In a calorimetry experiment, a calorimeter is calibrated to determine its heat capacity. After calibration, a substance is added that causes the calorimeter to reach thermal equilibrium at a new stable temperature. Which of the following statements best explains the importance of achieving thermal equilibrium in this context?

Achieving thermal equilibrium ensures that all heat released by the substance is shared evenly between the calorimeter and the surroundings, so the measured temperature rise truly reflects the energy change of the reaction. When equilibrium is reached, the temperature is constant, indicating that no heat is being transferred elsewhere, which means the calorimeter’s heat capacity can be accurately determined. This allows the experimenter to calculate the reaction’s enthalpy change from the observed temperature rise using the known heat capacity. For example, if a 10 kJ reaction raises the temperature by 5 °C in a calorimeter with a 2 kJ/°C heat capacity, the enthalpy change is 10 kJ, confirming the calibration. Thus, equilibrium is essential for reliable, reproducible calorimetric measurements.