Intermolecular Interactions and Physical Properties Flashcards
what is the relationship between the molecular masses of alkanes and their boiling points
an increase in their molecular forces results in an increase in heir boiling points
is the relationship proportional
- no
- the graph for it curves until its nearly horizontal
what is the most significant intermolecular interaction between the alkane molecules
london forces
what are the two main factors that result in this increased boiling temperature as their chain length increases
- the increase in the electrons per molecule
- the increase in the number of points of contact between adjacent molecules
why does an increase to the electrons per molecule lead to an increased boiling temp
- the number of instantaneous and therefore induced dipoles would increase
- as there are more carbon atoms and therefore more bonds
- leading to more london forces of attraction between the molecules
why does an increase in the number of points of contact between adjacent molecules increase the boiling temp
- instantaneous dipole-induced dipole forces would exist at each point of contact between the molecules
- more point of contact means more intermolecular (london) forces of attraction
why do branch chain alkanes have lower boiling temps than their unbranched isomers
- because branching means fewer points of contact between adjacent molecules
- this leads to a decrease in the number of london forces they can produce between them
- leading to an overall decrease in the intermolecular force of attraction between them
what two main intermolecular forces do alcohols form and why
- london forces for the same reason anyone can
- hydrogen bonds because they have the OH group
between CH3OH and CH3CH3 which both have 18 electrons, which one has the higher boiling temp and why
- the CH3OH (methanol)
- methanol can form hydrogen bonds between adjacent alcohol molecules which ethane cant
- this additional force of attraction needs more energy to break and therefore has a higher boiling temp
what intermolecular force’;s strength should remain similar between them anyway
- the london forces strength
- because they have the same number of electrons
- therefore they can create similar numbers of instantaneous-induced dipole forces
what is the enthalpy change of vaporisation a measure of
- the amount of energy required to completely separate the molecules of a liquid
- converting it into a gas at the same temperature
what can the enthalpy change of vaporisation therefore dictate
the strength of intermolecular interactions between molecules
what are two if waters most important anomalies
- it has a relatively high melting and boiling temp for a molecule with so few electrons
- the density of ice at 0C is less than that of water at 0C
why does water have a high boiling temp compared to other molecules with similar electron counts
- a lot of waters intermolecular bonds are hydrogen bonds which are strong
- due to it just being hydrogen and oxygen
- this unusually high number of strong forces of attraction are what give it its high boiling temp
despite HF and H2O having 10 electrons (so london forces should be similar) and HF having slightly stronger hydrogen bonds, what is the main reason why H2O’s boiling temp still higher
- HF can only form an average of one h bond per molecule
- whereas H2O can from an average of 2
- so hydrogen bonds are much more extensive in water
- leading to higher boiling temps than it despite each h bond of HF being slightly stronger
what is a characteristic of HF itself that also prevents its boiling temp from being higher than waters
- not all of the h bonds in HF are broken on vaporisation
- because HF is substantially polymerised even in the gas phase
what is the structure of the water molecules in ice
- they are arranged in rings of 6
- held together by hydrogen bonds
why is the density of ice therefore less than waters
- the structure creates large areas of open space inside the rings
- when it melts the ring structure is destroyed
- causing the average distance between the molecules to decrease
- therefore leading to an increase in density
what two conditions need to be met in order for a substance to dissolve
- the solute particles must be separated from each other
- then become surrounded by solvent particles
- secondly the forces of attraction between the solute and solvent particles must be strong enough to overcome the solvent-solvent and solute-solute forces
where does (some or all of) the energy to dissolve an ionic solid by separating the ions come from
the hydration of the ions by water molecules
how are cations separated from the crystal lattice of an ionic solid
- the partially negatively charged end of the water molecules (the oxygen atom) attract the cations strongly enough to remove them from the lattice
- the cations become surrounded by water molecules
- with the oxygen end of the H2O molecules facing it
what is this interaction between cations and water molecules called
ion-dipole interaction
how are anions separated from the crystal lattice of an ionic solid
- the o+ end of the water molecules (hydrogen atoms) attract the anions strongly enough to remove them
- the anions become surrounded bu water molecules
- with the hydrogen end of the H2O molecules facing it
what is the energy released from this manner of hydration called
hydration energy
what new bonds can alcohols make in water and why
- hydrogen bonds between them and water molecules
- because they both have oxygen atoms and the H2O has hydrogen atoms
- allowing a bond between the O of the alcohol and the H of the water
why does the solubility of alcohols in water decrease with increasing hydrocarbon length
- because london forces predominate between the alcohol molecules
- making it harder for them to be separated by water molecules
are alkanes soluble in water
no
why
- because theyre non polar
- the attraction between them and H2O molecules isnt strong enough to overcome the hydrogen bonding between H2O molecules
- therefore the bonds between water cant be broken to make new ones with alkanes
what are the two reasons polar molecules would have a limited solubility in water
- they dont form hydrogen bonds with water
- the hydrogen bonds are weak compared to H2Os original h bonds
what is the general rule of thumb with non-aqueous solvents
- like dissolves like
- molecules in the same homologous series usually dissolve within each other
