7B Flashcards
Two reasons for the increase in boiling temperature with increasing molecular mass
- As relative molecular mass increases, the number of electrons per molecule and size of molecule increases, larger electrons cloud, greater fluctuation in electron density and therefore larger instantaneous and induced dipoles created, stronger London forces.
- As the length of the carbon chain increases, the number of contact points between adjacent molecules increases. Instantaneous dipole-induced dipole forces exist at each point of contact between the molecules, so the more points of contact there are, the greater the overall intermolecular (London) forces of attraction
how was a lower boiling point branched or unbranched hydrocarbons
Branched alkanes have lower boiling points than their unbranched isomers
why do Branched alkanes have lower boiling points than their unbranched isomers
Branched alkanes cannot pack closely together and their molecular surface contact is small compared to straight-chain alkanes isomers.
Fewer London forces can form.
Weaker London forces, less heat energy required to overcome them, lower boiling point.
Describe the trend in boiling points of alcohols
As the relative molecular mass of the alcohols increases the boiling point also increases.
Two reasons for the increase in boiling temperature with increasing molecular mass:
As relative molecular mass increases, the number of electrons per molecule and size of molecule increases, larger electrons cloud, greater fluctuation in electron density and therefore larger instantaneous and induced dipoles created, stronger London forces.
As the length of the carbon chain increases, the number of contact points between adjacent molecules increases. Instantaneous dipole-induced dipole forces exist at each point of contact between the molecules, so the more points of contact there are, the greater the overall intermolecular (London) forces of attraction.
Explain with examples why alcohols boiling points are higher when compared with the equivalent alkane.
Alcohols have a higher boiling point than equivalent alkanes because they have London forces and hydrogen bonding.
Methanol and ethane have a similar chain length and the same number of electrons.
If the intermolecular interactions in each were only London forces, then their boiling points would be almost identical.
However, the boiling point of methanol is higher than ethane because it also has hydrogen bonding between molecules whereas ethane does not.
More energy is needed to overcome the additional force of attraction.
Therefore methanol has a higher boiling point.
is alcohol boiling points higher when compared with the equivalent alkane
yes they are
when doe the London forces become predominant in the alcohol
as the length of the chain increase
what is the Enthalpy of vaporization
is a measure of the amount of energy that is required to completely separate the molecules of a liquid and convert it into a gas at the same temperature.
It is therefore a direct measure of the strength of the intermolecular interactions.
Greater enthalpy change of vaporization…..
greater the forces of attraction between the molecules.
how do you find whether the bonding is London forces predominant or hydrogen bond predominant
by subtracting the enthalpy change of the alcohol and the enthalpy change of alkane then dividing the product by the enthalpy change of the alcohol
what is the trend of hydrates as you go down groups 4, 5, 6, 7
as you go down the group the boiling point increases
what is the anomaly in group 5
NH3
what is the anomaly in group 6
H2O
what is the anomaly in group 7
HF
why does the boiling point of dipole-dipole increase as the number of electrons increase
because a greater dipole would form
what is the reason for the anomalies of HF, water, and ammonia
The boiling points of water, Hf, and ammonia are all higher than those of the hydrides of the other elements in their group, whereas you would expect them to be lower if only London forces were operating.
This is because H-bonding is present between the molecules in each of these compounds and these stronger intermolecular forces of attraction make the molecules more difficult to separate.
what are the two reasons for H2O having a higher boiling point than HF even though the hydrogen bond strength in HF is greater
- HF only forms two hydrogen bonds per molecule, whereas water can up to 4 hydrogen bonds per molecule. This means that the hydrogen bonding is more extensive in water
- not all of the hydrogen bonds in HF are broken on vaporization, that is because HF is substantially polarized even in the gas phase
why does ammonia form the lowset boiling temperature out of H2O and HF
that is because Nitrogen can only form one hydrogen bond with its lone pair and one with the positively charged hydrogen ions as well nitrogen has the weakest hydrogen bonds present as it is less electronegative than F
another than water high boiling point what another anomloy does it have
Ice at 0 c is less dense than liquid water at 0 c
why is ice less dense than liquid water
In ice, the water molecules are arranged so that there is the maximum number of hydrogen bonds. The lattice structure formed in this way wastes a lot of space.
In ice water molecules are held apart by hydrogen bonds. This causes ice to have an open lattice structure
When ice is heated, the hydrogen bonds break and the lattice breaks down.
The water molecules can move closer to each other to form liquid water which is denser
There are two main types of solvent:
Polar solvents – made of polar molecules, such as water. Water molecules bond to each other with hydrogen bonds. But not all polar solvents can form hydrogen bonds. For example, propanone is a polar solvent but can only form London forces and permanent dipole-dipole bonds,
Non-polar solvents – such as hexane. Hexane molecules bond to each other by London forces.
For a substance to dissolve, the following two conditions must be met:
The solute particles must be separated from each other and then become surrounded by solvent particles.
The forces of attraction between solute and solvent particles must be strong enough to overcome the solvent-solvent and solute-solute forces.
why are polar molecules soluble in polar solvents
This is because polar molecules are able to interact with polar solvents and form intermolecular forces with them
The energy required to separate the ions in the solid comes form
either completely or partially supplied by hydration of the ions.
what is the process of hydration in NaCl
The slightly negative end of water molecules attracts the sodium ions sufficiently to remove them from the lattice.
The sodium ions then become surrounded by water molecules
The slightly positive end of the water molecules attracts the chloride ions. Once they are in solution, the chloride ions become surrounded by water molecules, as shown, The chloride ions are hydrogen-bonded to the water molecules.
the energy released from hydration is known as
hydration energy
what is the interaction between sodium and water molecules in hydration called
ion-dipole interactions
do alcohol dissolve in water
Alcohols are covalent but they dissolve in water.
Alcohols contain polar –OH groups and can therefore form hydrogen bonds with water.
Polar –OH bond in alcohol is attracted to the polar –OH bond in water.
Hydrogen bonds form between the lone pairs on the slightly negative oxygen atoms and the slightly positive hydrogen atoms
as the hydrocarbon chain increases an alcohol solubility…
Solubility of alcohols in water decreases with increasing hydrocarbon chain length as London forces predominate between the alcohol molecules.
why cant non-polar molecules like alkane dissolve in water
the attraction between the alkane and the molecules of water formed is not strong enough to disrupt the hydrogen bond system in between the water molecules
why cant some polar molecules like halogenoalkanes and ethoxyethane dissolve in water
- because they do not form hydrogen bonds with water
2. the hydrogen bonds formed are weaker than the hydrogen bonds in between the water molecules
halogenoalkanes are more soluble in
ethanol thats whys one of their reactions are carried out in aqueous ethanol
Examples of non-polar solvents
alkanes
Examples of simple non-polar molecules
H2, O2,N2,Cl2,F2,I2,Br2 and ethene
Simple non-polar molecules are soluble in
non-polor solvents
why is it more convenient that Br water is used for testing saturations
because alkanes are not soluble in water but soluble in Br water
