Question

How is it possible to find the value of $\Delta U$ and not the value of U?

Answer 1

As we know that the internal energy of a thermodynamic system is defined as the energy contained within it. In simple words, it can be described as the energy necessary to create or prepare the system in any given internal state. It is denoted by ‘U’.

Complete answer:
Let us first discuss about internal energy as follows:-
-Internal energy: It is defined as the energy contained within it and it is denoted by ‘U’. In simple words, we can say that it is the energy required to create or prepare the system in any given internal state.
-It is an extensive property and also it cannot be measured directly. The thermodynamic processes that tell us about the internal energy are transfers of matter or energy as heat and thermodynamic work.
-We can find the value of change in internal energy ($\Delta U$) but we cannot measure internal energy (U) because there are various energies that contribute toward the internal energy. These energies are given below:-
Intermolecular energy
Energy associated with bonds within the nucleus
Translational energy, rotational energy and vibrational energy of the molecules
Potential energy of the molecules
Since it is not possible to determine the absolute values of the above energies as all these are microscopic energies of the molecules therefore only change in the internal energy from one state to another can be determined easily.

Note:
-Remember that change in internal energy ($\Delta U$) is calculated by using a bomb calorimeter. This experimental technique is known as calorimetry. This technique gives us the information regarding the amount of heat used in physical changes, chemical reactions or transition of phase.
-Afterwards we use the following formula for calculations of $\Delta U$:-
$\Delta U=m{{C}_{V}}\Delta T$
where,
m = mass of water
${{C}_{V}}$ = specific heat capacity at constant volume
$\Delta T$ = temperature difference