In a constant-volume calorimeter, the measured heat relates to which thermodynamic quantity?

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Multiple Choice

In a constant-volume calorimeter, the measured heat relates to which thermodynamic quantity?

Explanation:
In a constant-volume calorimeter, the vessel is rigid, so no PV work is done during the reaction. According to the first law, ΔU = q + w, and with w = 0 at constant volume, the heat exchanged with the surroundings equals the change in internal energy. So the measured heat corresponds to ΔU_rxn. This differs from constant-pressure calorimetry, where the heat measured corresponds to ΔH_rxn. The other options don’t match because q_rev is the heat for a reversible path, not the general heat transfer in this setup, and ΔG_rxn is a Gibbs energy change, not the heat exchanged in a constant-volume process.

In a constant-volume calorimeter, the vessel is rigid, so no PV work is done during the reaction. According to the first law, ΔU = q + w, and with w = 0 at constant volume, the heat exchanged with the surroundings equals the change in internal energy. So the measured heat corresponds to ΔU_rxn.

This differs from constant-pressure calorimetry, where the heat measured corresponds to ΔH_rxn. The other options don’t match because q_rev is the heat for a reversible path, not the general heat transfer in this setup, and ΔG_rxn is a Gibbs energy change, not the heat exchanged in a constant-volume process.

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