Liquids and Solids

Use the Force Luke!

Learning objectives

Describe origins of surface tension and meniscus

Describe different types of cohesive force

Identify type of cohesive force based on molecular formula

Describe origin of hydrogen bonding

Explain unique properties of water

Determine concentrations of solutions using the common concentration scales

Liquids and solids exist because of attractive

forces between molecules

Cohesive forces are attractive forces

between molecules of the same substance

Adhesive forces are attractive forces

between molecules of different substances

Surface tension results from

cohesive forces

Molecules on surface are

drawn inwards by

cohesive forces

Surface behaves like a

shrink-wrap film

Tendency to form sphere

Liquids form into

spherical drops

Water beading up on a

waxed car

Miracles explained: surface tension

and walking on water

Denser objects can

“float” on the surface

tension

The insect exerts too

little force on the

water to break the

surface tension

Consequences of surface tension:

What will these hands ne’er be clean?

Cohesive forces in water are

strong

Adhesive forces between

grease and water are weak

Water and grease don’t mix

Detergent acts as go-

between

Micelles encapsulate

hydrophobic tails and

grease in hydrophilic shell

Washing up done Hydrophilic head

sticks to water

Hydrophobic tail

sticks to non-polar material

Cohesive forces and meniscus

Adhesive forces pull

H2O molecules to

maximize coverage

Cohesive forces

between H2O

molecules drag

liquid up

Gravity pushes

liquid down

Solid: strong intermolecular forces

Translational energy

molecules less than

cohesive forces

Fixed shape

Not compressible

Rigid

Dense

Solids melt when molecules

overcome intermolecular forces

Molecules in solids rotate

and vibrate but don’t

move

Melting occurs when

translational energy of

molecules overcomes

intermolecular forces

Usually this is a very well

defined point (freezing pt

H2O = 0oC)

With amorphous solids or

large molecules it can be

smeared out – softening

of fats

Liquid: medium intermolecular

forces

Translational energy

greater than cohesive

forces

Molecules move

Not rigid

Assumes shape of

container

Not compressible

Dense

Vapour pressure and boiling

Molecules don’t all have same energy

Evaporation: High energy molecules escape the liquid – vapour pressure

When vapour pressure = atmospheric pressure boiling occurs – all liquid becomes gas

Even solids have vapor pressure

Sublimation is direct transition of solid to gas (dry ice)

The Four Forces of the Apocalypse Some substances are gases and others are solids: it’s about

intermolecular forces

All intermolecular forces involve electrostatic attractions between

positive and negative charges

They arise in different ways

Name of force Origin Strength

Ion-dipole Between ions and

molecules (NaCl in

H2O)

Quite strong

Dipole-dipole Between permanent

dipoles (NF3) Weak

Hydrogen bonds Polar bonds with H

and (O,N,F) (H2O)

Quite strong

London dispersion

forces

Fluctuating dipoles in

non-polar molecules

(CF4)

V weak in small

molecules

Stronger in large ones

London dispersion force:

Only force in nonpolar molecules

Arises from fluctuations of electrons in atoms/molecules

On average center of gravity of electrons coincides with nucleus

Electron motion means that charge can be unbalanced

Instantaneous dipole

Present in all molecules

Strength of dispersion force

increases with atomic size Only cohesive

force in nonpolar molecules

Increases with size of atoms/molecules

Boiling point increases with molar mass

Dipole-dipole force:

Dominant force in polar molecules Present in all polar

molecules

Usually stronger than dispersion forces

Polar molecules higher boiling point than nonpolar molecules

Molecule Molar mass

(g/mol)

Boiling point

(ºC)

Ethane (C2H6)

Non-polar 30.0 -88.0

Formaldehyde (CH2O)

Polar

30.0 -19.5

Polar or nonpolar: that is the

question? Polar molecules contain

polar bonds Determine bond polarity

from electronegativity

The polar bonds must not cancel out Determine molecular shape

Examples: • O2 nonpolar (no polar

bond)

• HCl polar (one polar bond)

• CHCl3 polar (asymmetry: three polar bonds)

• CCl4 nonpolar (symmetry: four polar bonds but they all cancel)

