chemistry xl-14a physical equilibrium
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Chemistry XL-14A Physical Equilibrium. August 13, 2011Robert Iafe. Final Information. 3 hours - Equivalent to 2 midterms Cheat sheet – one (1) page, single sided Graphing calculators ok. Chapter Overview. Phases and Phase Transitions Solubility Colligative Properties - PowerPoint PPT PresentationTRANSCRIPT
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CHEMISTRY XL-14A
PHYSICAL EQUILIBRI
UMAugust 13, 2011 Robert Iafe
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Final Information
3 hours - Equivalent to 2 midterms Cheat sheet – one (1) page, single sided Graphing calculators ok
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Chapter Overview Phases and Phase Transitions Solubility Colligative Properties Binary Liquid Mixtures
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Phases and Phase Transitions Vapor Pressure Volatility
Intermolecular Forces (Chapter 4) Ion-Dipole Forces Dipole-Dipole Forces Londen Forces Hydrogen Bonding
Variation of Vapor Pressure with Temperature Boiling Freezing and Melting Phase Diagrams Critical Properties
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Origin of Intermolecular Forces
Intermolecular forces are responsible for phases of matter
Phase: form of matter that is uniform in both chemical composition and physical state Gas Liquid Solid
All intermolecular interactions can be traced back to the coulombic interaction between charges
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Origin of Intermolecular Forces
Coulombic interaction
Deep well indicates a bond between atoms
Shallow well shows small attractive forces between molecules, even though no bonds are formed
€
E p = Q1Q2
4πεor
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Ion-Dipole Forces Ionic solids dissolve in water when water
molecules become attached to each ion and separate it from the other ions
Hydration due to dipole of water Partial positive interacts with anion Partial negative interacts with cation Ion-Dipole interaction
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Potential Energy Potential energy for the interaction of a full charge and
partial charges Potential energy is lowered by interaction with polar
solvent Distance of the ion and dipole play a larger role than
distance between the two ions Results:
Small cations are more extensively hydrated than large cations Smaller metals form hydrated salts Larger charges on metals cause hydration
€
E p ∝−z μr2
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Dipole-Dipole Forces
Polar molecules attract each other by the interaction between the partial charges of their electric dipoles
Dipole-Dipole interaction
€
E p ∝−μ1μ2
r3
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London Forces Attractive interactions are also found in nonpolar molecules Evidence: noble gases can be liquefied even though they
are nonpolar Electrons are not stationary Nonpolar molecules can have instantaneous dipoles Attractive interactions are called London interactions
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London Forces What determines strength of London
interactions? Distance Polarizability (alpha) Shape
€
E p ∝−α 1α 2
r6
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London Forces - Polarizability
More electrons = More polarizableMore polarizable = more stabilization
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London Forces - Shape
Bp: 36 oC
Bp: 10 oC
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Trends in London Forces
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Hydrogen Bonding Ammonia (NH3), water (H2O), and HF have
abnormally high boiling points Strong attractive forces due to hydrogen
bonding
Hydrogen bonding only occurs with N-H, O-H, F-H
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Vapor Pressure The vapor pressure of a
substance is the pressure exerted by its vapor when the vapor is in dynamic equilibrium with the condensed phase
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Vapor Pressure and IM Intermolecular forces play a large role in physical
properties of liquids and solids General rule:
Molecules with stronger intermolecular forces have lower vapor pressures (higher boiling points and melting points)
Molecules with weaker intermolecular forces have higher vapor pressures (lower boiling points and melting points)
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Vapor Pressure and IM
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Vapor Pressure and Temperature
Vapor pressure increases with temperature Higher temperature =
greater energy to overcome attractive forces of the liquidClausius-Clapeyron equation
€
ln P2
P1
=ΔHvap
o
R1T1
− 1T2
⎛ ⎝ ⎜
⎞ ⎠ ⎟
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Boiling Boiling occurs when the vapor pressure of a liquid
is equal to the external (atmospheric) pressure Normal boiling point = boiling point at 1 atm (760 torr)
Boiling point is dependent on: Intermolecular forces External Pressure
Thinking questions Does water boil at a higher or lower temperature on the top
of Mt. Everest? How does a pressure cooker work?
