NEET Study Notes for Redox Reactions: Redox Reactions are defined as those reactions in which oxidation and reduction occur simultaneously. Redox Reactions are accompanied by change of energy in the form of heat, light, or electricity.

• The term redox, refers to the process of “Reduction and Oxidation”. In a Redox reaction, substances that gain electrons which reduce them to a low valency state are known as oxidising agents whereas substances that lose electrons thereby oxidizing to a higher valency state are called reducing agents.
• NEET Syllabus is divided broadly into topics from Class 11th and Class 12th. Redox Reactions forms part of Class 11 Chemistry Syllabus. Check Detailed NEET Chemistry Syllabus
• Redox Reaction is a unit of Physical Chemistry that has an overall weightage of around 32% in the chemistry section of NEET 2022 

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NEET 2022 Important Topics from Redox Reactions

NEET Study Notes for Redox Reaction

Redox Reactions are those reactions in which oxidation and reduction occur simultaneously and cannot happen independently of each other. Oxidation and reduction alone are called half reactions because two half-reactions together form one whole reaction. 

Example: SnCl₂ + 2HgCl₂ \(\rightarrow\) SnCl₄ + Hg₂Cl₂

Oxidation

As per the classical concept of oxidation, it is defined as the addition of oxygen or any electronegative element and simultaneous removal of hydrogen or any electropositive element. 

Example: Mg(Magnesium) - 2e^_-→ Mg⁺² (Magnesium Ion)

Oxidation can also be defined in the following terms 

• Addition of Oxygen where 2Mg + O₂ \(\rightarrow\) 2MgO
• Removal of Hydrogen where 3O₂ + 4NH₃ \(\rightarrow\) 2N₂ + 6H₂O
• Addition of electronegative element Cu + Cl₂ \(\rightarrow\) CuCl₂
• Removal of electropositive element H₂S + Cl₂ \(\rightarrow\) 2HCl + S
• De electronation 

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Reduction 

In terms of classical concept, reduction can be defined as addition of hydrogen or any electropositive element and the simultaneous removal of oxygen or any electronegative element .

Example: Br₂+ H₂S → 2HBr + S

CuO + H₂ → Cu + H₂O

Reduction may also be defined in the following terms 

• Addition of Hydrogen N₂ + 3H₂ \(\rightarrow\) 2NH₃
• Addition of electropositive element CuCl₂ + Cu \(\rightarrow\) Cu₂Cl₂
• Removal of Oxygen CuO + H₂ \(\rightarrow\) Cu + H₂O
• Removal of electronegative element 2HgCl₂ + SnCl₂ \(\rightarrow\) Hg₂Cl₂ + SnCl₄
• Electronation M + ne^– \(\rightarrow\) Mn^–

What is Oxidant or Oxidising Agent?

Oxidant may be defined as the substance that supplies oxygen or electronegative element , removes hydrogen or electropositive element and accepts electrons. Oxidants show decrease in oxidation number.

Examples: KMnO₄, H₂O₂, Cl₂, Br₂, KClO₃, FeCl₃ etc.

What is Reductant or Reducing Agent?

Reducing Agents may be defined as those substances that supply hydrogen or electropositive element, remove oxygen or electronegative element, and have the ability to donate electrons. Reductants show an increase in oxidation number. 

Examples: H₂, H₂S, Mg, FeSO₄, H₂C₂O₄, H₂SO₃.

Also Read:

• NEET Study Notes for Chemical Equilibrium
• NEET Study Notes for Dual Nature of Matter and Radiation

NEET Study Notes on Oxidation Number

When an element changes from free state in a particular compound , certain electrons are lost and gained by the element. This number is referred to as the oxidation number. It can also be dined as the residual charge present in atom when the atom is combined with other atoms. 

Different values of the oxidation number of an element depend upon the nature of the compound in which it is present. Oxidation numbers may be positive, negative, fraction and even zero.

Determining Oxidation Number

• O.N. of free state elements is zero.
• Hydrogen O.N. is always +1 whereas for ionic metal hydrides is -1
• Oxygen O.N. is -2, for OF₂ it is +2, In peroxides O.N. is -1.
• Metals O.N. is always positive. However for elements belonging to IA group it is +1, and elements belonging to IIA element, it is +2.
• Halogens O.N. is -1 in metal hydrides.
• Ion O.N. is the number of electrons, it gains or loses to acquire a position of neutral. It is equal to electric charge. Eg  = -2
• The O.N. of an atom within a compound can be positive, negative, integer, zero or fraction.
• The algebraic sum of O.N. of elements is equal to zero.
• The algebraic sum of O.N. of all elements in an ion is equal to net charge of ion.
• Maximum O.N. of of element is equal to group number of valence electrons.
• Except metals, the minimum O.N. of element = 8- group number.
• The O.N. of metal corbonyl and amalgams of metals is zero.

