Measuring Change in Internal Energy and Change in Enthalpy

Calorimetry is an experimental technique that involves the measurement of heat changes associated with physical (or) chemical processes.

The experimental technique of Calorimetry actually involves two steps.

• The determination of heat capacity of a calorimeter.
• Involves the determination of the change in temperature during the completion of the reaction.

The measure of a bomb calorimeter's heat absorbing property is called its heat capacity.

Heat produced:
         q_v = C_v x ΔT
             Where,
                       C_v = heat capacity of the calorimeter

Molar heat of combustion = C_v x ΔT x M/W

Where , W = mass of the substance (in grams)
            M = Molecular weight of that substance (in grms)

              q_v = ΔU (Internal Energy Change)

Heat evolved is the heat of combustion at constant volume and is a measure of internal energy change ∆U.

In an exothermic reaction, heat is evolved and the system loses heat to the surroundings. Therefore enthalpy change ∆H is negative.

In an endothermic reaction, heat is absorbed by the system, from the surroundings and therefore, the enthalpy change ∆H will be positive.