Chemistry / Chem 0681 · Atom · ~20 seconds
Sign Convention for Enthalpy Change
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A negative enthalpy change marks an exothermic reaction; a positive one marks endothermic, and reversing flips this sign.
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Hydrogen and oxygen forming liquid water release 286 kJ per mole, so ΔH is −286 kJ; decomposing water back carries ΔH = +286 kJ.
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"The reaction gets cold, so it releases heat" inverts this convention — a falling temperature marks endothermic, a positive ΔH.