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Chapter 12: Phase Changes and Equilibrium
Grade Level Indicator
Describe how a physical, chemical, or ecological system in
equilibrium may return to the same state of equilibrium if the
disturbances it experiences are small. Large disturbances may
cause it to escape equilibrium and eventually settle into some
other state of equilibrium.
Essential Question
How do various factors combine to cause a substance to be a given
phase of matter?
I can...
list and define all phase changes (boiling, freezing, etc.)
relate phase changes to energy transfers and attractive forces
explain how the concept of dynamic equilibrium relates to phase
changes
discuss the relationship between temperature, vapor pressure,
and the boiling point of a substance
analyze heating and cooling curves and use them to determine
the amount of energy associated with phase change
compare molar heat of fusion and molar heat of vaporization,
and explain the difference in their magnitudes
analyze and interpret phase diagrams
Key Words
equilibrium dynamic equilibrium
phase condensation
evaporation boiling
freezing sublimation
deposition Le Chateliers principle
vapor pressure volatile
nonvolatile freezing point
boiling point molar heat of vaporization
molar heat of fusion phase diagram
triple point critical point
critical temperature critical pressure
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Chapters 12: Phase Changes and Equilibrium
Section 1: Mathematics of Phase Changes
Different phases exist because of the relationship between a substances temperature,
pressure, and intermolecular forces
Every phase change is accompanied by a change in energy
Energy is either released in the process(exothermic), or absorbed in theprocess(endothermic)
The energy change associate with these processes can be thought of in both thequalitative and quantitative sense
Qualitatively, this can be determined by applying a little logic
SAMPLE EXERCISE: For each of the following processes, determine if the process isendothermic or exothermic
a) H2O(s) H2O(l) _________________________
b) H2O(g) H2O(l) _________________________
c) CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) _________________________
Quantitatively, the energy change associated with a phase change is found using the
molar heat of fusion and the molar heat of vaporization
The molar heat of fusion is the amount of heat energy(in kJ) that must be absorbed tomelt 1 mole of a substance, or the amount of heat released when 1 mole of a substance
freezes
The molar heat of vaporization is the amount of heat energy(in kJ) that must beabsorbed to vaporize(evaporate) 1 mole of a substance, or the amount of heat released
when 1 mole of a substance condenses
The three important points to remember with molar heats of fusion and vaporization
are:
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1) Heat of fusion is used with melting and freezing; heat of vaporization is used with
evaporating and condensing
2) Each substance has its own heat of fusion and heat of vaporization
The heat of fusion of water is different than the heat of fusion of pudding
3) The heats of fusion and vaporization and fusion are the number of kJ for every one
mole of the substance: if there is more or less than 1 mole, this must be taken intoaccount
SAMPLE EXERCISE 1: What quantity of energy is needed to melt 500.0 grams of
ice, H2O(l)?
SAMPLE EXERCISE 2: How much heat is given off when 37.0 g of C6H6(g) isconverted to C6H6(l)?
Substance Heat of Fusion Heat of Vaporization
Water, H2O 6.02 kJ/mol 40.7 kJ/mol
Butane, C6H6 5.0 kJ/mol 24.0 kJ/mol
Ethanol, C2H5OH 5.02 kJ/mol 38.56 kJ/mol
Oxygen, O2 0.22 kJ/mol 3.41 kJ/mol
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Chapters 12: Phase Changes and Equilibrium
Section 2: Phase Changes
The phase of a sample of matter(solid, liquid, gas) depends in part on the relationship
between kinetic energy
What happens to a sample of H2O when the temperature reaches 0C(at a pressure of 1
atm)?
What happens to a sample of H2O when the temperature reaches 100C(at a pressure of
1 atm)?
From this, it is seen that at the temperature of the phase change, the two phases exist in
dynamicequilibrium
Dynamic equilibrium exists when two opposite processes occur at identical rates
At a constant temperature(and pressure of 1 atm)) of 0C, H2O is melting and freezing
at the same rate
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H2O(s) + heat energy H2O(l)
H2O(l) H2O(l) + heat energy
So while ice is constantly melting, and water is constantly freezing, the amount of eachremains constant
H2O(s) + heat energy H2O(l) At a constant temperature(and pressure of 1 atm) of 100C, H2O is evaporating and
condensing at the same rate
H2O(l) + heat energy H2O(g)
H2O(g) H2O(l) + heat energy
So while water is constantly evaporating, and steam is constantly condensing, the
amount of each remains constant
H2O(l) + heat energy H2O(g)
Equilibrium is a part of all virtually every chemical process; at some point in a reaction,a dynamic equilibrium is reached in which the rate of the forward process equals the rate
of the reverse reaction
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Chapters 12: Phase Changes and Equilibrium
Section 3: Mathematics of Phase Changes II
This gives a way to calculate the amount of energy involved in, for example, themelting of 25.00 g of ice, H2O(s)
But think about what would need to happen if this sample of ice is initially at atemperature of -10.00C
Would the energy initially added to the ice cause the ice to melt?
