5.4.4
Heat of Formation
Introduction
Thermochemistry is the study of heat
changes in chemical reactions
 Exothermic reactions produce heat:ΔH is negative
 Endothermic reactions use heat:ΔH is positive

of Combustion – heat change
when 1 mole of a substance is burned
completely in excess oxygen
 Heat
Introduction
of Reaction – heat change
when the number of moles in
the balanced equation react
 Heat
Heat of Formation
Heat change when 1 mole of a
compound is formed / from its
elements / in their Standard
States [as found in nature: at RTP]
C(s) + O2(g) = CO2(g)
H = - 393 kJ mol-1
C(s) + 1/2 O2(g) = CO(g) H = - 110 kJ mol-1
2C(s) + 2H2(g) = C2H4(g) H = - 123 kJ mol-1
6C(s) + 6H2(g) + 3O2(g) = C6H12O6(aq)
H = + 1256 kJ mol-1
Most elements are one atom [mono-atomic]
And solids
Exceptions
Hg(l)
[ He, Ne, Ar, Kr, Xe, Rn – gases but don’t react]
Some are diatomic [2 atoms joined] and gases
H2(g), O2(g) , N2(g), F2(g), Cl2(g), Br2(l), I2(s)
[A few are other things e.g. Ozone O3(g)]
Is the following equation correct for the heat of
formation of ammonia?
N2(g) + 3 H2(g) = 2 NH3(g)
H = - 92.4 kJ
Elements are in their standard states
but
Above equation is wrong because
two moles of ammonia have been formed
So it is a heat of reaction
½ N2(g) + 1½ H2(g)= NH3(g)
H = - 46.2 kJ mol-1
Above equation is correct because
Elements : in standard states and 1 mole formed
Examples
Write equations for the heat of formation of
the following substances.
 SO2(g)
 NaOCl(s)
 HNO3(l)
 C12H22O11(s)
 MgSO4(s)
 CuSO4.5H2O(s)
 (NH4)2SO4(s)
