20 multiple-choice questions in College Board exam style. Each has four choices and a full explanation of why each option is right or wrong.
Always check the sign of ΔH first. Negative means exothermic (releases energy); positive means endothermic (absorbs energy).
When using Hess's law, track every reversal and scaling. Reversing a reaction flips the sign of ΔH; multiplying by a coefficient multiplies ΔH by that same factor.
Remember enthalpy is a state function. Bond enthalpy, enthalpy of formation, and Hess's law are three different paths to the same ΔH — use whichever method matches the data you're given.
Read the explanations even when you got it right. Each one teaches a small fact that often returns in a different form on the exam.