CBSE Class 11 Chemistry Revision Notes Chapter 8
Class 11 Chemistry Revision Notes for Chapter 8 – Redox Reactions
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Access Revision Notes for Class 11 Chemistry Chapter 8 – Redox Reaction
This chapter intends to ensure that students can identify redox reactions as a class
of reactions in which oxidation and reduction reactions occur simultaneously. Redox refers to an integral area of Chemistry. It is an essential reaction that concerns physical as well as biological phenomena.
The process that involves the addition of oxygen or any electronegative element or the removal of any electropositive element or hydrogen is called oxidation.
The process that involves the addition of hydrogen or any electropositive element or the removal of any electronegative element or oxygen is called reduction.
Since oxidation and reduction always take place simultaneously, the term “redox” was developed to refer to this class of chemical events.
- When an element transitions from its free elemental state to its combined form in molecules, it develops a seeming charge over each atom.
- The oxidation number is determined using a set of rules.
- A more practical way of determining the oxidation number is to keep track of shifts in electrons in a chemical reaction involving the synthesis of compounds that have been created.
- In this procedure, the complete transfer of electrons from a less electronegative atom to a more electronegative atom is always expected.
Rules Governing Oxidation Number
The rules used to determine the oxidation number of elements in different compounds are based on the element’s electronegativity. You can have a better understanding and clarity of concepts by going through the notes provided by Extramarks.
Atom of Fluorine
The most electronegative of all the known elements is Fluorine. It has an oxidation number of –1 in all of its compounds.
Atom of Oxygen
The atoms of oxygen and their oxides have an oxidation number of –2.
The oxidation number of a hydrogen atom is +1 in general. However, it is –1 in the case of metallic hydrides.
All halogen atoms have an oxidation number of –1. If a halogen atom is connected to an atom more electronegative than it, the oxidation number will be positive.
- +1 is the oxidation number of alkali metals.
- +2 is the oxidation number of alkaline earth metals.
- +3 is the oxidation number of aluminium.
- The oxidation number of a metal can also be negative or 0.
- The number of oxidations in an element’s free state or allotropic forms is always zero.
- A molecule’s total number of oxidation states for all of its atoms is zero.
- The sum of the oxidation numbers of all the atoms in the ion is equal to the charge on an ion.
- The oxidation number of an element with group number n in the periodic table can range from (n – 10) to (n – 18). This, however, mostly applies to p-block components.
Calculation of Average Oxidation Number: Solved Examples
Example 1: Q.Find out the oxidation number of underlined element: Na2S2
Ans: Let the oxidation number of S-atoms be y. Now work according to the rules stated before.
Individual Oxidation Number Calculation
It is crucial to understand the structure of any compound before attempting to determine the particular oxidation number of an element inside it.
Individual oxidation number calculation formula:
Oxidation number = Number of electrons in the valence shell – Number of electrons taken up after bonding
Q.Calculate the individual oxidation number of each S-atom in Sodium Thiosulphate (Na2S2O3) with the help of its structure.
Ans: Number of electrons in the valence shell of the middle S-atom = 6
After bonding, the number of electrons left = 0
The oxidation number of central S-atom = 6 − 0 = +6
Number of electrons in the valence shell of terminal S-atom = 6
After bonding, the number of electrons left = 8
The oxidation number of terminal S-atom=
6 − 8 = −2
Now, even the Average Oxidation number of S can be calculated, where, S=[6+(−2)]/2=+2 (as we have calculated before).
Q.To calculate the individual oxidation number we need to take help from the structures of the molecules. Find out the oxidation of Cr in CrO5.
Ans: It is evident that there are two peroxide linkages and one double bond in CrO5 from its structure. The contribution of each peroxide linkage is −2. Now, let the oxidation number of Cr be y.
∴y+(−2)2+(−2)=0 or y=6
∴ The oxidation number of Cr=+6
The paradox of Fractional Oxidation Number
The structural parameters show that the fractional oxidation number represents the average oxidation state of all atoms of the element under study. It also shows that the atoms of the element for which the fractional oxidation state is realised are actually present in different oxidation states. The structure of C3O2, for instance, exhibits similar bonding possibilities. Each element’s element with an asterisk (*) has an oxidation number that is distinct from the rest of the element’s atoms.
Oxidising and Reducing Agent
Oxidising Agent or Oxidant :
Oxidants are substances that oxidise others while reducing themselves during a chemical process. They cause an element’s oxidation number to decrease or cause an element to gain electrons in a redox process.
Reducing Agent or Reductant :
Reductants are substances that can both oxidise and reduce other molecules during a chemical reaction. They increase the oxidation number or cause the element to lose electrons in a redox reaction.
Redox reactions are chemical processes that involve simultaneous oxidation and reduction.
A disproportionation reaction is a redox process in which the same element that is present in a molecule in a particular oxidation state is oxidised and reduced simultaneously.
List of Some Important Disproportionation Reactions
- H2O2 → H2O + O2
- P4 + OH− → PH3 + H2PO2–
- HNO2 → NO + HNO3
Balancing of Redox Reactions
Two conditions must be met by any balanced equation.
- Atom balancing (mass balance): There should be an equal amount of atoms of each species on the reactant and product sides.
- Charge balance: The total of the real charges in the equation must be equal on both sides.
A redox equation can be balanced in the following two ways:
- Number change technique of oxidation
- Half-cell method or ion-electron method
Students are recommended to try the second method (Ion electron method) to balance redox reactions because the first strategy is not especially effective in doing so.
Method of Ion Electrons
Using this technique, redox equations in two different mediums are changed.
