In constant-pressure calorimetry, which quantity is measured using a calorimeter?

• A. The change in internal energy (ΔU) of a reaction
• B. The change in enthalpy (ΔH) of a reaction
• C. The change in entropy (ΔS) of a reaction
• D. The change in Gibbs free energy (ΔG) of a reaction

Answer: (B)

Verified step by step guidance

1. Understand that a calorimeter is a device used to measure heat changes during chemical reactions or physical processes.
2. Recognize that in constant-pressure calorimetry, the pressure is held constant throughout the experiment.
3. Recall the thermodynamic relationship that at constant pressure, the heat exchanged (q_p) is equal to the change in enthalpy (\$\Delta H\$) of the system: \$q_p = \Delta H\$
4. Note that the change in internal energy (\$\Delta U\$) is related to heat at constant volume, not constant pressure, so it is not directly measured in this setup.
5. Conclude that the quantity measured by a constant-pressure calorimeter is the change in enthalpy (\$\Delta H\$) of the reaction.