The Heating Curve

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The Heating Curve
Mr. Shields Regents Chemistry U07 L03
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Phase Changes
Lets review what weve learned previously about
PHASE CHANGES. First What is a phase
change? A change from one state of matter to
another. What does endothermic and exothermic
mean? Absorbs heat
/ Releases heat
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Phase Change Overview
What are the names of the phase changes weve
studied?
ENDOTHERMIC Phase changes
EXOTHERMIC Phase Chages
a.
Sublimation
d.
Deposition
b.
Fusion
e.
Condensation
c.
Vaporization
f.
Solidification
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Phase Changes
What actually happens to E as matter moves from
Solid ? Liquid ? Gas Energy is absorbed
(endo) Temp increases KE increases molecules
move further apart PE increases
(Why?) Remember PE is a function of the
position of two bodies relative to one
another So PE inc as molecular separation
increases
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Heating Curve
What happens when we keep adding energy to a
solid ? Solid becomes a liquid then the liquid
becomes a gas As this energy is added KE inc
and so does Temp. but does temp Uniformly
inc. over time?
The answer is NO!
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Heating Curve
So what does happen if the temperature does not
Uniformly increase? What happens is described
by what is called the HEATING CURVE

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Heating Curve
If we heat a solid its temperature increases
Steadily until we finally reach the
temperature at which the solid begins to
melt It begins to go thru a phase
change solid ? Liquid This phase change is
called
fusion
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Solid/Liquid Phase Transition
As the solid begins to melt something Unusual
Happens. As we continue to add heat to the
solid the temperature stops rising as the
solid continues to melt.
But why does this happen?
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Solid/Liquid Phase Transition
Solids exist in a rigid, closely packed,
highly structured pattern Liquids however have
no such rigid structure. As we reach the solids
m.p. there is just enough energy to begin
overcoming the intermolecular forces
between molecules holding them together in the
solid state... Molecules begin to separate
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Heat of Fusion
Added additional heat energy goes into Separating
more and more molecules As molecules move from
solid to liquid the PE Increases but since the
temperature doesnt rise The KE remains
constant. The energy necessary to melt 1 GRAM of
a solid is called the It is unique for every
substance. For water, the heat of fusion 334
Joules/gram
HEAT OF FUSION (Hf)
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Solid/Liquid Phase transition
Remember heat ALWAYS flows from hot to
cold Until the last piece of solid melts the
temperature the Solid/liquid mixture remains
constant Any excess heat in the liquid
immediately flows back into the colder solid
Once theres no solid left all additional heat
added begins to increase the temperature of the
liquid.
C
H
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Solid/Liquid Phase transition
When heat is added what happens to KE, T PE ?
Only a single solid (s) phase present KE (T)
increases PE is Constant Dual L/S Phase
present KE (T) is Constant PE increase Only a
single liquid (l) phase present KE (T)
increases PE is constant
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Liquid/Vapor Phase transition
Any additional heat added To the liquid inc.
Temp. and Vapor Pressure. Once VP Patm the
liquid Boils. As in the transition from
solid To liquid, 2 phases are now present the
Temp. of the boiling water remains constant
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Heat of Vaporization
Molecules in the liquid phase form a close
but loosely organized structure Molecules in the
gas Phase have no structure And are widely
separated. To separate these molecules this much
takes lots of energy. This energy is called the
HEAT OF VAPORIZATION (Hv)
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Heat of Vaporization
For water, Hv 2,260 Joules/g Note Hv gt Hf
As long as the liquid is boiling T and KE will
be Constant PE will Increase as molecules Move
further apart In the gas phase
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Summary
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A Review Specific Heat
Recall from our earlier discussions that
The SPECIFIC HEAT of a substance is the amount of
heat required to raise the temperature of 1 g of
the material by 1 degree centigrade. Each
substance has its own unique specific heat
The lower the specific heat the better the
conductor
And . q Cp x m x (Tf Ti)
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Specific Heat
One of the variables in Specific heat
calculations Involves Temperature Change. What
phases of the The heating curve Involve changes
in T? Phase 1, 3, 5
The Heating Curve
Phase V
Phase III Phase IV
Phase 1 Phase II
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Specific Heat
Its in these phases that specific heat
calculations are used to determine how much heat
us needed to raise the temperature of the
sample But how do we find the heat necessary
to to fully melt or vaporize a sample of
matter In regions where T does Not change (phase
II IV)
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Melting/Vaporization Calculations
Its even easier than specific heat calculations
since Temperature is not a variable. In phase
II Q m x Hf mtotal mass Hfheat of
fusion In phase IV Qm x Hv
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Melting/Vaporization Calculations
Problem 1 How much heat is necessary to melt
100g of ice? Problem 2 How much heat is
necessary to vaporize this water? Q m x Hf Q
100g x 334 J/g Q 33,400 J Q m x Hv Q 100g
x 2,260 J/g Q 226,000 J
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Melting/Vaporization Calculations
Note that these calculations are true for all
sorts of matter, not just water. For example
here are some Hv as a function of the
intermolecular force H2O Hydrogen
Bonding 2260 J/g NH3 Hydrogen Bonding 1276
J/g H2S Dipole-Dipole 553 J/g F2 London
Dispersion 155 J/g
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Melting/Vaporization Calculations
Problem What is the heat of vaporization of a
liquid if it takes 6500 J to totally vaporize
8g? Q m x Hv 6500 J 8g x Hv Hv 6500 / 8
812.5 J/g
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Heat of Fusion and Vaporization from heating
graphs
If heat is added at a constant rate over time to
a specific mass we can determine both Hf and Hv
by noting both the initial onset of melting or
vaporization and the completion point.
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Heat of Fusion and Vaporization from heating
graphs
For example in this graph the onset of
vaporization occurs after 875 Joules have been
added and is complete after 3100 J have been
added. Therefore It took 3100-875 KJ to Vaporize
this sample. If the mass of the Sample is 300g
then the Hv 2225KJ/300g 7.417 KJ/g The
same can be done To calculate the Hf
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Heat of Fusion and Vaporization from heating
graphs
If we add heat to a sample at a fixed rate, say
422.4 J/min, we Can then calculate both Hf and Hv
if we know the mass Of the sample. In this
example the water starts to boil at 3.8 min And
is complete at 14.5 min So weve added
422.4J/min For 10.7min 4520 J Since the mass
is 2 gram then Hv. 4520/2 2260 J
(Mass 2 gram)
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Cooling Curve
The opposite of a heating Curve is a
COOLING CURVE.
Brrrrr
Since the sample is cooling it must be releasing
heat. As Temp decreases KE dec and as a sample
goes from gas To liquid to solid the PE must be
decreasing. Instead of Heat of vaporization and
heat of fusion we have the Heat of condensation
and the Heat of solidification. They Are equal to
Hv and Hf but are opposite in value -Hv Hc
-Hf Hs
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Cooling Curve
Phase I T dec KE dec PE constant Phase
II T constant KE constant PE dec
Hc -Hv Phase III T dec KE dec PE
constant Phase IV T constant KE constant
PE dec Hs -Hf Phase V same as I and III
Note This is the mirror image of a heating curve
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Cooling Curve Calculations
Problem How much heat is released when 100g Of
water solidifies? How much heat is released
when 100g of water condenses? a) Q m x Hs
(i.e. Hf ) Q 100g x -334 J/g Q -33,400 J
(i.e. 33,400 joules are released) b) Q 100g
x -2260 J/g -226,000 J
The negative sign means this much heat is
released (and not absorbed)