Topic 1 - inter molecular forces Flashcards
What are intermolecular forces?
Intermolecular forces are forces that exist between molecules.
There are 3 types:
van der Waals forces - acts between ALL molecules
Dipole-dipole forces - acts only between certain types of molecules
Hydrogen bonding - acts only between certain types of molecules
All are much weaker than a covalent bond
What are van der Waals forces?
Imagine an argon atom:
There are 18 electrons around the nucleus.
These are constantly moving around.
At anyone moment there could be more electrons at one end of the atom just through chance:
This results in the formation of a very slight negative charge at one end of the atom, δ- ..
The other side of the atom has a very slight positive charge where there are less electrons, δ+
We call this an instantaneous dipole.
What will happen if…
…along comes another atom of argon?
The instantaneous dipole can cause the formation of an instantaneous dipole in other atoms.
This can cause a weak attractive force between the δ- and the δ+ of different atoms.
The weak force of attraction between the neighbouring oppositely charged ends of two instantaneous dipoles is called a van der Waal’s force.
These forces exist all the time but the dipoles are constantly moving around with the electrons..
All atoms and molecules possess these forces.
Noble gas boiling points
As boiling point increases:
atomic number increases
electrons increase
size of van der wall force increases
number of electrons increase: van der walls force increases
Dipole moments
Some covalent bonds are polar.
The arrangement of polar
bonds can result in a molecule becoming polar overall.
We say the molecule has a dipole moment.
Dipole – dipole forces
Dipoles on different molecules can interact.
This causes an attractive force between different molecules.
Dipole forces are permanent in a molecule.
This makes dipole-dipole forces stronger than van der Waals forces.
Polar bonds definition
formed between 2 atoms when one of them has a higher electronegativity value
Electronegativity definition
power of an atom to attract the bonding electrons towards itself in a covalent bond
Dipole moment definition
overall charge separation of a molecule
Hydride boiling points
A substances boiling point is a measure of the intermolecular forces present between particles.
Noble gases show a gradual increase in boiling points as the atom is larger, has more electrons hence van der Waals increases.
Hydrides (element joined to H) should also show this trend.
Structure of water
Nitrogen, oxygen and fluorine are all very electronegative.
When a hydrogen atom covalently bonds to them they form a dipole.
As hydrogen is so small, the slight charge on it has a very high charge density.
The oxygen atom also has two lone pairs of electrons.
Hydrogen bonding
The lone pair of electrons on oxygen atoms from other water molecules can interact with this hydrogen.
This forms a very strong intermolecular attraction called a hydrogen bond.
Hydrogen bonds will form whenever hydrogen is joined to nitrogen, oxygen or fluorine.
The bond is always between the lone pairs of N, F or O and a Hδ+.
Importance of hydrogen bonding
Ice is less dense than water.
Water has a much high melting and boiling point than would be expected.
Protein folding.
DNA base pairing.
Enzyme reactions.
what effects electronegativity
atomic radius
nuclear charge
screening/sheilding
