Chem 162 - Free Energy and Spontaneity

Chapter 1: A Spontaneous Reaction

• Free Energy and Spontaneity

  • Definition of spontaneity: Will this reaction proceed on its own?

  • Gibbs Free Energy (ΔG):

    • If ΔG < 0 (negative), the reaction is spontaneous (energetically favorable).

    • If ΔG > 0 (positive), the reaction is non-spontaneous (requires added energy).

  • Keywords to recognize: favorable (ΔG negative) vs. unfavorable (ΔG positive).

Chapter 2: A Different Temperature

• Units of Measurement

  • ΔG and ΔH are measured in kilojoules per mole.

  • T must be in Kelvin (K).

  • ΔS is measured in joules per mole Kelvin.

  • Important to watch the signs of these values, as they indicate energy flow.

• Example Calculation:

  • Reaction: Oxidation of ethyl alcohol to acetic acid and water.

  • Standard conditions: Temperature is always 25 °C or 298 K.

  • Understanding standard conditions is crucial if temperature is not directly provided.

Chapter 3: Kilojoules or Kilojoules

• Given Values:

  • Standard enthalpy (ΔH): -495.2 kJ

  • Standard entropy (ΔS): -136 J/K, converting this to -0.136 kJ/K for consistency.

  • Use the formula: ΔG = ΔH - TΔS

• Plugging in Values:

  • Calculating ΔG:

    • ΔG = -495.2 kJ - (298 K)(-0.136 kJ/K)

    • Result: ΔG = -454.7 kJ

  • The ΔG value indicates a spontaneous reaction (ΔG < 0).

Chapter 4: Put Any Energy

• Process Specifics:

  • Ethyl alcohol (found in wine) oxidizes to acetic acid (vinegar) and water when exposed to air.

  • This oxidation process explains why open wine turns into vinegar if left for a long time.

  • Key takeaway: This spontaneous reaction does not require additional energy input.