alleryStart learning →In a Hess's law problem a given step is reversed and then halved. What happens to its enthalpy change?
AThe sign flips and the value is halved
BThe sign flips and the value is doubled
CThe sign is kept and the value is halved
DThe sign is kept and the value is doubled
Answer & Solution
Correct answer: A. The sign flips and the value is halved
1. Reversing a reaction reverses the arithmetic sign of its enthalpy change.
2. Multiplying or dividing a reaction scales its enthalpy change by the same number.
3. Halving the reaction therefore halves the value.
4. Applying both operations flips the sign and halves the magnitude.
5. Starting from minus 49.4 kJ, reversing gives plus 49.4 kJ and halving gives plus 24.7 kJ.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_
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