Redox Reactions chemistry Assignment Help




Redox (shorthand for oxidation-reduction) reactions describe all chemical reactions in which atoms have their oxidation number (oxidation state) changed. This can be either a simple redox process, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of sugar (C6H12O6) in the human body through a series of complex electron transfer processes.

The term comes from the two concepts of reduction and oxidation. It can be explained in simple terms:

  • Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
  • Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.


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Redox Reactions Examples

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Introduction to redox reactions examples:

 Two chemical reactions always occurring at the same time are known as oxidation-reduction reactions or redox reactions. Chemical reactions involving the change in the oxidation number of the atoms that are involved in the reaction are called redox reactions. The valency of the atom carrying the sign that represents the nature of the charge denotes the oxidation number. Oxidation occurs either due to loss of hydrogen atoms or electrons, or gain of oxygen or an increase in the oxidation state. Reduction reactions occur due to gain of hydrogen atoms or electrons, loss or oxygen or a decrease in the oxidation state.

Examples of Redox Reactions:

A few redox reactions that are commonly known are

The formation of hydrogen fluoride involving the oxidation of hydrogen and reduction of fluorine is an example of redox reaction.

H2 + F2 `->` 2HF

H2 `->` 2H+  + 2e- (Oxidation reaction)

F2+ 2e- `->` 2F- (Reduction reaction)

Reaction between Potassium permanganate and Sodium sulphite

                        2 KMnO4 + 3 Na2SO3 + H2O → 2 MnO2 + 3 Na2SO4 + 2 KOH

Rusting or tarnishing reaction: Oxidation of Fe(II) to Fe(III) by hydrogen peroxide and an acid

                        2 Fe2+ + H2O2 + 2 H+ → 2 Fe3+ + 2 H2O

Cellular respiration involving oxidation of glucose to carbon dioxide

                        C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

An acid and a base involved in the redox reaction

                        3 CuS + 8 HNO3 → 3 CuSO4 + 8 NO (g) + 4 H2O

Combustion is a redox reaction occurring rapidly producing light and heat every time any material burns. During the combustion of coal, a nearly pure carbon oxidation occurs rapidly releasing energy. C + O2 `->` CO2

Corrosion is a redox reaction involving the oxidization of a metal and reduction of oxygen that occurs generally in the presence of moisture.

Biological processes: A number of changes taking place in the living organisms are redox reactions.

Conclusion on Redox Reactions Examples:

Chemical reactions involving oxidation and reduction reactions at the same time are called redox reactions. The redox reactions are the chemical reactions that are most essential and most common reactions in everyday life involving biological processes like cellular respiration, digestion, etc. During redox reactions, one of reactant is reduced and the other oxidized.

Redox Reactions Oxidation Numbers

Redox reactions or in other words ‘oxidation-reduction’ reactions are reactions that involve simple electron transfer processes. In redox reactions, oxidation numbers change; that is, both oxidation and reduction processes occur in a redox reaction. The oxidation part of the reaction involves the loss of electrons by a chemical species whereas the reduction part of the reaction involves the gain of electrons by the species that accepts the electrons from the species that is getting oxidized. An oxidizing agent is the chemical species that cause the oxidation process and accepts electrons. The reducing agent is tat chemical species that is the cause of the reduction reaction and loses electrons.

Introduction to redox reactions oxidation numbers

Single displacement reactions are examples for redox reactions and single displacement reaction is one in which one element replaces another similar element in a compound. These reactions provide a very simple introduction to the theoretical definitions of oxidation and reduction we have just seen above.

The coupled reduction-oxidation reactions also called redox reactions occur in some other instances too.  As in metallurgy, the term reduction is associated with producing metals from their compounds and the term “oxidation” includes reactions with oxygen such as the wide range of combustion and corrosion reactions.

An oxidation-number is a number that corresponds to the apparent charge that an element in a molecule or ion would have if the electron pair in covalent bonds belonged entirely to the more electronegative atom. To systematically identify oxidation numbers of elements in a compound we simply count electrons. Therefore, the sum of the oxidation numbers in a compound or ion must equal the charge – zero for neutral compounds, and the ion charge for ions.

How do we Find the Oxidation Number of an Element in a Compound or Ion?

When we try to find the oxidation number of an atom in a compound, we need to consider the following rules.

Rule 1: The sum of the oxidation numbers of the elements in a molecule or ion is equal to the value of the net charge on the molecule or the ion.

a)    When it comes to a compound, the sum of the oxidation numbers is a zero.
b)    In the case of a polyatomic ion, the sum of the oxidation numbers equals the charge on the ion.

Rule 2: Any unknown oxidation number in a compound or ion can be determined algebraically from the sum of the known oxidation numbers and the net charge on the entity.

Rule 3: All elements in an atom have an oxidation number of zero.

Rule 4: The hydrogen in all compounds except for those in hydrides has an oxidation number of +1. The hydrogen in hydrides is assigned an oxidation number of -1.
   
