Unit 9 Explanation 1

Unit 9: Applications of Thermodynamics

9.1 Gibbs Free Energy and Spontaneity

Gibbs Free Energy (ΔG) determines if a reaction is thermodynamically favorable (spontaneous).

ΔG = ΔH – TΔS

• ΔG < 0: Reaction is spontaneous (thermodynamically favorable)
• ΔG > 0: Reaction is non-spontaneous
• ΔG = 0: Reaction is at equilibrium

Signs to Remember:

• ΔH < 0 = Exothermic (heat released)
• ΔS > 0 = Increased disorder (more entropy)

Temperature Trick:

• When ΔH and ΔS have the same sign, temperature determines spontaneity
• +ΔH, +ΔS: Spontaneous at high T
• –ΔH, –ΔS: Spontaneous at low T

Standard Free Energy and K

ΔG° = –RT ln K

• ΔG° < 0 → K > 1 → Product-favored
• ΔG° > 0 → K < 1 → Reactant-favored
• ΔG° = 0 → K = 1 → Equilibrium

R = 8.314 J/mol·K
T = Kelvin

💡 AP Tip: ln K means the natural log of K, not log base 10. Students often mix this up!

Units of ΔG:

Always in kJ/mol in final answer (convert if needed from J to kJ).

KEY IDEAS:

• ΔG is the master controller: ties together enthalpy (ΔH), entropy (ΔS), and temperature (T)
• Links Thermodynamics to Equilibrium (through K)
• Don’t forget: signs matter more than numbers in MCQs

⚠️ Common Mistake: Plugging in 298 instead of converting Celsius to Kelvin (T = °C + 273).