Thermodynamics 11 Class Notes Fighter JEE 2025.pdf

FIGHTER JEE 2025 PHYSICAL CHEMISTRY Lecture - 11 THERMODYNAMICS

Instructor: Mohan Sir (CMS Sir)

Platform: Physics Wallah

Topics to be Covered

  1. Thermochemistry

    • Study of heat changes that occur during chemical reactions and changes of state.

    • Importance in understanding energy transfers in physical and chemical processes.

  2. Enthalpy of Formation

    • Defines the heat change when one mole of a compound is formed from its elements in their standard states.

    • Essential for calculating reaction enthalpies using Hess's Law.

  3. Enthalpy of Neutralization

    • Specific enthalpy change when one equivalent of an acid reacts with one equivalent of a base to form water and a salt.

    • Usually negative for strong acids and bases indicating exothermic nature.

  4. Enthalpy of Combustion

    • Measures the heat released when a compound undergoes complete combustion in oxygen.

    • Important for energy content calculations in fuels.

Determining Spontaneity of a Process

Conditions for Spontaneous Processes:

  1. ΔH > 0 and ΔS < 0 - Non-spontaneous at all temperatures.

  2. ΔH < 0 and ΔS > 0 - Always spontaneous (exothermic processes).

  3. ΔH < 0 and ΔS < 0 - Spontaneous at low temperatures (exothermic but with a decrease in disorder).

  4. ΔH > 0 and ΔS > 0 - Spontaneous at high temperatures (endothermic processes with an increase in disorder).

Temperature for Spontaneity

Example Calculation: Given: ΔH = 200 J/mol, ΔS = 40 J/K·mol To find Minimum Temperature (T) for spontaneity:

ΔG = ΔH - TΔS For spontaneity, ΔG must be less than 0.

Choices for Minimum Temperature:

  • (a) 20K

  • (b) 4K

  • (c) 5K

  • (d) 12K

Enthalpy Changes in Reactions

Reaction Representation:

  • A general reaction can be represented as:

aA + bB ⇌ cC + dD

  • ΔH: Total enthalpy change during the reaction can be either positive (endothermic) or negative (exothermic).

Types of Reactions

  1. Exothermic Reactions

    • Heat is released to the surroundings, indicated by ΔH < 0.

    • Examples: Combustion (e.g., burning of hydrocarbons), Neutralization reactions.

  2. Endothermic Reactions

    • Heat is absorbed from the surroundings, indicated by ΔH > 0.

    • Examples include photosynthesis and certain dissolution processes.

Factors Affecting ΔH of Reactions

  • Reaction Conditions:

    • Most reactions are conducted under constant temperature and pressure or volume.

    • Under constant T and P, we have.

    • Enthalpy of Reaction (ΔH) is a key indicator of heat exchange during chemical processes.

Physical State of Reactants and Products

  • The enthalpy change for reactions can vary significantly depending on the physical state of the reactants and products:

    • Example: H₂(g) + O₂(g) → H₂O(l)

    • The physical state (gas vs liquid) influences ΔH values due to the intermolecular forces involved.

Modes of Representation of Reactions

  • Notation and Enthalpy Changes:

    • Different representations can indicate ΔH changes based on various states and conditions of reactants.

    • Kirchoff's Equation: Shows relationship between temperature changes and enthalpic changes in reactions.

Standard Enthalpy of Reaction

  • Defined as the enthalpy change for a reaction when all reactants and products are in their standard states (1 bar pressure for gases and specific concentrations for solutions).

Types of Enthalpy

  1. Enthalpy of Formation:

    • Significant for determining stability and reactivity as it measures energy change when forming one mole of a compound from its elements.

    • Reference states for elements are defined specifically for thermodynamic calculations.

Example of Enthalpy Calculation

  • The calculation method of enthalpy changes using standard values:

    • Formula: ΔH = {(ΣΔH products) - (ΣΔH reactants)}

Application of the Enthalpy of Formation

  • Includes practical examples to calculate enthalpy changes in specific reactions, example:

    • N₂(g) + 3H₂(g) → 2NH₃(g)

Heat Calculation at Constant Pressure

  • An example of calculating the overall heat of formation:

    • Reaction: F₂O(g) + H₂O(g) → O₂(g) + 2HF(g)

    • Demonstrates the heat required defined by the heat values of reactants.

Heat Requirement for Reaction Formation

  • Calculation example for determining the heat for forming a specific mass of a compound:

    • Example: 1.28 kg of CaC₂ requiring 2240 kcal, including reactant values for CaO, CaC₂, and CO.

Enthalpy of Neutralization

  • Defining the enthalpy change associated with neutralizing a defined amount of acid and base, typically one equivalent:

    • The example discusses diverse strong acid/base reactions leading to salt and water.

Example of Neutralization Reaction

  • Reaction Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

    • Standard values of enthalpy change for the reaction are noted for strong acids and bases evaluated.

Homework Assignment

  • DPP-08 homework assigned to reinforce topics covered in this lecture.

Conclusion

  • Closure and acknowledgment of participants for engaging in the lecture, encouraging further exploration of thermodynamic principles.