Rule 5: Oxygen in all compounds except for those in peroxides has an oxidation number of -2. Those in peroxides have an O.N of +2.

Rule 6: All monoatomic ions are assigned the charge on ion.

A few Examples of Ocidation Number

Let us now assign the oxidation numbers for the following(assuming the unknown oxidation number to be ‘x):
1.    S in SO3 2-

        x + 3(-2) = -2
        x -6 = -2
        x= 6-2
        x = +4


2.    Mn in MnO3 2–

        x + 3(-2) =-2
        x -6 = -2
        x = +4


3.    Cl in HClO2

      +1 + x+ 2(-2) = 0
      +1 +x -4 = 0
       x -3 = 0
       x = +3


4.    Determine the oxidation number of nitrogen in

a)    N2O (g)

     2x + (-2) = 0
     2x = +2
     x = +1


b)    NO (g)

      x + (-2) =0
      x= +2

c)    NO2 (g)
       x + 2(-2) = 0
       x -4 = 0
       x= +4


5.  Determine the oxidation number of carbon in sodium bicarbonate.

The molecular formula of sodium bicarbonate is:  NaHCO3

+1 + 1 + x + 3(-2) = 0
2 + x -6 = 0
x -4 =0
x = +4


6. Determine the oxidation number of iron in Fe3O4.

3x + 4(-2) = 0
3x -8 =0
3x = +8
x = +8/3
 

Examples of Single Displacement Reactions as Redox Reactions:

Let us try assigning the oxidation number for the species in the following reactions and analyze the species that gets oxidized and gets reduced.
 
a)    Cu (s) + 2AgNO3 (aq) → 2 Ag (s) + Cu(NO3)2  (aq)

In the reaction given, Cu is getting oxidized to Cu 2+. Ag +1 is getting reduced to Ag. The oxidizing agent being silver ions of silver nitrate solution and reducing agent is the copper metal.  The oxidation number INCREASES of the species that gets oxidized, namely copper (from 0 to +2) and the oxidation number of the reduced species DECREASES as in the case of Ag+ (+1 to 0).


b)    Is this reaction a typical redox reaction? Analyze.

Pb (NO3)2 (aq) +2 KI  → PbI2 (s) +2 KNO3 (aq)

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There is no change in the oxidation number. So this is not a redox reaction.

c)    Cl2 (aq) +2KI (aq) →  I2 (s) +2KCl (aq)

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The oxidation number of Cl DECREASES from 0 (in elemental chlorine) to -1 (in the chloride ion) so chlorine is getting reduced during the course of the reaction. Iodine on the other hand is oxidized as the oxidation number INCREASES during the reaction from -1 to 0.

Oxidation Reduction Electron Transfer

Introduction to oxidation reduction electron transfer:

Oxidation and reduction are the basic reactions in chemistry.

They are used in industrial production of chemicals,pharmaceutical industries,laboratories,electrictrochemical cells etc.

As the name itself implies oxidation involves something to do with oxygen.

So in real sense oxidation is gain of oxygen.

e.g. In a water formation reaction,oxygen is added to hydrogen ,hence it is oxidation of hydrogen.

In other sense,which is old now,oxidation is also loss of hydrogen.

e.g In the case of propanol to propanal conversion.

CH3CH2CH2OH---------------> CH3CH2CHO

propanol                                      propanal

Here propanol  is loosing hydrogen,so it is getting oxidised.

This reaction cannot simply take place but need other chemicals which would either add oxygen or 'snatch' the hydrogen.Such reagents are called as oxidising reagent.

A few of such reagents are oxygen itself,K2Cr2O7 in sulphuric acid etc.

Oxidation

Previously only addition of oxygen was termed as oxidation.

Oxygen is strongly electronegative element.In a compound,it would snatch the electron from the entity to which it is added,if it is a ionic bond.

If it is a covalent bond,it will pull the electron more towards itself in the bond.

So in effect,the definition tells us about electron position with relevance to oxygen.

As such a more transparent ,more definitive definition for oxidation evolved,which simply states that,oxidation is loss of electrons.

e,g.

In the reaction,

Fe 2+ ----------> Fe 3+ + e-

Fe 2+ is loosing electron,so it is getting oxidzed.

Reduction of Electron Transfer:

Gain of electrons is reduction.

Hint : OIL-RIG [Oxidation is loss-Reduction is gain]

e.g. If the above reaction is reversed,

Fe 3+ + e- -------------> Fe 2+

It is reduction.

Redox Reactions of Electron Transfer:

It is observed generally that oxidation reactions do not take place independently but along with reduction.

The reactions where such both reactions take place simultanously are called redox reaction.

e.g. CuO +Mg----------->Cu +MgO

The half reactions,as they are called,can be represented as under

CuO ----------->Cu+2 + 2e-     Oxidation

Mg +2 + 2e- ----> MgO

There is always change in the oxidation number in redox reaction.


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