Energy and change. Endothermic and exothermic processes, heat transfer and thermal equilibrium, calorimetry, energy of phase changes, bond enthalpies, enthalpy of formation, and Hess's law.
Defining whether a process absorbs or releases energy relative to its surroundings.
Topic 6.2
Energy Diagrams
Graphing the energy of a system before and after a process to visualize energy change.
Topic 6.3
Heat Transfer & Thermal Equilibrium
Heat flows spontaneously from hotter to colder objects until thermal equilibrium is reached.
Topic 6.4
Heat Capacity & Calorimetry
Using q = mcΔT and calorimetry experiments to measure heat transfer.
Topic 6.5
Energy of Phase Changes
Heat absorbed or released during phase transitions, without a temperature change.
Topic 6.6
Introduction to Enthalpy of Reaction
Enthalpy (H) as a state function, and ΔH as the heat released or absorbed at constant pressure.
Topic 6.7
Bond Enthalpies
Estimating ΔH from the energy needed to break bonds in reactants versus form bonds in products.
Topic 6.8
Enthalpy of Formation
Using standard enthalpies of formation (ΔHf°) to calculate ΔH for a reaction.
Topic 6.9
Hess's Law
Combining known reactions algebraically to determine the enthalpy of a reaction that wasn't measured directly.
About Unit 6
Unit 6 examines the energy changes that accompany physical and chemical processes. You'll learn to classify processes as endothermic or exothermic, use calorimetry and the equation q = mcΔT to measure heat transfer, and connect heat absorbed during phase changes to intermolecular forces from Unit 3. The unit then introduces enthalpy (H) as a state function and three methods for finding a reaction's enthalpy change: from bond enthalpies, from enthalpies of formation, and by combining known reactions using Hess's law.
This unit is roughly 7–9% of the AP Chem exam and takes about 10–11 class periods. The concept of enthalpy as a state function — meaning ΔH depends only on initial and final states, not the path taken — is the throughline that connects every calculation method in this unit, and it carries forward directly into Unit 9 (Gibbs free energy).
Two core themes run through this unit:
Theme
Transformations — every physical and chemical process involves an energy transformation
Theme
Scale & quantity — enthalpy calculations connect microscopic bond energies to measurable heat