STANDARD MOLAR
ENTHALPY OF FORMATION
• Enthalpy change when 1 mol of species is formed in its Standard State at a
Specified Temperature from the most stable forms of its constituent elements in their standard forms (at the same temperature).

MOST STABLE FORM OF
ELEMENT
• = Form favored in Equilibrium at 1
Atmosphere and specified temp.
(usually 298.15 K)
• e. g.
for C at 298.15 K and 1 atmos., most stable form is GRAPHITE ( not diamond!)

STANDARD ENTHALPY OF
FORMATION FOR CO
2
• H f
0 CO
2 (gas)
= Standard Enthalpy of
Reaction for:
• C
(s, graphite)
+ O
2 (g)
CO
2(g)
•
Nomenclature:
– H f
0 Std. State at 25 0 C understood
Formation

ENTHALPY OF FORMATION
• N.B. H f
0 for an ELEMENT in its
Standard State = 0
– If not in its Standard State = 0
– e.g. For C
(s, graphite)
H 0 = 1.895 kJ mol –1
C
(s, diamond)

CALCULATION OF H FROM
TABLE OF H VALUES
• 1. Break down steps of reaction into:
– (a) Decomposition of Reactants into
Elements in Standard Forms
– (b) Formation of Products from Elements in
Standard States
– 2. Apply HESS’S LAW

HESS’S LAW
Add 2 (or more) Reactions to give New
Reaction, then Add Enthalpies in same manner to give Enthalpy of New Reaction

HESS’S LAW

CALCULATE ENTHALPY OF
FORMATION FOR CO
• Need Reaction:
C
(s, graphite)
+ ½ O
2
CO
2 (g)
H= ??

I.
CALCULATE ENTHALPY OF
FORMATION FOR CO (cont.)
C
(s, graphite)
+ O
2 (g)
CO
2 (g)
,
II. CO
H 0 = -393.5 kJ mol –1
2 (g)
– ½ O
2 (g)
CO
(g) ,
H 0 = +283.0 kJ mol –1

APPLICATION OF HESS’S
LAW
• ADD I + II:
C
(s, graphite)
+ O
2 (g)
CO
2 (g)
,
H 0 = -393.5 kJ mol –1
II. CO
2 (g)
– ½ O
2 (g)
CO
(g) ,
H = + 283.0 kJ mol –1
C
(s, graphite)
+ ½ O
2 (g)
CO
(g)
H 0 = -393.5 kJ + 283.0 kJ = -110.5 kJ mol -1

HESS’S LAW

•
DIFFERENT ALLOTROPIC &
PHYSICAL FORMS OF
ELEMENTS e.g.
red , white and black P
– different forms of S
– C (graphite and diamond)
– Spacing and arrangement of atoms is different in Graphite and Diamond, and requires energy input to effect the transition from one form to the other.

BOND ENTHALPIES
• Energy used to BREAK Specific Bond in
Gas Phase Reaction
• N.B.
Bond Enthalpies are ALWAYS +.