Intermolecular Interactions

Intermolecular Interactions

What are intermolecular interactions?

Forces of attraction between molecules that are weaker than typical covalent and polar covalent bonds.

What are intermolecular interactions also called?

Non-bonded interactions because they occur between molecules rather than being covalent bonds within molecules.

What are the three main types of intermolecular interaction?

London forces, permanent dipole–permanent dipole interactions and hydrogen bonds.

What are London forces also sometimes called?

Dispersion forces.

What causes London forces?

Attraction between an instantaneous dipole in one molecule and an induced dipole in a neighbouring molecule.

Are London forces present in polar or non-polar molecules?

Both; London forces occur between all molecules.

How can a non-polar molecule develop an instantaneous dipole?

Electron density fluctuates, so at a particular instant electrons may be distributed unevenly, producing temporary δ+ and δ− ends.

What is an instantaneous dipole?

A temporary separation of charge caused by fluctuations in electron density within a molecule.

How does an instantaneous dipole induce a dipole in a neighbouring molecule?

It causes the electron density in the neighbouring molecule to become unevenly distributed, producing an induced dipole.

What is an induced dipole?

A temporary dipole produced in a molecule by the presence of a nearby dipole.

What is a London force?

An attractive intermolecular force between an instantaneous dipole and an induced dipole.

Why do an instantaneous dipole and an induced dipole attract?

The induced dipole becomes aligned so that opposite partial charges are close together, producing attraction.

Are instantaneous and induced dipoles permanent?

No, they form and disappear very rapidly as electron density continually fluctuates.

Why can molecules continue to experience London forces while moving and rotating?

New instantaneous and induced dipoles are continually produced, allowing attractive interactions to keep forming.

How does the number of electrons affect the strength of London forces?

The more electrons a molecule has, the greater the fluctuations in electron density and the stronger the London forces.

Why do larger atoms and molecules generally have stronger London forces?

They contain more electrons, allowing larger instantaneous and induced dipoles to form.

What happens to the boiling temperature of the noble gases from helium to radon?

It increases because the number of electrons increases and London forces become stronger.

Why does stronger London attraction cause a higher boiling temperature?

More energy is required to overcome the intermolecular attractions and separate the particles.

How does molecular shape affect London forces?

Molecules with more points of contact between them experience greater overall London forces.

Why can long or less-branched molecules have stronger London forces than compact molecules?

They can have a larger area of contact with neighbouring molecules, producing stronger overall intermolecular attractions.

Are London forces present when molecules also have permanent dipoles?

Yes. London forces are always present, even when other intermolecular forces also occur.

Are London forces present in molecules that form hydrogen bonds?

Yes. London forces occur between all molecules, including molecules that hydrogen bond.

What is a permanent dipole–permanent dipole interaction?

An attraction between the oppositely charged δ+ and δ− ends of neighbouring polar molecules.

When do permanent dipoles produce an attractive interaction?

When the molecules are aligned so that opposite partial charges are close together.

Can permanent dipoles cause repulsion?

Yes. Unfavourable alignment can place like partial charges close together, causing repulsion.

Why are permanent dipole interactions averaged out to be weaker than might be expected?

Random molecular movement means permanent dipoles are not always aligned favourably.

Can a permanent dipole induce a dipole in another molecule?

Yes, producing a permanent dipole–induced dipole interaction.

What does the term van der Waals force refer to?

The sum of intermolecular interactions such as London forces and permanent dipole–permanent dipole interactions.

Why is it better to avoid the term van der Waals forces in exam answers?

It is more precise to name the specific interaction, such as London forces or permanent dipole–permanent dipole interactions.

What is a hydrogen bond?

An intermolecular interaction involving partial bond formation between a hydrogen atom bonded to a highly electronegative atom and another electronegative atom with a lone pair.

Which elements commonly allow strong hydrogen bonding when bonded to hydrogen?

Nitrogen, oxygen and fluorine.

Why must hydrogen be bonded to a highly electronegative atom for hydrogen bonding?

The electronegative atom strongly attracts the bonding electrons, leaving hydrogen strongly δ+.

What feature must the atom attracting the δ+ hydrogen possess?

A lone pair of electrons.

Is a hydrogen bond simply an extreme permanent dipole–permanent dipole interaction?

No. It also involves some partial bond formation using a lone pair of electrons.

Can hydrogen bonding occur between different molecules?

Yes; this is intermolecular hydrogen bonding.

Can hydrogen bonding occur within the same molecule?

Yes; this is intramolecular hydrogen bonding.

What molecules highlighted in this topic commonly form hydrogen bonds?

H₂O, NH₃ and HF.

Why does water form hydrogen bonds?

