Lecture 8 CH131 Summer 1 2020 6/4/2020 12:35 PMgenchem.bu.edu/ch131-summer-1-2020/lectures/08-as-given.pdf · Lecture 8 CH131 Summer 1 2020 6/4/2020 12:35 PM Copyright © 2020 Dan

Lecture 8 CH131 Summer 1 2020 6/4/2020 12:35 PM

Copyright © 2020 Dan Dill [email protected] 1

Lecture 8 CH131 Summer 1Thursday, June 4, 2020

• Complete: IMFs determine relative boiling points• Practice: Boiling points and enthalpy of vaporization• Vapor pressure: Liquid-vapor equilibrium• Vapor pressure and boiling pointNext: Phase diagrams; Ch11: Solutions

Why is HCl < HBr < HI?Copyright © 2020 Dan Dill [email protected] 8 CH131 Summer 1 2020

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TP The correct order of polarity of HBr, HCl and HI is least to most …

1. HBr HCl HI2. HBr HI HCl3. HCl HBr HI4. HCl HI HBr5. HI HCl HBr6. HI HBr HCl

Copyright © 2020 Dan Dill [email protected] 8 CH131 Summer 1 2020

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TP The correct order of boiling point of HBr, HCl and HI is lowest to highest …

1. HBr HCl HI2. HBr HI HCl3. HCl HBr HI4. HCl HI HBr5. HI HCl HBr6. HI HBr HCl


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Permanent dipolesSketch a second molecule arranged so that it …

attracts maximally repels maximally neither attracts nor repels

Lecture 8 CH131 Summer 1 2020

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Copyright © 2020 Dan Dill [email protected]

Induced dipolesSketch the induced dipole in each case:


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Why is HCl < HBr < HI?Copyright © 2020 Dan Dill [email protected] 8 CH131 Summer 1 2020

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Why is NH3 > PH3 < AsH3 < SbH3?Copyright © 2020 Dan Dill [email protected] 8 CH131 Summer 1 2020

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Why is OH2 > FH > NH3?Copyright © 2020 Dan Dill [email protected] 8 CH131 Summer 1 2020

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Why is FH > NH3?Copyright © 2020 Dan Dill [email protected] 8 CH131 Summer 1 2020

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TP Identify the compound with the smallest energy of vaporization kJ/mol .

1. water2. ethanol3. dimethyl ether4. hexane


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TP The correct order of boiling point of CH3OH, CH4, and S C Olowest to highest is …

1. CH3OH CH4 S C O2. S C O CH4 CH3OH 3. CH4 S C O CH3OH 4. CH4 CH3OH S C O5. some other order


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TP The correct order of boiling point of CH3Br, CH2Br2, CH3Cl, and CH2Cl2lowest to highest is …

1. CH2Br2 CH2Cl2 CH3Br CH3Cl2. CH2Cl2 CH2Br2 CH3Cl CH3Br3. CH3Cl CH3Br CH2Cl2 CH2Br2

4. CH3Br CH3Cl CH2Br2 CH2Cl2

5. CH3Cl CH3Br CH2Br2 CH2Cl2

6. some other order


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Liquid-vapor equilibriumLecture 8 CH131 Summer 1 2020

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TP Bubbles in a pot of boiling water contain …

1. Only O2 𝑔 and N2 𝑔 2. Only H2O 𝑔3. Only air 4. H2O 𝑔 and air5. All of the above equally6. None of the above


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Equilibrium vapor pressureLecture 8 CH131 Summer 1 2020

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Sketch what is going on at the surface at this point.

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Sketch what is going on at the surface now.


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Sketch how pressure must change with temperature.

Equilibrium vapor pressure

Vapor pressure must rise with temperature.


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Equilibrium vapor pressure

Vapor pressure must rise with temperature.Why is this so?


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EvaporationVapor pressure rises with temperature.


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EvaporationVapor pressure rises with temperature.

𝑇 60.0℃,𝑃 351 Torr𝑇 78.3℃, 𝑃 760 Torr


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EvaporationA 0.750-gram sample of ethanol CH CH OH is placed in a sealed 400-mL container. Is there any liquid present when the temperature is held at 60℃?

