroxide (H2O2), Tin & hydrochloric acid (Sn  + HCl), Sodium & alcohol (Na + C2 H5OH), Metallic salts (ous) like SnCl2 , FeSO4 etc.


Oxidation Number OR Oxidation State:

  • The donation of electrons is called the oxidation and the gain of electrons is called the reduction. oxidation and reduction can further be explained by a knowledge of “Oxidation number”.
  • The oxidation state of an atom in its free or ground state is taken as zero. When the atom loses electrons its oxidation state increases and when the atom gains electrons its oxidation state decreases. The term oxidation-number represents the positive or negative character of the atom in a compound.
  • Oxidation number is defined as the charge an atom appears to have when electrons are assigned in accordance with the following arbitrary rules.
  • Electrons shared by two like atoms are divided equally between the two atoms. Electrons shared between two unlike atoms are assigned to the more electronegative atom of them.

Conventions Used in Assigning Oxidation Number:

  • The oxidation number of an element in a free atomic state (Na, H, Cl, O, P etc) or in its poly-atomic state (graphite, H2, Cl2, O2 etc) is always zero.
  • The oxidation number of hydrogen is always +1 in its compounds.  However, in metal hydrides like NaH, MgH etc. the oxidation number of hydrogen is -1 because metals are more electropositive than hydrogen.
  • O.N. of oxygen is always -2 in its compounds.  However, in peroxides like H2O2, Na2O2, BaO2 etc. the oxidation number of oxygen is -1. In OF the oxidation number of oxygen is +2 because F is more electronegative than O.
  • O.N. of group IA element i.e. Li, Na, K etc is always +1 in their compounds.
  • O. N. of group IIA elements i.e. Be, Mg, Ca, Sr and Ba are always +2 in their compounds.
  • O. N. of F is always -1 in its compounds because it is most highly electronegative.  Oxidation O. N. of other elements of group VIIA. (17) i.e. Cl, Br and I are also generally –1.
  • In an ion, the sum of the oxidation numbers of different atoms is equal to charge over the ion.
  • In a complex compound (involving co-ordination by ligands) it is more convenient to use oxidation number of group (ligand) as a whole instead of oxidation number of individual atoms. For example, in HCN the oxidation number of CN- ion is –1. Here CN-  as a whole is considered and not of individual C or N.
  • on the basis of above standard oxidation numbers, which may be taken as rules, the oxidation, a number of a particular given atom in a compound can be determined.

Valency and Oxidation Number:

  • Valency is a different term than oxidation number though sometimes the valency and the oxidation number of an element are same in a compound.
  • Valency of an element is given by the number of electrons it actually loses or gains or shares during the formation of a compound, Whereas oxidation number is just the apparent charge (not necessarily actual) over the atom when the electrons are counted according to the arbitrary rules given earlier.
  • In most of the cases, the valency of an element is constant whereas the oxidation state of an element may vary in its different compounds.
  • Valency and oxidation states of carbon in its different compounds give a good example of this:
  • In CH4, CH3Cl, CH2Cl2, CHCl3 and CClthe valency of carbon is always four (due to sharing of four electrons) but its oxidation number is – 4, -2, 0, +2 and +4 respectively.


Oxidation-Reduction in Terms of Oxidation Number:

  • On the basis of oxidation number a reaction involving the increase in oxidation number is called as oxidation while a reaction involving the decrease in oxidation number is called as reduction (Remember increase in O.N. means increase in positive O.N. or decrease in negative O.N., while decrease in O.N. means decrease in positive O.N. or increase in negative O.N.).
  • For example, in the reaction, 2Mg + O2 →  2MgO, The O.N. of Mg increase from 0 to +2.while the O.N. of O decreases from 0 to -2.  Thus, magnesium is oxidised while oxygen is reduced.

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