Unit 9: Applications of Thermodynamics - AP Chemistry Notes

1. Gibbs Free Energy (\Delta G)

• Definition: Indicates the spontaneity of a reaction.

• Equation: \Delta G = \Delta H - T\Delta S

  • \Delta G: Free energy change (kJ/mol)

  • \Delta H: Enthalpy change (kJ/mol)

  • T: Temperature (Kelvin)

  • \Delta S: Entropy change (J/mol*K)

• Spontaneity:

  • \Delta G < 0: Spontaneous

  • \Delta G > 0: Non-spontaneous

  • \Delta G = 0: Equilibrium

2. Entropy (\Delta S)

• Definition: Measure of disorder or randomness in a system.

• Units: J/mol*K

• Increase in Entropy:

  • More gas particles formed.

  • Solid to liquid, liquid to gas.

  • Dissolving a solid into a solution.

3. Enthalpy (\Delta H)

• Definition: Heat change at constant pressure.

• Exothermic: \Delta H < 0 (releases heat)

• Endothermic: \Delta H > 0 (absorbs heat)

4. Temperature and Spontaneity

• Temperature affects spontaneity:

  • High T: Favor reactions with increasing entropy (\Delta S > 0)

  • Low T: Favor reactions with decreasing entropy (\Delta S < 0)

5. Thermodynamic Favorability

• Favorable Conditions:

  • \Delta H < 0 (exothermic)

  • \Delta S > 0 (increasing entropy)

• If \Delta H and \Delta S have opposite signs, \Delta G will determine spontaneity.

• If \Delta H and \Delta S have same sign, T will determine spontaneity.

6. Standard Free Energy Change (\Delta G^\circ)

• Equation: \Delta G^\circ = -RTlnK

  • R = 8.314 J/mol*K

  • T = Temperature (K)

  • K = Equilibrium constant

• Interpretation:

  • K > 1: \Delta G^\circ < 0, reaction is spontaneous.

  • K < 1: \Delta G^\circ > 0, reaction is non-spontaneous.

7. Coupled Reactions

• Definition: Reactions that are linked so that one non-spontaneous reaction is driven by a spontaneous one.

• Example: Cellular respiration coupled with ATP synthesis.

8. Reaction Quotient (Q) and Equilibrium (K)

• Q = Reaction quotient (current state)

• K = Equilibrium constant (at equilibrium)

• If Q < K: Reaction shifts right (forward)

• If Q > K: Reaction shifts left (reverse)

• If Q = K: System is at equilibrium

9. Phase Changes and Thermodynamics

• Melting/Boiling: Increase in entropy, requires energy (endothermic)

• Freezing/Condensation: Decrease in entropy, releases energy (exothermic)

10. Summary of Spontaneity Conditions

11. Key Concepts to Remember

• Systems tend toward lower energy (exothermic) and higher disorder (higher entropy).

• Free energy (\Delta G) is the final determinant of spontaneity.

• Temperature plays a crucial role when \Delta H and \Delta S oppose each other.