unit 5 states of matter part a: phase...
TRANSCRIPT
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Unit 5
States of MatterPart A: Phase Changes
AP Chemistry
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States of Matter
Many scientists now believe that the temperature becomes warm enough in Mars equatorial regions to melt subsurface
ice and cause small gullies to form in the soil.
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Phase ChangesPhase changes involve a change in the potential energy of a
material brought about by either the strengthening or weakening of the interactions between representative particles.
The table below lists the six phase changes of water and the potential energy change of each.
Phase Change States Involved
Change in PE
Change in Tsurr
Endo or Exothermic
Melting s --> l +6.0 kJ/mol - Endo
Evaporation l --> g +41 kJ/mol - Endo
Condensation g --> l -41 kJ/mol + ExoFreezing l --> s -6.0 kJ/mol + Exo
Sublimation s --> g +47 kJ/mol - EndoDeposition g --> s -47 kJ/mol + Exo
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Phase ChangesEndothermic phase changes occur when the molecules have enough kinetic energy to overcome the coulombic attractions
between the particles.
Examining the melting of water (ice)
Potential Energy
solid
liquid
Since the kinetic energy of the molecules was converted to potential energy, the Tsurr will decrease.
When a substance melts, the bonds are only stretched thus requiring less energy than boiling where the bonds are broken.
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Phase ChangesExothermic phase changes occur when the molecules have lost
enough kinetic energy to permit coulombic attractions to form between the particles.
Examining the condensing of water
Potential Energy
liquid
gas
Since the potential energy of the molecules was converted to kinetic energy, the Tsurr will increase.
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Phase Changes
At a certain temperature and pressure, two phase changes will occur at equal rates resulting in equilibrium.
Standard Melting Point of H2O(0 C at 1 atm)
H2O(s) <--> H2O(l) = equilibrium
Temperature above 0 C at 1 atm
H2O(s) --> H2O(l) = not at equilibrium
Temperature below 0 C at 1 atm
H2O(s) <-- H2O(l) = not at equilibrium
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Phase ChangesThe standard (1 atm) melting and boiling point temperatures are
dependent on the types of interactions found within the substance.
Substance Example Particle Interactions
Influenced by
StandardMP
Ionic MgO Ionic (intra) charge and ionic radii 2852 C
Covalent Network C(diamond) Covalent
(intra) atomic radii 3550 C
Metallic Au Covalent (intra) ionic radii 1064 C
Molecular I2LDF's (inter)
polarizability (# of e-) 114 C
Note the big difference in melting point between substances with intra vs. inter molecular interactions.
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1 Which of the following phase changes would be exothermic?
A Na(s) --> Na(l)
B Na(l) --> Na(g)
C Na(s) --> Na(g)
D Na(g) --> Na(l)
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1 Which of the following phase changes would be exothermic?
A Na(s) --> Na(l)
B Na(l) --> Na(g)
C Na(s) --> Na(g)
D Na(g) --> Na(l)
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Ans
wer
D
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2 Which of the following would be true when ethyl alcohol evaporates?
A I only
B II only
C III only
D I and III only
E I, II, and III
I. The potential energy of the substance increases.
II. The kinetic energy of the surroundings increases.
III. Coulombic attractions are broken.
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2 Which of the following would be true when ethyl alcohol evaporates?
A I only
B II only
C III only
D I and III only
E I, II, and III
I. The potential energy of the substance increases.
II. The kinetic energy of the surroundings increases.
III. Coulombic attractions are broken.
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Ans
wer
D
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3 Which of the following changes would be endothermic?
A sublimation
B deposition
C condensation
D freezing
E All of these
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3 Which of the following changes would be endothermic?
A sublimation
B deposition
C condensation
D freezing
E All of these
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Ans
wer
A
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4 For which of the following are covalent bonds breaking?
A CO2(s) --> CO2(l)
B CO2(l) --> CO2(g)
C C(s) --> C(g)
D C(s) --> C(l)
E NaCl(l) --> NaCl(g)
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4 For which of the following are covalent bonds breaking?
