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Lecture 11

CHEM 217 Lecture Notes - Lecture 11: Dry Ice, Phase Diagram, Sinad
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18 Pages
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Spring 2018

Department
Chemistry
Course Code
CHEM 217
Professor
kate
Lecture
11

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Unit 11 Liquids
solids, intermolecular
forces
11 . 1 G E C K OS A N D I N T E R MO L EC U LA R FO R C E S
Respondible for existence of condesnsed states -> l,g,s depends on magnitude of intermolecular forces
between constituent particles relative to amount of thermal energy in the sample
Constant random motion of atoms and molecules that incresaes wih temperature – elenergy associated
wioth this motion is calle dthermal energy.
oWhen thermal energy is high relative to intermolecular forces, matter tends o be gaseous and
when it’s low it tends to be l, s
11 . 2 S , L , G M O L E CU L A R CO M PA R I SO N
Densities of s,l reater than of gas and also more similar in density and molar volume to each other than
they are to gases
18.0mL of water would occupy 31 L in gas state at 100C with standard pressure
Low density of gaseous water is a direct result if this large separation between molecules
For H2O, solid slightly less dense than the liquid and this is atypical
Major difference between solid and liquid is freedom of movement
oIn liquids, thermal energy partially voercomes attractions between them allowin them to move
freely
oSoli can only virbate at a fixed pointi
oThat’s why liquids assume sape ofcontainer and not easily compressed like a gas
oLike liquids, solids have a efinite volume and generally can
o‘t be compressed
oSolid may be crystalline – molecules or atoms are arragned in well rodered 3D arrangememtn
oThey may be amorphous in which case atoms and molecules have no long range order
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CHANGE BETWEEN STATES
Can transform by changing temperature or pressure or both
SOLID <cool | heat> LIQUID < cool or increase P | heat or reduce P > GAS
11 . 3 F O R C ES HO L DI N G C O N D EN S ED ST A T E S T O GE T HE R
Intermolecular forces originate from interactions between charges, partial charges and temporary charges
on molecules anda toms
Remember according to coulom’s law the potential energy (E) of two oppositely charged particles
(charges q1q2) decreases (becomes more negative) with increasing magnitude of charge and with
decrasing separation ®:
E= (1/4πε0)(q1q2/r)
oWhere q1, q2 are opposite in sign, E is negative and E of the system decreases as the two
opposite charges get closer
oP+ and e- are attracted becaue their potential energy decrases as they get close together
Similarly, molecules with partial or temporary charges are attracted because their
potential energy decreases as they get closer
oHowever, intermolecular forces are generally much weaker than bonding ones
Bonding forces are the result of large charges interacting at close distances
ION INDUCED DIPOLE FORCES
When an ion approaches a nonpolar atom or molecule, it can cause a distort of negative electron cloud in
The nonpolar atom or molecule -> since the electron cloud is distorted to one side, it has a small dipole
induced by the presence of an ion
The magnitude of ion-induced diple forces depends on charge if the ion and how easily the electrons in
the atom or molecule can move or polarize (a capability of electron clouds called polarizability) in
response to ion presence
oThe magnitude of polarizability of an atom or molecule is in part dependent on size or volume of
the electron cloud
A larger electron cloud results in a greater dispersion force because electrons are held
less tightly by nucleus and easier to polarize
DISPERSION FORCE
the one intermolecular force present among all neutral atoms and molecules is the dispersion force
dispersion forces are a result of fluctuations in the electron distribution within atoms or molecuesl in the
absence of an ion
th electron in an atom or molecule may at any instant be unevenly distributed resulting in slight charges
to anuy side of the atom (partial – or partial +)
othis fleeting charge separation is called an instantaneous dipole or temporary dipole
a temporary dipole on one atom induces a temporary dipole on the next
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Like the ion induced dipole, the magnitude of dispersion force depends on how easily the electrons in the
atom can move of polarize in response to an instantaneous dipole, which partly depends on the size of
the electron cloud
oLarger electron cloud means greater dispersion forces because electrons are held less tightly by
molecules and therefore are distorted – polarize – more easily
If all other variables are constant, the dispersion force increases with increasing number of erlectrons
which typically increases the size of the cloud
Number of electrons alone however does not determine magnitude of the dispersion force
