Intermolecular forces Flashcards
What are London/van der Waals forces?
London forces act as an induced dipole between molecules
How strength varies:
* Higher number of electrons = stronger London forces, because dipoles can be larger
* Straighter chain molecules = stronger london forces, because there is more surface area contact, and can pack closer together
How do London forces form between molecules?
- There is an uneven distribution of electrons
- resulting in an instantaneous dipole being set up
- this induces a dipole on the other molecule
- the molecules are attracted to each other because of the difference in charge
What are permenant dipole forces?
between polar molecules, where different atoms of that molecule have different delta charges
the delta positive and delta negative regions of neighbouring polar molecules attract each other and hold the molecules together in a lattice-like structure
What are hydrogen bonds?
A hydrogen bond is a particularly strong permenant dipole-permenant dipole attraction between the lone pair of electrons on a very electronegative atom (N, O or F) and a hydrogen atom directly covalently bonded to another very electronegative N, O or F atom
Hydrogen bonding in H2O, NH3, and HF
Hydrogen bonds only act between hydrogen and the most electronegative atoms:
* Nitrogen
* Oxygen
* Fluorine
The lone pair of these atoms form a bond with a delta positive hydrogen from another molecule
Water’s anomalous properties: why high melting and boiling temperature?
Despite water having weakish London forces between molecules due to low electron number,
Water has hydrogen bonds between molecules, which are stronger than both london and permenant dipole forces, requiring more energy to overcome
Water’s anomalous properties: density of ice compared to water?
- Hydrogen bonds between water molecules in ice hold it in a lattice structure, with more space between molecules
- H-bonds are also longer than covalent bonds so ice is less dense than water
Predicting the presence of hydrogen bonding in molecules
Criteria for hydrogen bonding:
* H and either O, N or F
* O, N or F have lone pairs
* the molecules involved are polar
Trends in the boiling temperatures of alkanes with increasing chain length
- Alkanes with increased chain length have more electrons per molecule
- Meaning the London forces between these molecules are stronger
- Requiring more energy to overcome
What is the effect of branching in the carbon chain on the boiling points of alkanes?
- Branching of alkane chains weakens the London forces between molecules because they are less able to pack tightly together
- The distance over which the London forces act is increased, weakening them
Why do alcohols have higher boiling temperatures compared to alkanes with similar numbers of electrons?
Alcohols have the OH functional group, which introduces hydrogen bonding between molecules
Hydrogen bonds are much stronger than both London and permenant dipole forces
Trends in boiling temperatures of hydrogen halides (HF –> HI)
- HF has the highest b.pt because it has hydrogen bonds between its molecules
- HCl –> HI, b.pt increases because the number of electrons in each molecule increase and so do the London forces
What is hydration?
Hydration is the process of adding water molecules to an ion to form a hydrated ion
Why is water chosen to dissolve ionic compounds? (In terms of hydration)
- Because of its hydrogen bonding capabilities, they allow it to dissolve some ionic compounds
Why is water chosen to dissolve simple alcohols? (In terms of hydrogen bonding)
water can dissolve some alcohols by forming hydrogen bonds with their hydroxyl group
