The change in enthalpy is

Study for the Medical Chemistry Test. Explore multiple choice questions with hints and explanations. Get exam-ready with engaging quizzes and comprehensive study materials.

Multiple Choice

The change in enthalpy is

The change in enthalpy reflects the heat that enters or leaves a system when the process occurs at constant pressure and only pressure-volume work is involved. Since enthalpy H equals U plus pV, for a constant-pressure process the differential dH simplifies to dU plus p dV. From the first law, the heat added at constant pressure equals dU plus p dV (the PV work term), so the heat flow at constant pressure equals the enthalpy change. That makes the statement about heat exchanged at constant pressure the best description of ΔH. Heat added at constant volume, by contrast, equals the change in internal energy, not the enthalpy, because there is no PV work at constant volume. The work done on or by the system is not, in itself, the enthalpy change (enthalpy relates to heat under constant pressure, not to work directly). And enthalpy generally changes with temperature (and composition), so it is not independent of temperature.

The change in enthalpy reflects the heat that enters or leaves a system when the process occurs at constant pressure and only pressure-volume work is involved. Since enthalpy H equals U plus pV, for a constant-pressure process the differential dH simplifies to dU plus p dV. From the first law, the heat added at constant pressure equals dU plus p dV (the PV work term), so the heat flow at constant pressure equals the enthalpy change. That makes the statement about heat exchanged at constant pressure the best description of ΔH.

Heat added at constant volume, by contrast, equals the change in internal energy, not the enthalpy, because there is no PV work at constant volume. The work done on or by the system is not, in itself, the enthalpy change (enthalpy relates to heat under constant pressure, not to work directly). And enthalpy generally changes with temperature (and composition), so it is not independent of temperature.

More practice questions