Equilibrium 6
Liquid - Vapour Equilibria (IB Topic 17.1)
http://brainyandthescientist.blogspot.com/2010/03/equilibrium.html
Reference: Higher Level Chemistry, p. 252-259
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IB Assessment Statements
17.1: Liquid–vapour equilibrium
17.1.1
Describe the equilibrium established between a liquid and its
own vapour and how it is affected by temperature changes.
17.1.2
Sketch graphs showing the relationship between
vapour pressure and temperature and explain them
in terms of the kinetic theory.
17.1.3
State and explain the relationship between enthalpy of
vaporization, boiling point and intermolecular forces.
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Liquid-Vapour Equilibrium
When a liquid is in a closed system,
two reactions occur:
vapourization
condensation
In a liquid-vapour equilibrium,
the rate of vapourization = the rate of condensation.
This type of equilibrium is also known as a phase equilibrium.
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Liquid-Vapour Equilibrium
Write an equation showing the equilibrium systems when
these liquids are placed in closed containers.
water
methanol
H2O(l) ⇌ H2O(g)
CH3OH(l) ⇌ CH3OH(g)
one other
volatile liquid
Sketch and annotate a
rate vs time graph for this
type of equilibrium.
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Vapour Pressure
http://www.kidsgeo.com/geography-for-kids/0104-water-vapor-saturation.php
Pressure is exerted by the particles in the vapour phase
when they collide with the walls of the container and the
surface of the liquid.
The pressure exerted by the vapour AT EQUILIBRIUM
is called vapour pressure.
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Factors Affecting Vapour Pressure
1. Temperature
Particles in the liquid phase need
kinetic energy to “escape”.
This energy is used to break the
intermolecular forces of attraction
between the liquid molecules.
*
Sketch a Maxwell Boltzmann diagram
showing the effect of temperature.
* = kinetic energy needed to break IMFs
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Factors Affecting Vapour Pressure
1. Temperature
Consider LeChatelier’s Principle
H2O(l) ⇌ H2O(g)
∆H is +
Increasing temperature favours the endothermic reaction.
The equilibrium shifts to the right, increasing the vapour pressure.
http://www.md.rcm.upr.edu/physiology/respir-notes.php
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Factors Affecting Vapour Pressure
1. Temperature
Boiling Point
Boiling point is the temperature at which the vapour pressure
equals the external atmospheric pressure.
This graph shows how boiling
point changes with changing
atmospheric pressure:
Green and Damji, Chemistry 3rd Edition, p. 193
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Factors Affecting Vapour Pressure
2. Nature of the Liquid
The vapour pressure is related to the ease with which the molecules can
evaporate. This is related to the strength of the intermolecular forces.
substance
ethanol
ethoxyethane
formula
C2H5OH
(C2H5)2O
ethanoic acid CH3COOH
structure
type of
intermolcuar
force
hydrogen
bonds
van der Waals
more hydrogen
bonds
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Factors Affecting Vapour Pressure
2. Nature of the Liquid
The stronger the intermolecular force, the more
difficult it is for liquid molecules to escape.
This decreases the vapour pressure.
weak IMF
van der Waals forces
high vapour pressure
strong IMF
dipole dipole forces
hydrogen bonds
low vapour pressure
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Factors Affecting Vapour Pressure
2. Nature of the Liquid
Identify the strengths of the IMFs for
the four liquids.
Compare the effect of
intermolecular forces in a liquid on
boiling point.
http://www.chem.ufl.edu/~itl/2045/lectures/lec_f.html
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Factors Affecting Vapour Pressure
2. Nature of the Liquid
Simulation:
http://www.media.pearson.com.au/schools/cw/au_sch_lewis_cw2/int/VaporPressure/1104.html
Choose two different pairs of liquid and record the results.
Explain the results.
(You may need to look up the formula and structures of the liquids.)
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Factors Affecting Vapour Pressure
3. Enthalpy of Vapourization
H2O(l) ⇌ H2O(g)
∆Hºvap = + 45 kJ mol-1
compound
enthalpy of
vapourisation
(kJ mol-1)
boiling point
(K)
IMF
methane
9.0
109
VDW
methoxymethane
27.2
248
VDW
ethanol
38.6
352
hydrogen bonds
water
+45
373
hydrogen bonds
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Factors Affecting Vapour Pressure
Factors that do NOT affect VP:
1. volume of liquid
2. surface area
Explain why!
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Factors Affecting Vapour Pressure
Summary
weak IMF
van der Waals forces
strong IMF
dipole dipole forces
hydrogen bonds
high vapour pressure
low vapour pressure
low boiling point
high boiling point
low ∆Hºvap
high ∆Hºvap
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