Electrochemistry And Chemical Kinetics (LAQs)

Chemistry-2 | 3. Electrochemistry and Chemical Kinetics – LAQs:
Welcome to LAQs in Chapter 3: Electrochemistry and Chemical Kinetics. This page includes the key FAQs for Long Answer Questions. Answers are provided in simple English, with a Telugu explanation, and formatted according to the exam style. This will help in thorough preparation and securing top marks in your final exams.

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LAQ-1 : State and explain Kohlrausch’s law of independent migration of ions.

Introduction

Kohlrausch’s Law is a cornerstone principle in the realm of electrolytes, shedding light on the conductivity behaviors of ionic solutions and the individual migration patterns of ions.

Understanding Kohlrausch’s Law

1. Definition: Kohlrausch’s Law asserts that the limiting molar conductivity (Λ_m⁰​) of an electrolyte at infinite dilution equals the sum of the individual molar conductivities of its cations (λ₊⁰​) and anions (λ₋^0​).
2. Formula Representation: $$\Lambda_m^0 = \lambda_+^0 + \lambda_-^0$$
   Here, Λ_m^0​ denotes the limiting molar conductivity, λ₊⁰​ the molar conductivity of the cation, and λ₋⁰​ the molar conductivity of the anion.
3. Example: For NaCl, the relationship is illustrated as: $$\Lambda_{(NaCl)}^0 = \lambda_{(Na^+)}^0 + \lambda_{(Cl^-)}^0$$
   This equation demonstrates that the total conductivity of a NaCl solution is the aggregate of the conductivities of sodium (Na⁺) and chloride (Cl⁻) ions.

Applications of Kohlrausch’s Law

1. Finding Conductivities: The law facilitates the determination of limiting molar conductivities for both strong and weak electrolytes.
2. Determining Degree of Dissociation: It enables the calculation of the degree of dissociation (�α) for weak electrolytes using: $$\alpha = \frac{\Lambda_m}{\Lambda_m^0}​​$$
   where α signifies the degree of dissociation.
3. Calculating Dissociation Constant: Kohlrausch’s Law is instrumental in deriving the dissociation constant (K) of weak electrolytes: $$K = \frac{C\alpha^2}{1-\alpha}$$
   K represents the dissociation constant, providing insight into the dissociation behavior of weak electrolytes in solution.

Summary

Kohlrausch’s Law illuminates the intricate dynamics of ion migration in electrolyte solutions, positing that the overall conductivity is the cumulative contribution of individual ions. This foundational concept in solution chemistry has broad applications, from calculating conductivities to assessing dissociation characteristics of electrolytes, making it a pivotal study area for students and professionals in the field.

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LAQ-2 : What are Galvanic cells? Explain the working of Galvanic cell with a neat sketch taking Daniel cell as example.

Introduction

Galvanic cells are pivotal in electrochemistry, converting chemical energy from spontaneous reactions into electrical energy. These cells harness the energy from redox reactions to generate electricity.

What is a Galvanic Cell?

Definition: A Galvanic cell is an electrochemical cell that produces electrical energy through spontaneous redox reactions.

Construction of a Daniel Cell

1. Two Half Cells: The Daniel Cell, a classic Galvanic cell example, comprises two distinct half cells: the anode and the cathode half cells.
2. Salt Bridge: A salt bridge connects these half cells, containing a saturated KNO₃ solution in agar-agar gel.
3. Anode: Features a zinc rod in a ZnSO₄ solution.
4. Cathode: Includes a copper rod in a CuSO₄ solution.
5. Representation: $$\text{Zn(s)}/\text{Zn}^{2+}(\text{aq}) \, || \, \text{Cu}^{2+}(\text{aq})/\text{Cu(s)}$$

Working of a Daniel Cell

1. Initiation: When the two half cells are connected externally (e.g., via a voltmeter), electrical current flows due to the potential difference between them.
2. Chemical Reactions:
   • At the Anode (Oxidation): Zinc is oxidized, releasing Zn²⁺ ions and two electrons: $$\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$$
   • At the Cathode (Reduction): Cu²⁺ ions accept electrons and get reduced to solid copper: $$\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$$
   • Net Reaction: The overall cell reaction is: $$\text{Zn(s)} + \text{Cu}^{2+}(\text{aq}) \rightarrow \text{Zn}^{2+}(\text{aq}) + \text{Cu(s)}$$
     This demonstrates the interaction between zinc metal and copper ions, resulting in zinc ions and copper metal.

