Chemistry: Van der Waals Forces and Alkane Boiling Points

What are intermolecular forces?

Attractive forces between molecules.

What type of compounds do intermolecular forces apply to?

Only covalently bonded compounds.

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?

8- represents a slightly negative charge, while 8+ 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 IN A CLOSE BY MOLECULE; electrons move to other side of 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

build up of electrons give this side a partial negative charge

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 and the strength of Van der Waals forces?

More electrons lead to larger instantaneous dipoles 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 is the significance of the term 'temporary dipole'?

It refers to the fleeting charge imbalance that occurs due to electron movement.

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

They contribute to the overall intermolecular interactions in covalent compounds.