What is Enthalpy?
Enthalpy (H) is the total heat energy of a substance. The change in enthalpy (ΔH) during a reaction tells us whether heat is released (exothermic) or absorbed (endothermic). It's one of the most useful quantities in chemistry for predicting reaction behavior.
Enthalpy (H) is a state function that measures heat content. The heat of reaction ΔH = H_products − H_reactants; negative ΔH means exothermic (releases heat), positive means endothermic (absorbs heat).
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Step-by-step worked examples
Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = −890 kJ/mol. Is this exothermic?
ΔH = −890 kJ/mol (negative) Negative ΔH = exothermic (releases heat) Answer: Yes, heat is released.
Melting ice at 0°C absorbs 6 kJ/mol. What is ΔH?
Energy absorbed = positive ΔH ΔH = +6 kJ/mol (endothermic) Answer: ΔH = +6 kJ/mol
A neutralization releases 57 kJ/mol. Standard formation gives ΔH_neutralization ≈ −57.3 kJ/mol. Why negative?
Reaction releases heat → heat flows out Heat out = negative ΔH Answer: Heat released = ΔH < 0
Flashcards
Quick quiz
Q1.ΔH = −45 kJ/mol. Exothermic or endothermic?
Q2.What does ΔH measure?
Q3.Boiling water at 100°C, ΔH is…
Q4.Freezing water, ΔH is…
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Common mistakes
Confusing ΔH with temperature change. — Correct: ΔH is heat content change; temperature is molecular kinetic energy (related but different).
Thinking exothermic means the system is hot. — Correct: Exothermic releases heat; system can be cool.
Forgetting the sign of ΔH. — Correct: Sign matters: negative = exothermic, positive = endothermic.
ΔH is not a state function (depends on path). — Correct: Enthalpy IS a state function — only start/end states matter.
FAQ
What is enthalpy formula?
ΔH = H_products − H_reactants. Also, ΔH = q at constant pressure.
Why is enthalpy useful?
Most reactions occur at constant pressure; ΔH directly gives the heat released or absorbed.
Difference between ΔH and ΔU?
ΔH includes pressure–volume work; ΔU is internal energy. At constant pressure, ΔH ≈ ΔU + Δ(PV).
Can ΔH be zero?
Yes, for some phase changes where heat in = heat out over a full cycle (adiabatic reversible).