Therefore, to calculate the energy needed to melt 25.00 grams of ice that had an initial
temperature of -10.00C, it is also necessary to calculate the amount of energy needed toget the ice to its melting point at 0.00C
Unless told otherwise, it is always assumed that the temperature for a phase change isthe normal(at 1 atm) temperature for that phase change
To help organize and plan the work for these questions, heating curves and cooling
curves are very helpful
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To see how this works, consider the amount of energy needed to take 25.00 grams of
ice, H2O(s) and melt it all. That is, convert it all to H2O(l).
H2O(s) H2O(l)
Heat of Vaporization, H2O: 40.67 kJ/mol
Heat of Fusion, H2O : 6.008 kJ/mol
Specific Heat, H2O(s) : 2.092 J/gK
Specific Heat, H2O(l) : 4.184 J/gK
Specific Heat, H2O(g) : 1.841 J/gK
Note the specific heat(c) also depends on the phase of a substance
SAMPLE EXERCISE 1: Calculate the enthalpy change upon converting
18.02 grams of ice at -25.0C to water vapor(steam) at 125C undera constant pressure of 1 atm.
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56.0 kJ or 56,000 J
SAMPLE EXERCISE 2 : What is the energy change during the process in
which 100.0 g of water at 50.0C is cooled to ice at -30.0C? Is
this an endothermic or exothermic process?
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60.6 kJ or 60,600 J
Chapters 12: Phase Changes and Equilibrium
Section 4: Vapor Pressure and Vapor Pressure Diagrams
Imagine, if you will, a sealed bottle of water: what are the H2O(l) molecules at the
surface of the liquid constantly doing?
How will this cause the pressure inside the bottle do change?
When this dynamic equilibrium has been reached, the constant pressure inside the bottleis known as the equilibrium vapor pressure, although it is more commonly just called
vapor pressure
Vapor pressure is normally defined as the pressure exerted by a vapor in equilibrium
with its liquid or solid phase
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In the chart above, the curves represent the vapor pressures of variousliquids---------------------------------------------------------------------------------------------------
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Answer the following questions based on the vapor pressure diagram above:
1) What is the relationship between temperature and vapor pressure?
2) What is the relationship between atmospheric pressure and the vapor pressure of the
liquid at the liquids boiling point?
3) What is the normal boiling point(boiling point at 1 atm) for diethyl ether? For ethyl
alcohol?
4) Which of these three substances has the strongest intermolecular forces? Which hasthe weakest? Middle?
Chapters 12: Phase Changes and Equilibrium
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Section 5: Phase Diagrams
Shown above is the phase diagram for water; this shows the characteristic appearance of
a phase diagram
All phase diagrams have essentially the same appearance, with only a few smalldifferences
In looking at the graph, what conditions determine the phase(solid, liquid, or gas) of a
substance?
So when it is said water boils at 100C, is this always true?
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Notice the diagram is separated into three regions; each of these regions represents one
of the phases of matter
Which region goes with which phase?
The same phase goes with the same region in every single phase diagram you will eversee
Water at 0.5 atm and -20C would be at what phase?
Water at 0.5 atm and 50C would be at what phase?
So water at 0.5 atm and -20C would undergo what phase change when the pressure
remains unchanged and the temperature increases to 50C?
The lines between the phases regions show the pressure and temperature conditions at
which a given phase change will occur
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At any set of temperature and pressure data that are a point along a line, this is where a
phase change occurs, and the two phases exist in equilibrium
What happens where all three of these line segments come together?
The other point to be aware of is the critical point: this point represents the criticaltemperature and the critical pressure
Beyond the critical point, the liquid and the gas phases are indistinguishable
Critical temperature is the highest possible temperature at which a substance can still bea liquid
The critical pressure is the pressure at the critical temperature
SAMPLE EXERCISE: Describe any changes in the phases present when H2O is kept at1000C while the pressure is increased from 6.0 x 10-3 atm to 10 atm.
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