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Lecture 11

CHEM 217 Lecture Notes - Lecture 11: Dry Ice, Phase Diagram, SinadPremium

18 pages97 viewsSpring 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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