Intermolecular Forces Section 3.4 Pg. 105-117

1) Explain intermolecular forces, London (dispersion) forces, dipole-dipole attractions and hydrogen bonding

2) Relate properties of substances to the predicted intermolecular bonding in the substance.

BACKGROUND• All chemical changes (reactions) are accompanied by energy

changes

▫ Energy is mostly heat, light, or electrical energy

▫ Energy can be released slowly (battery) or quickly (fireworks)

▫ Two types of energy changes are possible:

EXOTHERMIC – energy is released into the surroundings - the product’s bonds have less energy than the reactant’s bonds

ENDOTHERMIC – energy is absorbed from the surroundings - the product’s bonds have more energy than the reactant’s

bonds

▫ Bond Energy – the energy required to break a chemical bond or the energy released when a bond is formed

BACKGROUND• There are three types of forces in matter:

1) Intranuclear force (bond) – bonds within the nucleus between protons and neutrons (very strong)

2) Intramolecular force (bond) – bonds between atoms within the molecule or between ions within the crystal lattice (quite strong)

3) Intermolecular force (bond) – bonds between molecules (quite weak); are electrostatic (involve positive and negative charges)

There are 3 types of intermolecular bonds:

a) London Force (a.k.a. London Dispersion Force, Dispersion Force)

b) Dipole-Dipole Forces (a.k.a. Polar Forces)

c) Hydrogen BondingNote: “Van der Walls force” – includes London and dipole-dipole forces

Weakest

Medium

Strongest

1) London Force• Simultaneous attraction between a momentary dipole

in a molecule and the momentary dipoles in surrounding molecules

momentary dipole: an uneven distribution of electrons around a molecule, resulting in a temporary charge difference between its ends

They last for just the instant

that the electrons are

not distributed perfectly even.

1) London Force• Fritz London also showed that momentary dipoles

occurring in adjacent molecules would result in an overall attraction

• The strength of the London force is directly related to the number of electrons in the molecule, and inversely related to the distance between the molecules.

▫ Increase electrons = Increase force (directly related)..▫ Increase distance = Decrease force (inversely related)

1) London Force

• The key point is that:▫ the more electrons a molecule has, the more

easily momentary dipoles will form, and the greater the effect of the London force will be.

• London forces are present between all molecules, whether any other type of attraction is present.

2) Dipole-Dipole Force•The simultaneous attraction between

oppositely charged ends of polar molecules.

▫ Simply put, the attraction between diploes

▫Dipole-dipole forces are among the weakest intermolecular forces, but still control important properties (i.e. Solubility because water is polar))

Dipole: a partial separation of positive and negative charges within a molecule, due to electronegativity differences

2) Dipole-Dipole Force

In a liquid, polar molecules can move and rotate to maximize attractions and minimize repulsions. The net effect is greater overall attraction.

The strength of the dipole-dipole force is dependent on the overall polarity of the molecule

Note: If a molecule is polar it will be soluble in water? Why?

2) Dipole-Dipole Forces

In a liquid: In a solid:

Why do we care about intermolecular forces?

• We can use Dipole-Dipole and London Forces to predict Boiling Points

Compound (at SATP)

Electrons Boiling Point (°C)

CH4(g) 10 -164

SiH4(g) 18 -112

GeH4(g) 36 -89

SnH4(g) 54 -52

Remember (if all other factors are equal):

1) The more polar the molecule = The stronger the dipole-dipole force

2) Increase the number of electrons = Increase the strength of London Force

A higher boiling point temperature means more energy has to be added, thus we assume the intermolecular

forces are stronger. (see Learning Tip pg. 109)

Example #1• Use Intermolecular force theory to predict which of

the following hydrocarbons has the highest boiling point:

▫ methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10)

1) Are the molecules polar or non-polar? non-polar (no dipole-dipole force)

2) Which has more electrons? butane: greatest # of e-’s = greatest London force

Check:

Alkane Boiling Point (°C)

methane -162

ethane -89

propane -42

butane -0.5

Example #2• Use Intermolecular force theory to predict which of the following

has the highest boiling point:

▫ bromine (Br2 ) or iodine monochloride (ICl)

1) Which has more electrons?

