Notes-Class-11-Science-Chemistry-Chapter-6-Redox Reactions-Maharashtra Board

Redox Reactions

Maharashtra State Board-Class-11-Science-Chemistry-Chapter -6

Notes

Topics to be Learn :

  • Introduction
  • Oxidation number
  • Balancing of redox reactions
  • Redox reaction and electrode potential

Introduction :

Term Redox : Redox is an abbreviation used for the terms 'oxidation and reduction'.

A large number of phenomena such as respiration, rusting, combustion of fuel involve redox reactions.

Daily use examples :

  • Apple turn brown when exposed to air : Catechol present in apple undergoes oxidation when exposed to air and imparts brown colour.
  • Old car bumper changes colour : Metallic bumper undergoes oxidation by air and changes colour.
  • New batteries become useless after some days : The batteries involve redox reactions during their use and consume the chemical like H2SO4. Hence they become useless after some days.

Classical ideas of redox reactions :

Classically oxidation refers to combination of an element or a substance with oxygen.

Example :

Oxidation of carbon : C (s) + O2 (g) → CO2 (g)  

Oxidation of magnesium : 2Mg + O2 → 2MgO(s)  

In above reactions the carbon and magnesium are oxidized on reacting with oxygen.

Classically reduction refers to combination of an element or a substance with hydrogen or removal of oxygen from its compound.

Example :

2H2 (g) + O2 (g) → 2H2O (g)

In this reaction, hydrogen is oxidised and oxygen is reduced.

Some more examples of oxidation and reduction :

Oxidants/ Oxidising agent :

A reagent/substance which itself undergoes reduction and causes oxidation of another species is called oxidant /oxidising agent.

Example :

Mg (s) + F2 (s) → MgF2(s)

In the reaction fluorine is oxidant which oxidises Mg.

Reductant\reducing agent :

According to classical definition reductant is a substance which itself undergoes oxidation and causes the reduction of another substance.

Example :

Mg (s) + S(s) → MgS (s)

In the reaction Mg is reductant.

Explanation of oxidation and reduction with the help of electropositive and electronegative elements :

Oxidation : Combination of a substance with electronegative element or removal of electropositive element is called oxidation.

Reduction: Combination of a substance with electropositive element or removal of electronegative element is called reduction.

For example,

2Mg + O2 → 2MgO(s)  

In this Mg undergoes oxidation while O undergoes reduction.

Key points :

Redox reaction in terms of electron transfer :

Redox reaction can be described as electron transfer.

Example :

Mg + ½ O2  → Mg2+ + O2

Development of charges on the species produced suggest the reactions can be written as :

When Mg is oxidised to MgO, the neutral Mg atom loses electrons to form Mg2+ in

MgO. The elemental oxygen gains electrons and forms O2 in MgO.

Each of the above steps represents a half reaction which involves electron transfer (loss or gain). Sum of these two half reactions or the overall reaction is a redox reaction.

Consider the following half reactions.

Fe(s) → Fe2+(aq) + 2e  ……(1)

Cu2+ (aq) + 2e → Cu(s)  ……(2)

Fe(s) + Cu2+ (aq) → Fe2+ (aq) + Cu(s)  ……(3)

In the half reaction (1) Fe acts as a reducing agent.

In half reaction (2) Cu2+ acts as oxidising agent which accepts electrons.

The half reaction involving loss of electrons is called oxidation reaction and that involving gain of electrons is called reduction.

Thus eq. (1) is oxidation, eq. (2) is reduction and eq. (3) is a redox reaction.

Key points :

  • Oxidant / oxidizing agent : A reagent / substance which itself undergoes reduction and causes oxidation of another species is called oxidant or oxidizing agent. This is an electron acceptor.
  • Reductant / Reducing agent : A reagent / reducing agent is defined as a substance / reagent which itself undergoes oxidation and brings about reduction of another species. A reductant is electron donor.

Displacement reaction :

This is a redox reaction in which an ion (or an atom) in a compound is replaced by an ion (or an atom) of another element.

For example,

X + YZ → XZ + Y

Fe + CuSO4 → FeSO4 + Cu

Q. Identify oxidising and reducing agents involved in the following displacement reactions.

(i) Zn (s) + Cu2+(aq) Zn2+(aq) + Cu(S)

(ii) Cu(s) + 2Ag+(aq) Cu2+(aq)  + 2Ag(S)

(iii) 2Co (s) + 3Ni2+ 2Co3+(aq) + 3 Ni (s)

Answer :

Oxidation number or oxidation state : The oxidation number (or oxidation state) of an atom in a molecule or an ion is defined as the number of charges it would carry if the electrons were completely transferred.

