General Chemistry Principles, Patterns, and Applications, 2011

Equation 5.23
elements ® compound
For example,
C s
DHrxn = DHf
( ) +O2( g) ® CO2( g) DHrxn = DHf[CO2 g
( )]
The sign convention for ΔHf is the same as for any enthalpy change: ΔHf < 0 if heat is released when
elements combine to form a compound and ΔHf > 0 if heat is absorbed. The values of ΔHvap and ΔHfus for
some common substances are listed in . These values are used in enthalpy calculations when any of the
substances undergoes a change of physical state during a reaction.
Table 5.1 Enthalpies of Vaporization and Fusion for Selected Substances at Their Boiling Points and
Melting Points
Substance ΔH vap (kJ/mol) ΔH fus (kJ/mol)
argon (Ar) 6.3 1.3
methane (CH 4) 9.2 0.84
ethanol (CH 3CH 2OH) 39.3 7.6
benzene (C 6H 6) 31.0 10.9
water (H 2O) 40.7 6.0
mercury (Hg) 59.0 2.29
iron (Fe) 340 14
Note the Pattern
The sign convention is the same for all enthalpy changes: negative if heat is released by the system and
positive if heat is absorbed by the system.
Standard Enthalpies of Formation
The magnitude of ΔH for a reaction depends on the physical states of the reactants and the products (gas,
liquid, solid, or solution), the pressure of any gases present, and the temperature at which the reaction is
carried out. To avoid confusion caused by differences in reaction conditions and ensure uniformity of
data, the scientific community has selected a specific set of conditions under which enthalpy changes are
measured. These standard conditions serve as a reference point for measuring differences in enthalpy,
much as sea level is the reference point for measuring the height of a mountain or for reporting the
altitude of an airplane.
The standard conditions for which most thermochemical data are tabulated are apressure of 1 atmosphere
(atm) for all gases and a concentration of 1 M for all species in solution (1 mol/L). In addition, each pure
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