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Introduction to Enthalpy of Reaction

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The enthalpy of reaction, ΔH, is the heat a reaction releases or absorbs at constant pressure. Its sign follows the convention (negative for exothermic, positive for endothermic), and its magnitude scales with amount: react twice as much and you release twice the heat.

UNIT 6 TOPIC 6.6 • INTRODUCTION TO ENTHALPY OF REACTION ENTHALPY OF REACTION WHAT ΔHrxn MEANS At constant pressure, ΔHrxn = heat absorbed or released for the reaction AS WRITTEN (at constant P). enthalpy reaction progress reactants products ΔHrxn < 0 HEAT SCALES WITH MOLES q = n·ΔHrxn q scales with moles n = moles of reaction (as written) that occur EXAMPLE If ΔHrxn = −100 kJ (per mole of rxn) q = (2.00 mol)(−100 kJ) = −200 kJ → 2.00 mol releases 200 kJ (exothermic) products end up below reactants (heat given off) NEGATIVE ΔHrxn → EXOTHERMIC : releases heat products below reactants surroundings warm up POSITIVE ΔHrxn → ENDOTHERMIC : absorbs heat products above reactants surroundings cool down CED ANCHOR The enthalpy change of a reaction, ΔHrxn, is the heat exchanged at constant pressure for the reaction as written. Its sign gives the direction of heat flow; its magnitude scales with the moles reacted. AP Chemistry · Unit 6 · Thermodynamics
The enthalpy of reaction (ΔH) is the heat released or absorbed at constant pressure. Its sign follows the convention (negative = exothermic), and its magnitude scales with the amount that reacts — twice the moles releases twice the heat.
Enthalpy Ledger · Open the sandbox →

The traps mishandle the sign convention and treat ΔH as a fixed number regardless of how much reacts. ΔH per the balanced equation scales directly with the moles — it is an amount-dependent quantity, not a fixed constant.

The work

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Lesson
Enthalpy of Reaction ›

ΔH is the heat at constant pressure, with a sign convention and a magnitude that scales with amount. The lesson tracks both, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 6.6 misconceptions: ΔH sign convention misread, ΔH treated as fixed regardless of amount, heat equated with temperature, and signs mishandled when manipulating equations.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
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