Cracking the AP Chemistry Exam
Content Review for the AP Chemistry Exam
Big Idea #3: Chemical Reactions, Energy Changes, and Redox Reactions
Enthalpy Change, ∆H
The enthalpy of a substance is a measure of the energy that is released or absorbed by the substance when bonds are broken and formed during a reaction.
The Basic Rules of Enthalpy
When bonds are formed, energy is released.
When bonds are broken, energy is absorbed.
The change in enthalpy, ∆H, that takes place over the course of a reaction can be calculated by subtracting the enthalpy of the reactants from the enthalpy of the products.
∆H = Hproducts − Hreactants
If the products have stronger bonds than the reactants, then the products have lower enthalpy than the reactants and are more stable; in this case, energy is released by the reaction, or the reaction is exothermic.
All substances are the
most stable in their
lowest energy state.
This means that, in
processes are more
likely to be favorable
If the products have weaker bonds than the reactants, then the products have higher enthalpy than the reactants and are less stable; in this case, energy is absorbed by the reaction, or the reaction is endothermic.