Compare the molar masses and boiling points of isomeric hydrocarbons pentane and 2,2-
dimethylpropane
oSame # of electrons but different shape
The pentante are long and can interact with one another along the entire length
Contrast, bulky shape of 2,2-dimethylporpane molecules result in smaller area of interaction between
neighnouring molecules, resulting in lower boiling point
Although molecular shape and other factors can be considered in determining magnitude of dispersion
force,s # of electrons can act as a guide when comparing dispersion forces within a family of similar
elements or compounds
DIPOLE-DIPOLE FORCES
Exist in all molecules that are polar
Polar molecules have permanent dipoles that interact with the permanent dipoles of neighbouring
molecules as you can see in fig. 11.8
o+ of one attracts – of another molecule
polar molecuels therefore have higher melting and boiling points than nonpolar molecules of the same
size and number of electrons
remember, all molecules have dispersion forces
opolar molecules have dip-dip + dispersion forces
polarity of molecules composing liquids is also important in determining miscibility (ability to mix without
separating intwo two states) in liquids
ogenerally, polar liquids are miscible with other polar liquids but not with nonpolar liquids
However, dip-dip interactions aren’t always responsible for trends in physical properties of compounds
that might seem related
HYDROGEN BONDING
Polar molecules containing H atoms bonded directly to small electronegative atoms – notably, F,O,N –
exhibit intermolecular force called H bonding
A sort of super dip-dip force
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Description
Unit 11 Liquids solids, intermolecular forces 11.1 GECKOS AND INTERMOLECULAR FORCES Respondible for existence of condesnsed states -> l,g,s depends on magnitude of intermolecular forces between constituent particles relative to amount of thermal energy in the sample Constant random motion of atoms and molecules that incresaes wih temperature elenergy associated wioth this motion is calle dthermal energy. o When thermal energy is high relative to intermolecular forces, matter tends o be gaseous and when its low it tends to be l, s 11.2 S,L,G MOLECULAR COMPARISON Densities of s,l reater than of gas and also more similar in density and molar volume to each other than they are to gases 18.0mL of water would occupy 31 L in gas state at 100C with standard pressure Low density of gaseous water is a direct result if this large separation between molecules For H2O, solid slightly less dense than the liquid and this is atypical Major difference between solid and liquid is freedom of movement o In liquids, thermal energy partially voercomes attractions between them allowin them to move freely o Soli can only virbate at a fixed pointi o Thats why liquids assume sape ofcontainer and not easily compressed like a gas o Like liquids, solids have a efinite volume and generally can o t be compressed o Solid may be crystalline molecules or atoms are arragned in well rodered 3D arrangememtn o They may be amorphous in which case atoms and molecules have no long range order CHANGE BETWEEN STATES Can transform by changing temperature or pressure or both SOLID LIQUID < cool or increase P | heat or reduce P > GAS 11.3 FORCES HOLDING CONDENSED STATES TOGETHER Intermolecular forces originate from interactions between charges, partial charges and temporary charges on molecules anda toms Remember according to couloms law the potential energy (E) of two oppositely charged particles (charges q1q2) decreases (becomes more negative) with increasing magnitude of charge and with decrasing separation : E= (1/40)(q12 /r) o Where q1, q2 are opposite in sign, E is negative and E of the system decreases as the two opposite charges get closer o P+ and e- are attracted becaue their potential energy decrases as they get close together Similarly, molecules with partial or temporary charges are attracted because their potential energy decreases as they get closer o However, intermolecular forces are generally much weaker than bonding ones Bonding forces are the result of large charges interacting at close distances ION INDUCED DIPOLE FORCES When an ion approaches a nonpolar atom or molecule, it can cause a distort of negative electron cloud in The nonpolar atom or molecule -> since the electron cloud is distorted to one side, it has a small dipole induced by the presence of an ion The magnitude of ion-induced diple forces depends on charge if the ion and how easily the electrons in the atom or molecule can move or polarize (a capability of electron clouds called polarizability) in response to ion presence o The magnitude of polarizability of an atom or molecule is in part dependent on size or volume of the electron cloud A larger electron cloud results in a greater dispersion force because electrons are held less tightly by nucleus and easier to polarize DISPERSION FORCE the one intermolecular force