rate of evaporation condensation chemistry/ap chem lectures/… · chapter 9: liquids and solids...
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CHAPTER 9: LIQUIDS AND SOLIDS
Section 9.1 Liquid/Vapor Equilibrium Vaporization – process in which a
liquid vapor open container
- evaporation continues until all liquid evaporates
closed container
1) Liquid evaporate. 2) Vapor particles collect and
Condense. 3) Eventually,
Rate of Evaporation
Rate of Condensation
DYNAMIC EQUILIBRIUM
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Vapor Pressure ● At equilibrium, # molecules/volume is constant. ● Pressure of gas over liquid is constant. ● As long as both liquid and vapor are present,
the pressure exerted bythe vapor is independent of the volume of the container.
Vapor Pressure is dependent on:
a) Characteristics of liquid b) Temperature
Important As long as BOTH liquid and solid are present, the vapor pressure will be constant.
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If volume
More liquid will evaporate
Equilibrium Re-establish
Vapor Pressure vs. Temperature
In general, Vapor pressure as Temp. For example, H2O Vapor Pressure of H2O Temp 24 mmHg 25°C 92 mmHg 50°C 760 mmHg 100°C
Higher the Temperature
More Molecules Vaporize
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A plot of pressure vs. temperature does not produce a straight line.
This is not a direct relationship!
● Graphs of curves are often difficult to
interpret.
The solution to this problem: Graph manipulated variables of pressure and temperature that will represent a
straight line.
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Instead of P vs. T, graph ln P vs. 1/T
● Recall that the general equation of a straight line is y = mx + b (m = slope & b = y-intercept) Here, y = 1n P x = 1/T m = -∆Hvap/R Therefore,
ln P = - TR
H vap 1 + b
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If 2 different temps are evaluated:
at T2 : ln P2 = -2
1
TR
Hvap
+ b
at T1 : ln P1 = -1
1
TR
Hvap
+ b
Clausius- Clapeyron Equation
121
2
12
11lnlnln
TTR
H
P
PPP
vap
where R = 8.314 J/K●mole
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Boiling Point
Vapor Pressure of Liquid
EQUALS
Pressure Above the Surface of the Liquid
Normal Boiling Point = the temperature a liquid boiling at 1 atm of pressure above the liquid.
● The boiling point of any liquid can be
lowered by reducing pressure above liquid. Varies with altitude.
Critical Temperature – the
temperature above which the liquid state of a pure substance cannot exists regardless of the pressure.
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Critical Pressure – the pressure that be applied to cause the condensation of a pure liquid at the critical temperature.
Section 9.2 Phase Diagrams ● Phase Diagram – a graphical way to summarize the conditions under which the different states of a substance are stable.
Water’s Phase Diagram
Liquid
Vapor
Solid
Pre
ssu
re (
atm
)
Temperature (oC)
Critical Point
Normal Boiling Pt.
1.00
.0060
217.75
NormalFreezing Point
Triple Point
0.00 0.01 100.00 373.99
*Not to Scale
D C
B
A
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● Melting-Point Curve - Observe the solid/liquid states at different
pressures. - Along the curve both phases are in
equilibrium.
- Special Note: When conditions indicate that a substance is in the liquid or solid state, the vapor of that substance is also present (in equilibrium).
- The question one should ask is “How much
vapor is present?”
- Answer: It depends!!! Sublimation – Transformation of a solid
directly into a vapor. Melting Point – The opposite process
freezing.
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Triple Point – the point on a phase diagram representing the temperature and pressure at which three phases of a substance coexist in equilibrium.
For example, water’s triple point is
0.1°C, 0.00603 atm and all phases coexist.
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Section 9.3 Molecular Substances: Intermolecular Interactions
1. Nonconductors (when pure) examples: I2, C3H8, C2H5OH
● Most water solutions are also nonconductors ● Some polar molecules form ions when they react with H2O
conduct electricity
For example: HF(g) H+1 (aq) + F-1(aq)
2. Generally, molecular compounds are insoluble in water. 3. They have low melting & boiling points. ● Many are gases (N2, O2, …) ● Some are liquids with melting points <25oC (like H2O, mp = 0oC).
