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The enthalpy of reaction

The enthalpy of a reaction is not a fixed label like a boiling point — it is a per-amount quantity. React twice as much and you release twice the heat. Sign and scale are the whole game.

§1

Heat that scales with amount.

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The enthalpy of reaction, ΔH, is the heat a reaction releases or absorbs at constant pressure. It carries a sign (negative for exothermic, positive for endothermic) and is usually reported per the balanced equation.

Its magnitude scales with the amount that reacts. If the balanced equation releases 100 kJ, then reacting twice as many moles releases 200 kJ — ΔH is an extensive, amount-dependent quantity.

So to use a ΔH you must track two things: the sign (which direction the energy flows) and the amount (how much reacts). Doubling the reaction doubles the heat; reversing it flips the sign.

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
Fig. 6.6.1 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.
§2

Using a reaction enthalpy.

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Apply the sign and scale to the amount.

  1. Read the ΔH per the balanced equation. The value is for the moles shown in the equation.
  2. Scale to your amount. Multiply ΔH by the ratio of your moles to the equation's moles.
  3. Apply the sign. Negative means heat released (exothermic); positive means absorbed (endothermic).
  4. Reverse if needed. Reversing the reaction flips the sign of ΔH.
§3

The pieces you'll meet.

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Sign and scale run every enthalpy problem.

ΔH_rxn
Enthalpy of reaction
Heat at constant pressure, per the balanced equation.
sign
Sign
Negative exothermic, positive endothermic.
scale
Scaling
ΔH is proportional to the amount that reacts.
extensive
Extensive
Depends on amount, unlike an intensive property.
reverse
Reversing
Flips the sign of ΔH.
per equation
Per equation
Reported for the moles in the balanced equation.
§4

Worked example: scale a reaction enthalpy.

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Given. A reaction as written releases 200 kJ (ΔH = −200 kJ) for 2 mol of product.

Scale up. For 6 mol of product (three times as much), ΔH = 3 × (−200) = −600 kJ.

Scale down. For 1 mol of product (half the equation), ΔH = ½ × (−200) = −100 kJ.

Reverse. The reverse reaction has ΔH = +200 kJ per 2 mol — the sign flips. ΔH tracks both amount and direction.

§5

Mistakes that cost real points.

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Pitfall · 01

"ΔH is a fixed value regardless of how much reacts."

ΔH scales with amount. The value reported for the balanced equation applies to those moles; react more and the heat is proportionally larger. Treating ΔH as a fixed constant ignores that it is an extensive quantity.

Fix. Scale ΔH by the amount that reacts, in proportion to the balanced equation's moles.

Pitfall · 02

"The sign of ΔH doesn't matter."

The sign carries the direction of energy flow — negative for exothermic (released), positive for endothermic (absorbed). Dropping or flipping the sign reverses the physical meaning of the result.

Fix. Always carry the sign: negative = released (exothermic), positive = absorbed (endothermic).

Pitfall · 03

"Reversing the reaction keeps ΔH the same."

Reversing a reaction reverses the energy flow, so it flips the sign of ΔH (same magnitude, opposite sign). An exothermic forward reaction has an endothermic reverse.

Fix. Flip the sign of ΔH when reversing the reaction; keep the magnitude.

§6

Skill Check.

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