Hydrogen is bonded to highly electronegative oxygen, and oxygen has lone pairs that can interact with δ+ hydrogen atoms on neighbouring molecules.

How many lone pairs does the oxygen atom in a water molecule have?

Two.

How many other water molecules can the oxygen atom of one water molecule accept hydrogen bonds from?

Two, because it has two lone pairs.

What is the hydrogen bond in water formed between?

The δ+ hydrogen of one water molecule and a lone pair on the oxygen atom of another water molecule.

Do alcohols form intermolecular hydrogen bonds?

Yes, because they contain O–H groups.

How does ethanol form hydrogen bonds?

The δ+ hydrogen in its O–H group interacts with a lone pair on the oxygen atom of another ethanol molecule.

Why are hydrogen bonds described as directional?

Their strength depends on the relative positions and bond angles of the atoms involved.

What bond angle is often favourable for a hydrogen bond?

Approximately 180°, although hydrogen bonds do not always have this exact angle.

Why is hydrogen bonding significant in ammonia?

NH₃ contains N–H bonds and a lone pair on nitrogen, allowing hydrogen bonds to form between ammonia molecules.

What is the hydrogen bond in ammonia formed between?

A δ+ hydrogen bonded to nitrogen in one NH₃ molecule and the lone pair on nitrogen in another NH₃ molecule.

Which fluorine compound forms intermolecular hydrogen bonds?

Hydrogen fluoride, HF.

Why does HF form hydrogen bonds?

Hydrogen is bonded to highly electronegative fluorine, and fluorine has lone pairs that can attract the δ+ hydrogen of another HF molecule.

What is the hydrogen bond in HF formed between?

The δ+ hydrogen of one HF molecule and a lone pair on the fluorine atom of another HF molecule.

Why does ethanol have a much higher boiling temperature than methoxymethane?

Ethanol forms hydrogen bonds between its O–H groups, whereas methoxymethane cannot form hydrogen bonds between its own molecules, so less energy is needed to separate methoxymethane molecules.

Why can methoxymethane not hydrogen bond with itself even though it contains oxygen?

It has no hydrogen atom directly bonded to oxygen, so it cannot provide the δ+ hydrogen required for hydrogen bonding between its molecules.

What intermolecular forces are present between ethanol molecules?

London forces, permanent dipole–permanent dipole interactions and hydrogen bonds.

What intermolecular forces are present between methoxymethane molecules?

London forces and permanent dipole–permanent dipole interactions.

Why do substances with hydrogen bonding often have unusually high boiling temperatures?

Extra energy is required to overcome the hydrogen bonds between molecules.

Does the presence of hydrogen bonding always mean it is the most significant intermolecular force?

No. In larger molecules, London forces can become increasingly important and hydrogen bonding may become relatively less significant.

Why does hydrogen bonding become relatively less significant as an alcohol carbon chain gets longer?

The increasing number of electrons and larger molecular surface increase London forces, while the molecule still has only one O–H group.

What is meant by dimerisation through hydrogen bonding?

Two molecules associate with one another by forming hydrogen bonds between them.

How can ethanoic acid molecules form a dimer?

Two ethanoic acid molecules form hydrogen bonds between their carboxylic acid groups, producing a pair of associated molecules.

Can hydrogen bonding occur between different types of molecule?

Yes, provided there is a suitable δ+ hydrogen and an electronegative atom with a lone pair.

What generally happens to hydrogen-bond strength as the electronegativity of the atom bonded to hydrogen increases?

Hydrogen-bond strength generally increases.

What trend in hydrogen-bond strength is given for H₂S, NH₃, H₂O and HF?

H₂S < NH₃ < H₂O < HF.

Why is hydrogen bonding involving sulfur usually weaker than hydrogen bonding involving N, O or F?

Sulfur is less electronegative, so the S–H bond produces a less strongly δ+ hydrogen.

Are hydrogen bonds generally stronger or weaker than full covalent bonds?

Generally much weaker than full covalent bonds.

Can hydrogen bonds to ions be stronger than hydrogen bonds to neutral molecules?

Yes, hydrogen bonding to ions can be much stronger.

What determines which intermolecular forces are present in a substance?

Its molecular structure, polarity, presence of suitable H–N, H–O or H–F bonds, lone pairs, number of electrons and molecular shape.

How should you explain a difference in boiling temperatures using intermolecular forces?

Identify the intermolecular forces present, compare their strengths, and explain that stronger attractions require more energy to overcome, giving a higher boiling temperature.

Define a hydrogen bond.

An intermolecular interaction in which there is some evidence of bond formation between a hydrogen atom bonded to an atom more electronegative than hydrogen and another electronegative atom in the same or a different molecule.