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● Some are solids with melting points <300oC (like I2, mp = 114oC).
The boiling point and melting point of molecular substances is
directly related to the strength of their INTERMOLECULAR ATTRACTIVE FORCES
among molecules.
Intermolecular Forces 1. (London) Dispersion Forces ● Found in all molecular substances.
involves a temporary or induced dipole.
Consider the H2 molecule
● Nonpolar bond equal sharing of electrons
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For an instant, the electrons within the molecule can concentrate closer to one atom in the molecule.
● Produces a +/ - (dipole) within the
molecule. ● This temporary dipole induces a similar dipole
within another molecule. ● These temporary dipoles result in the two molecules attracting each other.
This attraction is the Dispersion Force! The attraction is dependant on:
1. The # of electrons in the molecules involved.
2. The ease of the electrons in the molecules to be dispersed within the individual molecules.
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Larger atoms/molecules
Produce Greater Temporary Dipoles.
In general,
As Molar Mass ,
The dispersion forces ,
The bp & mp of nonpolar
molecules .
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2) Dipole Forces ● These interactions occur in polar
molecules. ● The +/ - (dipole) of one polar molecule
lines up with +/ - (dipole) of another polar
molecule (opposites attract). The greater the dipole moments (the
measure of the polarity of a molecule) of the molecules, the stronger the attractive force. ● This interaction (attraction) really only works when the molecules are close together. ● When the molecules are in the gas phase,
the dipole forces of attraction are negligible (as is the case for dispersion forces).
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3) Hydrogen Bonding
This attraction occurs in polar molecules, HOWEVER, only in molecules that have X – H bonds where X = N, O, or F.
This attractive force is an unusually strong dipole force of attraction.
Why is the hydrogen bonding such a powerful attractive force?
2 – Reasons: 1. There is a large difference in the
electronegativity of the X and H H(2.2) F(4.0)
H(2.2) O(3.5) H(2.2) N(3.0)
The H – atom almost behave as a “naked” proton.
2. The H – atom is very small.
The lone pair of electrons on F, O, and N can get really close to H.
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Let’s look at the pattern of boiling points.
bp(oC) bp(oC) bp(oC)
NH3 -33 H2O 100 HF 19
PH3 -88 H2S -60 HCl -85
AsH3 -63 H2Se -42 HBr -67
SbH3 -18 H2Te -2 HI -35
Note the effect of hydrogen bonding in the first row of boiling points.
IMPORTANT Although these intermolecular force are very important, they are very weak compared to a covalent bond.
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Section 9.4: Network Covalent, Ionic, and Metallic Solids
● Most Molecular substances are gases or
liquids at room temperature. ● Most NON-molecular substances (network
covalent, ionic, and metallic) are solids at room temperature.
1. Network Covalent Solids
These solids are made of atoms joined by a continuous network of covalent bonds.
In general, these solids are: a. High melting (over 1000oC)
- In order for this type of solid to melt, bonds need to be broken
- Remember: When molecular solids to
melt, only interactions need to be broken!
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b. This type of solid is typically insoluble in all common solvents.
- Why? Bonds need to be broken!
NOT EASY TO DO!!!
c. These solids are poor conductors of electricity.
- Why? No mobile electrons are available
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Example: Carbon 2 types of solids exist: Graphite & Diamond Both have very high melting points >3500oC.
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Ionic Solids These solids are held together by very strong electrostatic attractive forces (ionic bonds).
1. They are composed of cations/anions. 2. They are non-volatile (do not become gases
very easily). 3. They are high melting (600 – 2000oC). 4. They do not conduct electricity (they only do
when they form aqueous solutions or they are molten).
5. Many (but not all) are soluble in water.
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Metallic Solids A structural unit of electrons and metal cations.
Positive metal ions anchored in position with electrons moving around from one metal ion to another.
Metals are highly conductive - Very mobile electrons – metals have very low
electronegativities.
● Metals have a high thermal conductivity - Very mobile electrons vibrate
● Metals are ductile and mobile.
● Metals have high luster.
- Electrons within a metal can absorb and emit light energy very easily.
● Metals are insoluble in common solvents.