Heat in Changes of
State
Section 17.3
Standards
7.c. Students know energy is released
when material condenses or freezes
and is absorbed when material
evaporates or melts.
7.d. Students know how to solve
problems involving heat flow and
temperature changes, using known
values of specific heat and latent heat
of phase changes.
Heats of fusion and
solidification
All solids absorb heat as they melt to
become liquids
Molar heat of fusion: the heat absorbed
by one mole of solid substance as it
melts to a liquid (ΔHfus)
Molar heat of solidification: the heat
lost when one mole of liquid solidifies
(ΔHsolid)
Heat of fusion and
solidification
The quantity of heat absorbed by a
melting solid is exactly the same as the
quantity of heat released when the
liquid solidifies.
OR ΔHfus = ΔHsolid
Figure
Problem 15
Problem 16-18
Heat of vaporization and
condensation
Molar heat of vaporization: the amount
of heat necessary to vaporize one mole
of a given liquid (ΔHvap))
H2O (l)  H2O (g) Δhvap = 40.7 kJ/mol
Molar heat of vaporization
and condensation
Molar heat of condensation: the amount of heat
released when 1 mol of vapor condenses (ΔHcond)
The quantity of heat absorbed by vaporizing a
liquid is exactly the same as the quantity of heat
released when the vapor condenses
ΔHvap = -ΔHcond
H2O (g)  H2O (l) ΔH = -40.7 kJ/mol
Heating curve
Answer questions a, b, and c.
Sample Problem #19
Questions 20-22
Molar Heat of Solution
During the formation of a solution, heat
is either released or absorbed
Molar heat of solution: the enthalpy
change caused by dissolution of one
mole of substance
Sample Problem
Problems #23-25
TO DO
In Class: Pg 25 #27-31, Pg 27 #5556
HW: Pg 25 #17-20