How much energy does it take?
Changes in Phase
 It takes energy to go from solid to liquid and liquid to
gas. The change from one phase to another takes a
little extra push of energy in order to get to that next
phase.
Energy Diagram for Phase Changes
Changes in Phase
 The temperature does not change (along the plateaus),
but notice that the heat is still being supplied. This
allows for the change from one phase to another phase.
 This is due to the intermolecular forces of attraction.
These small forces take energy to allow the molecule to
move from the other molecules
Changes in Phase
 H2O (s)  H2O (l)
 ∆Hfus = 6.01kJ/mol
 H2O (l)  H2O (s)
 ∆Hsolid = - 6.01 kJ/mol
 Notice the heats of enthalpies are just opposite in
charge.
 When water changes from solid to liquid it must absorb
heat (+)
 When water changes from liquid to solid it must lose
heat (-)
Changes in Phase
 There are four types of energy phases changes
 Molar Heat of Fusion- The heat absorbed by one mole
of a substance in melting from solid to a liquid at
constant temperature (∆Hfus)
 Molar Heat of Solidification- The heat lost when one
mole of a liquid solidifies (∆Hsolid)
 Molar Heat of Vaporization- The amount of heat
needed to vaporize one mole of a substance (∆Hvap)
 Molar Heat of Condensation- The heat lost when one
mole of a vapor condenses (∆Hcond)
Here is an example!
 How much heat is needed to change 28.0 grams of
water at 0ºC to liquid water? ∆Hfus = 6.01 kJ
1 mol H2O
6.01 kJ
28.0 g H2O 

 9.35 kJ
18.0 g H20 1 mol H2O
Another
Example
 How much heat is needed to change 200.0 grams of
water from 85ºC to 100ºC and change the water to
steam? (Specific Heat of Water = 4.184 J/gºC ; ∆Hvap =
40.7 kJ/mol)
 This requires two calculations:
Slide 3
Another Example (Cont.)
 First you must calculate how much heat is needed to
change the temperature from 85ºC to 100ºC.
q = m x C x ∆T =
(200.0g) x (4.184 J/gºC) x (15 ºC)
= 12552 = 12600 J or 12.6 kJ
Slide 3
Another Example (Cont.)
 Next you need to calculate how much heat is needed
to convert the water from liquid to vapor.
1 mol H2O
40.7 kJ
200.0 g H2O 

 452 kJ
18.0 g H20 1 mol H2O
Now you add the two values together:
452 kJ + 12.6 kJ = 464.6 kJ = 465 kJ
Energy Diagram for Phase Changes
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