1. CHM 102
Sinex
Enthalpy
CHM 102 Sinex

2. 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

3. 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

4. Standard State Conditions
Temperature - 25oC or 298K
Pressure – 1.00 atm
Element in its stable state
O (g)
O2 (g)
O2 (l)
O3 (g)
Br2 (g)
Br2 (l)
Br2 (s)
C (diamond)
C (graphite)
CHM 102 Sinex

5. C (graphite) + O2 (g)  CO2 (g) + 394 kJ
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

6. Combustion of methanol
2CH3OH (l) O2 (g)  2CO2 (g) + 4H2O (g)
DHof:
-239 kJ/mol
-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:
-239 kJ/mol
-201 kJ/mole
CHM 102 Sinex

7. Describe all processes for the following:
I2 (s)  2I (g)
I2 (s)  I2 (g)
DHsubl =
DHof: 62
I2 (g)  2I (g)
DHdiss =
DHof: 62
107
Bond energy
overall: I2 (s)  2I (g)
DHrxn =
CHM 102 Sinex

8. Energy Diagram 2I (g) 2I (g)  I2 (s) I2 (s)  2I (g) Exothermic rxn
Endothermic rxn
I2 (g)
I2 (s)
CHM 102 Sinex

9. From an energetics point of view, what is happening in each case for the water? Is the reaction endothermic or exothermic?
H20
2H O
H OH-
H O2
Breaking both O-H bonds
Breaking one O-H bonds
Breaking both
O-H bonds and forming H-H and O=O bonds (need bond energies)
endothermic
CHM 102 Sinex
answer

10. 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

11. 6C (graphite) + 3H2 (g)  C6H6 (l)
Write the reaction used for the measurement of the standard molar enthalpy of formation of benzene.
6C (graphite) H2 (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