Resonance Chemistry Equivalent Concept & Titrations

 Resonance in Chemistry & Equivalent Concept in Titrations

 Resonance in Chemistry

Resonance is a concept in chemistry where a molecule or ion can be represented by two or more Lewis structures, which differ only in the position of electrons but have the same atomic arrangement.

Example: Benzene (C₆H₆)

• Benzene has two equivalent resonance structures, where the double bonds shift positions.
• The actual structure is a hybrid of these resonance forms, making benzene more stable than expected from a single Lewis structure.

 Key Points About Resonance:

Delocalization of electrons occurs, increasing stability.
 The actual structure is a resonance hybrid, not a flipping between structures.
More resonance structures = More stability (e.g., carbonate ion CO₃²⁻ has three equivalent resonance forms).

 Equivalent Concept in Titration (Chemistry)

The Equivalent Concept is crucial in acid-base titrations, redox reactions, and precipitation reactions. It helps in simplifying complex calculations involving molarity, normality, and titration reactions.

 1. Equivalent Weight Formula:

Equivalent weight=Molecular weightn-factor\text{Equivalent weight} = \frac{\text{Molecular weight}}{\text{n-factor}}Equivalent weight=n-factorMolecular weight​

Where n-factor depends on the type of reaction:
Acid-Base Titration → n-factor = Number of H⁺ or OH⁻ ions exchanged
Redox Reactions → n-factor = Number of electrons exchanged

 2. Normality (N) Formula:

Normality=Moles of solute×n-factorVolume of solution in Liters\text{Normality} = \frac{\text{Moles of solute} \times \text{n-factor}}{\text{Volume of solution in Liters}}Normality=Volume of solution in LitersMoles of solute×n-factor​

Or

$$N=M\times n(\text{Normality}=\text{Molarity}\times \text{n-factor})$$

 Acid-Base Titration Example:

In the reaction:

$$\text{H₂SO₄}+2\text{NaOH}\to \text{Na₂SO₄}+2\text{H₂O}$$

H₂SO₄ (Sulfuric Acid) has n-factor = 2 (it gives 2 H⁺ ions).
NaOH has n-factor = 1 (it gives 1 OH⁻ ion).

Using the Normality Equation:

N1V1=N2V2N_1V_1 = N_2V_2N1​V1​=N2​V2​

Where:

• N1,V1N_1, V_1N1​,V1​ = Normality & Volume of Acid
• N2,V2N_2, V_2N2​,V2​ = Normality & Volume of Base

This helps in finding unknown concentrations in titrations.

 Redox Titration Example:

For the reaction:

$$\text{KMnO₄}+5\text{Fe²⁺}+8\text{H⁺}\to \text{Mn²⁺}+5\text{Fe³⁺}+4\text{H₂O}$$

KMnO₄ (Potassium Permanganate) has n-factor = 5 (Mn⁷⁺ → Mn²⁺, gaining 5 electrons).
Fe²⁺ to Fe³⁺ has n-factor = 1 (losing 1 electron).

Here, we apply:

N1V1=N2V2N_1V_1 = N_2V_2N1​V1​=N2​V2​

to determine the concentration of Fe²⁺ solution.

 Key Takeaways:

Resonance increases stability by electron delocalization.
Equivalent weight = Molecular weight / n-factor.
Normality (N) = Molarity (M) × n-factor.
Titration formula: N1V1=N2V2N_1V_1 = N_2V_2N1​V1​=N2​V2​ helps find unknown concentrations.

Let me know if you need more examples!

Resonance Chemistry Equivalent Concept & Titrations

Some Basic Concepts of Chemistry

CONTENT Download

Redox-Reaction-Notes.pdf

Here is a student-friendly guide on two important topics from Chemistry:

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Resonance Chemistry (Equivalent Concept) & Titrations

For: Class 11–12, NEET, JEE, and Competitive Exams

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PART 1: Resonance Chemistry – Equivalent Concept

What is Resonance?

• Resonance is a way to describe molecules where one structure is not enough to explain the bonding.
• In such cases, multiple structures (called resonance structures) are written, and the actual structure is a hybrid of all.

Example: Benzene (C₆H₆)

• Benzene has alternating double and single bonds.
• Two main structures (Kekulé structures) exist:

  Structure 1      Structure 2
 C=C–C=C–C=C  ⇄  C–C=C–C=C–C

• The real structure is a resonance hybrid with equal bond lengths between all carbon atoms.

Resonance Rules:

1. Only π electrons and lone pairs move.
2. σ bonds (single bonds) never move.
3. All resonance structures must be valid Lewis structures.
4. The hybrid is more stable than individual contributors.

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What is the Equivalent Concept in Chemistry?

The Equivalent Concept is used in acid-base and redox reactions and titrations to relate substances based on the amount that reacts.

Equivalent =

The amount of a substance that reacts with or supplies 1 mole of H⁺ ions (acid)
or OH⁻ ions (base) or 1 mole of electrons (in redox reactions).

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Equivalent Mass Formulae

Substance Type	Formula
Acid	Molar Mass / Basicity
Base	Molar Mass / Acidity
Salt	Molar Mass / Total positive or negative charge
Oxidizing/Reducing Agent	Molar Mass / n-factor

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PART 2: Titrations – Volumetric Analysis

What is Titration?

Titration is a laboratory method to determine the unknown concentration of a solution by reacting it with a known volume/concentration of another solution.

Basic Formula:

N₁V₁ = N₂V₂

Where:

• N₁, V₁ = Normality & Volume of Solution 1
• N₂, V₂ = Normality & Volume of Solution 2

You can also use:

M₁V₁ / n₁ = M₂V₂ / n₂

Where:

• M = Molarity
• n = Number of H⁺ or OH⁻ or electrons exchanged

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Types of Titrations:

Type	Example	Indicator
Acid–Base	HCl vs NaOH	Phenolphthalein
Redox	KMnO₄ vs FeSO₄	Self-indicator
Complexometric	EDTA vs Metal ions (Ca²⁺, Mg²⁺)	Eriochrome Black T
Precipitation	AgNO₃ vs NaCl	Potassium chromate

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Indicators Summary:

Indicator	Color in Acid	Color in Base
Phenolphthalein	Colorless	Pink
Methyl Orange	Red	Yellow

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Quick Practice Question:

Q: 20 mL of 0.1 N HCl is completely neutralized by 25 mL of NaOH. Find the Normality of NaOH.

A:
Use N₁V₁ = N₂V₂
→ (0.1)(20) = N₂(25)
→ N₂ = 2 / 25 = 0.08 N

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Summary Chart

Topic	Key Formula / Idea
Resonance	Real molecule = Resonance Hybrid
Equivalent Mass	Molar Mass / n-factor
Acid-Base Titration	N₁V₁ = N₂V₂ or M₁V₁/n₁ = M₂V₂/n₂
Redox Reaction	e⁻ transferred determines n-factor

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Resonance Chemistry Equivalent Concept & Titrations

EQUIVALENT CONCEPT AND VOLUMENTRIC ANALYSIS