Unit 5 States of Matter Part A: Phase...

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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)

[This object is a pull tab]

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.


Ans

wer

D


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


Ans

wer

A


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)


Ans

wer

C


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


Ans

wer

B


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


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


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


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?


Ans

wer

38,090 J/mol


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)


Ans

wer

14.09 grams


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.


Ans

wer

9.9 kJ/mol


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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A heating curve provides information about the strength of particle interactions.

T

time

ss <--> l

l

l <--> g

g


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


Boiling Point

CH3OH Cp = 1.7 J/gC

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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


Ans

wer

B


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


Ans

wer

C


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


Ans

wer

D


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


Ans

wer

C


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


Ans

wer

368,000 J or 368 kJ


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


Ans

wer

C


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


Ans

wer

A


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


Ans

wer

E


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Unit 5 States of Matter Part A: Phase...

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