Intermolecular Forces Flashcards
What are intermolecular forces?
Attractions between neighbouring molecules
What are the 3 types of intermolecular forces?
Hydrogen bonding, London forces and permanent dipole-dipole forces
What are dipole-dipole attractions?
Dipole-dipole attractions only occur between polar molecules which have a permanent dipole. The positive pole of one molecule attracts the negative pole of another molecule and visa versa.
Are dipole-dipole attractions strong?
No, they are weak
What is hydrogen bonding?
A special case of dipole-dipole attractions.
What 2 things are needed for hydrogen bonding to occur?
Hydrogen must be attached to an atom with high electronegativity (O, N, F).
There must be an unshared pair of electrons on the electronegative atom.
In water what atoms does a hydrogen bond form between and why?
The oxygen from one water molecule and the hydrogen from another water molecule.
The oxygen in a water molecule is slightly more negative due to its electronegative nature (attracts the electrons more than hydrogen does) and so attracts the slightly positive hydrogen.
How many hydrogen bonds can form on an oxygen atom in a water molecule and why?
2 due to its 2 lone electrons.
How strong are hydrogen bonds?
About 1/10th the strength of a covalent bond.
What happens as a result of the weak hydrogen bonds in water?
The hydrogen bonds are constantly broken and reformed in liquid water.
What is the double helix in DNA held together by?
Hydrogen bonds between the 4 different bases
What are London Forces?
The attractive forces between non-polar substances due to temporary dipoles.
How strong are London forces?
1/10th - 1/100th the strength of covalent bonds, weakest of the intermolecular forces
Where are London forces found?
Non-polar molecules
What produces temporary dipoles?
The uneven spread of electrons - at one point in time, one part of a molecule may have a higher density of electrons than the other side. This side becomes more negative.
What are the spread of electrons like in temporary dipoles?
One side is electron rich, the other side is electron deficient
What can this instantaneous dipole cause?
It can induce a dipole in a nearby particle
What allows the creation of a temporary dipole?
Electrons are mobile
What happens to the strength of a London force when the size of a molecule increases and why?
A greater induced dipole attraction will occur as the number of electrons increases making the molecule more polarisable
What is the order of intermolecular bond strength? (1 being the highest and 3 being the lowest)
- Hydrogen bond
- Dipole to dipole
- London forces
