In a calorimetry experiment, which relations are used to determine ΔHrxn from calorimeter data?

• A. Use q = mcΔT for the solution; qrxn = qcal; ΔHrxn = qrxn/n
• B. Use q = mcΔT for the calorimeter; ΔHrxn = qcal/n
• C. Use q = mcΔT for the solution; qrxn = -qcal; ΔHrxn = qrxn/n
• D. Use qrxn = -qcal; ΔHrxn = qrxn/n

Answer: (C)

The key idea is energy conservation: the heat released or absorbed by the chemical reaction is transferred to the surroundings, which in a calorimeter is the solution (and the calorimeter itself). Because the temperature change you measure comes from the solution, you determine the heat exchanged with the surroundings using q = m_sol c_sol ΔT. Since the surroundings gain heat when the reaction releases heat, the heat of the reaction has the opposite sign: qrxn = -qcal, where qcal represents the heat absorbed by the calorimeter. Finally, to find the molar enthalpy change, you divide the heat of the reaction by the number of moles that reacted: ΔHrxn = qrxn / n. This is why the best approach is to use the solution’s heat via q = m c ΔT, relate that to the reaction with qrxn = -qcal, and then compute ΔHrxn by dividing by n.

The other options distract by either tying qrxn to qcal with the wrong sign, or by using the calorimeter’s heat as the source of q instead of the solution’s heat (or by skipping the relationship between the measured ΔT and the solution’s heat).