A CO2(s) --> CO2(l)
B CO2(l) --> CO2(g)
C C(s) --> C(g)
D C(s) --> C(l)
E NaCl(l) --> NaCl(g)
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Ans
wer
C
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5 Which of the following BEST characterize the standard boiling point of a material?
A Liquid and vapor phases are in equilibrium at any pressure
B Liquid and vapor phases are in equilibrium at1 atm pressure
C The rate of vapor formation exceeds that of liquid formation at any pressure
D The rate of liquid formation exceeds that of vapor formation at 1 atm
E The potential energy of the material is decreasing
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5 Which of the following BEST characterize the standard boiling point of a material?
A Liquid and vapor phases are in equilibrium at any pressure
B Liquid and vapor phases are in equilibrium at1 atm pressure
C The rate of vapor formation exceeds that of liquid formation at any pressure
D The rate of liquid formation exceeds that of vapor formation at 1 atm
E The potential energy of the material is decreasing
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Ans
wer
B
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6 Which of the following correctly ranks the materials from lowest to highest melting point?
A Na < Li < K
B CaO < MgO < NaF
C CH4 < NH3 < H2O
D I2 < Br2 < F2
E Na < H2O < I2
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6 Which of the following correctly ranks the materials from lowest to highest melting point?
A Na < Li < K
B CaO < MgO < NaF
C CH4 < NH3 < H2O
D I2 < Br2 < F2
E Na < H2O < I2[This object is a pull tab]
Ans
wer
C
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Phase ChangesThe heat of fusion is the amount of energy that is required to
melt a given quantity of a material. The same amount of energy is released when that substance freezes.
The heat of fusion depends on the strength of the particle interactions within the solid.
Substance Substance Type
Particle Interactions Heat of Fusion
NaCl Ionic Ionic Bonds (Intra) 28 kJ/mol
NaF Ionic Ionic Bonds (Intra) 32 kJ/mol
H2O Molecular H-Bonds, LDF's (Inter) 6 kJ/mol
CH4 Molecular LDF's (Inter) 1 kJ/mol
Ag Metallic Covalent Bonds (Intra) 11.3 kJ/mol
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Phase ChangesThe heat of vaporization is the amount of energy that is required to vaporize a given quantity of a material. The same amount of
energy is released when that substance condenses.
The heat of vaporization depends on the strength of the particle interactions within the solid.
Substance Substance Type
Particle Interactions Heat of Fusion
CH3OH Molecular LDF's, H-Bonds 35 kJ/mol
CH3CH2CH3 Molecular LDF's 16 kJ/mol
H2O Molecular H-Bonds, LDF's 41 kJ/mol
CH3COCH3 Molecular LDF's, DDF's 31 kJ/mol
I2 Molecular LDF's 42 kJ/mol
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Phase ChangesCalculating the heats of fusion or vaporization requires
measuring the energy absorbed or released as the material undergoes a phase change.
Example: What is the heat of fusion for water (kJ/mole)assuming that the temperature of a 100 gram sample of water
(Cp = 4.2 J/gC) decreased by 14.2 degrees when 18.0 grams of ice is melted in a container surrounded by the water.
1. Find the heat absorbed from water:
Heat absorbed (q) = m* T*cp = 100 g x 14.2 C x 4.2 J = 5,960 J
gC 2. Find kJ/mol:
18 g H2O x 1 mol = 1 mole H2O
18 g
5.96 kJ/ 1mole = 5.96 kJ/mol
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7 Which of the following would have the highest heat of fusion?
A Na
B Li
C K
D Mg
E Be
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7 Which of the following would have the highest heat of fusion?
A Na
B Li
C K
D Mg
E Be[This object is a pull tab]
Ans
wer
E
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8 Which of the following would have the lowest heat of vaporization?
A C4H10
B CH3CH2CH2CHO
C I2
D H2O
E C2H4
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8 Which of the following would have the lowest heat of vaporization?
A C4H10
B CH3CH2CH2CHO
C I2
D H2O
E C2H4 [This object is a pull tab]
Ans
wer
E
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9 What is the heat of vaporization (J/mol) of ethanol assuming that 419 Joules of energy are needed to vaporize 0.5 grams of the material?
