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Physical Chemistry 46

Specific enthalpy changes

A number of chemical and physical processes are given specific names in order to aid
concise discussion. Thermodynamically, there are no differences between the processes,
and the only reason for the use of these specific terms is convenience and brevity. A
selection of the more important processes is listed in Table 1.

Hess’s law
Because enthalpy is a state function, it follows that the absolute enthalpy associated
with the reactants and products in a reaction are independent of the process by which they
were formed. Consequently, the enthalpy change during the course of a reaction, given by
ΣH reactants−ΣH products is independent of the reaction pathway. Hess’s law of constant heat
summation is a recognition of this fact, and states that:

‘The overall enthalpy change for a reaction is equal to the sum of the
enthalpy changes for the individual steps in the reaction measured at the
same temperature’.

The law is particularly useful when measurement of a specific enthalpy change is
impractical or unfeasible. This may be illustrated by measurement of the

Table 1. Definitions of some commonly encountered
enthalpy changes

Quantity Enthalpy associated with: Notation Example
+
−
Enthalpy of Electron loss from a species in the ∆H i Na(g)→Na (g)+e (g)
ionization gas phase
−
−
Enthalpy of The gain of an electron by a species ∆H ea F (g)+e (g)→F (g)
electron affinity in the gas phase
Enthalpy of The vaporization of a substance ∆H v H 2 O (l)→H 2 O (g)
vaporization
Enthalpy of The sublimation of a substance ∆H sub CO 2 (s)→CO 2 (g)
sublimation
Enthalpy of Any specified chemical reaction ∆H Fe 2 O 3 +3Zn→2Fe+3ZnO
reaction
Enthalpy of Complete combustion of a substance ∆H c H 2 +½O 2 →H 2 O
combustion
Enthalpy of The formation of a substance from ∆H f 2Fe+3S→Fe 2 S 3
formation its elements in their standard state