Download Change of state - Mrs. Coyle`s College Chemistry

Changes of State
College Chemistry
Changes of State
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Change of state or
phase change –
conversion of a
substance from one
of the three physical
states of matter to
another
Always involves a
change of energy
Change of State
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In order to move “up”
from solids  liquids
 gases, you must
add more energy
(endothermic)
In order to move
“down”, energy is
given off
(exothermic)
Vaporization, Condensation, and
Evaporation
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Vaporization – change of state from a liquid
to a gas
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Occurs when heat and/or pressure is added to a
system
Broad term – includes evaporation
Condensation – change of state from a gas
to a liquid
Evaporation – process by which molecules
escape from the SURFACE of a liquid to
become a gas
Vaporization, Condensation,
and Evaporation
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Why do liquids evaporate?
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Look to the kinetic-molecular theory –
energy of a liquid depends on
temperature, a rapidly moving molecule
near the surface of the liquid occasionally
possesses enough energy to overcome
the intermolecular attractions and escape
as a gas
Rate of evaporation depends on
temperature (inc temp = inc rate)
Vapor Pressure
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When a liquid evaporates, gaseous molecules exert
a vapor pressure
As the concentration of molecules in the vapor state
increases, some molecules condense and become
trapped by intermolecular forces in the liquid
Equilibrium vapor pressure – equilibrium
achieved between rate of condensation and rate of
evaporation
Liquid-Vapor Equilibrium
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Molecules that have gone through
vaporization, can return to the liquid
state by colliding with the liquid surface
At equilibrium, Rate of condensation =
rate of evaporation
There IS still condensation and
evaporation going on  the rates of
both are equal
Liquid- Vapor Equilibrium
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Equilibrium vapor pressure –the
pressure of the molecules in the
vapor state at equilibrium in a
CLOSED container!
Boiling Point
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Extension of vapor pressure equilibrium…
As you wait for boiling to occur, small
bubbles of vapor appear. This pressure of
the vapor is the vapor pressure of water at
that temperature.
As long as the vapor pressure is less than
the atmospheric pressure, the bubbles
collapse. Once they gain enough heat and
the vapor pressure equals the atmospheric
pressure, the bubbles of vapor escape
Heat of Vaporization
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Heat of vaporization – amount of heat necessary
to vaporize a given amount of liquid
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We found the heat of vaporization for water
How close was your value to 80 cal/g??
Heat is needed to vaporize
For condensation, heat is released  heat of
condensation
The energy does NOT change during a phase
change

When you were melting your ice, was there a period
where the temperature changed very little?
Molar Heat of Vaporization
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Molar heat of vaporization, DHvap –
energy (usually kJ) required to
vaporize 1 mole of a liquid
usually directly related to strength of
intermolecular forces holding liquid
together
Enthalpy of Vaporization
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This relationship between temperature
and vapor pressure is expressed by the
Clausius- Clapeyron equation
ln Pvap= (-∆Hvap/R) x (1/T) + A
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Pvap= vapor pressure (torr) or mmHg
T=temperature (K)
∆Hvap= enthalpy of vaporization (J/mol)
R=8.31 J/mol*K
A=constant, each liquid has a specific value
Clausius- Clapeyron Equation
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You can set this equation to solve for
the vapor pressure of the liquid at
different temperature
ln (P1/P2) = DHvap/R [(1/T2 – 1/T1)]
Question 1
At 100°C the vapor pressure of ethylene
glycol is 14.9 torr and at 125°C it is 49.1
torr. What is the enthalpy of vaporization?
 Use ∆Hvap=-[(ln Pvap2- ln Pvap1)/(1/T2)(1/T1)] *8.314J/mol*K
 ∆Hvap=-[(ln 49.1 – ln 14.9)/(1/398)(1/373)] *8.314J/mol*K
 ∆Hvap= 58.75 kJ/mol
Pressure should be in mmHg or torr!
Freezing and Melting
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Freezing point/melting point – point
where a liquid freezes or ice melts
Heat of fusion – energy needed to
melt a solid
Heat released when freezing a solid is
EXACTLY the same as the heat of
fusion  just a different sign
Molar Heat of fusion
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Molar heat of fusion, DHfus – energy
(usually in kJ) required to melt 1 mole
of a solid
Molar heats of fusion are generally
much smaller than molar heats of
vaporization (liquid molecules are
packed closer together and more
energy need to rearrange from a solid
to liquid)
Supercooling
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Supercooling – a liquid is temporarily
cooled below its freezing point
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Occurs when heat is removed so rapidly
that the molecules have no time to
assume the order of a solid structure
Generally unstable
Superheating
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The same thing can occur when
heating a solid or liquid
For example, if you heat water in the
microwave, it heats up really quickly
and can easily burn you when you take
it out
Sublimation and Deposition
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Some solids can actually go straight to
a gaseous phase without becoming a
liquid (sublimation)
The gases of these solids can convert
back to a solid state without going
through a liquid (deposition)
Ex: dry ice (CO2), snow on the ground
Molar heat of sublimation
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Molar heat of sublimation, DHsub –
energy (usually in kJ) required to
sublime 1 mole of a solid
Equal to heats of fusion + vaporization
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DHsub = DHvap + DHfus
Enthalpy is the same whether you go
straight from a solid to gas or go
through a liquid
Example 11.8
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Calculate the energy needed to heat 346 g of liquid
water from 0°C to 182°C. Assume that the specific
heat of water is 4.184 J/g*°C and the specific heat
of steam is 1.99 J/g*°C
1. Heat water from 0°C to 100°C
2. Evaporating 346 g of water at 100C (phase
change)
3. Heating steam from 100°C to 182°C
Example 11.8
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1. q = mcDT
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2. DHvap is 40.79 kJ/mol for water
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q = (346 g)(4.184 J/g*C)(100 – 0)
q = 145 kJ
q = (346 g water) x (1 mol water/ 18.02 g water) x (40.79
kJ/1 mol water)
q = 783 kJ
3. q = mcDT
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(346 g)(1.99 J/g*C)(182 – 100)
q = 56.5 kJ
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Add them up: 145 + 783 56.5 = 985 kJ
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Heating Curves
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Heating curves – explain how
temperature is changing throughout
phase changes for a certain compound
or element
Ex of water:
Critical Temperature and
Pressure
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Critical temperature – temperature
above which its gas phase CANNOT
be made to liquefy, no matter how
much pressure is applied
Critical pressure – minimum pressure
needed to bring about liquefaction at
the critical temperature
Phase Diagrams
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Phase diagrams – relates the states
of a substance to their temperature
and pressure
Water
Carbon Dioxide