Enthalpy
CHM 102 Sinex
Enthalpy (H) ~ heat content (q) @ constant pressure
DH = thermal (heat) energy change = q
Physical process
H2O (l) + energy  H2O (g)
DHvap = 44 kJ/mole
heat of vaporization
What happens for the reverse process?
DH = - 44 kJ/mole and exothermic
CHM 102 Sinex
What does the sign of the enthalpy tell you?
Endothermic process or reaction
DH > 0 or positive
Exothermic process or reaction
DH < 0 or negative
Endothermic or exothermic?
H20 (l)  H2O (s)
CHM 102 Sinex
Standard State Conditions
• Temperature - 25oC or 298K
• Pressure – 1.00 atm
• Element in its stable state
O (g)
O2 (g)
O2 (l)
O3 (g)
CHM 102 Sinex
Br2 (g)
Br2 (l)
Br2 (s)
C (diamond)
C (graphite)
Chemical process
C (graphite) + O2 (g)  CO2 (g)
+ 394 kJ
DHof = -394 kJ/mole
This is the standard molar enthalpy of formation
for the formation of one mole substance from
its elements in their standard states.
DHof = 0 kJ/mole for free elements
CHM 102 Sinex
Combustion of methanol
2CH3OH (l) + 3O2 (g)  2CO2 (g) + 4H2O (g)
DHof: -239 kJ/mol
0
-394
-242
DHrxn = SnDHof,products - SnDHof,reactants
where n is the number of moles
How about just vaporizing the methanol?
CH3OH (l)  CH3OH (g)
DHof:
CHM 102 Sinex
-239 kJ/mol
-201 kJ/mole
Describe all processes for the following:
I2 (s)  2I (g)
I2 (s)  I2 (g)
DHof:
0
62
I2 (g)  2I (g)
DHof:
62
107
overall: I2 (s)  2I (g)
CHM 102 Sinex
DHsubl =
DHdiss =
Bond energy
DHrxn =
Energy Diagram
2I (g)
2I (g)  I2 (s)
I2 (s)  2I (g)
Endothermic rxn
I2 (g)
I2 (s)
CHM 102 Sinex
Exothermic rxn
From an energetics point of view, what is
happening in each case for the water? Is
the reaction endothermic or exothermic?
H20
2H + O
Breaking
both O-H
bonds
CHM 102 Sinex
H+ + OHBreaking
one O-H
bonds
endothermic
answer
H2 +
0.5O2
Breaking both
O-H bonds and
forming H-H and
O=O bonds (need
bond energies)
Compare the DH for this reaction calculated
by DHf data with that calculated from bond
energies.
Bond Energy Calculator - an interactive Excel spreadsheet - Click here
CHM 102 Sinex
Write the reaction used for the measurement
of the standard molar enthalpy of formation of
benzene.
6C (graphite) + 3H2 (g)  C6H6 (l)
answer
Do you think this reaction is possible?
Reacting graphite in the presence of
hydrogen gas will not produce benzene.
So how do we get the DHf for benzene?
answer
CHM 102 Sinex