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Forming 1 mol of liquid water from its elements releases 286 kJ, but forming gaseous water releases only 242 kJ. What does this show?
AEnthalpy change depends on the physical states shown
BEnthalpy change depends on how fast the reaction runs
CEnthalpy change depends on the container volume used
DEnthalpy change depends on which catalyst is chosen
Answer & Solution
Correct answer: A. Enthalpy change depends on the physical states shown
1. The same amounts of hydrogen and oxygen are used in both cases.
2. Only the state of the water product differs, liquid in one and gas in the other.
3. The released heat differs by 44 kJ per mole because of that state difference.
4. So states of matter must always be written in a thermochemical equation.
5. Reaction speed and catalysts do not alter an enthalpy change, since enthalpy is a state function.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_
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