Lecture Notes on Thermodynamics

Overview

• Today's session focuses on an important chapter of physical chemistry: Thermodynamics.
• We will cover the concepts of internal energy, enthalpy, and their applications in chemical reactions.

Key Concepts

Internal Energy (U)

• Internal energy at temperature T1 is denoted as U1.
• 90 grams of water corresponds to 5 moles.
• If a system releases heat, its stability increases.

Enthalpy of Reaction

• Enthalpy of reaction is calculated using the formula:

  [ \Delta H_{reaction} = \Delta H_{products} - \Delta H_{reactants} ]_

Thermodynamics Basics

• Understanding stability and entropy is crucial in physical chemistry.
• Entropy (S) is a measure of disorder; for a reaction:
  • [ \Delta S_{reaction} = S_{products} - S_{reactants} ]
• Important to note:
  • Higher temperature generally leads to higher entropy.
  • At absolute zero (0 K), the entropy of a perfect crystalline structure is zero._

Types of Thermodynamic Processes

1. Isothermal Process:

   • Temperature remains constant.
   • [ W = - P_{external} \Delta V ]
   • [ \Delta S = \frac{Q}{T} ] for heat transfer.

2. Adiabatic Process:

   • No heat exchange with surroundings.
   • [ \Delta U = W ]
   • The work done is equal to the change in internal energy.

3. Isobaric Process:

   • Pressure remains constant.
   • [ W = P \Delta V ]
   • [ \Delta S = \frac{Q}{T} ] for heat transfer.

4. Isochoric Process:

   • Volume remains constant.
   • No work done, [ W = 0 ]
   • [ \Delta U = Q ]_

Gibbs Free Energy (G)

• Gibbs free energy is defined as:
  • [ G = H - TS ]
• For spontaneous processes, [ \Delta G < 0 ] (negative change).

Calculating Changes

• To find the change in internal energy, enthalpy, or Gibbs energy for reactions, the following is used:
  • Hess's Law:
    • The enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction.

Examples and Applications

• Example 1: Calculating the enthalpy change for combustion reactions.
• Example 2: Calculating Gibbs free energy for reactions at constant temperature and pressure.

Important Formulas

• For Gibbs Energy Change:

  [ \Delta G = \Delta H - T \Delta S ]

• Entropy Changes:

  • For reactions, use:

  [ \Delta S = S_{products} - S_{reactants} ]

Conclusion

• Understanding these concepts is crucial for mastering the subject of thermodynamics in chemistry.
• Regular practice of problems related to these concepts will enhance understanding and retention.