viscosity variations with molecular structure
DESCRIPTION
In this presentation, I explain why viscosity varies with molecular structure using a homologous series of straight chained alcohols. Hint: London dispersion forces!TRANSCRIPT
Charlotte Chaze η
The phenomenon that applied shearing force that produces flow in a fluid is resisted by a force that is proportional to the gradient of flow velocity in that fluid1
In other words: viscosity is resistance to flow
Olive oil shows greater resistance to flow than water does:It has a higher viscosity
Determine viscosities of homologous series of compounds Ethanol 1-propanol 1-butanol 1-hexanol 1-octanol
Show a useful correlation between viscosity and physical properties Molecular weight Density Boiling point Enthalpy of vaporization Entropy of vaporization
The cause of differential viscosity between compounds
Remember freshman year?
Attraction felt between polar molecules due to opposite charges In 1° alcohols: hydrogen bonds▪ H on electronegative atom + lone e- pair on
another electronegative atom
Hydrogen is a small atom, allowing a small bond length between it and oxygen▪ strong dipole-dipole interaction
H
H
H
H H
H
HO
HO
H
HH
H
δ+
δ+
δ-
δ-
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
O
OH
H
H
H
H
H
H
H
H
H
H
H
H
δ+
δ+
δ-
δ-
ethanol 1-octanol
H-bonding = Resistance to flow
Homologous compounds = similar H bonding
Images from http://www.uic.edu/classes/bios/bios100/lectures/chemistry.htm
• In ice, water molecules form a crystal lattice• In water, no crystal lattice forms• Ice is less dense than water• Which has a higher viscosity?
Ice, ice baby
The net E of interaction of permanent dipoles is given by:
Note: net E is inversely proportional to r6
Energy of interaction falls off rapidly with distance between dipoles
When electrons are distributed unsymmetrically, molecules develop temporary (instantaneous) dipoles
Image from http://www.chem.purdue.edu/gchelp/liquids/disperse.html
A second molecule can then be distorted by the dipole in the first molecule, leading to an electrostatic attraction between the two molecules
Momentary uneven electron distribution causes partial charges
Electron density of a nearby molecule is attracted to the positive charge
Image from http://www.chem.purdue.edu/gchelp/liquids/disperse.html
Larger, heavier molecules exhibit stronger dispersion forces In larger molecules, the valence electrons are
farther from the nuclei▪ They are less tightly held and can more easily form
temporary dipoles The ease with which the electron
distribution around a molecule can be distorted is called polarizability
Larger molecules are more polarizable and thus more viscous
H
H
OH
HH
H
Size of molecule defines size of electron cloud
Longer chain = larger electron cloud = more polarizability & more London dispersion forces
When molecules are more polarizable, the δ+ and δ- of different molecules is stronger, and tightly packs the molecules into a more viscous solution
1-octanol should be most viscous, ethanol should be least viscous
Measuring a homologous series of compounds
Warning: measuring viscosity requires long periods of waiting
Ostwald viscometer Calibrated by water (known viscosity)▪ Thermostat used to control temperature
Measure efflux time of compound Meniscus flowing from upper mark to
lower mark using only force of gravity
Image from http://www.tpub.com/fluid/ch1k.htm
Determines rate of flow through viscometer:
and p1-p2 is proportional to density (ρ), so:
Use this equation to solve for viscosity!
r, radiusL, length of tubep1-p2, pressure difference between two ends of tube
A, calibration constantt, time for meniscus to fall
Viscosity and Error Estimates using Water
Viscosity…It’s as easy as η = Aρt
A is the calibration constant used for all other viscosity calculations
A= 0.003499 mPa cm3 g-1
t= 411.5 sρ= 789300 g m-3
η= Aρtη=(0.003499 mPa cm3 g-1) (789300 g
m-3)(411.5 s)
η= 1.136 g m-1 s-1
Δ%A = (ΔA /A) x 100Δ%A = (1.3996 x 10-6 mPa cm3 g-1 / 3.499 x 10-3 mPa cm3
g-1) x100Δ%A= 0.04%
Δ%t = (Δt /t) x 100Δ%t = (0.1 s / 255.1 s) x100Δ%t = 0.04%
Δ%η= √[(0.04%)2 + (0.04%)2]Δ%η= 0.05657%
Experimental vs. literature viscosity values and factors contributing to viscosity values
Name Molecular Weight (g mol-1)
Efflux Time (s)
Experimental Viscosity (g m-1 s-1)
Literature Value Viscosity2 (g m-1 s-1)
Percent Error
ethanol 46.07 411.5 ± 0.3
1.136 ± 0.001
1.074 5.7
1-propanol 60.10 706.2 ± 0.4
1.976 ± 0.001
1.945 1.6
1-butanol 74.12 911.3 ± 0.3
2.581 ± 0.001
2.544 1.5
1-hexanol 102.17 1551.2 ± 1.7
4.416 ± 0.005
4.578 3.5
1-octanol 130.23 2711.9 ± 0.4
7.840 ± 0.003
7.288 7.6
• High MW = larger compound, more polarizable• Higher
viscosity
• R2
• Increases with increasing MW
• More polarizable molecules are more difficult to send into gas phase due to higher intermolecular forces
• R2
• Higher increase in entropy of vap for larger molecules
• They are more difficult to vaporize due to intermolecular forces
• R2
• High density: high viscosity
• Lowest R2 • Density least
accurate model
1-octanol:•Highest boiling point bc:• Most
polarizable• Binding forces
are the strongest
•More energy is needed to separate the molecules and send them into gas phase•R2
Name Molecular Weight (g mol-
1)
Enthalpy of Vap. at 25 C (kJ mol-1)
Entropy of Vap. at 25 C (kJ mol-1 K-
1)
Density (g cm-1)
Boiling Point (K)
Exp.Viscosity (g m-1 s-1)
ethanol 46.07 42.32 0.120 0.7893 351.351.136 ± 0.001
1-propan
ol60.10 47.45 0.128 0.7997 370.35
1.976 ± 0.001
1-butanol
74.12 52.35 0.134 0.8095 390.852.581 ± 0.001
1-hexanol
102.17 61.61 0.143 0.8136 430.754.416 ± 0.005
1-octanol
130.23 70.98 0.152 0.8262 468.257.840 ± 0.003
Dipole-dipole interactions London dispersion forces Hydrogen bonding It’s more polar
Unsymmetrical distribution
Symmetrical Distribution
Longer chain = larger electron cloud = more polarizability = more London dispersion forces!
Intermolecular forces and viscosity
Strength of dispersion forces increase with increasing molecular weight Causes increasing boiling point, enthalpy of
vaporization, and entropy of vaporization The greater the intermolecular force,
the greater the viscosity Main difference in viscosity for each
compound is London dispersion forces
Viscosity increases with increasing: Molecular weight Enthalpy of vaporization Entropy of vaporization Density Boiling point
Mostly due to London dispersion forces
1. Garland, C.; Nibler, J.; Shoemaker, D. Spectroscopy. Experiments in Physical Chemistry; McGraw-Hill Higher Education: New York, NY, 2009; pp. 129-130, 320-326.
2. Physical Constants of Organic Compounds. Handbook of Chemistry and Physics, Lide, D., Ed.; CRC Press: Boca Raton FL, 2008; 89th edition, pp. 3-4 to 3-522.