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9 What is the heat of vaporization (J/mol) of ethanol assuming that 419 Joules of energy are needed to vaporize 0.5 grams of the material?
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Ans
wer
38,090 J/mol
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10 How many grams of water must have been present if 3210 J of energy were needed to vaporize the sample? (The heat of vaporization of water is 41 kJ/mol)
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10 How many grams of water must have been present if 3210 J of energy were needed to vaporize the sample? (The heat of vaporization of water is 41 kJ/mol)
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Ans
wer
14.09 grams
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11 Calculate the heat of fusion (kJ/mol) for benzene (C6H6) based on the following data. When a 12 gram sample of benzene was melted in in a 500 gram sample of water, the temperature of the water dropped by 0.73 degrees.
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11 Calculate the heat of fusion (kJ/mol) for benzene (C6H6) based on the following data. When a 12 gram sample of benzene was melted in in a 500 gram sample of water, the temperature of the water dropped by 0.73 degrees.
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Ans
wer
9.9 kJ/mol
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Heating/Cooling Curves
A heating curve plots the change in temperature over time as a material is heated through it's phase changes.
T
time
ss <--> l
l
l <--> g
g
Melting/Freezing Point
Boiling PointMore time/heat is needed to
vaporize the material than melt it. This is due to the heat of
vaporization being much higher than the heat of fusion.
Note how the temperature (average kinetic energy) remains stable during a phase change. This is because kinetic energy is being converted to potential energy when a substance changes state.
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Heating/Cooling Curves
A heating curve provides information about the strength of particle interactions.
T
time
ss <--> l
l
l <--> g
g
Melting/Freezing Point
Boiling Point
Assuming a constant amount of added heat, the shallower the slope, the higher the specific heat, indicating stronger particle
interactions.
H2O Cp = 4.2 J/gC
T
time
ss <--> l
l
l <--> g
g
Melting/Freezing Point
Boiling Point
CH3OH Cp = 1.7 J/gC
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Heating/Cooling Curves
A cooling curve plots the change in temperature over time as a material is cooled through it's phase changes. .
time cooled
Temperature
(C)
boiling point100
0 freezing point
KE
KE
KE
PE
PE
super-cooled liquid
H2O Cooling Curve
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12 At which point(s) would the solid and liquid phase be in equilibrium?
A A
B A & B
C C
D C & D
E E
T
time
AB
CD
E
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12 At which point(s) would the solid and liquid phase be in equilibrium?
A A
B A & B
C C
D C & D
E E
T
time
AB
CD
E
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Ans
wer
B
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13 At which point would kinetic energy be transferred to potential energy?
A A only
B A & C & E
C B & D
D A & B
E E only
T
time
AB
CD
E
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13 At which point would kinetic energy be transferred to potential energy?
A A only
B A & C & E
C B & D
D A & B
E E only
T
time
AB
CD
E
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Ans
wer
C
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14 What best characterizes what is occurring at point D on the heating curve?
A The distance between the molecules is decreasing
B The kinetic energy of the molecules is increasing
C The potential energy of the molecules is decreasing
D Coulombic attractions are being broken
E None of these
T
time
AB
CD
E
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14 What best characterizes what is occurring at point D on the heating curve?
A The distance between the molecules is decreasing
B The kinetic energy of the molecules is increasing
C The potential energy of the molecules is decreasing
D Coulombic attractions are being broken
E None of these
T
time
AB
CD
E
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Ans
wer
D
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15 The cooling curve for an unknown substance is found below. What is its freezing point?
A 94 C
B 100 C
C 80 C
D 78 C
E 70 C
T
time
100
90
80
70
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15 The cooling curve for an unknown substance is found below. What is its freezing point?
