Intermolecular Forces Flashcards
Describe structure and bonding of Simple Molecular compounds.
The molecules of these covalent compounds are held together by weak intermolecular force of attraction which are electrostatic in nature. They melt and boil if sufficient energy is provided to overcome the weak IMF forces. Covalent bonds between atoms in molecules remain intact.
Occurrence of Instantaneous dipole - induced dipole interactions in elements.
Instantaneous dipole - induced dipole (id - id) interactions exist for all covalent non-metal molecules with simple molecular structure and between atoms of noble gases.
How do id-id interactions arise?
As electrons are always in random motion, at any point in time, there is an uneven distribution of electrons in a molecule.
The separation of chargers creates id in the molecule and the id in one molecule can induce the formation of dipoles in the neighbouring unpolarised molecules.
As a result, forming id-id interactions which has weak electrostatic forces of attraction between 2 dipoles.
State the factors affecting the strength of id-id interactions.
Size of electron cloud:
The bigger the size of the electron cloud, the more polarisable the electron cloud, the stronger the id - id interactions between the molecules.
Surface area of contact of the molecule:
The greater the surface area of contact of the molecule, the more extensive and hence stronger id-id interactions between the molecules.
eg. straight-chain molecule, which is linear, has a greater surface area of contact than a branched molecule, which is spherical.
What are the physical properties of covalent substance with id-id interactions?
Low melting and boiling point.
Non-conductor of electricity.
Insoluble in water but soluble in non-polar solvents
eg. The energy evolved when iodine molecules form instantaneous dipole-induced dipole interactions with water molecules is insufficient to overcome the hydrogen bonds between water molecules and the instantaneous dipole-induced dipole interactions between the iodine molecules.
Occurrence of pd-pd interactions in covalent molecules
pd-pd interactions exist only for polar covalent molecules with simple molecular structure.
How do pd-pd interactions arise?
As there is uneven distribution of electrons in polar bonds, permanent separation of charges (dipole) is found within polar molecules.
pd in neighbouring polar molecules attract each other. As a result forming a weak electrostatic attraction between these molecules, also known as pd-pd interaction.
What are the factors that affect the strength of a pd-pd interaction?
Magnitude of molecule’s dipole moment:
The greater the magnitude of the dipole moment , the more polar the molecule and hence the stronger the pd-pd interaction.
What are the physical properties of covalent molecules with pd-pd interactions?
Low melting and boiling point but molecules with pd-pd interactions tend to have a greater boiling and melting point than non-polar molecules with only id-id interactions as more energy is required to overcome the pd-pd interactions as compared to the id-id interactions between polar molecules.
Occurrence of hydrogen bonding in covalent substances.
Hydrogen bonding is a specific type of pd-pd that is applicable to certain polar molecules with simple molecular structure only.
Criteria that must be satisfied for hydrogen bonds to be formed in covalent substances.
One molecule must have a H atom directly bonded to a small and highly electronegative atom such as F, O or N.
Other molecules must have at least one lone pair of electrons present on a small and highly electronegative atom such as N, O or F.
How do hydrogen bonds arises? (are formed)
F, O or N atoms being more electronegative than H atom attracts the bonding electrons in H-Y bond to itself. The Y atom gets a partial negative charge while the H atom gets a partial positive charge.
The strong partial positive charge on H atom is strongly attracted to the partial negative charge of lone pair of electrons on F,O or N of another molecule forming hydrogen bonds.
State the factors affecting the strength of Hydrogen bonds.
Dipole moment of H-Y bond:
greater difference in electronegativity -> greater
magnitude of dipole moment -> stronger the hydrogen bond.
Extensiveness of hydrogen bonds formed:
The more intermolecular hydrogen bonds a molecule is able to form, the stronger the hydrogen bonding.
What are the physical properties of Hydrogen bonds?
Unusually high melting and boiling points:
Due to presence of intermolecular hydrogen bonding.
Relative densities of ice and water:
Hydrogen bonds between water molecules are positioned roughly tetrahedrally around each O atom resulting in very open and regular tetrahedral arrangement.
