1. Heat of Vaporization. (loosely based on Chap
Heat of Vaporization (loosely based on Chap. 12 Sec 8 of Jespersen 6th Ed)
Dr. C. Yau
Spring 2014

2. Molar Heat of Vaporization =
amt of heat needed to vaporize 1 mole of liquid
p.552

3. For water, Hvap = +43.9 kJ mol-1 What exactly does this mean?
H2O (l)  H2O (g) H = kJ Or
H2O (l) kJ  H2O (g) 3

4. How is Hvap Determined?
Clausius, a German physicist, and Clapeyron, a French engineer, determined that
The relationship between ln P and T
where P is the vp and T is the temp in K.
Mathematically, as an equation, it is…
ln P = k (1/T)
where k is the proportionality constant and it turns out to be a negative value.

5. ln P is proportional to 1/T
ln P = k (1/T)
For P = vapor pressure of liquid
T = temp in deg Kelvin
where R = J mol-1 K-1
This is an eqn for a straight line: y = mx + b

6. Graphical Determination of Hvap
y = m x b
Plot ln P vs. 1/T
Slope =
Rearrange eqn to calculate Hvap = - (slope)(R)

7. ln P vs. 1/T Are the slopes positive or negative?
Ans. Negative. Slope for water is more negative than that for acetone.

8. Hvap must be positive, as expected for vaporization (endothermic)
slope =
Since the slope is negative,
Hvap must be positive, as expected for vaporization (endothermic)
Slope of water being more negative than acetone means
|Hvap| (water) > |Hvap | (acetone)
(Why is this so? What does this mean?)

9. Graphical Determination of Hvap
Measure vp at various temp.
Plot ln P versus 1/T (kelvin)
Calculate the slope from the graph
4) From the slope calculate Hvap
Hvap = - (slope)(R)

10. Two-Point Method to Determine Hvap
Instead of measuring a set of P and T values, Hvap can also be determined by measuring only two sets of P and T.
You need only vp at two different temp:
(P1, T1) and (P2, T2)
See lecture notes for derivation of
Clausius-Clapeyron Equation:

11. Example 12.3: Methanol, CH3OH, experiences hydrogen bonding, dipole-dipole interactions, and London forces. At 64.6 oC, it has a vp of 1.00 atm, and at 12.0oC, it has a vp of atm. What is the heat of vaporization for methanol? Given: R = J mol-1 K-1 Write out the eqn & see what is known & what is unknown.

12. See page 554 for calculation. Ans 35.2 kJ/mol
T = 64.6 oC, P =1.00 atm,
T =12.0oC, P = atm
Which is the unknown?
R = J mol-1K-1
T1 = = K
T2 = = K
P1 = 1.00 atm
P2 = atm
See page 554 for calculation. Ans 35.2 kJ/mol
Do Practice Exercises & 12.11