Explanation :

  • The oxidation number or oxidation state does not always imply ionic charges on the species.
  • The oxidation number of monoatomic ion is equal to charge of the ion. For example, Na+ has + 1 charge while Cl has −1 charge, Ca2+ has + 2 charge and so on (But the oxidation states are 1+ , 1−, 2+ respectively).
  • In case of a neutral molecule, the sum of the oxidation numbers of all the constituent atoms is always zero.
  • The charge on a polyatomic ion is equal to the algebraic sum of the oxidation numbers of all the constituent atoms of the ion.
  • From the change in the oxidation numbers in a redox reaction, an oxidant and a reductant can be identified.

Remeber : Oxidation number or oxidation state is represented as 1+, 2+, etc. while charge is represented as +1, +2, etc.

Rules to assign oxidation number :

Stock notation : Oxidation number represents the oxidation state of an atom and is also denoted by Roman numeral in parentheses after the chemical symbol of the concerned element in the molecular formula.

This representation is called Stock notation after the German Scientist Alfred Stock.

Example :

  • Au1+ Cl1 → Au(I)Cl
  • Au3+Cl31  → Au(III)Cl3

Redox reaction in terms of oxidation number :

Oxidation : An increase in the oxidation number of an element in a given substance.

Reduction : A decrease in the oxidation number of an element in a given substance.

Oxidizing agent : A substance which increases the oxidation number of an element in a given substance, and itself undergoes decrease in oxidation number of a constituent element.

Reducing agent : A substance that lowers the oxidation number of an element in a given substance, and itself undergoes an increase in the oxidation number of a constituent element in it.

Know This :

Some elements in a particular compound may possess fractional oxidation number. For example : C3O2, Br3O8, Na2S4O6, C8H18, etc. In these compounds oxidation number of C, Br, S, C are 4/3, 16/3, 2.5, 9/4, respectively. These oxidation numbers are actually the average oxidation number of all the atoms of elements in that compound. Different atoms of the element in such species exhibit different oxidation states. For example : Tertra thionate ion has two S atoms with oxidation number +5 and two with zero (0). Therefore, the average oxidation number of S in these species is 10/4 = 2.5

Balancing of redox reactions :

Two methods are used to balance chemical equation for redox processes :

  • Oxidation number method
  • Half reaction method or Ion electron method.

The Oxidation number method :

Half reaction method :

Redox reaction and electrode potential :

Redox reaction with the help of Daniel cell :

Consider the following displacement reaction,

The electron transfer from Zn atom to Cu2+ ions can be demonstrated with the help of Daniel Cell reactions.

Construction :

  • Daniell cell consist of two half cells. One half cell consists of a beaker containing ZnSO4 (1 M) solution in which a polished strip of metallic zinc is immersed while the second half cell consists of a beaker containing CuSO4 (1 M) solution in which a polished strip of metallic copper is immersed.
  • Two half cells are electrically connected by a U-shaped salt bride containing a gel of KCl or NH4NO3 in agar-agar.

Working of Daniell cell : When the circuit is complete, following reactions take place :

At zinc electrode : Zinc atoms from zinc plate lose electrons spontaneously which flow in the external circuit from zinc plate to copper plate through wire.

Zn(s) → Zn2+(aq) + 2e  ..(oxidation)

At copper electrode :

  • Cu2+ from solution receive these electrons through copper plate and are reduced to copper atoms which are deposited on the copper plate.

Cu2+(aq) + 2e  → Cu(s) …(reduction)

  • The net reaction in the cell is an electrochemical redox reaction represented as,

Zn(s) + Cu2+(aq)  → Zn2+(aq) + Cu(s)

  • In the cell zinc electrode is an anode while copper electrode is a cathode. In the external circuit electrons flow from anode (Zn) to cathode (Cu). An electrical potential called electrode potential is established at two electrodes of the electrochemical cell.

Electrode potential :

An electrical potential established at the electrode of an electrochemical cell is called an electrode potential. The magnitude of the electrode potential depends upon the nature of metal and ions, concentration of ions and temperature.

Electrode reaction :

The reaction associated with an electrode involving metal and ions is called electrode reaction.

Redox couple :

The two chemical species which are linked by transfer of electrons represents a redox couple. For example, Zn(s) | Zn2+(aq)

Standard electrode potential : The observed electrode potential is referred to as standard electrode potential (E0) when the concentration of each species involved in the electrode reaction is unity and the temperature is 298 K.

Significance of standard electrode potential :

Q. On the basis of standard electrode potential explain :

(a) the reducing power of alkali metals and

(b) oxidising power of fluorine.

Answer :

Standard electrode protentianls of some redox couples :

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