present among all neutral atoms and molecules is the dispersion force dispersion forces are a result of fluctuations in the electron distribution within atoms or molecuesl in the absence of an ion th electron in an atom or molecule may at any instant be unevenly distributed resulting in slight charges to anuy side of the atom (partial or partial +) o this fleeting charge separation is called an instantaneous dipole or temporary dipole a temporary dipole on one atom induces a temporary dipole on the next Like the ion induced dipole, the magnitude of dispersion force depends on how easily the electrons in the atom can move of polarize in response to an instantaneous dipole, which partly depends on the size of the electron cloud o Larger electron cloud means greater dispersion forces because electrons are held less tightly by molecules and therefore are distorted polarize more easily If all other variables are constant, the dispersion force increases with increasing number of erlectrons which typically increases the size of the cloud Number of electrons alone however does not determine magnitude of the dispersion force Compare the molar masses and boiling points of isomeric hydrocarbons pentane and 2,2- dimethylpropane o Same # of electrons but different shape The pentante are long and can interact with one another along the entire length Contrast, bulky shape of 2,2-dimethylporpane molecules result in smaller area of interaction between neighnouring molecules, resulting in lower boiling point Although molecular shape and other factors can be considered in determining magnitude of dispersion force,s # of electrons can act as a guide when comparing dispersion forces within a family of similar elements or compounds DIPOLE-DIPOLE FORCES Exist in all molecules that are polar Polar molecules have permanent dipoles that interact with the permanent dipoles of neighbouring molecules as you can see in fig. 11.8 o + of one attracts of another molecule polar molecuels therefore have higher melting and boiling points than nonpolar molecules of the same size and number of electrons remember, all molecules have dispersion forces o polar molecules have dip-dip + dispersion forces polarity of molecules composing liquids is also important in determining miscibility (ability to mix without separating intwo two states) in liquids o generally, polar liquids are miscible with other polar liquids but not with nonpolar liquids However, dip-dip interactions arent always responsible for trends in physical properties of compounds that might seem related HYDROGEN BONDING Polar molecules containing H atoms bonded directly to small electronegative atoms notably, F,O,N exhibit intermolecular force called H bonding A sort of super dip-dip force o The large electronegativity difference between H and any of the electronegative elements causes the H to have fairly large partial positive charge Additionally, since these atoms are all quite small, the H atom on one molecule can approach the F,O,N atom on another molecule very closely the result is strong attraction between the H atom on one molecule and the F,O,N of another Hydrogen bond H bonds shouldnt be confused with chemical bonds that occur between individual atoms in a molecule H bonds occur between molecules Weak intermolecular forces : H brongs are stronger than dispersion or dip-dip bonds Directionality : H bonds have an orientation unlike other intermolecular interactions o O-H bond adapts a nearly collinear geometry with the negative end of the molecule to which it is bound That is, an O-H-O angle is nearly 180 degrees for the H bond Why water is so special: two lone pairs to accept H bonds and two O-H bonds to act as donors o Can have HH bonds simultaneously DIPOLE INDUCED DIPOLE FORCES Between molecules that have a permanent dipole and an atom or molecule that is nonpolar These intermolecular forces occur in mixtures and are important when we discuss solutions When nonpolar atom or molecule to molecule with a permanent dipole approach each other, the nonpolars electron cloud is distorted (polarized) o Since nonpolar now has an induced dipole, a small one, it can interact with the polar molecule o Magnitude depends on size of the dipole of the polar molecules and polarizability of the nonpolar atom or molecule ION-DIPOLE FORCE Ionic compound mixes with a polar compound. Especially important in aqueous solutions of ionic compounds Strongest types of intermolecular forces discussed here SUMMARY Ion induced dipole forces occur between an ion and nonpolar molecule as in a dilute solution of an ionic compound in a nonpolar solvent Dispersion forces and present in all. Weak between small atoms and molecules but can be significant with a large # of electrons Dipole=dipole forces occur between polar molecules H bonds are present in molecules containing H bonds directly to F,O,N Dipole induced dipoule forces occur in dilute mixtures of polar and nonpolar
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