Unit 6 is where the exam starts asking not just what happens in a reaction, but how much energy moves around while it does. It's also one of the more forgiving units, since most of it comes down to careful bookkeeping once you understand the underlying ideas.
What's actually in Unit 6
Endothermic and exothermic processes
Every process either absorbs energy from its surroundings, endothermic, or releases energy into them, exothermic. An energy diagram visualizes this as the relative energy of reactants and products, with the difference between them showing the overall enthalpy change.
Heat transfer and calorimetry
Heat and temperature are related but not the same thing. Heat is energy transferred because of a temperature difference; temperature measures the average kinetic energy of particles. Calorimetry uses the heat capacity of a substance to calculate how much heat was absorbed or released during a process.
Enthalpy of reaction, bond enthalpies, and formation
The enthalpy of a reaction can be estimated by comparing the energy needed to break bonds in the reactants against the energy released forming bonds in the products. Enthalpy of formation values, tabulated for many compounds, offer another route to the same kind of calculation.
Hess's Law
Hess's Law says the total enthalpy change for a reaction is the same regardless of how many steps you break it into. That means a target reaction's enthalpy can be found by combining known reactions, reversing a reaction flips the sign of its enthalpy, and scaling a reaction scales its enthalpy proportionally.
Where students actually lose points here
Treating heat and temperature as the same thing
Two substances can absorb the exact same amount of heat and end up at very different temperatures, depending on their heat capacity. Free response questions specifically probe this distinction.
Sign errors on enthalpy
A negative enthalpy change means energy left the system, exothermic. A positive one means energy entered, endothermic. Losing track of that sign convention when flipping a reaction for Hess's Law is one of the most common errors in this unit.
Forgetting to scale enthalpy when scaling a reaction
If a reaction gets multiplied by two to make a Hess's Law problem work, its enthalpy has to be multiplied by two as well. Students sometimes remember to flip signs on a reversed reaction but forget to scale a doubled one.
Energy bookkeeping, taught carefully
Seven plus years as an AP Chemistry educator, 500 plus students, and 65 percent scoring a 5. Hess's Law problems especially benefit from working through a few together before trying one alone under time pressure.
Questions I get asked about Unit 6
What percentage of the AP Chemistry exam is Unit 6?
7 to 9 percent, and it lays the groundwork for the more advanced thermodynamics unit near the end of the course.
What is the difference between heat and temperature?
Heat is energy transferred between objects because of a temperature difference. Temperature is a measure of the average kinetic energy of particles in a substance. Two objects can transfer the same amount of heat and end up at very different temperatures depending on their heat capacity.
How does Hess's Law work?
It says the total enthalpy change for a reaction is the same no matter how many steps you break it into. That means you can add up the enthalpy changes of a series of known reactions, flipping signs when a reaction is reversed and scaling values when it's multiplied, to find the enthalpy of a reaction you never measured directly.