Chemistry 1LC Practice Test 2026 – Complete Exam Prep

The main idea is balancing heat flow in the calorimeter. In a coffee-cup calorimeter, the heat released or absorbed by the reaction equals the heat gained by the solution plus the heat absorbed by the calorimeter. If you pick a reaction with a known enthalpy change, you know exactly how much heat qrxn was released or absorbed (qrxn = n ΔH). You then measure the temperature change ΔT. The heat gained by the solution is m c ΔT, where m is the solution mass and c is its specific heat (about 4.18 J/g·°C for water). The heat gained by the calorimeter is Ccal ΔT, where Ccal is the calorimeter’s heat capacity. So you have qrxn = - (m c ΔT + Ccal ΔT). From this, you can solve for the calorimeter constant: Ccal = - qrxn / ΔT - m c. This calibration lets you determine Ccal under the same conditions you’ll use in actual measurements.

That’s why using a reaction with a known ΔH and measuring the resulting ΔT is the reliable way to determine Ccal. It wouldn’t work to use a reaction with unknown ΔH, since qrxn would be unknown; simply measuring ΔT without knowing qrxn or calibrating under a known reference wouldn’t give Ccal either. A vague calibration with a standard solution lacks the necessary known heat change to pin down Ccal.