Study Guide Unit 6 AP chemistry

Unit 6: Thermochemistry – The Study of Heat in Chemical Reactions

🧠 Concept Overview

Thermochemistry is the branch of chemistry that deals with energy changes—especially heat—during chemical and physical processes.

• Why some reactions release heat while others absorb it
• How to measure and calculate heat changes
• How energy is stored in chemical bonds

⚗️ Theory Breakdown

6.1 Endothermic and Exothermic Processes

• Exothermic: Releases heat (ΔH < 0)
• Endothermic: Absorbs heat (ΔH > 0)

6.2 Energy Diagrams

Visualize activation energy and ΔH with reaction coordinate diagrams.

6.3 Heat Transfer and Thermal Equilibrium

Use the formula q = cmΔT to calculate heat transfer.

6.4 Heat Capacity and Calorimetry

• Specific heat of water: 4.18 J/g°C
• Use calorimeters to measure heat exchange

6.5 Energy of Phase Changes

• During phase change: temperature stays constant
• Use latent heat formulas: q = mHvap, q = mHfus

6.6 Enthalpy of Reaction (ΔH_rxn)

ΔH_rxn is the energy released/absorbed per mole of reaction at constant pressure.

6.7 Bond Enthalpies

ΔH = Σ bonds broken – Σ bonds formed

6.8 Enthalpy of Formation

ΔH_rxn = Σ ΔH_f°(products) – Σ ΔH_f°(reactants)

6.9 Hess’s Law

• Enthalpy is a state function—path doesn’t matter
• Flip and scale equations as needed

💜 Real-World Analogy

Think of heat like money: Exothermic gives it away, endothermic spends it to make something happen.

📘 Worked Example

Problem: A student mixes 50 mL of 1.0 M NaOH with 50 mL of 1.0 M HCl. ΔT = 5.5°C

Step 1: q = cmΔT = 4.18 × 100 × 5.5 = 2299 J

Step 2: 0.050 mol → scale to 1 mol: ΔH = -2299 / 0.050 = -46 kJ/mol

❌ Common Misconceptions

• Heat (q) ≠ Enthalpy (ΔH)
• Temperature is not the same as heat
• q depends on the amount; ΔH is per mole

🧩 Review Questions

1. Differentiate between endothermic and exothermic reactions.
2. Calculate q for heating 100 g water from 20°C to 50°C.
3. Draw and label an energy diagram.
4. Use bond enthalpies to estimate ΔH for combustion of methane.
5. Explain Hess’s Law with an example.

🗂️ Flashcard Points

• Enthalpy (ΔH): Heat at constant pressure
• Exothermic: ΔH < 0
• Endothermic: ΔH > 0
• Calorimetry: Experimental heat measurement
• Hess’s Law: Enthalpy is path-independent
• Bond enthalpy: Energy to break/form bonds