Chapter 14

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

Tro, Chemistry: A Molecular Approach 2

Properties of the 3 Phases of Matter


•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


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


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


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


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



+ - + - + - + -

+++

+

____

+ + + + + + +

- - - - - - -

++ + +

+

--

- --

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


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



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


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



Water

Dichloromethane(methylene chloride)

Ethanol(ethyl alcohol)


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



HF

Practice – Choose the substance in each pair that is more soluble in watera) CH3OH CH3CHF2

b) CH3CH2CH2CH3 CH3Cl


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


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



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

Chapter 14

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