3.1.3.7 Forces between molecules

vdw perm dip dip hydro bonding

What are intermolecular forces?

Attractive forces between molecules.

What type of compounds do intermolecular forces apply to?

Only covalently bonded compounds and atoms in noble gases.

Are intermolecular forces considered bonds?

No, they are attractions, not bonds.

What determines the boiling and melting points of simple covalent compounds?

The strength of intermolecular forces.

What are Van der Waals' forces?

electrostatic attractions between temporary partial opposite charges on different molecules (much weaker than covalent bonds)

What are other names for Van der Waals' forces?

Induced dipole-dipole interactions, dispersion forces, London forces.

Why does distribution of electrons around atom change at every instant?

There’s electron movement

What causes an instantaneous dipole?

When electrons are located at one end of the molecule, creating a temporary charge imbalance.

What do these 2 symbols represent in dipole notation?

δ- represents a slightly negative charge, while δ+ represents a slightly positive charge.

What happens when an instantaneous dipole occurs?

It induces a dipole in a neighboring molecule, resulting in induced 2nd dipole

The б- in the instantaneous dipole (one that induced other thing) REPELS ELECTRONS TO MOVE TO THE OTHER SIDE IN A CLOSE BY MOLECULE

The Molecule has uneven electron density and becomes dipole

How do Van der Waals forces arise?

random movement of electrons

electrons can sometimes build up on one side of an atom/molecule creating a dipole and creating a partial negative charge on this side

partial negative charge repels electrons on nearby atoms/molecules creating temporary dipole so side gains a partial positive charge

What is the relationship between the number of electrons as well as shape of molecule and the strength of Van der Waals forces?

More electrons lead to larger instantaneous dipoles and stronger Van der Waals forces.

Longer chain molecules have larger surface area of contact between molecules for Van der Waals to form and stronger Van der Waals forces.

What effect does chain length have on boiling points of alkanes? (REFER TO VDW FORCES)

• Higher boiling point as chain length increases

• A larger molecule has more electrons

Therefore the instantaneous dipole will be larger •

Therefore more and greater Van der Waals forces between the molecules • Therefore greater attraction between molecules.

Therefore more energy is required to break this attraction

Why is more energy required to break Van der Waals forces in larger molecules?

Larger molecules have stronger attractions due to greater instantaneous dipoles.

What is an induced dipole?

A dipole that forms in a neighboring molecule due to the influence of an instantaneous dipole.

What is the significance of Van der Waals forces in molecular interactions?

They act between all atoms or molecules at all times and so do other intermolecular forces.

What does the term 'instantaneous dipole' refer to?

A temporary dipole that arises due to the uneven distribution of electrons at a given moment.

What type of interactions do Van der Waals forces contribute to?

They contribute to the overall intermolecular interactions in covalent compounds.

T or F: hydrogen bonding is weakest intermolecular force and permanent dipole-dipole interactions is medium strongest intermolecular false

FALSE bc 2nd Statement true but hydrogen bonding is strongest

for Dipole-dipole interactions molecules MUST have:

Be Polar molecule

Polar bonds

True or false: all molecules with polar bonds are polar

FALSE I.e not co2!!! Its partial charges cancel out!!!

permanent dipole- dipole interactions are caused by:

electronegativity differences

since permanent dipole-dipole interactions are between ALL polar molecules

There’s Attraction between atoms of OPPOSITE PARTIAL CHARGE ON DIFFERENT MOLECULES

How do you know molecule is non polar and why?

Molecule is symmetrical (has multiple planes of symmetry)

SO dipoles are equal and opposite to each other SO cancels out

Molecules with plane of symmetry but not symmetrical in all directions are called:

Asymmeticral

Symmetrical shapes

All 5 basic shapes and square planar so the rest is asymmetrical

T or F: hydrogen bonding are bonds

Hydrogen bonding is an INTERMOLECULAR FORCE and they are much weaker than any covalent bond

When does hydrogen bonding occur?

occurs in compounds that have a H atom covalently bonded to one of nitrogen, oxygen and fluorine, which must have an available lone pair of electrons.

Why does hydrogen bonding occur here? Why are the three elements so electronegative?

large electronegativity difference between H and N/O/F.

Little shielding because one filled electron shell

draw the hydrogen bonding between 2 HF molecules

Bond angle of all hydrogen bonds

180^o

Why is this the bond angle for all hydrogen bonds?

Hydrogen has two areas of electron density

4 Properties hydrogen bonding gives water

•Surface tension

•Excellent solvent

•Liquid at room temperature

•Highly polar molecule

(Compared to increase in boiling point from H₂S to H₂Te)

The anomalously high boiling points of H2O, NH3 and HF are caused by the hydrogen bonding between the molecules

general increase in boiling point from H2S to H2Te is caused by increasing Van der Waals forces between molecules due to an increasing

number of electrons.

Hydrogen bonding in ice

In ice, each water molecule is hydrogen bonded to 4 other water

molecules. (So on average, the are two H-bonds per molecules) - Ice has an "open structure" with lots of spaces between

molecules - As a result the molecules are held further apart than in liquid

water - When the ice melts, the structure is lost and the molecules

collapse closer together. - This means ice is less dense than water (the same mass takes up

more volume)

- So ice floats on water.

EQ criteria to answer (7 steps)

Explain why the melting point of (structure A) is so much higher than that of (structure B)

1. What is the bonding?

2. What is the structure?

3. What must be overcome to melt the substance?

4. Compare the strengths. 5. If the same type of bonding/IMF is being

overcome, explain the difference in strengths.

6. Which requires more energy to overcome

7. Link to melting point