- Acidic Medium
- Basic Medium
Balancing in Acidic Medium
For balancing redox reactions, students are advised to use the ion-electron technique in an acidic medium. Refer to Chapter 8 Chemistry Class 11 Notes for understanding this in detail.
Balancing in Basic Medium
Bases dissolve into OH− ions in solution; therefore, balancing redox reactions in basic conditions requires OH−. Refer to Chemistry Chapter 8 Class 11 Notes for understanding this in detail.
Concept of Equivalents:
Equivalent Mass of Element
The number of parts by mass of an element that displaces or reacts from a compound containing 8 parts by mass of oxygen, 1.008 parts by mass of hydrogen, and 35.5 parts by mass of chlorine is known as the equivalent weight of that element.
Equivalent Weight (E)
An equivalent weight, also known as gram equivalent, is the mass of one equivalent, i.e. the mass of a given substance that will combine with or displace a fixed quantity of another substance.
- For a solution, the number of equivalents is equal to N1V1, where N denotes normality and V denotes volume in litres.
- When expressed in grammes, equivalent mass—a pure number—becomes known as gramme equivalent mass.
- The comparable mass of a substance may vary depending on the circumstances.
- The molecular mass will not always be less than the corresponding weight, however, this is not an absolute rule.
- Number of Equivalents = mass of specieseq. wt. of that species
Valency Factor Calculation
The valency factor equals the element’s valence for elements. For acids, the number of hydrogen ions that can be replaced per acid molecule is the valence factor.
Acid – Base Reaction
The actual number of hydrogen or hydroxide ion-exchanged in the reaction is the valence factor in an acid-base reaction. More hydrogen or hydroxide ions may be replaceable in the acid or base than what is actually replaced in the reaction. The number of hydrogen ions replaced by each molecule of the base from the acid is denoted by v. f.
ln Non-Reacting Condition
The total amount of negative or positive charges present in the chemical is known as the valency factor.
Normality is the number of equivalents of solute present in one litre or 1000 millilitres of a solution.
Law of Equivalence
According to the law, one equivalent of one element must combine with one equivalent of another. A chemical reaction results in the same number of equivalents or milliequivalents of products when equivalents or milliequivalents of reactants react in the same proportion.
The process of estimating a solution’s concentration by allowing a precisely measured volume to react with a known-concentration standard solution of another substance is called titrations. The standard solution so used is known as Titrant.
Titrants are of two types:
- Primary Titrants/Standards
- Secondary Titrants/Standards
Indicators are auxiliary materials added to allow physical detection of titration completion. When the titration is finished, the colour of the indicator usually changes.
In both acid and base mediums, Hydrogen Peroxide can act as a reducing agent as well as an oxidising agent.
Hardness of water (Hard water does not give lather with soap)
Calcium and Magnesium bicarbonates cause temporary hardness while calcium and Magnesium chlorides and sulphates cause permanent hardness. The water sample can be softened by boiling or by using washing soda.
Measurement of Hardness
Hardness is measured in terms of Parts Per Million (ppm) of Calcium Carbonate (CaCO3)
or its equivalent.
Q1. A fresh H2O2 solution is labelled 11.2V. It has the same concentration as a solution which is:
- None of these
Explanation: Molarity of H2O2 = vol. strength 11.2=11.211.2=1
Now, %(w/v)= wt. of solute in g wt. of solution in mL×100
= Molarity × Mol. wt. of solute ×110
Q2. Find out the normality of a solution containing 13.4g of sodium oxalate in a 100mL solution.
Ans: Normality = (wt. in g/eq. wt)/vol of solution in a litre
Here, eq. wt. of Na2C2O4=134/2=67
Class 11 Redox Reactions Revision Notes
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Sub-Topics Covered Under Redox Reactions
- Balance Redox Reactions
- Classical Idea of Redox Reactions
- Oxidation Number
- Redox Reactions & Electrode Potential
- Types of Redox Reactions
- Redox Reactions as the Basis of Titrations
- Redox Reactions
Some Important Points to Determine the Oxidation Number
- In an uncharged compound, the algebraic sum of the oxidation numbers of the atoms is zero. The algebraic sum of an ion’s charge is equal to its charge.
- All of the elements’ oxidation numbers in their elementary states are zero.
- Metal in amalgams has no oxidation at all.
Importance of Redox Reactions Revision Notes
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Tips to Use Notes of Chapter 8 Chemistry Class 11
- Students are advised to read the chapter first from the NCERT Chemistry book and highlight the sections they have trouble understanding after having a basic idea of the chapter.
- They must carefully go through the Class 11 Chemistry Chapter 8 Notes to understand the challenging sections identified earlier to avoid any silly mistakes and be confident.
- Students can apply what they have learned by practising the questions that are provided at the end of the chapter in the textbook.
FAQs (Frequently Asked Questions)
1. Explain the different types of Redox Reactions.
The different types of Redox Reactions are:-
- Decomposition Reactions – It is a reaction where a compound breaks into two or more simple substances.
- Combination Reactions – It is a type of reaction where two or more substances (compounds or elements) combine to form a single substance.
- Displacement Reactions – A reaction in which one ion or atom in a compound is replaced by another ion or atom of another element is known as a displacement reaction.
2. Explain some of the applications of redox reactions.
Some of the real-time applications of redox reactions are:
- They are used in the study of electrode processes and cells.
- The redox reactions are used in industrial pharmaceutical, biological, metallurgical, and agricultural areas.
- They are needed in the study of various phenomena such as the “Hydrogen economy” and the ozone hole.
- Used in the manufacture of chemical compounds such as caustic soda, corrosion of metals, operation of wet and dry batteries, etc.