Constant Pressure Calorimetry

For a chemical reaction carried out under constant pressure, such as atmospheric pressure, the heat exchanged during the process is measured as the change in enthalpy ΔH. The enthalpy change for the reaction is manifested as a change in temperature, which is measured using a technique called calorimetry.

In calorimetry, the reaction is executed in a sealed and calibrated container called a calorimeter. The calorimeter is well insulated and prevents any heat flow between itself and its environment.

Thus, heat exchanged between the reactants and products (which constitute the system) and the calorimeter (which is the surroundings), can be accurately calculated by monitoring the subsequent changes in the temperature in the calorimeter.

If the reaction is exothermic, heat flows from the system into the surroundings, and the temperature rises. Conversely, if the reaction is endothermic, heat flows from the surroundings into the system, causing the temperature of the calorimeter to fall.

A simple coffee cup calorimeter measures change in enthalpy of a reaction occurring in a solution, under constant pressure conditions.

The calorimeter consists of two Styrofoam coffee cups nested together and equipped with a thermometer and a stirrer. The calorimeter is closed with a loose-fitting cork lid to maintain conditions of constant pressure that is open to the atmosphere.

Suppose 50.0 milliliters each of 1.0 molar aqueous hydrochloric acid and 1.0 molar aqueous potassium hydroxide, react within the calorimeter, to raise the temperature of the solution by 6.9 °C.

The heat absorbed by the solution, q_solution, is equal to its specific heat, C_s, (4.18 J/g·°C) — just below that of water — multiplied by its total mass, m (100.0 g), and the change in temperature.

The heat of the solution is 2.9 × 10³ joules or 2.9 kilojoules. The heat of the reaction, q_reaction, has the same value but with an opposite sign. Because the pressure is constant, the enthalpy change is the same as the heat of the reaction.

To find the enthalpy change of the reaction per mole, the enthalpy is divided by the number of moles of hydrochloric acid. The moles of hydrochloric acid are found by multiplying the volume in liters (0.05 L) with the molarity (1 molar). Therefore, negative 2.9 kilojoules divided by 0.050 moles gives negative 58 kilojoules per mole.