Class 11 Chemistry · Chapter 7 NotesRedox Reactions
Learn about redox reactions in Class 11 Chemistry, including oxidation and reduction, oxidation numbers, types of redox reactions, balancing equations, and practical…
Redox reactions are a fundamental concept in chemistry where oxidation and reduction occur simultaneously. This chapter explores the classical, electronic, and oxidation number approaches to understanding redox processes. You will learn how to identify redox reactions, assign oxidation numbers, balance redox equations, and understand the practical applications of redox reactions in daily life. The chapter also introduces the concept of electrode potentials and their significance in redox processes.
What you'll learn
1Understand the concepts of oxidation and reduction in redox reactions
2Assign oxidation numbers to elements in compounds using the given rules
3Classify redox reactions into combination, decomposition, displacement, and disproportionation types
4Balance redox equations using the oxidation number method and the half-reaction method
5Identify the oxidising and reducing agents in a redox reaction
6Apply the concept of redox reactions to practical applications in daily life
7Understand the concept of electrode potentials and their role in redox processes
Chapter at a glance
01Oxidation and Reduction: Concept and Definition
02Oxidation Number and Its Rules
03Balancing Redox Equations: Methods and Techniques
04Applications of Redox Reactions in Daily Life
Detailed chapter notes
01
Oxidation and Reduction: Concept and Definition
Oxidation and reduction are complementary processes. Historically, oxidation was defined as the addition of oxygen or removal of hydrogen, while reduction was the removal of oxygen or addition of hydrogen. Modern definitions involve a change in oxidation numbers: oxidation is an increase in oxidation number, and reduction is a decrease. In a redox reaction, one substance is oxidised while another is reduced.
OxidationIncrease in oxidation number, loss of electrons, gain of oxygen, or loss of hydrogen
ReductionDecrease in oxidation number, gain of electrons, loss of oxygen, or gain of hydrogen
Redox reactionA reaction where both oxidation and reduction occur simultaneously
02
Oxidation Number and Its Rules
Oxidation numbers help track electron transfers in redox reactions. The rules for assigning oxidation numbers include: elements in their free state have an oxidation number of zero, monatomic ions have an oxidation number equal to their charge, and oxygen typically has an oxidation number of -2. Fluorine always has an oxidation number of -1, and hydrogen is usually +1 unless combined with metals. The sum of oxidation numbers in a compound must be zero, and in a polyatomic ion, it must equal the ion's charge.
Oxidation number rules for elements, ions, and compounds
Common oxidation numbers for oxygen, hydrogen, and fluorine
Calculating oxidation numbers for complex compounds and ions
03
Redox Reactions: Types and Classification
Redox reactions can be classified into four main types: combination, decomposition, displacement, and disproportionation. Combination reactions involve the combination of two substances to form a single product. Decomposition reactions involve the breakdown of a single substance into two or more products. Displacement reactions involve the displacement of one element from a compound by another. Displacement reactions are further divided into metal displacement and non-metal displacement. Disproportionation reactions involve an element in an intermediate oxidation state being simultaneously oxidised and reduced.
Combination reactionsA + B → C
Decomposition reactionsAB → A + B
Displacement reactionsA + BC → AC + B
Disproportionation reactions2A → A + A
04
Balancing Redox Equations: Methods and Techniques
Balancing redox equations involves ensuring the same number of atoms and charges on both sides. The oxidation number method involves identifying changes in oxidation numbers and balancing the increase and decrease. The half-reaction method splits the reaction into oxidation and reduction half-reactions, balances each, and then combines them. Both methods can be used in acidic or basic solutions, with adjustments made for H+ and OH- ions.
Oxidation number methodIdentify oxidation number changes and balance them
Half-reaction methodSplit into oxidation and reduction half-reactions and balance each
Adjusting for acidic or basic solutions
05
Applications of Redox Reactions in Daily Life
Redox reactions have numerous practical applications. They are essential in electrochemical cells, such as batteries, where redox reactions generate electrical energy. Redox reactions are also crucial in corrosion processes, where metals react with oxygen and water to form oxides. In biological systems, redox reactions are involved in processes like photosynthesis and respiration. Industrial applications include the extraction of metals from their ores and the manufacture of chemicals.
Electrochemical cells and batteries
Corrosion of metals
Biological processes like photosynthesis and respiration
Industrial applications in metal extraction and chemical manufacturing
Want the complete chapter resources?Topic notes, quizzes and flashcards for Redox Reactions.
Which of the following statements best defines oxidation in modern chemistry?
ALoss of oxygen from a substance
BLoss of electrons from a substance
CGain of hydrogen by a substance
DIncrease in mass of a substance
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Answer: (B) Loss of electrons from a substance
In modern chemistry, oxidation is defined as the loss of electrons from a substance, regardless of whether oxygen is involved. This definition applies to all redox reactions, not just those involving oxygen.
Question 02
What is the oxidation number of oxygen in most of its compounds?
A+2
B-2
C+1
D-1
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Answer: (B) -2
Oxygen has an oxidation number of -2 in most compounds because it is highly electronegative and gains 2 electrons to complete its octet.
Question 03
Which of the following is a redox reaction?
