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8/13/2019 Chapter 9 Chat Long
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TẬP ĐOÀN DẦU KHÍ VIỆT NAM
TRƯỜNG ĐẠI HỌC DẦU KHÍ VIỆT NAM
Lecturer : Assoc. Prof. Pham Hong Quang
Email : [email protected]
Fundamental of Physics
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PGS. TS. Phạm Hồng Quang Fundamentals of Physics 2
Chapter 9: Liquids
1. Properties of Liquids and the Kinetic-Molecular Theory
2. Intermolecular Forces
3. Surface Tension
4. Measuring Surface Tension
5. Wettability6. Vapor Pressure
7. Evaporation
8. Boiling
9. Viscosity
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The Kinetic-Molecular Theory states thatparticles of a liquid have no fixed space,and move about constantly.
Fluid-is a substance that can flow andtakes the shape of its container- used forliquids and gases both
9.1 Properties of Liquids and the Kinetic-Molecular Theory
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Relatively High
Density
The liquids are very dense because the particles of liquids
are extremely close together. Also, different liquids havedifferent densities.
Relative
Incompressibility
Liquids are much less compressible because they have
tightly packed particles, and also transmit pressure equally.
Ability to Diffuse The liquids diffuse with most liquids, but at a slower rate
than gases because the particles are more tightly packed,
and there are many attractive forces between the particles.
9.1 Properties of Liquids and the Kinetic-Molecular
Theory
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Intermolecular forces are attractive forcesbetween molecules
Intramolecular forces hold atoms together in a
molecule
Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter )
• 930 kJ to break all O-H bonds in 1 mole of
water (intra)
Generally, inter molecular forces are much weaker
than intramolecular forces.
9.2 Intermolecular Forces
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Dipole-Dipole Forces
Attractive forces between polar molecules
9.2 Intermolecular Forces
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7
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
9.2 Intermolecular Forces
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8
Hydrogen
Bond The hydrogen bond is a special dipole-dipole interactionbetween their hydrogen atom in a polar N-H, O-H, or F-H
bond and an electronegative O, N, or F atom.
A-H..B or A-H..A
A & B are N, O, or F
9.2 Intermolecular Forces
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9
Various intermolecular forcesdraw the liquid particles
together. Along the surface, the
particles are pulled toward the
rest of the liquid, as shown in
the picture to the right.Surface tension (denoted with
the Greek variable gamma) is
defined as the ratio of the
surface force F to the length d along which the force acts:
gamma = F / d
The higher the attractionforces (intermolecular
forces), the higher the
surface tension.
9.3 Surface
Tension
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Surface tension has the dimension of force
per unit length, or of energy per unit area.
The two are equivalent—but when referring
to energy per unit of area, people use theterm surface energy—which is a more
general term in the sense that it applies also
to solid and not just liquids.
Unit of the Surface tension are N/m, J/ m2 ,D/cm
9.3 Surface Tension
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L
Soap film
Force =2Lγ
Force =mg
9.3 Surface Tension
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Surface Tensions of Pure Liquids at 293
KSubstance / (10
-3 N/m)
Acetone 23.7
Benzene 28.8
CarbonTetrachloride 27.0
Methylene Iodide 50.8
Water 72.8
Methanol 22.6
n-Hexane 18.4
9.3 Surface Tension
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The meniscus is the curve in the upper surface of a liquid close
to the surface of the container or another object, caused by
surface tension It can be either convex or concave. A convex
meniscus occurs when the molecules have a stronger attraction
to each other (cohesion) than to the material of the container
(adhesion). Conversely, a concave meniscus occurs when the
molecules of the liquid attract those of the container's, causing
the surface of the liquid to cave downwards.
9.3 Surface
Tension
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If a capillary tube of inside radius=r immersed in a liquid that wet itssurface, the liquid continues to rise inthe tube due to the surface tension,until the upward movement is just
balanced by the downward force ofgravity due to the weight of theliquid.
