Liquid-Vapor Equilibrium
(Intermolecular Forces and Liquids and Solids)
Green/Damji: 17.1
Chang: Chapter 11
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Write an equation for the
equilibrium between liquid
water and its vapor:
Is evaporation an
endothermic or exothermic
process?
Condensation
Focus on
Evaporation and
Condensation
Evaporation
Least
Order
Greatest
Order
11.8
Before
Evaporation
At
Equilibrium
11.8
The equilibrium vapor pressure is the vapor pressure
measured when a dynamic equilibrium exists between
condensation and evaporation
H2O (l)
H2O (g)
Dynamic Equilibrium
Rate of
Rate of
= evaporation
condensation
11.8
Factors affecting the vapor pressure
•
SURFACE AREA
increasing surface area only means
equilibrium is reached faster
•
TEMPERATURE
What is the name of this
diagram? Why is it
important?
T2 > T1
11.8
Vapor Pressure Versus Temperature
Patterns? Why?
11.8
Enthalpy of vaporization (DHvap) is the energy required to
vaporize 1 mole of a liquid at its boiling point.
Enthalpy of vaporization : physical process (phase change) as
______________________: chemical reaction
11.8
The relationship between vapor pressure and temperature
can be expressed mathematically – NOT req’d by IB
Clausius-Clapeyron Equation
DHvap
ln P = +C
RT
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
C = integration constant
Vapor Pressure Versus Temperature
11.8
Heating Curve
Why are there plateaus on this graph?
11.8
The boiling point is the temperature at which the
(equilibrium) vapor pressure of a liquid is equal to the
external pressure.
The normal boiling point is the temperature at which a liquid
boils when the external pressure is 1 atm.
11.8
What are the three primary intermolecular forces?
List in order of increasing strength.
The boiling point and enthalpy of vaporization are
influenced by intermolecular forces…
Compound
methane
CH
Enthalpy of
vaporization
in kJ mol-1
Boiling Point
in K
9.0
109
27.2
248
38.6
352
4
methoxy
methane
CH OCH
3
3
ethanol
CH CH OH
3 2
Intermolecular
Forces
State the relationship between intermolecular
forces and enthalpy of vaporization.
State the relationship between intermolecular
forces and boiling point.
A phase diagram summarizes the conditions at which a
substance exists as a solid, liquid, or gas.
Phase Diagram of Water
11.9
Effect of Increase in Pressure on the Melting Point
of Ice and the Boiling Point of Water
11.9
Textwork 17.1
Read 17.1 on pp 192-193
Address each of the learning objectives on p.
192 by taking notes that directly relate to the
objectives.
Due on __________________