ACaCO₃ → CaO + CO₂
B2Na + Cl₂ → 2NaCl
CHCl + NaOH → NaCl + H₂O
DAgNO₃ + NaCl → AgCl + NaNO₃
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Answer: (B) 2Na + Cl₂ → 2NaCl
In 2Na + Cl₂ → 2NaCl, sodium is oxidized from 0 to +1 and chlorine is reduced from 0 to -1, making it a redox reaction. The other reactions are decomposition, neutralization, and precipitation reactions respectively.
Question 04
In the oxidation number method for balancing redox equations, what is the first step?
AAssign oxidation numbers to all elements in the equation
BIdentify the oxidizing and reducing agents
CBalance the equation by inspection
DAdd water molecules to balance oxygen
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Answer: (A) Assign oxidation numbers to all elements in the equation
The oxidation number method begins by assigning oxidation numbers to all elements to identify which species undergo oxidation and reduction. This foundational step enables all subsequent balancing steps.
Question 05
Which of the following is an example of a redox reaction used in daily life?
ABurning of fuel in vehicles
BDissolution of salt in water
CMelting of ice
DMixing of oil and water
Show answer
Answer: (A) Burning of fuel in vehicles
Burning of fuel involves oxidation of hydrocarbons and reduction of oxygen, making it a redox reaction. The other options are physical processes, not chemical redox reactions.
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Q1. Define oxidation and reduction in terms of electron transfer. Explain how these two processes are always linked in a redox reaction.
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Model answer
Oxidation is the loss of electrons from an atom, ion, or molecule, while reduction is the gain of electrons. These processes are always linked because in any redox reaction, the electrons lost by one substance must be gained by another. The substance that loses electrons is oxidized (oxidizing agent), and the substance that gains electrons is reduced (reducing agent). They occur simultaneously, making redox reactions inseparable processes.
Sample question3 marks
Q2. Define oxidation number and state any three rules used to assign oxidation numbers to atoms in a compound or ion.
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Model answer
Oxidation number is a number assigned to an element in a chemical compound that represents the number of electrons lost or gained by an atom of that element in the compound. Three important rules are: (1) The oxidation number of an element in its elemental form is zero. (2) The oxidation number of a monatomic ion equals the charge on the ion. (3) Oxygen has an oxidation number of -2 in most compounds, except in peroxides where it is -1. (4) The algebraic sum of oxidation numbers of all atoms in a neutral compound is zero, and in a polyatomic ion, it equals the charge on the ion.
Sample question3 marks
Q3. Define redox reactions and explain how oxidation and reduction occur simultaneously in such reactions. Give one example to illustrate your answer.
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Model answer
A redox reaction is a reaction in which both oxidation and reduction occur at the same time. Oxidation is the loss of electrons (or gain of oxygen/loss of hydrogen), while reduction is the gain of electrons (or loss of oxygen/gain of hydrogen). These two processes always occur together because electrons lost by one species must be gained by another. For example, in the reaction between zinc and copper sulfate, zinc is oxidized (loses 2 electrons) and copper ions are reduced (gain 2 electrons), producing zinc sulfate and copper metal.
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Q4. What is the oxidation number method for balancing redox equations? Explain its main steps briefly.
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Model answer
The oxidation number method is used to balance redox equations by tracking changes in oxidation states of elements. The main steps are: first, assign oxidation numbers to all atoms in the equation; second, identify which elements are oxidized (increase in oxidation number) and which are reduced (decrease in oxidation number); third, balance the increase and decrease in oxidation numbers by adjusting coefficients so that total electrons lost equals electrons gained. This ensures both mass and charge are balanced in the final equation.
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Q5. Explain how redox reactions are used in the process of electroplating. Why is electroplating important in daily life?
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Model answer
Electroplating involves a redox reaction where a metal object (cathode) is coated with another metal by passing electric current through an electrolyte solution. At the cathode, reduction occurs as metal ions gain electrons and deposit on the surface, while at the anode, oxidation occurs as the coating metal loses electrons. Electroplating is important in daily life because it provides corrosion resistance, improves appearance of objects like jewelry and utensils, and extends the life of metal items by protecting the underlying metal from rust.
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What is the difference between oxidation and reduction?
Oxidation is a process where a substance loses electrons, resulting in an increase in oxidation number. Reduction is the gain of electrons, leading to a decrease in oxidation number. Both processes occur simultaneously in a redox reaction.
How do you assign oxidation numbers to elements in a compound?
Oxidation numbers are assigned using rules such as: elements in their free state have an oxidation number of zero, monatomic ions have an oxidation number equal to their charge, and the sum of oxidation numbers in a compound must be zero.
What are the types of redox reactions?
Redox reactions are classified into combination, decomposition, displacement, and disproportionation reactions. Each type involves different processes of electron transfer between substances.
How do you balance a redox equation?
Redox equations can be balanced using the oxidation number method or the half-reaction method. The oxidation number method involves identifying changes in oxidation numbers and balancing them, while the half-reaction method splits the reaction into oxidation and reduction half-reactions and balances each.
What are some practical applications of redox reactions?
Redox reactions have applications in electrochemical cells, corrosion, biological processes like photosynthesis and respiration, and industrial manufacturing processes such as metal extraction and chemical production.
Why are redox reactions important in chemistry?
Redox reactions are fundamental in understanding chemical processes, including energy production, biological functions, and material synthesis. They are crucial for studying electrode potentials, electrochemical processes, and various industrial applications.