(1) Enthalpy of Formation : It is a form of standard enthalpy of reaction in which one mole of compound is formed from its elements. The standard enthalpy change for the formation of one mole of a compound from its elements. The standard enthalpy change for the formation of one mole of a compound from its elements in their most stable states of aggregation is called Standard Molar Enthalpy of Formation. Its symbol is ∆H°.
N2(g) + O2(g) ➝ 2NO(l) ; ∆fH° =+180 kJ
(2) Phase Transition Enthalpy: The change in phase of a substance takes place at a very small rate. Thus, it is not possible to measure the heat change for phase transition experimentally. But, phase transition enthalpy can be calculated with the help of Hess’s Law. The heat of transition of diamond to graphite can be calculated from the heat of combustion data. For diamond and graphite, which is -395.4 kJ and -393.5 kJ respectively.
The thermochemical equations showing the combustion reaction of diamond and graphite are:
C(diamond) + O2(g) ➝ CO2(g) ∆H = -395.4 kJ …(i)
C(graphite) + O2(g) ➝ CO2(g) ∆H = -393.4 kJ …(ii)
The conversion that is required is :
C(diamond) ➝ C(graphite) ∆H = ?
This can be obtained by subtracting the second equation from the first one.
(3) Entropy : Entropy is a measure of the molecular disorder, or randomness, of a system. The concept of entropy provides deep insight into the direction of spontaneous change for many everyday phenomena. Its introduction by the German physicist Rudolf Clausius in 1850 is a highlight of 19th-century physics.
There is increase in entropy during melting of a solid and vaporisation of a liquid, in solids, there is a definite crystal lattice whereas in gases and liquids, this arrangement is very less. Therefore, with increase in entropy in gases and liquids, disorderlness also increases. Thus, entropy increases in spontaneous processes. Therefore, entropy of a system is a measure of orderlness. In this way, entropy changes related to molecules take place.
(i) More the number of molecules, more will be the entropy.
(ii) More the number of energy levels available to molecules on heating, more will be the entropy.
4. Enthalpy of solution : Enthalpy of solution of a substance is the enthalpy change when one mole of it dissolves in a specified amount of solvent. The enthalpy of solution at infinite dilution is the enthalpy change observed on dissolving the substance in an infinite amount of solvent when the interactions between the ions (or solute molecules) are negligible.
When an ionic compound dissolves in a solvent, the ions leave their ordered positions on the crystal lattice. These are now more free in solution. But solvation of these ions (hydration in case solvent is water) also occurs at the same time. Thus, in dissolution of ionic solid, contributing of both the energies is necessary.