𝑇 60.0℃,𝑃 351 Torr, 𝑅 0.0820575

𝑛 ?, calculate from molar mass𝑃 ?, calculate from ideal gas law, 𝑃𝑉 𝑛𝑅𝑇𝑛 ?, calculate from ideal gas law, using actual pressure, 𝑃𝑛 𝑛 𝑛 ?


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𝑇 60.0℃,𝑃 351 Torr, 𝑅 0.0820575

𝑛 0.0163 mol, calculated from molar mass


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𝑇 60.0℃,𝑃 351 Torr, 𝑅 0.0820575

𝑛 0.0163 mol, calculated from molar mass𝑃 846 Torr, calculate from ideal gas law 𝑃 351 Torr


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𝑇 60.0℃,𝑃 351 Torr, 𝑅 0.0820575

𝑛 0.0163 mol, calculated from molar mass𝑃 846 Torr, calculated from ideal gas law 𝑃 351 Torr𝑛 0.00675 mol, calculated from ideal gas law, using 𝑃 351 Torr


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𝑇 60.0℃,𝑃 351 Torr, 𝑅 0.0820575

𝑛 0.0163 mol, calculated from molar mass𝑃 846 Torr, calculated from ideal gas law 𝑃 351 Torr𝑛 0.00675 mol, calculated from ideal gas law, using 𝑃 351 Torr𝑛 𝑛 𝑛 0.0163 mol 0.00675 mol 0.0096 mol


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𝑇 60.0℃,𝑃 351 Torr, 𝑅 0.0820575

𝑛 0.0163 mol, calculated from molar mass𝑃 846 Torr, calculated from ideal gas law 𝑃 351 Torr𝑛 0.00675 mol, calculated from ideal gas law, using 𝑃 351 Torr𝑛 𝑛 𝑛 0.0163 mol 0.00675 mol 0.0096 mol

Note, we have ignored the slight increase in vapor volume because of the decrease in liquid volume due to evaporation.


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Vapor pressure and boiling pointBoiling means “bubbles”.

Bubbles are pure vapor of the liquid.

Bubbles form only if they are able to hold back the liquid walls.

This means bubbles only exist if the vapor pressure is the same as the pressure acting on the liquid surface.


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Vapor pressure and boiling pointThe normal boiling point is the temperature at which bubbles form at 1 atm.

What do you predict for relative boiling points of these substances?


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TP The order of normal boiling points lowest to highest is …

1. diethyl ether acetone ethanol2. ethanol acetone diethyl ether3. acetone diethyl ether ethanol4. ethanol diethyl ether acetone5. something else


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Substance Vapor pressure at 25℃, kPa

Normal 1 atm boiling point/℃

Acetone, CH3C O CH3 30.8Diethyl ether, CH3CH2 2O 71.7

Ethanol, CH3CH2OH 7.87Water, H2O 3.17 100

Vapor pressure and boiling pointThe normal boiling point is the temperature at which bubbles form at 1 atm.

What do you predict for relative boiling points of these substances?


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Substance Vapor pressure at 25℃, kPa

Normal 1 atm boiling point/℃

Acetone, CH3C O CH3 3: 30.8 3: 56

Diethyl ether, CH3CH2 2O 4: 71.7 4: 35

Ethanol, CH3CH2OH 2: 7.87 2: 78

Water, H2O 1: 3.17 1. 100

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Relative normal boiling point, TbThe normal boiling point is the temperature at which bubbles form at 1 atm.

What do you predict for relative vapor pressures of these substances at 200°C?


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TP The substance with the lowest vapor pressure substances at 200°C is …

1. CH4

2. NH3

3. HCl4. N2


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Documents

Lecture 8 CH131 Summer 1 2020 6/4/2020 12:35 PMgenchem.bu.edu/ch131-summer-1-2020/lectures/08-as-given.pdf · Lecture 8 CH131 Summer 1 2020 6/4/2020 12:35 PM Copyright © 2020 Dan