These notes explain how to convert between the amount of reacting substance (moles) and the heat absorbed or released by a reaction using a molar enthalpy value and correct stoichiometry.

Core idea: scale heat by chemical amount

In AP Chemistry, reaction heat is commonly reported “per mole” of reaction as written. To find the actual heat change for a specific quantity, you scale the molar value by the number of moles that actually react.

Molar enthalpy as a proportionality constant

A key skill is identifying what the given $\Delta H$ is “per mole of”:

• Sometimes it is explicitly per mole of a named reactant/product (e.g., “$\text{kJ per mol of } \text{O}_2$ consumed”).

• Sometimes it is tied to the balanced thermochemical equation (meaning the stated $\Delta H$ corresponds to the stoichiometric amounts shown).

Relationship between heat and moles

Always keep units consistent: if $\Delta H_{\text{rxn}}$ is in $\text{kJ mol}^{-1}$, then $q_{\text{rxn}}$ comes out in kJ.

How to choose the correct “n”

Correctly determining $n$ is the most important step. The reaction may involve multiple reactants, and the enthalpy value is only directly proportional to the reaction progress defined by stoichiometry.

Use the balanced equation as the reference

Treat the balanced chemical equation like a “recipe.” The coefficients tell you mole-to-mole relationships:

• If the enthalpy is given for the reaction as written, then “1 mole of reaction” corresponds to the full set of coefficients.

• If you know moles of some species, convert that amount to the equivalent “moles of reaction” using the coefficient ratio.

Practical approach (no arithmetic shown here):

• Start from the amount you’re given (mass, particles, volume and molarity, etc.) and convert to moles.

• Use the mole ratio from the balanced equation to convert to the moles that match the enthalpy basis.

Limiting reactant determines heat actually produced/absorbed

If multiple reactants are provided, the heat is determined by the reactant that runs out first.

Use this logic:

• Convert each reactant amount to a possible extent of reaction (how many “moles of reaction” each could support).

• The smallest extent corresponds to the limiting reactant.

• Use that extent as $n$ in $q_{\text{rxn}} = n\Delta H_{\text{rxn}}$.

Sign conventions and interpreting the result

The sign on $\Delta H_{\text{rxn}}$ carries meaning and should be preserved when computing $q_{\text{rxn}}$:

Energy-level style thermochemical diagrams for an exothermic and an endothermic reaction, with the corresponding $\Delta H$ values labeled. The figure makes the sign convention concrete: exothermic processes have $\Delta H<0$ because enthalpy decreases, while endothermic processes have $\Delta H>0$ because enthalpy increases. Source

• Negative $\Delta H_{\text{rxn}}$: heat is released by the reaction; $q_{\text{rxn}}$ is negative for the amount reacted.

• Positive $\Delta H_{\text{rxn}}$: heat is absorbed; $q_{\text{rxn}}$ is positive.

Common interpretation checkpoints:

• If the problem asks “how much heat is released,” report the magnitude but keep your internal sign logic consistent.

• If the reaction amount is doubled (in moles), the heat change doubles; if halved, the heat change halves.

Common AP Chemistry pitfalls to avoid

• Forgetting stoichiometric scaling: Using moles of the wrong species directly with $\Delta H_{\text{rxn}}$ when the enthalpy is based on a different reactant/product or on the whole equation.

• Ignoring limiting reactant: Calculating heat from a reactant that is actually in excess.

• Unit mismatch: Mixing $\text{J}$ and $\text{kJ}$ or reporting $\text{kJ mol}^{-1}$ when the question asks for total heat in $\text{kJ}$.

• Sign errors: Changing the sign because the question uses words like “released” or “absorbed,” instead of letting the given $\Delta H_{\text{rxn}}$ determine the sign.

What you should be able to do on sight

• Identify whether $\Delta H_{\text{rxn}}$ is tied to a thermochemical equation or to 1 mol of a specific species.

• Convert given quantities to moles and relate them to the enthalpy basis using the balanced equation.

• Use $q_{\text{rxn}} = n\Delta H_{\text{rxn}}$ with correct sign and units to state heat absorbed or released for the amount that reacts.