Intermolecular forces Flashcards
What are intermolecular forces responsible for?
Physical properties like melting and boiling points, solubility as well as density of many covalent compounds with simple molecular structures.
Strength of intermolecular forces
Similar to the intramolecular covalent bond (i.e. attraction between atoms), these intermolecular forces
of attraction are also electrostatic in nature.
However, they are generally weaker than the conventional bonds such as ionic, covalent and
metallic bonds because the attraction is between particles having partial charges.
indication
of the strength of the intermolecular forces
This is because during the melting and boiling of a simple molecular substance, the bonds that are
broken are the intermolecular forces between molecules and NOT the intramolecular covalent
bonds within molecules.
instantaneous dipole - induced dipole interactions (id-id)
- Exist for all covalent molecules (polar and non-polar) with simple molecular structure between atoms of noble gases
Only form of intermolecular forces that is present in non-polar molecules
How id-id arises?
As electrons are always in random motion, at any point in time, there is an uneven distribution of
electrons in a molecule.
This separation of charges creates an instantaneous dipole in the CO2 molecule.
This instantaneous dipole can induce the formation of dipoles in the neighbouring unpolarised CO2 molecules.
As a result, a weak electrostatic forces of attraction, known as instantaneous dipole – induced dipole interactions, forms between these two dipoles.
id-id interactions are ..
○ Short-lived because the electrons keep moving and the dipoles vanish and reform
○ Usually very weak as the separation of charge is temporary and the magnitude of the partial charge formed is usually quite small
Factors affecting strength of id-id interactions
○ Size of electron cloud of the molecule (or number of electrons the molecule has)
§ The bigger the size of the electron cloud (or greater the number of electrons), the more polarisable is the electron cloud, the stronger is the id-id interactions between the molecules
§ This is because molecules with a larger electron cloud have a higher chance of an instantaneous dipole (more polarisable)
○ Surface area for contact of the molecule (only applicable for isomers)
§ The greater the surface area for contact of the molecules, the more extensive and hence stronger is the id-id interactions between the molecules
Physical properties of covalent substances with id-id interactions between molecules
○ Low melting and boiling points
§ Small amount of energy is required to overcome the weak id-id between the molecules
○ Does not conduct electricity
§ Valence electrons are used in the formation of covalent bonds between the atoms
○ Insoluble in water but soluble in non-polar solvents
Permanent dipole - permanent dipole interactions (pd-pd)
- Exist for only polar covalent molecules with simple molecular structure
- A molecule is considered polar only if,
○ There are polar covalent bonds within the molecule, and
○ The molecule has a net dipole moment
i.e. The dipole moments of all the polar bonds inside the molecule do not cancel one another vectorially
- A molecule is considered polar only if,
How pd-pd arises?
As there is an uneven distribution of electrons in polar bonds, permanent separation of charges
(dipole) is found within polar molecules.
The permanent dipoles in neighbouring polar molecules attract each other.
The weak electrostatic attraction forms between these dipoles, is known as permanent dipole – permanent interactions
Factors affecting strength of pd-pd
○ The magnitude of the molecule’s net dipole moment
§ The greater the magnitude of the dipole moment, the more polar is the molecule, the stronger is the permanent dipole - permanent dipole interactions between the molecules
§ The magnitude of the dipole, in turn, depends on the magnitude of the electronegativity difference between the bonding atoms. The greater the difference in electronegativity, the greater is the dipole moment, and the more polar is the covalent bond
Physical properties of covalent molecules with pd-pd between the molecules
○ Low melting and boiling points
§ Small amount of energy is required to overcome the weak permanent dipole - permanent dipole between the molecules
§ Polar molecules, with permanent dipole - permanent dipole interactions tend to have higher boiling and melting points than the non-polar molecules with only instantaneous dipole - instantaneous dipole interactions between the polar molecules
§ This is because, more energy is required to overcome the pd-pd interactions between the polar molecules.
○ Does not conduct electricity (except for acids in aqueous solution as H2SO4, HCl, HNO3, as they dissociate in aqueous solution to H+ and the corresponding anions which can act as charge carriers) as the valence electrons are used in the formation of covalent bonds between the atoms.
Hydrogen bonding
- Specific type of pd-pd interaction that is applicable to certain polar molecules with simple molecular structure only
- For 2 molecules to exhibit intermolecular hydrogen bonding, it must satisfy the following:
○ One molecule must have a H atom bonded directly to a small and highly electronegative atom such as F, O or N (protonic hydrogen); and
○ The other molecule must have at least one lone pair of electrons present on a small and highly electronegative atom such as F, O or N - [Note:] the terms “hydrogen bonding” and “pd-pd interactions” are non-interchangeable
- For 2 molecules to exhibit intermolecular hydrogen bonding, it must satisfy the following:
How hydrogen bonding arises
N atom, being more electronegative than H atom, attracts the bonding electrons in the H−N bond
closer to itself.
As a result, it is more “electron rich” and gains a partial negative charge (−).
The H atom, on the other hand, acquires a partial positive charge (+) and is almost a bare proton of the
hydrogen nucleus.
This electrostatic attraction between the H atom covalently bonded to N atom of one NH3 molecule
and the lone pair of electrons on N atom on the other NH3 molecule, is known as hydrogen bond.
Factors affecting strength of hydrogen bond?
○ Dipole moment of the H-Y bond
§ The greater the magnitude of the dipole moment, the stronger is the hydrogen bond between the molecules
§ Dipole moment of the H-Y bond: H-F > H-O > H-N
Since the electronegativity of F is greater than O which is in turn greater than N thus the electronegativity difference between the H atom and the Y atom decreases from H-F to H-O to H-N
○ The extensiveness of hydrogen bonds formed (or average number of hydrogen bonds formed per molecule)
§ The more intermolecular hydrogen bonds each molecule is able to form, the stronger is the hydrogen bonding
