BC.09 Weak bonds and Van der Waals forces

Describe at which geometry is a hydrogen bond at its strongest.

When the participating three nuclei (hydrogen-acceptor, hydrogen and hydrogen-donor) of a hydrogen bond lie in a straight line.

What is the other (weak) rule that determines the stability and strength of a hydrogen bond?

Dipoles of the hydrogen bond align which provides mor stability.

What type of dipoles do Van der Waals interactions involve?

Transient (induced) dipoles

Explain the Lennard-Jones potential graph.

* As you bring two atoms closer together, they begin to induce dipoles in each other resulting in attraction (Van der Waals interactions)
* However, the nuclei and electron clouds are also being brought closer, resulting in repulsion (electrostatic Coulomb repulsion)
* Initially, the interaction gets better, but then quickly becomes unfavourable

Draw a Lennard-Jones potential graph and note the **net favourable Van der Waals interactions**, the **optimal Van der Waals interactions** and the **sum of the Van der Waals radii.**

Precisely, when does steric clash occur?

When atoms are brought closer than the sum of Van der Waals radii.

Water forms a clathrate around hydrophobic substances, why is this unfavourable?

Disorganised water molecules are more favourable than organised water molecules due to entropy.

Compare the energies (KJ/mole) of covalent and hydrogen bonds, Van der Waals interactions and rotational conformations.

Covalent bonds = 350

Hydrogen bonds = 5-20

Van der Waals = 0.2-2

Rotational conformations = 10