Chemistry / Unit 4 Chem 0685 short lesson, about 20 seconds
Physical State in a Thermochemical Equation
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The idea
A reaction's enthalpy change depends on the physical states of every reactant and product, so a thermochemical equation must show them.
In practice
Forming liquid water from hydrogen and oxygen releases 286 kJ per mole, but forming water vapor from those elements releases only 242 kJ.
Where people slip
Omitting a state label from a thermochemical equation leaves its ΔH value ambiguous between two different quantities.
Your place on the trail
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