Heat in Changes of State
Chapter 17.3
Learning Objectives
 Understand how to calculate heat (q) when a
substance undergoes a phase change
 Know the definition of heat of fusion/solidification
 Know the definition of heat of
vaporization/condensation
 Calculate heat change as you heat substance up and
pass through a phase change (putting it all together)
Different types of heats
 Molar heats of … describe the
amount of heat absorbed/released
when a 1 mol of a substance changes
state




Fusion (Hfus) solid to liquid
Solidification (Hsolid) liquid to solid
Vaporization (Hvap) liquid to vapor
Condensation (Hcond) vapor to liquid
Endothermic/Exothermic
 Endothermic Events (+H)
 Fusion (melting)
 Vaporization
 Exothermic Events (-H)
 Solidification (freezing)
 Condensation
Pairings
 Hfus = -Hsolid
 Hvap = -Hcond
Units are sometimes kJ/mol
However, Mr. D does not feel we have to use these
units since heats (enthalpy) of fusion are often
reported for various substances as J/g or kJ/kg.
For instance water Hfus = 334 J/g or 79.72 cal/g
Hvap = 2260 J/g or 540 cal/g
Hsolid = -334 J/g or -79.72 cal/g
Hcond = -2260 J/g or -540 cal/g
Heating/Cooling Curve
Remember what is happening at each point
Heating/Cooling Curve: Phase Changes
Assume you have 1 gram of water
Water is vaporizing Hvap= 2260 J/g of H2O
Water is melting Hfus = 334 J/g of H2O
(J)
What if you don’t have 1 gram of water?
You just multiply H of fusion/vaporization by mass
Water is vaporizing Hvap= 2260 J/g of H2O
q = m x Hvap
Water is melting Hfus = 334 J/g of H2O
q = m x Hvap
(J)
Heating/Cooling Curve: No Phase Changes
What happens at A, C, and E??
We use our old formula q = m x C x T
q=m
q=m
x
Csolid
x
x
Cliq
x
T
q=m
T
(J)
x
Cgas
x
T
Heating/Cooling Curve: Putting it all Together
You should be able to calculate the total heat going all
the way from heating a substance from its solid to its gas
q=m
q=m
q=m
q=m
x
x
Csolid
x
Cliq
x
x
Hvap
q=m
T
Hfus
x
T
(J)
x
Cgas
x
T
Cool Down Problem
You have a 4.30 grams of ice at -13.2oC. You heat it until it
completely vaporizes. How much heat was needed to complete
this process? Here are some numbers you might need.
(Cice = 2.10 J/goC)
(Cwater = 4.18 J/goC)
(Csteam = 1.70 J/goC)
(Hfus = 334 J/g)
(Hvap = 2260 J/g)
To get answer you simply
add these 4 numbers together:
13071 J or 13.1 kJ (3 sig. figs)
T (4.30)(2.10)(0.00 - -13.2)= 119.2 J
q=m
x
Cice
q=m
q=m
x
Hfus (4.30)(334)= 1436.2 J
Cwat x T (4.30)(4.18)(100. - 0.00)= 1797.4 J
x
q=m
x
x
Hvap = (4.30)(2260)= 9718 J