3.7 forces between molecules Flashcards
What are the three intermolecular forces?
- Van Der Waals (induced dipole-dipole)
- Permanent dipole-dipole
- Hydrogen bonding
What is the strongest intermolecular force?
Hydrogen bonds
What is the weakest intermolecular force?
Van Der Waals
What are Van Der Waals forces?
Van der Waals forces are weak electrostatic forces that attract neutral molecules to one another. Also known as induced dipole-dipole interactions.
Where do VdW forces occur?
They occur between all simple covalent molecules and the separate atoms in noble gases.
How do they form?
In atoms electrons are moving constantly and randomly. As this happens electron density can fluctuate and parts of the atom become more or less negative which gives the atom a temporary dipole. This dipole will induce a temporary dipole in a neighbouring atom by attracting or repelling its electron charge cloud.
What factors affect Van Der Waals’ forces?
- The size (Mr) of the molecule
- The surface area of a molecule
How does the size of a molecule affect VdW forces?
Larger atoms/molecules have more electrons and therefore have stronger Van Der Waals’ forces between them than smaller molecules. Melting and boiling points for larger molecules will be greater.
How does the surface area of contact of a molecule affect VdW forces?
Molecules with a larger surface area of contact have stronger VdW forces between them as more of the electron cloud is exposed for interactions.
What are permanent dipole-dipole forces?
Permanent dipole-dipole forces are intermolecular forces of attraction between polar molecules.
Where do permanent dipole-dipole forces occur?
Permanent dipole-dipole forces occur when the partially negatively charged end of one molecule is attracted to the partially positively charged end of another molecule.
Permanent dipole-dipole forces occurs in addition to…
Van Der Waals’ forces.
Example of a permanent dipole-dipole force:
Gaseous hydrogen chloride (HCl)
- H-Cl bond is polar due to greater electronegativity of chlorine compared to hydrogen
- Hydrogen atoms have a partial positive charge (δ+) and the chlorine has a partial negative charge (δ-)
- HCl molecules align so the δ+ hydrogen of one molecule is attracted to the δ- chlorine of a neighbouring molecule.
What is hydrogen bonding?
Hydrogen bonding is a special type of permanent dipole-dipole force.
Where does hydrogen bonding occur?
Hydrogen bonding occurs in compounds that have a hydrogen atom attached to one of the three most electronegative atoms which are nitrogen, oxygen and fluorine which must have an available lone pair of electrons.
Hydrogen bonding occurs in addition to…
Van Der Waals’ forces.
What effect does hydrogen bonding have on properties of substances?
Greater solubility
- Substances that can form hydrogen bonds with water tend to be soluble while those that cannot are typically insoluble.
Higher melting + boiling points than those with no H bonds
- Extra energy is needed to overcome the strong hydrogen bonds.
Why do some molecules with polar bonds do not have a permanent dipole?
Some molecules with polar bonds do not have a permanent dipole because the molecule is symmetrical, causing the bond dipoles to cancel each other out. This makes the molecule nonpolar.
