5.7 Book Notes

📘 CLUE 5.7 — Gibbs (Free) Energy to the Rescue

The BIG question this section answers

Examples:

• Why does dye spread in water?

• Why does heat flow from hot → cold?

• Why does ATP release energy?

• Why don’t broken eggs reassemble?

Gibbs free energy gives us a single rule to predict this.

1⃣ Why energy alone is NOT enough

At first, chemists thought:

But that doesn’t explain everything.

Example:

• Ice melts at room temperature (absorbs energy) → still happens

• Some energy-releasing reactions don’t happen on their own

So we need more than energy.

2⃣ Two things control whether something happens

Every process depends on two factors:

🔥 Enthalpy (ΔH)

• Energy stored in chemical bonds

• “Heat content” of a system

If:

• ΔH < 0 → energy released (favorable)

• ΔH > 0 → energy absorbed (unfavorable)

🎲 Entropy (ΔS)

• How spread out energy and matter are

• Number of possible arrangements

Key idea:

Examples of higher entropy:

• Gas > liquid > solid

• Mixed > unmixed

• Warm > cold

3⃣ The problem Gibbs energy solves

Sometimes:

• ΔH is unfavorable but ΔS is favorable

• Or ΔH is favorable but ΔS is unfavorable

So which one “wins”?

👉 Gibbs free energy combines both.

4⃣ Gibbs Free Energy (ΔG)

The equation

ΔG=ΔH−TΔSΔG=ΔH−TΔS

Where:

• ΔG = Gibbs free energy change

• ΔH = enthalpy change (energy)

• T = temperature (in Kelvin)

• ΔS = entropy change

5⃣ What ΔG actually tells you

The rule (MEMORIZE THIS)

Spontaneous ≠ fast
It just means it can happen without added energy.

6⃣ Why temperature matters

Temperature multiplies entropy in the equation.

That means:

• At high T, entropy matters more

• At low T, energy matters more

Example:

• Ice melts at higher temperatures because entropy dominates

• Ice stays solid in the freezer because energy dominates

7⃣ How this explains real life

Example 1: Dye in water

• ΔH ≈ small

• ΔS ↑↑↑ (huge increase in arrangements)

→ ΔG < 0
→ Dye spreads spontaneously

Example 2: Protein folding

• ΔH decreases (favorable interactions)

• ΔS decreases (more order)

Balance determines if folding occurs

Example 3: ATP hydrolysis

ATP → ADP + Pi

• ΔH < 0 (energy released)

• ΔS > 0 (more particles)

→ Strongly negative ΔG
→ Powers life

8⃣ Why CLUE cares about Gibbs energy

CLUE emphasizes:

Gibbs energy explains:

• Metabolism

• Enzyme reactions

• Transport across membranes

• Muscle contraction

• Nerve signaling

Cells don’t “break rules” — they couple reactions.

9⃣ Coupled reactions (VERY IMPORTANT)

Some reactions have:

• ΔG > 0 (unfavorable)

Cells make them happen by coupling them to:

• A reaction with very negative ΔG (like ATP hydrolysis)

Total ΔG becomes negative → process happens

🔟 One-sentence CLUE takeaway

That’s Gibbs free energy.

🧠 Memory shortcuts

• ΔH = energy

• ΔS = spreading

• ΔG = decision maker

Or: