Unit 9 Explanation 2

Unit 9: Applications of Thermodynamics

9.2 ΔG and K – Predicting Reaction Direction

The link between thermodynamics and equilibrium is one of the most powerful ideas in AP Chemistry.

ΔG° = –RT ln K

This tells us how the **free energy change** determines the **equilibrium position**.

• ΔG° < 0 → K > 1: Products favored at equilibrium
• ΔG° > 0 → K < 1: Reactants favored at equilibrium
• ΔG° = 0 → K = 1: Perfect balance (not common)

💡 Tip: Use this to check if the reaction goes “forward” under standard conditions or needs help (like added energy).

Important Constants:

R = 8.314 J/mol·K     |     T = Temperature in Kelvin

Summary:

• This formula connects Thermo (ΔG°) to Equilibrium (K)
• If you calculate ΔG°, you can find K (and vice versa)
• Use natural logs (ln), not log base 10!

9.3 Coupled Reactions – Making Non-Spontaneous Reactions Work

Sometimes a reaction that isn’t spontaneous (ΔG > 0) can be driven by pairing it with one that is.

This is called a coupled reaction and it’s common in both biology and industrial chemistry.

ΔG_total = ΔG₁ + ΔG₂

• If overall ΔG is negative → the combined process is spontaneous
• ATP hydrolysis in cells is often used to power unfavorable reactions

⚙️ In labs: Electrical or thermal energy is often used to force non-spontaneous steps to occur (electrolysis, decomposition).

Real-World Link:

Photosynthesis is not spontaneous (needs sunlight). It’s a perfect example of a driven process using external energy to make glucose.

KEY TAKEAWAYS:

• Spontaneity is about energy favorability, not speed!
• Coupling lets cells or machines do useful work with the right reaction pairing
• Total ΔG must still be negative for the process to occur naturally