The total upward force around the
inside circumference of the tube is
θ= the contact angle between the
surface of the liquid and the capillary
wall
cos2
r a
Capillary Rise Method
9.4 Measuring Surface
Tension
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For water the angle Ө is
insignificant, i.e. the liquid
wets the capillary wall so
that cos Ө = unity
The downward force ofgravity is given by
g hr 2
At Maximum height, the
opposing forces are in
equilibrium
g rh 2
1
9.4 Measuring Surface
Tension
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Wtot = Wring + 4R
The method is simple and measures the detachment
force
(the surface tension multiplied by the periphery
2*2R)
The Ring Method (du Nouy
1919)
9.4 Measuring Surface
Tension
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Drop Weight and drop volume method
If the volume or weight of a
drop as it is detached from a
tip of known radius is
determined, the surface andinterfacial tension can be
calculated from
r
g V
r
mg
22
Where m = the mass of the drop V = the volume of the drop
ρ = the density of the liquid r = the radius of the tip g = the acceleration due togravity
Φ = a correction factor
Why is Φ needed
1)The drop does not completely
leave the tip.
2) The surface tension forces
are not completely vertical.
3) There is a pressure
difference across the
curved surface.
9.4 Measuring Surface
Tension
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9.5 Wettability
According to the nature of the liquid and the solid,a drop of liquid placed on a solid surface will
adhere to it or no. That is the wettability between
liquids and solids.
When the forces of adhesion are greater thanthe forces of cohesion, the liquid tends to wet the
surface and vice versa.
Place a drop of a liquid on a smooth surface of a
solid. According to the wettability, the drop will
make a certain angle of contact with the solid.
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A contact angle is lower than 90°, the solid is called
wettable
A contact angle is wider than 90°, the solid is named
non-wettable.
A contact angle equal to zero indicates completewettability.
9.5 Wettability
9 6
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9.6 Vapor Pressure
Vapor Pressure: The pressure exerted on the
surface of a liquid by the vapor that is in equilibrium
with the liquid is called as “vapor pressure”
Once equilibrium between a liquid and vapor is
reached, the number of molecules per unit volume in
a vapor does not change with time. Hence, the vapor
pressure over the liquid remains constant at a given
temperature.Vapor Pressure is independent of the volume of the
container.
9 6V P
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Vapor pressure increases
with the increase in
temperature.
9.6Vapor Pressure
9 7 E ti
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9.7 Evaporation
Evaporation- is the process where particles escape
from the surface of a non boiling liquid and enters the
gas state.
~Evaporation takes place because the particles of
liquids have different kinetic energies, therefore
some of the particles with higher kinetic energy
overcome the intermolecular forces and evaporate to
go in the gas phase.
9 8
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Boiling- is the change of a liquid to bubbles or vapor.
Boiling occurs when the vapor pressure becomes equals
atmospheric pressure.
A l iquid bo i ls at the temp. at wh ich i ts vapor pressu re
is equal to the pressu re above i ts surface. (usuallyatmospheric pressure)
If the pressure above the liquid’s surface is 1 atm, then
this temperature is called as its “Normal Boiling Point”
B.P. of a liquid is reduced by lowering the pressure
above it.
Why does it take longer to cook at high altitudes?
9.8
Boiling
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A liquid boils at a temp. when the vapor pressure P1
becomes equal to the external pressure P2 above theliquid
9.8 Boiling
9 9 Vi it
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9.9 Viscosity
Defined as “resistance to flow” of a fluid.
Viscous liquids move slower.
The greater the intermolecular forces the
more is the viscosity.
9 9 Vi it
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The dynamic viscosity(η) of a fluid is a measure ofthe resistance it offers to relative shearing motion.Viscosity (η) is defined as the ratio of shear stress
(τ)to shear rate (u/h)
η= F/ [A×(u/h)]
η= τ /(u/h) N-s/m²
9.9 Viscosity
9 9 Vi it
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Kinematic Viscosity :
It is defined as the ratio of absolute
viscosity to the density of fluid.ν= η/ρ m²/s ; ρ= density of
fluid
9.9 Viscosity
9 9 Vi it
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The viscosity of liquids
decreases with increase
the temperature.
The viscosity of gases
increases with the
increase the
temperature.
9.9 Viscosity
9 9 Viscosit
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For Newtonian fluids, shear stress linearly vary
with the shear rate as shown in Figure. Viscosity
is constant for this kind of fluid.
τ = η (u/h)
9.9 Viscosity
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Non Newtonian fluid doesn’t follow the linearrelation between viscosity and shear rate.
9.9 Viscosity