Since chemical changes are always accompanied by energy changes, it indicates that reactants and products must be having certain amounts of energy. A fixed quantity of any substance is associated with a definite amount of energy which depends upon chemical nature of the substance and its state of existence. This energy is called internal energy or intrinsic energy of the substance and is represented by U. Earlier, internal energy used to be represented by the symbol E. However U is latest notation as per the recommendations of IUPAC. Internal energy of the system is the energy possessed by all its constituent molecules. The various forms of energies which contribute towards the energy of the molecule are translational energy (Et) ,rotational energy (E r) vibrational energy (E v), electronic energy (E e), nuclear energies of constituent atoms (En) and potential energy due to interaction with neighbouring molecules (EPE). Internal energy is an extensive property and is also a state function. Its value depends upon state of the substance but does not depend upon how that state is achieved. For example, CO2 can be obtained by various methods such as by heating calcium (II) trioxocarbonate(IV) or by burning coal. However, one mole of CO2 at S. T.P. is associated with a definite amount of internal energy which does not depend upon the source from which it is obtained.
It may be noted that the absolute value of internal energy cannot be determined because it is not possible to determine the exact values for the constituent energies such as translational, vibrational, rotational energies, etc. However, we can determine the change in internal energy (ΔU) of the system when it undergoes a change from initial state (U l) to final state (U f) For example, the change in internal energy (ΔU) of a reaction is the difference between the internal energies of the products (ΣUP) and the reactants (ΣUR)
ΔU = ΣUP – ΣUR
Significant Features of Internal Energy (U)
• Internal energy depends upon the quantity of substance in the system and hence, it is an extensive property.
• Change in internal energy (.6U) represents the heat evolved or absorbed in a reaction at constant temperature and constant volume.
• For processes involving evolution of energy, UP < U R‘ Thus, ΔU < 0 or sign of .6U is negative.
• For processes involving absorption of energy, Up> UR. Thus, .6U > 0 or sign of .6U is positive.
We have studied that energy change occurring during the reaction at constant temperature and constant volume is given by internal energy change. However, most of the reactions in the laboratory are carried out in open beakers or test tubes, etc. In such cases, the reacting system is open to atmosphere. Since atmospheric pressure is almost constant, therefore, such reactions may involve the changes in volume. The energy change occurring during such reactions may not be equal to the internal energy change. In order to understand this, let us assume a chemical reaction involving gaseous substances which proceeds with the evolution of heat. When the reaction is carried out at constant pressure, two possibilities arise.
(a) If the reaction proceeds with the increase in volume the system has to expand against the atmospheric pressure and energy is required for this purpose. The heat evolved in this case would be little less than the heat evolved at constant volume because a part of the energy has to be utilised for expansion.
(b) If the reaction proceeds with decrease in volume at constant pressure, the work is done on the system and heat evolved will be greater than the heat evolved at constant volume.
Thus, it can be concluded from the above discussion that heat changes occurring at constant pressure and constant temperature are not simply due to the changes in internal energy alone but also include energy changes due to expansion or contraction against the atmospheric pressure. In order to study the heat changes of chemical reactions at constant temperature and pressure a new function, enthalpy is introduced. Enthalpy is the total energy associated with any system which includes its internal energy and also energy due to environmental factors such as pressure-volume conditions. This can be understood as follows:
A substance has to occupy some space in its surroundings depending upon its volume (V). It does so against the compressing influence of the atmospheric pressure (P). Due to this, the substance possesses an additional energy called PV energy.
The sum of internal energy and PV energy of any system, under given set of conditions, is called enthalpy. It is denoted by H and is also called heat content of the system.
Mathematically, it may be put as
Some important features of enthalpy are:
• It is a state function and is an extensive property,
• It is also called heat content of the system,
• Its value depends upon the amount of the substance, chemical nature of the substance and conditions of temperature and pressure.
It is not possible to determine the absolute value of enthalpy of a system because absolute value of internal energy (U) is not known. However, change in enthalpy (ΔH) taking place during the process can be experimentally determined. Change in enthalpy is equal to difference between the enthalpies of products ( ΣHP) and reactants ( ΣHR)
The change in enthalpy may be expressed as
ΔH = ΣHP – ΣHR