Intermolecular forces

What do simple molecular substances consist of

Relatively small molecules and each molecule has a fixed number of atoms

Note

Intermolecular forces of attraction ac between molecules

They are much weaker than covalent bonds and are easily broken by high temperature

What is the strength of intermolecular forces in relation to covalent bonds ?

They are much weaker than covalent bonds and are easily broken by high temperature

What happens when we heat a simple molecular substance

This causes the molecules to move faster. At certain temperatures, the intermolecular forces break and this allows the molecules to move away from each other

Why do simple molecular substances have low boiling points

Bc it takes very little energy to break the intermolecular forces

What are the 3 different types of intermolecular forces?

1) induced dipole-dipole interactions- ( van der waals forces)

2) permanent dipole dipole interactions

3) hydrogen bonds

What are the different names for van der waal forces

Instantaneous dipole/ London forces/ dispersion forces/ induced dipole-dipole interactions

What causes vdw forces?

Random movement of electrons which crate a temporary/ instantaneous dipole in one molecule , this induces a dipole in another , the dipoles in different molecules attract

Explain in detail how vdw forces occur

Electrons randomly move , for a second more electrons may be on the right side of the atom - now that atom has a dipole this causes the left side of the atom to have a slight positive charge and the right side to have a slight negative charge. This is formed instantaneously due to random electron movement so its called an instaeous dipole

The electrons in atom 1 repels the electrons in atom 2 causing the electrons in atom 2 to move to the right side this is known as an induced dipole, the dipole in atom 2 didn’t happen randomly it was caused by the dipole in atom 1

The dipole in atom 2 then repels the electrons in atom 3 causing electrons to move to the right and form an induced dipole

All atoms experience an attraction known as van der waals

How strong are vdw forces

Vdw forces are weak and easily broken

Do all atoms/molecules experience vdw forces ?

Vdw forces are caused by random electron movement meaning that every single atom or molecule will experience vdw forces even if they experience other intermolecular forces as well

What does the strength of vdw forces depend on?

The strength of van der waals depend on the number electrons,

Atoms with a greater number of electrons have a higher strength of vdw

Why do atoms/ molecules with a greater number of electrons experience a higher boiling point ?

Bc atoms with a greater number of electrons have a higher b.p bc they experience stronger van der waals forces

what is a permanent dipole dipole interaction

A permanent dipole-dipole interaction is an intermolecular force of moderate strength that occurs when polar molecules align themselves so that the partially positive (\(\delta+\)) end of one molecule attracts the partially negative (\(\delta-\)) end of a neighboring molecule

What does δ+ represent?

A partial positive charge.

What does δ− represent?

A partial negative charge.

Which ends of neighbouring molecules attract?

The δ+ end of one molecule attracts the δ− end of another.

Which molecules experience permanent dipole–dipole interactions?

Polar molecules with permanent dipoles.

Contains polar covalent bonds arranged in an asymmetrical shape

Explain why there is a permanent dipole between H and Cl

The chlorine atom has a greater electronegativity than the hydrogen atom - meaning that the electron pair in the covalent bond is more attracted to the chlorine atom- this makes the chlorine atom slightly negative compared to the hydrogen atom.

How can 2 hydrogen chlorides form a permanent dipole- dipole interaction

When two molecules of HCl get near enough then there dipoles can lead to an attraction. ( the slightly negative CL atom gets attracted to the slightly positive hydrogen atom) this is a permanent dipole-dipole interaction

NOTE

Only molecules with a permanent dipole can experience a permanent dipole dipole interaction

Why does tetrachloromethane have a higher b.p than trichloromethane?

Both molecules contain polar bonds, trichloromethane has a permanent overall dipole , tetrachloromethane is commpletely symmetrical so has no overall permanent dipole so is not a polar molecule- whereas trichloromethane has a permanent dipole- dipole interaction

Bc trichloromethane has permanent dipole dipole interactions we might expect it to have a higher b.p than tetrachloromethane however it doesn’t.

Tetrachloromethane has vdw forces acting- the size of vdw forces depend on the number of electrons. Trichloromethane has 58 electrons and tetrachloromethane has 74 electrons

This means that the vdw forces are stronger in tetrachloromethane - explaining why it had a higher boiling point

What does the strength of the permanent dipole decrease due to?

Decreasing electronegativity

Why is hydrogen fluoride polar

Fluorine is the most electronegative element so it strongly attracts the covalent bonded electron pair therefore its polar

How does a hydrogen bond form between 2 hydrogen fluorides

The positive hydrogen atom is attracted to the lone pair of electron on the fluorine atom - this attraction is called a hydrogen bond

State the strength of a hydrogen bond in relation to other intermolecular forces

Hydrogen bonds are the strongest type of intermolecular force

What are the 2 conditions needed for a hydrogen bond to take place

1) we need a hydrogen atom bonded to a strongly electronegative element

2) the electronegative atom must have at least one lone pair of electrons

What this means is that we find hydrogen bonding with fluorine, oxygen or nitrogen- all 3 of these elements are strongly electronegative and have at least 1 lone pair of electrons.

Examples of hydrogen bonding

Why does water( H2O) have a relatively high boiling point (& melting point) and why does hydrogen sulfide(H2S) have a low b.p?

H2O b.p 100o- hydrogen atoms are bonded to oxygen which is the 2nd most electronegative element. This means that water molecules undergo hydrogen bonding. Bc of this when we boil water it takes a great deal of energy to break the hydrogen bonds-meaning water has a relatively high boiling point.

H2S b.p -60- the hydrogen atoms are bonded to sulfur, the electronegativity of sulfur is very similar to hydrogen - meaning H2S cannot form hydrogen bonding - therefore has a low boiling point of -60

Why does ice float on water/ why is ice less dense than water

Bc as we freeze water, the water molecules arrange themselves in an ordered structure stabilised by a network of hydrogen bonds( fixed open lattice structure). in ice the water molecules are further apart than in liquid water making ice less dense than water so it floats