Introduction to Enthalpy of Reaction — AP Chemistry
1. Core Definitions of Enthalpy and Enthalpy of Reaction ★★☆☆☆ ⏱ 3 min
Enthalpy of reaction (abbreviated $\Delta H_{rxn}$) is the change in enthalpy, a state function related to heat transfer, that occurs when a chemical reaction proceeds to completion under constant pressure — the most common reaction condition in open labs and biological systems. This topic is foundational for all thermodynamic calculations in AP Chemistry Unit 6, which makes up 19-20% of total AP exam score.
Enthalpy (symbol $H$) is a state function defined as:
H = U + PV
Where $U$ is internal energy, $P$ is pressure, and $V$ is volume of the system. We can derive the key relationship used for all enthalpy of reaction calculations:
Exam tip: $\Delta H$ is only equal to $q$ at constant pressure, which is the standard condition for all enthalpy of reaction measurements tested on the AP exam.
2. Sign Conventions for Endothermic/Exothermic Reactions ★★☆☆☆ ⏱ 4 min
By universal AP Chemistry convention, we always measure enthalpy change from the perspective of the *system* (the reaction itself), not the surroundings. This gives the standard sign rules:
- If $\Delta H_{rxn} < 0$ (negative): the system releases heat to the surroundings = **exothermic reaction**
- If $\Delta H_{rxn} > 0$ (positive): the system absorbs heat from the surroundings = **endothermic reaction**
Exam tip: Always double-check which perspective the question asks for. If the question asks for $\Delta H$ of the reaction (system), never reverse the sign even if the question focuses on temperature change of the surroundings.
3. Thermochemical Equations and Standard Enthalpy ★★★☆☆ ⏱ 4 min
When $\Delta H$ is reported at standard state conditions (1 atm pressure, 1 M concentration for solutions, pure solids/liquids, typically 298 K), it is called the **standard enthalpy of reaction**, written $\Delta H^\circ_{rxn}$. Key AP-tested rules for working with thermochemical equations:
- $\Delta H$ is proportional to moles of reactant: if you multiply the entire balanced equation by a factor $n$, multiply $\Delta H$ by the same factor $n$
- If you reverse the reaction (swap products and reactants), reverse the sign of $\Delta H$; the magnitude remains identical
- $\Delta H$ depends on the physical state of reactants and products, so you must always include $(s), (l), (g), (aq)$ phase notation for all species
Exam tip: Always confirm the physical states of all species when interpreting a thermochemical equation. Changing H₂O from liquid to gas changes the $\Delta H$ value for combustion reactions by more than 10%, so AP questions explicitly test recognition of mismatched states.
4. Stoichiometric Calculations of Total Heat Transfer ★★★☆☆ ⏱ 5 min
The proportionality of $\Delta H_{rxn}$ to moles of reactant or product allows us to calculate the total heat absorbed or released for any measured amount of reactant consumed, a common calculation on both AP MCQ and FRQ sections. The general formula for total heat $q$ is:
q = n \times \frac{\Delta H_{rxn}}{\text{coefficient of the substance in the balanced equation}}
Where $n$ is the number of moles of the substance you are given. For problems that give mass of reactant instead of moles, first convert mass to moles using the substance's molar mass before applying the formula.
Exam tip: If a question asks "how much heat is released", they expect a positive value for the magnitude, but always keep the correct negative sign for $\Delta H$ if the question explicitly asks for the enthalpy change of the process.
Common Pitfalls
Why: Students confuse system vs surroundings perspective; ΔH is always defined for the system by convention, not the surroundings
Why: Students treat ΔH as an invariant property of the reaction, not a proportional quantity that scales with moles
Why: Students assume ΔH is the same regardless of state, but enthalpy is different for solid, liquid, and gas phases of the same substance
Why: Students mix up ΔH per reaction event vs total heat for the given amount of reactant
Why: Students associate "heat produced" with positive numbers, forgetting the convention is based on the system's energy change