1. STATE FUNCTIONS and ENTHALPY
To play the movies and simulations included, view the presentation in Slide Show Mode.

2. FIRST LAW OF THERMODYNAMICS
heat energy transferred
∆E = q + w
energy
change
work done
by the
system
Energy is conserved!

3. Chemical Reactivity
What drives chemical reactions? How do they occur?
The first is answered by THERMODYNAMICS and the second by KINETICS.
Have already seen a number of “driving forces” for reactions that are PRODUCT-FAVORED.
• formation of a precipitate
• gas formation
• H2O formation (acid-base reaction)
• electron transfer in a battery

4. Energy transfer allows us to predict reactivity.
Chemical Reactivity
Energy transfer allows us to predict reactivity.
In general, reactions that transfer energy to their surroundings are product-favored.
So, let us consider heat transfer in chemical processes.

5. ENTHALPY: ∆H ∆H = Hfinal - Hinitial Process is ENDOTHERMIC
If Hfinal > Hinitial then ∆H is positive
Process is ENDOTHERMIC
If Hfinal < Hinitial then ∆H is negative
Process is EXOTHERMIC

6. ∆H along one path = ∆H along another path
This equation is valid because ∆H is a STATE FUNCTION
Depend ONLY on the state of the system
NOT how it got there.
Can NOT measure absolute H,
ONLY measure ∆H.

8. Standard Enthalpy Values
Most ∆H values are labeled ∆Ho
Measured under standard conditions
P = 1 bar
Concentration = 1 mol/L
T = usually 25 oC
with all species in standard states
e.g., C = graphite and O2 = gas

9. Standard Enthalpy Values
∆Hfo = standard molar enthalpy of formation
— the enthalpy change when 1 mol of compound is formed from elements under standard conditions.
See Table 6.2 and Appendix L

11. ∆Hfo, standard molar enthalpy of formation
H2(g) + 1/2 O2(g) --> H2O(g)
∆Hfo (H2O, g)= kJ/mol
By definition,
∆Hfo = 0 for elements in their standard states.

16. HESS’S LAW ∆Horxn =  ∆Hfo (products) -  ∆Hfo (reactants)
In general, when ALL enthalpies of formation are known,
Calculate ∆H of reaction?
∆Horxn =
 ∆Hfo (products)
-  ∆Hfo (reactants)
Remember that ∆ always = final – initial

18. Using Standard Enthalpy Values
Calculate the heat of combustion of methanol, i.e., ∆Horxn for
CH3OH(g) + 3/2 O2(g) --> CO2(g) + 2 H2O(g)
∆Horxn =  ∆Hfo (prod) -  ∆Hfo (react)

19. Using Standard Enthalpy Values
CH3OH(g) + 3/2 O2(g) --> CO2(g) + 2 H2O(g)
∆Horxn =  ∆Hfo (prod) -  ∆Hfo (react)
∆Horxn = ∆Hfo (CO2) + 2 ∆Hfo (H2O)
- {3/2 ∆Hfo (O2) + ∆Hfo (CH3OH)}
= ( kJ) + 2 ( kJ)
- {0 + ( kJ)}
∆Horxn = kJ per mol of methanol