Modern inorganic chemistry

ENERGETICS 63

THE ENERGY FACTOR

A change can only take place if the energy factor is favourable. Most

simple laboratory reactions are carried out in vessels open to the

atmosphere and are therefore at constant pressure. Consequently

the most commonly met energy factor is the enthalpy. H; the

enthalpy change. A//, is a measure of the heat gained from, or lost

to, the surroundings during a chemical process, such that, at the

end of the reaction, the temperature and pressure of the system are

the same as before the reaction occurred. In an exothermic process,

the total enthalpy of the products H 2 is less than that of the reactants,

HI, and the enthalpy change, AH, is negative (Figure 3.1). For an

endothermic process the enthalpy change is positive.

J9-D

Reactants

A// = # 2 -//,

Products

Reaction coordinate

Figure 3.1

The enthalpy (strictly, the enthalpy change) for a reaction can
readily be calculated from enthalpies of formation A//f which can
often be obtained from tables of data.
AH values relate to defined conditions, usually to the standard
state of the substance at 298 K and 1 atm pressure, indicated by
AHf 98 * That is,
AHreaction = ZA//t products — ZAH, reactants
For example, for the reaction
C 2 H 6 (g) 4- 3iO 2 (g) -> 2CO 2 (g) + 3H 2 O(1)
A/freactlon = [2 x A//fC0 2 (g) + 3 x AHfH 2 (Xl)] - [A//fC 2 H 6 (g)]
(Note that A//f for an element in its standard state is zero.) Hence
Abaction = -1560 kJ mol ~l. (This is in fact an enthalpy of
combustion.)

• The temperature subscript 298 will be assumed in this book unless otherwise
  stated.