AP Chemistry Unit 6: Thermodynamics

TLDR

• Thermodynamics explains whether a reaction can occur, not how fast.

• Master enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG).

• Use ΔG = ΔH − TΔS to predict spontaneity.

• Connect free energy to equilibrium using ΔG° = −RT ln K.

• Understand energy diagrams, calorimetry, and Hess’s Law.

Why This Unit Matters

Unit 6 is where chemistry becomes predictive. Instead of memorizing reactions, you learn to decide whether a process is spontaneous, temperature-dependent, or nonspontaneous under all conditions.

AP Chemistry frequently tests your ability to justify spontaneity using multiple thermodynamic variables at once, especially on free-response questions. Students who rely on memorization struggle here. Students who reason with ΔH, ΔS, and ΔG score highly.

1. Systems, Surroundings, and Energy Flow

• System: the part of the universe being studied.

• Surroundings: everything else.

• Universe: system + surroundings.

Energy transfer occurs as:

• Exothermic reactions: energy released, ΔH < 0.

• Endothermic reactions: energy absorbed, ΔH > 0.

Thermodynamics tracks energy movement, not reaction speed.

2. Heat, Work, and the First Law

Energy is conserved.

Key relationships:

• Heat (q): energy transferred due to temperature difference.

• Work (w): energy transferred by force and volume change.

Core equations:

• q = mCΔT

• w = −PΔV

• ΔE = q + w

A negative w means work is done by the system.

3. Enthalpy and Calorimetry

Enthalpy (ΔH) measures heat flow at constant pressure.

• ΔH = Hproducts − Hreactants

• Negative ΔH → exothermic

• Positive ΔH → endothermic

Calorimetry experimentally measures heat transfer:

• qreaction = −qsolution

• Constant-pressure calorimeters measure ΔH directly.

4. Hess’s Law and Enthalpy of Formation

Hess’s Law states that total enthalpy change is independent of reaction pathway.

Rules:

• Reverse a reaction → flip sign of ΔH.

• Multiply coefficients → multiply ΔH.

Standard enthalpy of formation (ΔH°f):

• Enthalpy change when 1 mol of a compound forms from elements in standard states.

• ΔH°f for elements in standard state = 0.

Reaction enthalpy:
ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants)

5. Entropy (ΔS)

Entropy measures disorder or energy dispersal.

Entropy increases when:

• Solids become liquids or gases.

• The number of gas particles increases.

• Temperature increases.

• Substances mix.

AP shortcut:
More gas particles → higher entropy.

6. Gibbs Free Energy and Spontaneity

Gibbs free energy determines whether a reaction is spontaneous.

Key equation:
ΔG = ΔH − TΔS

Interpretation:

• ΔG < 0 → spontaneous

• ΔG = 0 → equilibrium

• ΔG > 0 → nonspontaneous

Mnemonic:
“Good Horses Take Sugar” (G = H − T·S)

7. Temperature Dependence of Spontaneity

Spontaneity depends on both ΔH and ΔS.

• ΔH negative, ΔS positive → always spontaneous

• ΔH positive, ΔS negative → never spontaneous

• ΔH negative, ΔS negative → spontaneous at low temperature

• ΔH positive, ΔS positive → spontaneous at high temperature

This concept is frequently tested conceptually rather than mathematically.

8. Free Energy and Equilibrium

Thermodynamics connects directly to equilibrium.

Relationship:
ΔG° = −RT ln K

Where:

• R = 8.314 J·mol⁻¹·K⁻¹

• T must be in Kelvin

Interpretation:

• Large K → negative ΔG° → product-favored

• Small K → positive ΔG° → reactant-favored

At equilibrium, ΔG = 0.

9. Energy Diagrams

Energy diagrams visualize reaction progress.

• Vertical axis = energy

• Peak = activation energy (Ea)

• Difference between reactants and products = ΔH

Catalysts:

• Lower activation energy

• Do not change ΔH, ΔG, or equilibrium position

Key distinction:

• Thermodynamics → can a reaction happen

• Kinetics → how fast it happens

Common Pitfalls

• Treating ΔH and ΔG as interchangeable.

• Forgetting temperature must be in Kelvin.

• Saying spontaneous reactions are fast.

• Ignoring entropy in spontaneity explanations.

• Forgetting that catalysts do not affect thermodynamics.

Tutor Tip

On AP free-response questions, always justify spontaneity using both enthalpy and entropy.

Strong answer example:
“Although ΔH is positive, the increase in entropy at high temperature makes ΔG negative, so the reaction is spontaneous at high temperatures.”

This reasoning-based explanation is exactly what AP graders reward.

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