Concept of enthalpy, entropy, spontaneity of reactions

1. Enthalpy (H)

  1. Enthalpy is the measure of total heat content in a system.
  2. It is a state function that depends only on the initial and final states of the system.
  3. The change in enthalpy (ΔH) is used to determine the heat absorbed or released during a process.
  4. ΔH = H(products) - H(reactants).
  5. When ΔH < 0, the process is exothermic, releasing heat to the surroundings.
  6. When ΔH > 0, the process is endothermic, absorbing heat from the surroundings.
  7. Standard enthalpy changes include:
    • ΔHf: Enthalpy of formation.
    • ΔHc: Enthalpy of combustion.
    • ΔHvap: Enthalpy of vaporization.
    • ΔHfus: Enthalpy of fusion.
  8. Enthalpy changes are crucial for understanding chemical reactions, phase transitions, and energy systems.

2. Entropy (S)

  1. Entropy is a measure of the degree of disorder or randomness in a system.
  2. It is also a state function, depending only on the system's state.
  3. The change in entropy (ΔS) is given by: 
    ΔS = Q/T, where Q is the heat exchanged, and T is the temperature in Kelvin.
  4. An increase in entropy (ΔS > 0) indicates greater disorder.
  5. Processes that increase entropy are typically spontaneous (e.g., melting, vaporization).
  6. The entropy of the universe always increases for irreversible processes.
  7. At absolute zero (0 K), the entropy of a perfect crystal is zero (Third Law of Thermodynamics).
  8. Entropy changes help in predicting the feasibility of processes.

3. Gibbs Free Energy (G)

  1. Gibbs free energy is the energy available to do work in a system at constant temperature and pressure.
  2. The change in Gibbs free energy (ΔG) determines the spontaneity of a reaction.
  3. ΔG = ΔH - TΔS, where T is the temperature in Kelvin.
  4. If ΔG < 0, the reaction is spontaneous.
  5. If ΔG > 0, the reaction is non-spontaneous.
  6. If ΔG = 0, the system is in equilibrium.
  7. The sign of ΔH and ΔS affects the value of ΔG and the reaction's spontaneity:
    • ΔH < 0, ΔS > 0: Always spontaneous.
    • ΔH > 0, ΔS < 0: Never spontaneous.
    • ΔH < 0, ΔS < 0: Spontaneous at low temperatures.
    • ΔH > 0, ΔS > 0: Spontaneous at high temperatures.
  8. Gibbs free energy is fundamental in predicting chemical reaction feasibility.

4. Spontaneity of Reactions

  1. A reaction is spontaneous if it occurs without external intervention.
  2. Spontaneity is governed by the changes in enthalpy, entropy, and Gibbs free energy.
  3. Exothermic reactions (ΔH < 0) are often spontaneous, but not always.
  4. Processes that increase entropy (ΔS > 0) are more likely to be spontaneous.
  5. Gibbs free energy (ΔG) provides the most reliable criterion for spontaneity.

Key Points

  1. Enthalpy (H) is the heat content of a system, with ΔH indicating heat absorbed or released.
  2. Entropy (S) measures disorder, with ΔS helping predict system randomness.
  3. Gibbs free energy (G) combines enthalpy and entropy to determine spontaneity.
  4. ΔG = ΔH - TΔS is the key equation for reaction feasibility.
  5. A negative ΔG signifies a spontaneous reaction.
  6. Exothermic reactions are not always spontaneous; entropy also plays a role.
  7. The second law of thermodynamics highlights the importance of entropy in natural processes.
  8. At equilibrium, ΔG = 0, and no net change occurs in the system.
  9. The third law explains the behavior of entropy at absolute zero.
  10. These concepts are essential for solving problems in physical chemistry and thermodynamics.
Category
Enthalpy, Entropy, and Gibbs Free Energy
Thermodynamics
Chemistry