2.3 intermolecular forces & hydrogen bonding Flashcards
where are intermolecular forces found?
intermolecular forces are forces found between molecules.
list the three main types of intermolecular forces.
- instantaneous dipole-induced dipole bonds (london forces)
- permanent dipole-permanent dipole bonds.
- hydrogen bonds.
london forces cause attraction between all atoms and molecules. explain how an instantaneous dipole may induce another instantaneous dipole.
- electrons in charge clouds are constantly moving, which generates an instantaneous dipole on an atom.
- this dipole can induce another instantaneous dipole in the opposite direction on a neighbouring atom.
- the two dipoles are then attracted to each other.
explain why larger molecules and molecules with greater surface areas have stronger london forces.
- larger molecules have larger electron clouds, resulting in stronger london forces.
- molecules with greater surface areas have a greater exposed electron cloud, resulting in stronger london forces.
why do molecules with stronger london forces have greater melting and boiling temperatures than molecules with weaker london forces?
- solids with stronger london forces have higher melting points as intermolecular forces must be overcome to melt the solid.
- liquids with stronger london forces will have higher boiling points, as more energy is required to overcome the intermolecular forces so that the particles can escape from the liquid surface.
describe the bonding and intermolecular forces found within alkanes.
- alkanes contain covalent bonds inside the molecules.
- between the molecules are london forces, which hold the molecules together.
as the carbon chain of an alkane becomes longer, the london forces become stronger. explain why this occurs.
- the longer the carbon chain, the stronger the london forces.
- this is because there is more molecular surface contact and more electrons to interact along a longer chain.
- as the chain grows longer, it becomes more difficult to separate the molecules because more energy is required to overcome the strong london forces.
explain why fewer london forces form in branched alkanes as opposed to straight chain alkanes.
- branched-chain alkanes are unable to pack closely together.
- additionally, their molecular surface contact is small compared to straight chain alkanes, resulting in the formation of fewer london forces.
what are permanent dipole-permanent dipole bonds?
permanent dipole-permanent dipole bonds are electrostatic forces of attraction between molecules, caused by δ+ and δ- charges on polar molecules.
why can hydrogen covalently bond to only fluorine, nitrogen or oxygen?
- fluorine, nitrogen and oxygen are highly electronegative so draw the bonding electrons away from the hydrogen atom.
- the bond is polarised.
- hydrogen’s high charge density causes hydrogen atoms to form weak bonds with lone pairs of electrons on the fluorine, nitrogen or oxygen atoms of other molecules.
organic molecules that form hydrogen bonds generally contain which functional groups?
-OH or -NH groups (alcohols and amines)
molecules of hydrogen fluoride are able to form hydrogen bonds with each other, resulting in a high boiling point. the other group 7 hydrides are unable to form hydrogen bonds. explain why the boiling points of group 7 hydrides increase from hydrogen chloride to hydrogen iodide.
- from HCl to HI, although the permanent dipole-permanent dipole interactions decrease in a molecule, the number of electrons in the molecule increases, so the strength of the london forces present in a molecule also increases.
- this effect overrides the decrease in strength of the permanent dipole-permanent dipole interactions, so the boiling points of the group 7 hydrides increase from HCl to HI.
with reference to hydrogen bonds, explain why ice is much less dense than water.
- ice is a simple molecular structure.
- in ice, the water molecules are arranged to form the maximum number of hydrogen bonds. in this way, the formation of the lattice structure leaves lots of free space.
- as the ice melts, some of the hydrogen bonds are broken and the lattice breaks down, allowing molecules to ‘fill’ this space.
hydrogen bonding give alcohols low volatilities (high boiling points) compared to non-polar compounds. explain how alcohols form hydrogen bonds, which results in a low volatility.
- all alcohols contain an -OH group with a δ- charge on the oxygen atom and a δ+ on the hydrogen atom.
- this polar group helps alcohols to form hydrogen bonds.
- a lot of energy is required to overcome these hydrogen bonds, which results in alcohols having low volatilities.