Read NEET Study Notes for Reaction Mechanism 

Concept of Oxidising Agent and Reducing Agent Oxidation Number 

The concept of Oxidation Number as per Oxidising Agent and Reducing agents are:

• Oxidizing Agent - If the oxidation number of one of the elements of a substance is maximum, then it can act as an oxidizing agent. Eg - HNO₃ wherein N= 5, which is the maximum value 
• Reducing Agent- If the oxidation number of one of the elements of a substance is minimum, then such substance can act as a reducing agent. Electronegative elements like I-, Br^–, N^3– are reducing agents. Eg- SnCl₂ ( where O.N. of Sn=2 of minimum value)
• Reducing and Oxidising agent- If O.N. of one of the elements of a substance is between maximum and minimum value , then such substance can act as both oxidisng agent as well as a reducing agent. Eg - HNO₂ where( O.N. of N is +3 which between the maximum and minimum values of -5 and 0 respectively)

What is Stock Notation?

The oxidation state of elements is represented by Roman Numerals within parentheses. This is known as Stock Notation. Stock Notations, introduced by German chemist Alfred Stock is used to represent the oxidation number of metals. Some Stock notations are tabulated below:

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NEET Study Notes for Balancing Redox Reactions

Redox Reactions can be balanced in two ways:

• Oxidation Number Method
• Ion Electron Method (Half Equation Method)

Steps for Oxidation Number Method

When using the Oxidation Number Method for Balancing Redox Reactions, certain rules are to be followed:

• Oxidation Number is to be assigned to the atoms that show a change in oxidation state.
• Balance the total atoms that undergo change in oxidation state.
• Balance the electrons that are gained or lost.
• Balance all atoms other than H and O. 
• Balance (O) on both sides by adding H2O
• Balance H atoms atoms by adding H+ ions.

Balancing in Acidic Medium:

• Balance O atoms by adding H2O
• Balance H+ ions to the side in which H atoms are deficient.

Balancing in Basic Medium;

• First balance oxygen atoms by adding H2O to the side that is deficient.
• Then in order to balance Hydrogen, add water molecules that is equal to the number of deficient H atoms.
• Add equal OH- ions to the opposite equation.

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Ion Electron Method (Half Reaction Method)

The rules for this method are tabulated below:

• Split the reactions into two half-reactions by showing the oxidation and reduction process separately.
• Balance the atoms that undergo a change in oxidation state.
• Ad electrons to that sde of the equation to balance difference in oxidation number in each half reaction.
• If the reaction occurs in acidic medium, balance the charge by adding H+ ions, If it occurs in basic medium add OH- ions.
• Balance oxygen atoms by adding by adding water molecules to the side of the equation that has deficient oxygen atoms.
• In acidic medium, add H+ ions to balance H atoms on the deficient side of the equation.
• In basic medium, add water molecules to balance H atoms.
• The two half reactions are then multiplied by suitable coefficient. This is done so that number of electrons gained in half-reaction becomes equal to the number of electrons lost in half reaction.
• Add both half reactions
• Veriffy and ensure that the total charge on either side is equal.

NEET Sample Questions for Redox Reactions

Ques. Name the reducing agent in the reaction Cu⁺²+ Zn →Cu + Zn⁺²

1. Zinc
2. Hydrogen
3. Oxygen
4. Copper

Ans. Zinc

Zinc is the reducing agent in this reaction. Reducing agent or Reductant is the substance that can give Hydrogen or electropositive element, removes oxygen or electronegative element and donate electrons. Reducing agent show increase in oxidation number.

Ques. The oxidation number of oxidant increases in Redox Reaction?

1. True
2. False

Ans. False

In a redox reaction, the oxidant number of oxidant decreases while the oxidant number of a reducing agent increases. Example of an Oxidant is K2Cr2O7

Ques. What is Hydrogen Peroxide?

1. Oxidisng Agent
2. Reducing Agent
3. Both Reducing and Oxidisng Agent
4. Neither Reducing nor Oxidising Agent 

Ans. Both Reducing and Oxidisng Agent

 Hydrogen Peroxide(H2O2 ) is a strong oxidizing agent as well as reducing agent. Since it contains, two hydrogen and two oxygen atoms, it looses hydrogen or oxyzen and oxidised and reduces respectively.

Ques. The oxidation number of which of the following element undergoes maximum change during a reaction of oxalic acid , potassium chlorate and suphuric acid ?

1. H
2. S
3. C
4. CI

Ans. CI

Since the oxidation number of CI undergoes maximum change, it acts as an oxiding agent in the reaction.

Ques. Which of the following is not an oxidising agent?

1. Magnesium Oxide
2. Ozone
3. Sodium Hydride
4. Carbon dioxide

Ans. Sodium Hydride

Some examples of Oxiding agents include molecules of electrogenative element, compounds with maximum Oxidation number, oxides of metals and non metals. Sodium Hydride is a metallic hydride so it is not an oxidizing agent.

Ques. Name the strongest oxiding agent from the following

1. O₃
2. K₂Cr₂O₇
3. KMnO₄
4. H₂O₂

Ans. O₃ is the strongest oxidising agent

Ques. What happened when when a zinc rod is kept in a copper nitrate solution?

1. Copper is deposited on zinc
2. Zinc is deposited on copper
3. Zinc is deposited in beaker
4. Copper is deposited in the beaker

Ans. copper is deposited on zinc

The intensity of blue color is produced when zinc is placed in copper nitrate solution and copper iron is deposited in on zinc. This is an example of Redox Reaction between zinc and aqueous solution of copper nitrate in a beaker. 

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