calorimeters

Meaning: A calorimeter is treated as an isolated container, so heat from the reaction (the system) goes into the water and the device (the surroundings), or comes out of them. A coffee-cup calorimeter is open to the atmosphere, so it runs at constant pressure and q equals ΔH. A bomb calorimeter is a sealed, rigid steel vessel in a water bath, used for combustion. It runs at constant volume, so no PV work is done and q equals ΔU.

Equation: q = mcΔT, where q is heat in J, m is mass in kg (grams are often used), c is specific heat capacity in J/(kg·K), and ΔT is the temperature change in K (the same size as in °C). For a whole device, q = CΔT, where C is heat capacity in J/K. Also qsystem = −qsurroundings.

On the MCAT: A passage gives masses, a specific heat (water is about 4.18 J/(g·K)) and initial and final temperatures. You calculate q, then divide by moles to get ΔH per mole, or predict a sign. You may also be asked what happens if heat leaks out.

Common confusion: Students flip the sign. If the water warms, the reaction released heat, so qrxn and ΔH are negative. Also, a bomb calorimeter gives ΔU, not ΔH. They differ only by the work from a change in moles of gas. Finally, do not ignore the calorimeter's own heat capacity when the problem gives it.