Copyright © 2020 Dan Dill [email protected] Lecture 3 CH102 ...genchem.bu.edu/ch102-summer-1-2020/lectures/03-as-given.pdf · Lecture 3 CH102 Summer 1 2020 5/26/2020 4:22 PM Copyright ©

Lecture 3 CH102 Summer 1 2020 5/26/2020 4:22 PM

Copyright © 2020 Dan Dill [email protected] 1

Lecture 3 CH102 Summer 1 2020Tuesday, May 26, 2020

• Units of pressure• Vapor pressure: Liquid-vapor equilibrium• Vapor pressure and boiling point• Phase diagramsChapter 16: Colligative properties of solutions• Vapor pressure lowering, 𝑃 𝑥 𝑃°Next: Continue: Colligative properties of solutions

Review: Units of pressure1 Pa

Lecture 3 CH102 Summer 1 2020

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Copyright © 2020 Dan Dill [email protected]

Review: Units of pressure1 Pa 1 /


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Review: Units of pressure1 Pa 1 / 1 /


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Review: Units of pressure1 Pa 1 / 1 / 1


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Review: Units of pressure1 Pa 1 / 1 / 1

1 bar 100 kPa exactly

1 atm 101.325 kPa exactly 760 Torr exactly


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Equilibrium vapor pressureLecture 3 CH102 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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EvaporationProblem 15-21: A 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

Copyright © 2020 Dan Dill [email protected] 3 CH102 Summer 1 2020

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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

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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Phase diagramsLines of 𝑝 versus 𝑇 at which different phases

are present at the same time.

That is, values of 𝑝 and 𝑇 at which different phasesare in equilibrium.


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Phase diagramsLecture 3 CH102 Summer 1 2020

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What about the intersection marked “A”Lecture 3 CH102 Summer 1 2020

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Triple pointLiquid tert-butyl alcohol, C CH3 3OH, can boil and freeze at the same time... http://goo.gl/4K1SR


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Supercritical fluidSupercritical transition of liquid Cl2 … http://goo.gl/xo2jU


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Changes of phaseCO2 gas at 1 atm, 0℃

is heated to 1 atm, 60℃; then compressed to 75 atm, 60℃;

then cooled to 75 atm, 0℃; then expanded to 65 atm, 0℃.


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TP CO2 gas at 1 atm, 0℃ is heated to 1 atm, 60℃;

then compressed to 75 atm, ℃; then cooled to 75 atm, 0℃;

then expanded to 65 atm, 0℃. At this point, the CO2 will be a …

1. gas2. supercritical fluid3. liquid4. solid


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TP CO2 gas at 1 atm, 0℃ is heated to 1 atm, 60℃;

then compressed to 75 atm, ℃; then cooled to 75 atm, 0℃;

then expanded to 65 atm, 0℃. At this point, the CO2 will haveundergone a phase transition …

1. once2. twice3. three times4. None of the above


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Phase diagrams are specific to each substanceWater CO2


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H2O phase diagramIs density of liquid H2O greater than that of solid H2O? How can we tell?


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Colligative propertiesNon-volatile solute negligible vapor pressure …

• lowers vapor pressure of solvent: 𝑃 𝑥 𝑃° , ∆𝑃 𝑃° 𝑃 𝑥 𝑃°𝑥 is the mole fraction of the solvent: 𝑥

Pure solvent means 𝑥 1 and so 𝑃 𝑃°𝑥 is the mole fraction of the solute: 𝑥 1 𝑥

Lecture 6 CH102 A2 (MWF 11:15 am) Spring 2020

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Effect of solute on vapor pressureThe vapor pressure of water at 32℃ is 4.76 kPa. Calculate the vapor pressure after 68.0 g of ethylene glycol 𝑀 62.07 is dissolved in 500. g of water at 32℃.

Answer: 4. 58 kPa

Lecture 6 CH102 A2 (MWF 11:15 am) Spring 2020

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Copyright © 2020 Dan Dill [email protected] Lecture 3 CH102 ...genchem.bu.edu/ch102-summer-1-2020/lectures/03-as-given.pdf · Lecture 3 CH102 Summer 1 2020 5/26/2020 4:22 PM Copyright ©