What is Enthalpy?
Enthalpy (H) is the total heat content of a system at constant pressure. It tells you whether a chemical reaction absorbs or releases heat — a key concept in thermochemistry.
Enthalpy is the sum of internal energy and the work done by/on the system at constant pressure: H = U + PV. The change in enthalpy ΔH determines if a reaction is exothermic (ΔH < 0, releases heat) or endothermic (ΔH > 0, absorbs heat).
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Step-by-step worked examples
The combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Reactants = 50 kJ/mol, Products = −200 kJ/mol. Calculate ΔH.
ΔH = H_products − H_reactants ΔH = (−200) − (50) = −250 kJ/mol Negative ΔH → exothermic (releases heat)
Melting ice: H₂O(s) → H₂O(l). Solid = −300 kJ/mol, Liquid = −250 kJ/mol. Find ΔH.
ΔH = (−250) − (−300) = +50 kJ/mol Positive ΔH → endothermic (absorbs heat)
A reaction has reactants at 180 kJ/mol and products at 140 kJ/mol. Is it exothermic or endothermic?
ΔH = 140 − 180 = −40 kJ/mol Negative value → exothermic (releases 40 kJ/mol)
Flashcards
Quick quiz
Q1.Which reaction is exothermic?
Q2.A reaction has ΔH = −150 kJ/mol. How much heat is released?
Q3.Reactants = 210 kJ/mol, Products = 180 kJ/mol. Calculate ΔH.
Q4.Melting and boiling are endothermic. Why?
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Common mistakes
Confusing ΔH with absolute enthalpy H. — Correct: ΔH is the CHANGE — the difference between products and reactants.
Thinking all combustion is endothermic. — Correct: All combustion is exothermic (ΔH < 0).
Using ΔH = H_reactants − H_products (reversed). — Correct: ΔH = H_products − H_reactants (products first).
Ignoring the sign of ΔH. — Correct: The sign (+ or −) is crucial — it defines if heat is absorbed or released.
FAQ
What is the formula for enthalpy change?
ΔH = H_products − H_reactants. Units: kJ/mol or kJ.
What is the difference between exothermic and endothermic?
Exothermic: ΔH < 0, releases heat. Endothermic: ΔH > 0, absorbs heat.
Is melting ice endothermic or exothermic?
Endothermic — ice absorbs heat from surroundings to melt.
Can enthalpy change be zero?
Yes, if products and reactants have equal energy (rare, usually ΔH ≠ 0).