As ΔH of a reaction varies with temperature and pressure conditions, therefore, for the sake of comparison, the values of ΔH for different reactions are expressed at their standard state conditions. The standard conditions usually chosen are 298 K temperature, and 1 bar or 100 kPa pressure; and 1 mol dm-3 concentration for solution. A standard state (also called reference state) of a substance is its most stable state at 100 kPa pressure and 298 K.
According to IUPAC recommendations, the enthalpy change at the standard state conditions is called standard enthalpy of the reaction and is denoted by Δ r HO The superscript (9) represents standard state. It is worthwhile to mention here, that unless other1-vise mentioned, the LlH values are the standard enthalpy changes of the reactions.
Note. Earlier, the notation used for standard enthalpy of reaction was Δ H r This is being used as such even now in large number of books.
Let us now, discuss the standard enthalpies of some chemical processes such as formation, combustion and neutralisation.
MOLAR ENTHALPY OF FORMATION ( Δ F H)
It is the enthalpy change accompanying the formation of one mole of a compound from its constituent elements. It is generally denoted by Δ F H. For example, enthalpy of formation of carbon dioxide (C02) and that of ethyl alcohol (C2HpH) are -393.5 and – T/7.0 kJ moi-1 respectively. These are expressed as follows:
C(s) + O2 (g) à CO2(g) ; ΔH = – 393.5 KJ
2C(s) + 3H2(g) + ½ O2(g) à C2H5 OH(l);ΔH = – 277.0 KJ.
Standard molar enthalpy of formation (Δ f H)
When all the species of the chemical reaction are in their standard states, the enthalpy of formation is called standard molar enthalpy of formation. The standard molar enthalpy of formation is denoted by. Thus, standard molar enthalpy of formation of a compound is defined as the enthalpy change accompanying the formation of one mole of a compound from its constituent elements, all the substances being in their standard states ( 1 bar or 100 kPa pressure and 298 K).
It may be noted that:
(i) Standard molar enthalpy of formation of the substance is also called its standard enthalpy and can also be denoted as If’.
(ii) By conventions, the standard molar enthalpies of free elements are taken to be zero.
(iii) Standard state of elements refers to the pure element in its most stable form at 298 K and 100 kPa pressure. For example, the standard state of carbon, oxygen, sulphur, mercury, bromine, and iodine are: C (graphite), O2(g), S8 (rhombic), Br2(l) and I2(s) respectively.
The values of standard molar enthalpies of formation of some substances at 298 K are given in Table 17.1.
Table 17.1. Standard Molar Enthalpies of Formation of
Some Substances at 298 K
Importance of standard molar entbalpies of formation. The knowledge of standard molar enthalpies of various substances can help us to calculate standard enthalpy change of any reaction. Standard enthalpy change. of a reaction is equal to the difference of the standard molar enthalpies of all the products and standard enthalpies of all the reactants.
Sum of standard Sum of standard
Standard molar entbalpies molar enthalpies
enthalpy – of formation of formation
of reaction of products of reactants
For example, consider a hypothetical reaction
aA + bB à lL + Mm