A 94 C
B 100 C
C 80 C
D 78 C
E 70 C
T
time
100
90
80
70
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Ans
wer
C
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16 How much energy (in Joules) would be needed to heat of 120 gram chunk of ice from -7 C to 112 C assuming the following:
Heat of Fusion = 6 kJ/mol Heat of vaporization = 41 kJ/mol
Cp (ice) = 2.1 J/gC Cp(water) = 4.2 J/gC Cp(steam) = 2.0 J/gC
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16 How much energy (in Joules) would be needed to heat of 120 gram chunk of ice from -7 C to 112 C assuming the following:
Heat of Fusion = 6 kJ/mol Heat of vaporization = 41 kJ/mol
Cp (ice) = 2.1 J/gC Cp(water) = 4.2 J/gC Cp(steam) = 2.0 J/gC
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Ans
wer
368,000 J or 368 kJ
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Phase Diagrams
Phase Diagrams are not officially a part of the AP Chemistry curriculum as they expect the student to have mastered the content in a first year chemistry course. For those students who did not master the material yet for whatever reason,
the slides from the first year course covering phase diagrams have been included directly following this one.
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Phase Diagrams
Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria
exist between phases.
In general, increasing the pressure strengthens bonds and tends to produce the solid state. In contrast, increasing the temperature will
weaken bonds and will promote the gaseous state.
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Phase DiagramsThe liquid-vapor interface ends at the critical point (C); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other.
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Phase Diagrams
The circled line in the diagram below is the interface between liquid and solid.
The melting point at each pressure can be found along this line.
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Phase DiagramsBelow the triple point the substance cannot exist in the liquid state.
Along the circled line the solid and gas phases are in equilibrium; the sublimation point at each pressure is along this line.
Sublimation and deposition only occur below the triple point.
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The circled line is the liquid-vapor interface.
It starts at the triple point (T), the point at which all three states are in equilibrium.
Phase Diagrams
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Phase Diagram of Water
Note the solid-liquid equilibrium line has a negative slope. This means
that an increase in pressure causes the liquid state to form.
Due to the hydrogen bonding between
water molecules, the solid state of water is less dense than the liquid so an increase in pressure will disrupt these hydrogen bonds and produce
the liquid.
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Comparison of Two Phase Diagrams
For carbon dioxide, the slope of the solid-liquid line is positive, as it is for most
other substances. This means that an increase in
pressure can cause substances to freeze.
For water, the slope of the solid-liquid line is negative.
This means that an increase in pressure can cause this substance to
melt.
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Phase Diagram of Carbon Dioxide
Notice that CO2 has a triple point at a pressure 5x normal atmospheric pressure. As a result, when solid CO2 is heated, it will sublimate directly to a gas. This produces a nice foggy
effect in theaters!
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17 Which point represents a freezing point of the
substance?
A
B
C
D
E
P
(atm)
T (C) -20 0 40 60
1
2
3 A
B
C
D
E
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17 Which point represents a freezing point of the
substance?
A
B
C
D
E
P
(atm)
T (C) -20 0 40 60
1
2
3 A
B
C
D
E
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Ans
wer
C
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18 If the substance has its pressure increased isothermally at 40 C, from 1 atm to 3 atm, what phase change will it undergo?
A condensation
B evaporation
C sublimation
D deposition
E melting
P
(atm)
T (C) -20 0 40 60
1
2
3 A
B
C
D
E
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18 If the substance has its pressure increased isothermally at 40 C, from 1 atm to 3 atm, what phase change will it undergo?
A condensation
B evaporation
C sublimation
D deposition
E melting
P
(atm)
T (C) -20 0 40 60
1
2
3 A
B
C
D
E
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Ans
wer
A
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19 In the phase diagram below, if the material is heated from -20 C to 60 C at a constant pressure of 1 atm, what phase change will occur?
A Evaporation
B Condensation
C Melting
D Freezing
E Sublimation
P
(atm)
T (C) -20 0 40 60
1
2
3 A
B
C
D
E
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19 In the phase diagram below, if the material is heated from -20 C to 60 C at a constant pressure of 1 atm, what phase change will occur?
A Evaporation
B Condensation
C Melting
D Freezing
E Sublimation
P
(atm)
T (C) -20 0 40 60
1
2
3 A
B
C
D
E
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Ans
wer
E
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