They are isoelectronic: have the same number of electrons (70 e-’s)

-Therefore the London force is the same (or nearly the same)

2) Are the molecules polar or non-polar?

-Bromine is non-polar (has no dipole-dipole force; only London forces)

- Iodine monochloride is polar (has dipole-dipole forces and London forces)

- This extra attraction between ICl molecules produces a higher boiling point

Check:

Substance Electrons Boiling Point (°C)

bromine 70 59

iodine monochloride

70 97

You cannot predict boiling points when:

• One molecule has a stronger dipole-dipole force and the other has a stronger London force

• The two molecules differ significantly in shape

• The central atom of either molecule has an incomplete octet

Practice

•Pg. 109 #1-4

• 4) a) Boron is stable with 6 valence e-

b) chloromethane (CH3Cl)

3) Hydrogen Bonding• Occurs when a hydrogen atom bonded to a

strongly electronegative atom, (N, O and F) is attracted to a lone pair of electrons in an adjacent molecule.▫ Hydrogen nucleus (proton) is simultaneously

attracted to two pairs of electrons; one closer (in the same molecule) and one further away (on the next molecule)

Why do you need a strongly

electronegative atom?

It pulls the hydrogen’s electron away making it

“unshielded”, so the lone pair on the other side can come much

closer

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3) Hydrogen Bonding•Hydrogen bonds are momentary attractive

forces between passing mobile molecules but are the strongest of the intermolecular forces.

•Hydrogen bonds only act as continuous bonds between molecules in solids, where the molecules are moving slowly enough to be locked into position.

•Hydrogen force would have been a better name.

3) Hydrogen Bonding

• In ice, hydrogen bonds between the molecules result in a regular hexagonal crystal structure.

• The ···H– represents a hydrogen nucleus (proton) being shared unequally between two pairs of electrons

3) Hydrogen Bonding• Do lakes freeze from the

bottom-up or the top-down?

• Top–down, because water is unique in that its solid form (ice) is less dense than its liquid form. Why??

• The hydrogen bonds hold water molecules in a hexagonal lattice with open space in the center, which explains the low density (mass/volume) of ice.

Hydrogen Bonding in DNA

• FYI: The double helix of the DNA molecule owes its unique structure largely to hydrogen bonding.

• The red bonds are hydrogen bonds.

• If the helix were held together by covalent bonds, the DNA molecule would not be able to unravel and replicate and life could not continue!!

• Explains surface tension, shape of a meniscus, volatility and capillary action

1) Surface Tension

▫ Molecules within a liquid are attracted by other molecules in all directions equally, but right at the surface, molecules are only attracted downwards and sideways. This means the net pull is downward so the surface tends to stay intact

▫ The stronger the intermolecular force the stronger the surface tension.

Why do we care about intermolecular forces?

This shows water adhering to the faucet gaining mass until it is stretched to a point where the surface tension can no longer bind it to the faucet.

It then separates and surface tension forms the drop into a sphere.

2) Capillary Action – due to adhesion (attraction between unlike molecules) and cohesion (attraction of like molecules)

▫ The adhesion between water and glass is greater than the cohesion between water molecules.

▫ The cohesion between mercury molecules is greater than the adhesion between mercury and glass

Why do we care about intermolecular forces?

In a sense, water is pulled up the tube by

the intermolecular forces between

water and glass

Hg clip

Meniscus

Practice

•Pg. 117 # 1, 2, 4, 5▫#1 – use pg. 99 table to determine polarity

▫#1 – look for NH2, NH, OH2, OH, to determine if hydrogen bonding is

possible

• Ex. CH3CHOHCH3 will it have hydrogen bonding?