Unit 9 Explanation 3

Unit 9: Applications of Thermodynamics

9.4 ΔG and Cell Potentials (Electrochem Link)

Free energy is directly related to the cell potential (E°) from redox reactions.

ΔG° = –nFE°

• n = number of moles of electrons transferred
• F = Faraday's constant = 96,485 C/mol e⁻
• E° = standard cell potential in volts

🔌 Exam Tip: If E° is positive → ΔG° is negative → reaction is spontaneous (galvanic cell)

Key Link:

• Positive E° → negative ΔG → product-favored
• Negative E° → positive ΔG → non-spontaneous
• This formula connects Thermodynamics to Electrochemistry!

9.5 Entropy and Molecular Distribution

Entropy (ΔS) measures the spread of energy and randomness in a system.

• ΔS > 0 → increased disorder (more possible arrangements)
• ΔS < 0 → decreased disorder (more ordered)

Biggest Entropy Trends:

• Solids → Liquids → Gases: entropy increases
• More moles of gas → higher entropy
• Mixing substances → increases entropy

ΔS°_rxn = Σ ΔS°_products – Σ ΔS°_reactants

📦 Memory Aid: More randomness = more entropy = higher ΔS

Entropy Summary:

• Entropy is a key player in ΔG
• Helps explain why reactions "feel natural" or not
• Even exothermic reactions can be non-spontaneous if entropy decreases too much

9.6 Thermodynamics in Biological Systems

In living cells, reactions must be controlled and efficient. Thermodynamic principles explain how energy is managed.

Examples:

• ATP hydrolysis: Releases energy (ΔG < 0) to power other processes
• Protein folding: Balance between ΔH and ΔS determines shape stability
• Cell respiration: Highly exergonic, used to produce ATP from glucose

🧬 Science + Life: Biology questions on the AP Chem exam often focus on ΔG and reaction coupling in cells!

Final Takeaways – Unit 9:

• ΔG connects Thermo, Equilibrium, Electrochemistry, and Life Systems
• Entropy is about options and probability
• Coupled reactions and E° give control over spontaneous behavior