Hydrogen bonding

The ultimate expression

of polarity

Small positive H atom

exerts strong attraction

on lone pair on O atom

Other H-bonding

molecules: HF, NH3

H2O is the supreme

example: two H atoms

and two lone pairs per

molecule

Something about water

Boiling points of hydrides

increase with molar mass

for periods 3 and up

Trend is same for all

groups

Hydrides in period 2

(NH3, H2O, HF) are

exceptions (except CH4)

Hydrogen bonding is to

blame

H2O

HF

NH3

CH4

H2O has optimum combination of

lone pairs and H atoms

Compound Number of lone

pairs

Number of H

atoms

HF 3 1

H2O 2 2

NH3 1 3

H bonding generates three-

dimensional network

The regularity of H-bonding in ice

ICE WATER

Ice floats!

Something so familiar we might believe all solids

float on their liquids. Not so. Water is the

exception.

Hydrogen bonding and life

hold the two strands of the DNA double helix

together

hold polypeptides together in such secondary

structures as the alpha helix and the beta

conformation

help enzymes bind to their substrate

help antibodies bind to their antigen

help transcription factors bind to each other

help transcription factors bind to DNA

Implications for life on earth

Without H-bonds

molecules like DNA

would not exist

H-bonds hold the two

strands together

Comparative

weakness of bonding

allows for DNA

replication dna

Terms in solution

Solute: the dissolved substance

Solvent: the dissolving substance

Describing concentration:

Molarity Concentration is usually expressed in

terms of molarity:

Moles of solute/liters of solution (M)

Moles of solute = molarity x volume of solution

Example

What is molarity of 50 ml solution containing

2.355 g H2SO4?

Molar mass H2SO4 = 98.1 g/mol

Moles H2SO4 = 0.0240 mol

Volume of solution = 0.050 L

Concentration = moles/volume

= 0.480 M

2.355 g

98.1 g/mol

1 L50 mL x

1000 mL

0.0240 mol

0.050 L

Working with molarity

Dilution

More dilute solutions are prepared from

concentrated ones by addition of solvent

Dilution: V2 > V1

Molarity of new solution

To dilute by factor of ten, increase volume by

factor of ten

1 1 2 2M V M V

1 1 2 2

2 2

M V M V

V V 1

2 1

2

VM M

V

Working with molarity

Trace quantities: ppm and mg/L

Percent means one in a hundred (1:100)

PPM measures trace amounts – 1 in a

million (1:106)

Iodized salt contains tiny amounts of KI – 7.6

x 10-5 g in 1 g of salt

grams soluteppm =

grams solution

6x10

57.6 10 KI100% 0.0076%

1 salt

x gx

g

5

67.6 10 KI10 76

1 salt

x gx ppm ppm

g

Working with ppm

3 3 6

2 2 2

1 X 1 1 1 1 X

1 H O 10 10 1 H O 10 H O

mg g L mL gx x x

L mg mL g g

Milligrams per liter Units for impurities in drinking water

Equivalent to ppm

0.38 mg lead in 250 mL water

Concentration in mg/L (convert mL → L)

milligrams solutemg/L =

liters solution

0.38 mg 1000 mL 0.38 mgmg/L = x = =1.5 mg/L

250 mL 1 L 0.250 L

Convert

to g

Convert to

L to mL

Convert to

mL to g

Water contamination

Biological Human and animal waste –

bacteria leading to hepatitis, cholera, typhoid, dysentery

Chemical Organic

• Benzene

• Chlorohydrocarbons

Inorganic • Asbestos

• Nitrates

• Lead

• Mercury

Radioactivity • Uranium

• Tritium spills

Legislating cleanliness: The Safe

Drinking Water Act 1974

Establish maximum

contaminant levels

(MCLs) for 84

contaminants

All water supplies must

pass

Periodic sampling

required

Too much or too little?

Water treatment costs

money

Don’t trust the EPA? Treat at home

Active carbon filters

Effective on organic contaminants

Need regular replacement

Water softeners

Specific to hard water

Ion exchange using zeolites

Osmosis Transport of water molecules from dilute solution

to more concentrated one

Imbalance of concentration provides driving force

Osmotic pressure is the pressure required to oppose this flow

Purification by reverse osmosis

Apply pressure >

osmotic pressure to

saline side

Water travels from

concentrated to dilute