In this presentation, I explain why viscosity varies with molecular structure using a homologous series of straight chained alcohols. Hint: London dispersion forces!

1. Charlotte Chaze

2. 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

3. Olive oil shows greater resistance to flow than water does:
It has a higher viscosity

4. Determine viscosities of homologous series
of compounds
 Ethanol
 1-propanol
 1-butanol
 1-hexanol
 1-octanol

5. Show a useful correlation between viscosity
and physical properties
 Molecular weight
 Density
 Boiling point
 Enthalpy of vaporization
 Entropy of vaporization

6. The cause of differential viscosity between compounds
Remember freshman year?

7. 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

8. δ+
δ+
δ-
δ-
δ+
δ+
δ-
δ-
ethanol 1-octanol
H-bonding = Resistance to flow
Homologous compounds = similar H bonding

9. 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

10. 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

11. When electrons are distributed
unsymmetrically, molecules develop
temporary (instantaneous) dipoles
Image from http://www.chem.purdue.edu/gchelp/liquids/disperse.html

12. 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

13. 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

14. Size of molecule defines size of electron cloud
Longer chain = larger electron cloud = more polarizability
& more London dispersion forces

15. 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

16. Measuring a homologous series of compounds
Warning: measuring viscosity requires long periods of waiting

17. 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

18.  Determines rate of flow through viscometer:
and p1-p2 is proportional to density (ρ), so:
Use this equation to solve for viscosity!
r, radius
L, length of tube
p1-p2, pressure
difference between
two ends of tube
A, calibration constant
t, time for meniscus to fall

19. Viscosity and Error Estimates usingWater
Viscosity…
It’s as easy as η = Aρt

20. 
A is the calibration constant used for all other viscosity calculations

21. A= 0.003499 mPa cm3 g-1
t= 411.5 s
ρ= 789300 g m-3
η= Aρt
η=(0.003499 mPa cm3 g-1) (789300 g m-
3)(411.5 s)
η= 1.136 g m-1 s-1

22. Δ%A = (ΔA /A) x 100
Δ%A = (1.3996 x 10-6 mPa cm3 g-1 / 3.499 x 10-3 mPa cm3 g-1) x100
Δ%A= 0.04%
Δ%t = (Δt /t) x 100
Δ%t = (0.1 s / 255.1 s) x100
Δ%t = 0.04%
Δ%η= √[(0.04%)2 + (0.04%)2]
Δ%η= 0.05657%


23. Experimental vs. literature viscosity values and factors contributing to
viscosity values

24. Name Molecular
Weight (g
mol-1)
EffluxTime
(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

25. • High MW = larger
compound, more
polarizable
• Higher viscosity
• R2

26. • Increases with
increasing MW
• More polarizable
molecules are
more difficult to
send into gas
phase due to
higher
intermolecular
forces
• R2

27. • Higher increase in
entropy of vap for
larger molecules
• They are more
difficult to vaporize
due to
intermolecular
forces
• R2

28. • High density:
high viscosity
• Lowest R2
• Density least
accurate model

29. 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

30. Name Molecular
Weight (g
mol-1)
Enthalpy
of Vap.
at 25 C
(kJ mol-1)
Entropy
ofVap. 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 351.35
1.136 ±
0.001
1-
propanol
60.10 47.45 0.128 0.7997 370.35
1.976 ±
0.001
1-butanol 74.12 52.35 0.134 0.8095 390.85
2.581 ±
0.001
1-hexanol 102.17 61.61 0.143 0.8136 430.75
4.416 ±
0.005
1-octanol 130.23 70.98 0.152 0.8262 468.25
7.840 ±
0.003

32. Dipole-dipole interactions
London dispersion forces
Hydrogen bonding
It’s more polar

33. Unsymmetrical
distribution
Symmetrical
Distribution
Longer chain = larger electron cloud = more polarizability = more London dispersion forces!

34. Intermolecular forces and viscosity

35. 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

36. Viscosity increases with increasing:
 Molecular weight
 Enthalpy of vaporization
 Entropy of vaporization
 Density
 Boiling point
Mostly due to London dispersion forces

37. 1. Garland, C.; Nibler, J.; Shoemaker, D. Spectroscopy. Experiments in Physical
Chemistry; McGraw-Hill Higher Education: NewYork, 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.