calorimetry

Meaning: A calorimeter is an insulated container in which a reaction or process changes the temperature of a surrounding substance, usually water. If no heat escapes, the heat given off by the reaction is absorbed by the water and the calorimeter, so qrxn = −(qwater + qcalorimeter). A coffee-cup calorimeter runs at constant pressure, so the heat measured equals ΔH. A bomb calorimeter is a sealed rigid vessel at constant volume, so the heat measured equals ΔU (no PV work is done). Its own heat capacity, Ccal, must be included.

Equation: q = mcΔT, where q is heat (J), m is mass (kg, or g if c uses grams), c is specific heat (J/(kg·K)), and ΔT is the change in temperature (K, or equivalently °C for a difference). For a calorimeter as a whole, q = CcalΔT, with Ccal in J/K.

On the MCAT: Passages give masses, a temperature rise and a specific heat (water is about 4.18 J/(g·K)). You calculate q, then divide by moles to get ΔH per mole. Questions may also ask you to predict how heat loss to the room would bias the result.

Common confusion: Signs. If the water warms, the reaction released heat, so qrxn is negative (exothermic). The water's q is positive. Do not report the water's q as the reaction's q without flipping the sign.