€
ln P2
P1
=ΔHvap
o
R1T1
− 1T2
⎛ ⎝ ⎜
⎞ ⎠ ⎟
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Freezing and Melting Freezing temperature is the temperature at
which the solid and liquid phase are in dynamic equilibrium with each other Normal freezing point = freezing point at 1
atm (760 Torr) Supercooling = when a liquid exists beyond
its freezing point
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Phase Diagrams Phase diagram of
water Phase boundaries
Equilibrium btwn phases
Triple point All 3 phases coexist
Solid-liquid boundary Negative slope =
solid is less dense than liquid
(ice floats in water)
Cond
ensa
tion
Evap
orat
ion
Depositio
nSublim
ati
onFr
eezin
g Mel
ting
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Water
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CO2
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Sulfur
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Critical Points
Supercriticalfluid
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Solubility The Limits to Solubility The “Like-Dissolves-Like” Rule Pressure and Gas Solubility: Henry’s Law Temperature and Solubility
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Limits of Solubility Terms:
Solvent: the substance that dissolves Solute: the substance that is dissolved Saturated: the solvent has dissolved the
maximum amount of solute it can, and undissolved solute remains
Saturation is a dynamic equilibrium with the undissolved solute
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Like Dissolves Like Substances have attractive cohesion
forces when the have the same intermolecular forces
Rule: Like Dissolves Like Polar solvents dissolve polar solutes Nonpolar solvents dissolve nonpolar solutes Polar liquids and nonpolar liquids are
immiscible
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Detergents Detergents have a hydrophobic tail
group and a hydrophilic head group Hydrophobic – does not dissolve in water
‘hydro’ = water; ‘phobic’ = afraid Hydrophilic – dissolved in water
‘hydro’ = water; ‘philic’ = loves
micelle
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Pressure and Gas Solubility: Henry’s Law
English chemist William Henry, 1801 The solubility of a gas is directly
proportional to its partial pressure, P Henry’s Law
€
s = kHP
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Temperature and Solubility Rate of dissolving, but not generally the solubility
of a substance, increases at higher temperatures. Most gasses are less soluble in warm water than
in cold water
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Colligative Properties Molality Vapor-Pressure Lowering Boiling-Point Elevation Freezing Point Depression Osmosis
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Molality Three measures of concentration
Mole fraction
Molarity
Molality
Molality and mole fraction are independent of temperature
€
xsolute = nsolutensolute + nsolvent
€
M = nsoluteLsolution
= molL
€
bsolute = nsolutemsolvent
= mol(kg)solvent
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Mole Fraction MolalityWhat is the molality of benzene, C6H6, dissolved in
toluene, C6H5CH3, in a solution for which the mole fraction of benzene is 0.150? Step 1. Find the amount of solute molecules in a
total of exactly 1 mol of solution molecules
Step 2. Find the mass of solvent present and convert to kg
Step 3. Calculate the molality
€
bsolute = nsolutemsolvent
= 0.150mol0.0783kg
=1.92molkg
€
n solute= xsolute × ntotal = 0.150 ×1mol = 0.150mol
€
msolvent = {(1− xsolute )mol} ×MMsolvent
msolvent = {(1− 0.150)mol} × 92.13 gmol
× 1kg1000g
= 0.0783kg
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Molarity Molality Find the molality of sucrose, C12H22O11, in 1.06 M
C12H22O11(aq), which is known to have density 1.14g/mL Step 1. Find the mass of exactly 1 L (103 mL) of
solution
Step 2. Find the mass of solute in exactly 1 L of solution
Step 3. Find the mass of water present in exactly 1 L of sol’n
Step 4. Calculate molality.
€
msolution = d × (103mL) =1.14 ×103g
€
b(C12H22O11 ) = 1.06mol0.78kg
=1.4 molkg
€
mwater = msolution −msolute =1140 − 363g = 0.78kg€
msucrose = nsoluteMMsolute = csolute × (1L) ×MMsolute
msucrose =1.06molL
× (1L) × 342.3 gmol
= 363g
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Vapor Pressure Lowering
French scientist Francois-Marie Raoult Spent most of his time measuring vapor
pressure Vapor pressure of a solvent is
proportional to its mole fraction in a solution
Ideal solutions follow Raoult’s law The vapor pressure of a solvent is
reduced by the presence of a nonvolatile solute
€
P = xsolventPpure
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Boiling Point Elevation Presence of nonvolative solutes raises
the boiling point of pure liquids
€
ΔTb = ikb ×bsolutei = van’t Hoff i
factor(# of particles)
eg. NaCl = 2CaCl2 = 3
Sucrose = 1
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Freezing Point Depression Presence of nonvolative solutes lowers
the freezing point of pure liquids
€
ΔTf = ik f ×bsolutei = van’t Hoff i
factor(# of particles)
eg. NaCl = 2CaCl2 = 3
Sucrose = 1
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Practice Problems What is the change in boiling point when you add
26 g of sucrose (C6H12O6) to 1.00 kg of water? What is change in freezing point when you add 10
g of NaCl to 1.00 kg of water?
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Osmosis Definition: the flow of solvent through a
membrane into a more concentrated solution The pressure
needed to stop the flow of solvent is called the osmotic pressure, Π
€
Π=iRTc solute
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Binary Liquid Mixtures Vapor Pressure of Binary Liquid Mixtures Distillation Azeotropes
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Vapor pressure of Binary Liquid Mixtures
The vapor pressure of a mixture of two volatile liquids is the sum of their individual partial pressures
€
PA = xA (l)PA*
PB = xB (l)PB*
P = PA + PB
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Distillation The vapor pressure of a
binary mixture is intermediate between the 2 volatile liquids
Temperature-composition diagram
Upper curve = vapor composition
Horizontal line = tie line Distillate = condensed
vapor
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Fractional Distillation
Continuous redistillation leads to purer and purer distillate
Becomes richers in the substance with the lower boiling point
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Azeotropes Most liquid mixtures are not ideal, so their vapor
pressures do not follow Raoult’s law
Negative deviation
Positive deviation
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Impact on Biology and Materials Colloids
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Colloids Colloid: a dispersion of large particles (1 nm
to 1 μm in diameter) in a solvent Colloids have properties between those of a
homogeneous solution and a heterogeneous mixture
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Tyndall Effect Because colloids are a dispersion of many
tiny particles in a solvent, we can see the Tyndall Effect The beam reflects off each individual particle
Laser beam through silver solution