The document discusses the concept of viscosity as the resistance to flow in fluids, comparing the viscosities of various alcohols and their molecular properties. It highlights the role of intermolecular forces such as hydrogen bonding and London dispersion forces in determining viscosity, as well as how molecular weight and density correlate with viscosity. Additionally, it outlines the methodology for measuring viscosity using an Ostwald viscometer and examines experimental results versus literature values for different alcohols.

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.