Summary

Galvanic cells, exemplified by the Daniel Cell, are foundational in transforming chemical into electrical energy. Operating on redox reactions, these cells enable the flow of electrons from the oxidized metal at the anode to the reduced metal at the cathode, generating an electric current. This process effectively illustrates the conversion of chemical energy into usable electrical energy.

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LAQ-3 : Give a detailed account of Collision theory of reaction rates of bimolecular gaseous reactions.

Introduction

The Collision Theory provides a framework for understanding the reaction rates of bimolecular gaseous reactions, focusing on the dynamics of molecular collisions.

Collision Theory Explained

1. Basic Premise: The theory integrates the kinetic theory of gases, proposing that reactions occur through collisions between reactant molecules.
2. Not All Collisions are Effective: A key tenet of the Collision Theory is that not all molecular collisions result in a reaction. Only collisions meeting specific energy and orientation criteria lead to product formation.
3. Importance of Orientation: For a reaction to be successful, colliding molecules must align with the correct orientation. Incorrect alignment, even if the energy criterion is met, may not trigger a reaction.
4. Threshold Energy: Reactant molecules must possess a minimum energy level, known as ‘Threshold energy’ (E_T​), to undergo a reaction.
5. Activated Molecules: Molecules meeting the threshold energy condition are termed activated molecules.
6. Activation Energy: Activation energy (E_a​) is pivotal in Collision Theory, representing the energy difference between the threshold energy (E_T​) and the average energy of reactant molecules (E_R​). It delineates the energy barrier that must be overcome for a reaction to proceed.
7. Successful Collisions: Activated collisions are those with the appropriate orientation and energy exceeding the activation energy, leading to product formation.
8. Collision Frequency: The collision frequency (Z) reflects the number of collisions per unit time, influenced by factors such as collision diameter (σAB​), reduced mass of molecules (μ), and the number densities of molecules (n_A​ and n_B​).
9. Rate of Reaction: The rate of a reaction (K) is contingent on collision frequency and energy, encapsulated in the formula: $$K = A \cdot e^{-E_a/RT}$$
   where A is the pre-exponential factor, R is the universal gas constant, and T is the temperature.

Summary

Collision Theory elucidates the microscopic interactions underpinning bimolecular gaseous reactions, underscoring the significance of energy, orientation, and collision frequency in determining reaction rates. This theoretical model aids in comprehending why certain reactions occur more rapidly than others and how reaction conditions can affect these rates, offering essential insights into the kinetics of chemical reactions.

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LAQ-4 : What is molecularity of a reaction? How is it different from the order of a reaction? Name one bimolecular and one trimolecular gaseous reaction.

Introduction

Understanding chemical reactions requires familiarity with key concepts such as ‘molecularity’ and ‘order of a reaction.’ These terms offer insights into reactant behavior and are instrumental in predicting reaction outcomes.

Understanding Molecularity

Molecularity refers to the number of molecules, atoms, or ions involved in the rate-determining step of a chemical reaction.

Understanding Order of a Reaction

The order of a reaction indicates the relationship between reactant concentrations and the reaction rate, as expressed in the rate equation.

Contrasting Molecularity and Order

1. Nature:
   • Molecularity is inherently a whole number, reflecting the precise count of reactants in the rate-determining step.
   • Order of a reaction can be whole, fractional, or zero, based on the rate equation’s concentration terms.
2. Determination:
   • Molecularity is inferred from the reaction mechanism.
   • Order is experimentally determined using concentration data.
3. Applicability:
   • Molecularity applies solely to elementary reactions.
   • Order pertains to both elementary and complex reactions.

Examples of Molecularity

1. Bimolecular Reaction: The dissociation of hydrogen iodide $$2HI \rightarrow H_2 + I_2$$​ exemplifies a bimolecular reaction.
2. Trimolecular Reaction: The formation of nitrogen dioxide $$2NO + O_2 \rightarrow 2NO_2$$​ is a trimolecular process.

Summary

Molecularity and order of a reaction serve to elucidate reaction dynamics from distinct vantage points: molecularity from the standpoint of reactant participation in the rate-determining step, and order through the lens of concentration’s influence on rate. Recognizing their nuances aids in accurately predicting and comprehending the complexities of chemical reactions.