Chemistry · Thermodynamics · NEET
It measures ΔH (enthalpy change). The reaction is open to the atmosphere, so pressure stays constant. NCERT proves that heat at constant pressure, qp, equals ΔH. So whatever heat this calorimeter records is directly the enthalpy change. A bomb calorimeter is different: it is sealed, so volume is constant and it measures ΔU instead.
At constant pressure the first law gives ΔU = qp − pΔV. Rearranging: qp = ΔU + pΔV = (U2 + pV2) − (U1 + pV1). Since enthalpy H = U + pV, this becomes qp = H2 − H1 = ΔH. So the heat you measure at constant pressure IS the enthalpy change. That is why NCERT says: when heat is absorbed at constant pressure, you are actually measuring ΔH.
Being open keeps the pressure equal to the room's atmospheric pressure, which stays constant. Constant pressure is the whole point, because only at constant pressure does the measured heat equal ΔH. If you sealed it (like a bomb), volume would be fixed instead and you would measure ΔU.
A coffee-cup calorimeter is the simplest constant-pressure calorimeter. It is just an insulated cup (often two foam cups) holding a known volume of liquid, with a thermometer and a stirrer. The reaction happens in the liquid at atmospheric pressure, and the temperature change tells you the heat, which equals ΔH.
Use heat = C × ΔT, where C is the heat capacity of the liquid plus the calorimeter and ΔT is the temperature change. The heat lost by the reaction equals the heat gained by the calorimeter (same size, opposite sign). Divide by the moles of the limiting reactant to get ΔH per mole. If temperature rises, the reaction is exothermic, so ΔH is negative.
NEET often gives you a calorimeter setup and asks for the enthalpy of reaction, or asks whether a device measures ΔH or ΔU. Knowing that a constant-pressure (open) calorimeter gives ΔH, while a bomb (sealed, constant volume) gives ΔU, lets you pick the right formula and avoid the classic trap.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. A coffee-cup calorimeter is the common simple example of a constant-pressure calorimeter. Both stay open to air, so pressure is constant and they measure ΔH.
It measures the heat of reaction at constant pressure, qp, which equals the enthalpy change ΔH (also called the heat of reaction or ΔrH).
A bomb calorimeter is sealed, so volume is constant and no work is done (ΔV = 0); it measures ΔU. A constant-pressure calorimeter is open, so pressure is constant and it measures ΔH.
Yes. If the temperature of the liquid rises, heat was released, so the reaction is exothermic and ΔH is negative. If the temperature falls, the reaction is endothermic and ΔH is positive.
q = C × ΔT, where C is the total heat capacity of the calorimeter and its liquid, and ΔT is the measured temperature change.
