Enthalpy Changes for Different Types of Reactions and Phase Transitions:
The heat or enthalpy changes accompanying chemical reactions is expressed in different ways depending on the nature of the reaction. These are discussed below.
The standard heat of formation of a compound is defined as “the change in enthalpy that takes place when one mole of a compound is formed from its elements, present in their standard states (298 K and 1 bar pressure)". By convention the standard heat of formation of all elements is assigned a value of zero.
Fe(s)+ S(s) → FeS(s)
ΔHf0 = – 100.42 kJ mol−1
ΔHf0= + 222.33 kJ mol−1
ΔHf0 = – 92.4 kJ mol−1
The standard heats of formation of some compounds are given in Table 7.4.