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What happens to the enthalpy change when the direction of a thermochemical equation is reversed?
AIts magnitude doubles but the sign stays the same
BIts magnitude halves and the sign stays the same
CIts magnitude stays the same but the sign flips
DIts magnitude and sign both stay entirely unchanged
Answer & Solution
Correct answer: C. Its magnitude stays the same but the sign flips
1. A negative enthalpy change marks an exothermic reaction and a positive one an endothermic reaction.
2. Running the same change backwards must undo the heat flow.
3. So the magnitude is unchanged and only the arithmetic sign is reversed.
4. Hydrogen plus chlorine giving 2HCl is minus 184.6 kJ, so the reverse is plus 184.6 kJ.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_
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