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Measurement of U and H

We can determine energy changes associated with chemical or physical processes by an experimental technique called calorimetry. When a system absorbs or releases energy in the form of heat, there is generally a change in temperature. This temperature change can be measured. A known amount of a liquid (often water) of known specific heat capacity is taken in a calorimeter in which the physical or chemical processes are carried out. The temperature change, which is due to heat released or absorbed during the process, is measured. Knowing the heat capacity of the liquid and the calorimeter, it is possible to determine the heat evolved in the process. Measurements or energy changes are made under two different conditions:
  1. At constant volume and
  2. At constant pressure.
At constant volume.
E = qv

i.e., heat absorbed gives the increase in internal energy.
At constant pressure, heat absorbed is equal to increase in enthalpy. i.e.
H = qp

Measurements at constant pressure are very common as many chemical and physical processes, including that in nature take place at constant pressure. For chemical reactions heat absorbed at constant volume, is measured in a bomb calorimeter.
A strong steel vessel (the bomb) is immersed in a large volume of water. A combustible sample is burnt in pure oxygen supplied in the steel bomb. Heat evolved during the reaction is transferred to the water around the bomb and only a small change in temperature occurs. In this measurement, oxygen and compound represent the system and bomb and water around it are the surroundings. In a bomb calorimeter, the energy changes associated with reactions are measured at constant volume and almost constant temperature. No work is done in this case as the reaction is carried out at constant volume in the bomb calorimeter. Even for reactions involving gases, there is no work done as V = 0.

Measurement of heat change at constant pressure (generally under atmospheric pressure) can be done in a calorimeter . We know H = qp (at constant P. T) and, therefore, heat absorbed, qp at constant pressure is also called the heat of reaction or enthalpy change,H. Here, we measure the heat absorbed in relation to initial temperature of the system.

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