Notes from Toppers

Chemical Thermodynamics Notes

1. Laws of Thermodynamics:

  • Zeroth law of thermodynamics (NCERT Class 11, Chapter 12): Defines thermal equilibrium as a state where no net flow of heat occurs between two systems in contact.

  • First law of thermodynamics (NCERT Class 11, Chapter 12): States that energy can neither be created nor destroyed but can only be transferred from one form to another. Mathematically, it can be expressed as:

    ΔU = q + w

where ΔU is the change in internal energy, q is the heat absorbed by the system, and w is the work done by the system.

  • Second law of thermodynamics (NCERT Class 12, Chapter 6): Describes the concept of entropy and spontaneity. It states that the total entropy of an isolated system always increases over time, and processes occur in the direction of increasing disorder.

2. Thermodynamic Systems and Processes:

  • Types of systems:
  • Open system: Exchanges both energy and matter with the surroundings.
  • Closed system: Exchanges energy but not matter with the surroundings.
  • Isolated system: Does not exchange any energy or matter with the surroundings.
  • Types of processes:
  • Isobaric: The pressure remains constant.
  • Isochoric: The volume remains constant.
  • Isothermal: The temperature remains constant.
  • Adiabatic: No heat is exchanged with the surroundings.

3. Heat and Work:

  • Forms of heat transfer:

  • Conduction: Heat transfer through direct contact between objects. -Convection: Heat transfer through the movement of fluids (liquid or gas). -Radiation: Heat transfer through electromagnetic waves.

  • Types of work (NCERT Class 11, Chapter 12):

  • Pressure-volume work (PV work): Work done by a system when it expands against an external pressure. It can be calculated as:

    w = -PΔV

where P is the pressure and ΔV is the change in volume.

4. State Functions and Equations of State:

  • State functions: Quantities that depend only on the current state of the system and not on the path taken to reach that state. Examples include internal energy (U), enthalpy (H), free energy (G), and entropy (S).
  • Equations of state (NCERT Class 11, Chapter 12): Mathematical equations that relate the pressure, volume, and temperature of a substance. Common equations of state include the ideal gas equation (PV = nRT) and the van der Waals equation ( [P + {n^2a \over V^2} ) ( V- nb ) = nRT )

5. Heat Capacities and Enthalpies:

  • Molar heat capacities (NCERT Class 12, Chapter 6):
  • Cp: Molar heat capacity at constant pressure.
  • Cv: Molar heat capacity at constant volume.
  • Standard enthalpies (NCERT Class 11, Chapter 12):
  • Standard enthalpy of formation: Enthalpy change when one mole of a substance is formed from its constituent elements in their standard states.
  • Standard enthalpy of reaction: Enthalpy change when one mole of reactants are completely converted to products in their standard states.
  • Standard enthalpy of combustion: Enthalpy change when one mole of a substance undergoes complete combustion with oxygen.

6. Entropy:

  • Microscopic and macroscopic interpretations: Microscopic interpretation of entropy involves the counting of microstates, while the macroscopic interpretation relates entropy to disorder or randomness.
  • Entropy changes in processes (NCERT Class 12, Chapter 6):
  • Entropy increases for processes such as melting, vaporization, mixing, and irreversible processes.
  • Entropy decreases for processes like freezing, condensation, and crystallization.
  • Third law of thermodynamics: As the temperature approaches absolute zero, the entropy of a perfect crystal approaches zero.

7. Gibbs Free Energy and Chemical Equilibrium:

  • Definition and significance: Gibbs free energy (G) combines enthalpy and entropy to determine the spontaneity of a process.
    ΔG = ΔH - TΔS

where ΔG is the change in Gibbs free energy, ΔH is the enthalpy change, ΔS is the entropy change, and T is the absolute temperature.

  • Criteria for spontaneity:
  • ΔG < 0: Spontaneous process.
  • ΔG > 0: Non-spontaneous process.
  • ΔG = 0: Equilibrium.

8. Solutions:

  • Thermodynamics of mixing (NCERT Class 12, Chapter 6):

  • Mixing involves an increase in disorder leading to a positive entropy change.

  • The enthalpy of mixing can be positive (endothermic) or negative (exothermic) depending on the interactions between the components.

  • Ideal and non-ideal solutions: Ideal solutions obey Raoult’s law and have zero enthalpy of mixing, while non-ideal solutions deviate from Raoult’s law and have non-zero enthalpy of mixing.

  • Colligative properties (NCERT Class 12, Chapter 6):

  • Colligative properties depend on the concentration of solute particles and not on their identity.

  • Boiling point elevation, freezing point depression, and osmotic pressure are colligative properties.

9. Phase Equilibria:

  • Phase diagrams: Graphical representations showing the conditions (temperature and pressure) at which different phases (solid, liquid, and gas) coexist in equilibrium.
  • Phase transitions: Changes in the phase of a substance, such as melting, freezing, and vaporization.
  • Applications of phase diagrams:
    • Determining melting points and boiling points.
    • Understanding phase behavior of mixtures (e.g., alloys, salt solutions).
    • Predicting the conditions for phase changes in industrial processes.

10. Chemical Reactions and Thermodynamics:

  • Thermodynamic favorability of reactions (NCERT Class 12, Chapter 6):
  • For a reaction to be thermodynamically favorable, it should have a negative Gibbs free energy change (ΔG < 0).
  • Equilibrium constant (K): The ratio of product concentrations to reactant concentrations at equilibrium.
  • Relationship between equilibrium constant and Gibbs free energy:
  • ( \Delta G^o = -RT \ln K )
  • ( K = e^{- \Delta G^o / RT } ) where ΔG° is the standard Gibbs free energy change, R is the ideal gas constant, and T is the absolute temperature.

11. Electrochemistry:

  • Redox reactions: Chemical reactions involving transfer of electrons between species.

  • Electrode potentials (NCERT Class 12, Chapter 3):

  • Potential difference between an electrode and a reference electrode when immersed in a solution containing its ions.

  • Standard electrode potential: Electrode potential measured under standard conditions (1 atm pressure, 1 M concentration, 298 K temperature).

  • Nernst equation (NCERT Class 12, Chapter 3): Relates the electrode potential to the concentration of reactants and products in an electrochemical reaction. ( E = E^o - \frac{ RT }{ nF } \ln Q) E: electrode potential under non-standard conditions (E^0 ): Standard electrode potential R: ideal gas constant ( 8.314 J(K^{-1} mol^{-1}) ) T: temperature (Kelvin) n: Number of moles of electrons transferred in the electrode reaction F: Faraday constant ( ( 96,485 \ C\ mol^{-1} ) ) Q: Reaction quotient

  • Electrochemical cells: Devices that convert chemical energy into electrical energy (galvanic cells) or electrical energy into chemical energy (electrolytic cells).

12. Statistical Thermodynamics:

  • Introduction: Applies statistics to understand the macroscopic properties of a system by considering the microscopic behavior of its particles.
  • Microstates and macrostates:
  • Microstates: Specific configurations of particles within a system.
  • Macrostates: Collection of microstates with similar properties.
  • Probability: The likelihood of a particular microstate occurring.
  • Partition function (NCERT Class 12, Chapter 14): Sum of the Boltzmann factors for all microstates of a system.
  • Relationship to thermodynamic properties:
  • Internal energy (U): Average energy of all microstates.
  • Entropy (S): Measure of the randomness or disorder of microstates.
  • Free energy (G): Measure of the spontaneity or equilibrium state of a system.

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