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Liquids and Solids
Comparisons of the States of Matterthe solid and liquid states have a much
higher density than the gas statetherefore the molar volume of the solid and
liquid states is much smaller than the gas statethe solid and liquid states have similar
densitiesgenerally the solid state is a little denser
notable exception: ice is less dense than liquid water
the molecules in the solid and liquid state are in close contact with each other, while the molecules in a gas are far apart
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Properties of the 3 Phases of Matter
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•Fixed = keeps shape when placed in a container •Indefinite = takes the shape of the container
State Shape Volume Compressible Flow Strength of Intermolecular
Attractions
Solid Fixed Fixed No No very strong
Liquid Indef. Fixed No Yes moderate
Gas Indef. Indef. Yes Yes very weak
Explaining the Properties of Solidsthe particles in a solid are packed close
together and are fixed in positionthough they may vibrate
the close packing of the particles results in solids being incompressible
the inability of the particles to move around results in solids retaining their shape and volume when placed in a new container; and prevents the particles from flowing
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Solidssome solids have their particles arranged in an orderly geometric pattern – we call these crystalline solidssalt and diamonds
other solids have particles that do not show a regular geometric pattern over a long range – we call these amorphous solidsplastic and glass
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Explaining the Properties of Liquidsthey have higher densities than gases
because the molecules are in close contact
they have an indefinite shape because the limited freedom of the molecules allows them to move around enough to get to the container walls
but they have a definite volume because the limit on their freedom keeps them from escaping the rest of the molecules
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Energy Requirements for the changes of StateIntramolecular (within the
molecule)forces : the bonding forces that hold the atoms of a molecule together
Intermolecular (between molecules) forces : The forces that occurs among molecules that cause them to aggregate to form a solid of a liquid
O
H
H
H
O
H
Intermolecular forces
intramolecular forces
Energy Requirements for the changes of StateMolar heat of fusion: The energy required
to melt 1 mol of a substancekJ/mol
Molar heat of vaporization: The energy required to change 1 mol of liquid to its vaporkJ/mol
Calculating Energy Changes: Solid to Liquid and Liquid to GasCalculate the energy required to melt 8.5 g of
ice at 0.0oC. The molar heat of fusion for ice is 6.02 kJ/mol
Calculate the energy (kJ) required to heat 25 g of liquid water from 25.0oC to 100.0oC and change it to steam at 100.0oC. The specific heat capacity of liquid water is 4.184 J/goC, and the molar heat of vaporization of water is 40.6 kJ/mol
Calculate the energy required to vaporize 35.0 g of water at 100.0oC. The molar heat vaporization of water is 40.6 kJ/mol
Why are molecules attracted to each other?
intermolecular attractions are due to attractive forces between opposite charges+ ion to - ion+ end of polar molecule to - end of polar molecule
H-bonding especially strongeven nonpolar molecules will have temporary charges
larger the charge = stronger attractionlonger the distance = weaker attractionhowever, these attractive forces are small relative
to the bonding forces between atomsgenerally smaller chargesgenerally over much larger distances
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Trends in the Strength of Intermolecular Attraction? the stronger the attractions between the
atoms or molecules, the more energy it will take to separate them
boiling a liquid requires we add enough energy to overcome the attractions between the molecules or atoms
the higher the normal boiling point of the liquid, the stronger the intermolecular attractive forces
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+ - + - + - + -
+++
+
____
+ + + + + + +
- - - - - - -
++ + +
+
--
- --
Dispersion Forcesfluctuations in the electron distribution in atoms
and molecules result in a temporary dipoleregion with excess electron density has partial (─)
chargeregion with depleted electron density has partial (+)
chargethe attractive forces caused by these temporary
dipoles are called dispersion forcesaka London Forces
all molecules and atoms will have themas a temporary dipole is established in one
molecule, it induces a dipole in all the surrounding molecules
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Size of the Induced Dipolethe magnitude of the induced dipole
depends on several factorspolarizability of the electrons
volume of the electron cloudlarger molar mass = more electrons =
larger electron cloud = increased polarizability = stronger attractions
shape of the moleculemore surface-to-surface contact = larger
induced dipole = stronger attraction
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Noble Gases are all nonpolar atomic elements.
As the molar mass increases, the number of electrons increase. Therefore the strength of the dispersion forces increases.
The stronger the attractive forces between the molecules, the higher the boiling point will be.
Dipole-Dipole Attractionspolar molecules have a permanent dipole
because of bond polarity and shapedipole momentas well as the always present induced dipole
the permanent dipole adds to the attractive forces between the moleculesraising the boiling and melting points relative
to nonpolar molecules of similar size and shape
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Attractive Forces and Solubility
Solubility depends on the attractive forces of solute and solvent moleculesLike dissolves Likemiscible liquids will always dissolve in each
otherpolar substance dissolve in polar solvents
hydrophilic groups = OH, CHO, C=O, COOH, NH2, Cl
nonpolar molecules dissolve in nonpolar solventshydrophobic groups = C-H, C-C
Many molecules have both hydrophilic and hydrophobic parts - solubility becomes competition between parts
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Water
Dichloromethane(methylene chloride)
Ethanol(ethyl alcohol)
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CH3
CH2
CH2
CH2
CH2
CH3
n-hexane
CH
CHCH
CH
CHC
CH3
toluene
C
Cl
ClClCl
carbon tetrachloride
Hydrogen BondingWhen a very electronegative atom is bonded to
hydrogen, it strongly pulls the bonding electrons toward itO-H, N-H, or F-H
Since hydrogen has no other electrons, when it loses the electrons, the nucleus becomes deshieldedexposing the H proton
The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules
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HF
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Practice – Choose the substance in each pair that is more soluble in watera) CH3OH CH3CHF2
b) CH3CH2CH2CH3 CH3Cl
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Ion-Dipole Attractionin a mixture, ions from an ionic compound are
attracted to the dipole of polar moleculesthe strength of the ion-dipole attraction is one
of the main factors that determines the solubility of ionic compounds in water
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SummaryDispersion forces are the weakest of the
intermolecular attractions. Dispersion forces are present in all molecules
and atoms. The magnitude of the dispersion forces
increases with molar massPolar molecules also have dipole-dipole
attractive forces
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Summary (cont’d)Hydrogen bonds are the strongest of the
intermolecular attractive forcesa pure substance can have
Hydrogen bonds will be present when a molecule has H directly bonded to either O , N, or F atomsonly example of H bonded to F is HF
Ion-dipole attractions are present in mixtures of ionic compounds with polar molecules.
Ion-dipole attractions are the strongest intermolecular attraction
Ion-dipole attractions are especially important in aqueous solutions of ionic compounds
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