By Sean Butner, Updated Aug 30, 2022
Hess’s Law underpins the conservation of energy in chemical reactions, asserting that the heat released or absorbed by a reaction equals the sum of the heats of its constituent steps. A calorimeter provides a closed system to capture this heat flow. While the instrument ideally records the true enthalpy change, it also absorbs a measurable amount of heat. Determining the calorimeter’s heat capacity—Qcal—allows us to correct the raw reading and obtain the reaction’s true heat change.
Apply a known heat source, such as a Bunsen burner with a calibrated power rating (Watts), to the empty calorimeter.
Measure the time (seconds) required for the calorimeter’s temperature to rise by exactly one degree Celsius.
Multiply the power (Watts) by the measured time (seconds). The product yields Ccal in joules per degree Celsius (J °C⁻¹).
Record the temperature change (ΔT in °C) that occurs during the reaction inside the calorimeter.
Multiply the previously determined Ccal by ΔT. For example, if Ccal = 3.5 J °C⁻¹ and the reaction raises the calorimeter’s temperature by 5 °C, the product is 17.5 J.
Qcal equals this product. A positive value indicates heat absorbed by the calorimeter, while a negative value indicates heat released. Incorporate Qcal into the overall energy balance to obtain the true reaction enthalpy.
