7B

Question 1

Two reasons for the increase in boiling temperature with increasing molecular mass

Answer 1

1. 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.
2. 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

Question 2

how was a lower boiling point branched or unbranched hydrocarbons

Answer 2

Branched alkanes have lower boiling points than their unbranched isomers

Question 3

why do Branched alkanes have lower boiling points than their unbranched isomers

Answer 3

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.

Question 4

Describe the trend in boiling points of alcohols

Answer 4

As the relative molecular mass of the alcohols increases the boiling point also increases.

Question 5

Two reasons for the increase in boiling temperature with increasing molecular mass:

Answer 5

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.

Question 6

Explain with examples why alcohols boiling points are higher when compared with the equivalent alkane.

Answer 6

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.

Question 7

is alcohol boiling points higher when compared with the equivalent alkane

Answer 7

yes they are

Question 8

when doe the London forces become predominant in the alcohol

Answer 8

as the length of the chain increase

Question 9

what is the Enthalpy of vaporization

Answer 9

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.

Question 10

Greater enthalpy change of vaporization…..

Answer 10

greater the forces of attraction between the molecules.

Question 11

how do you find whether the bonding is London forces predominant or hydrogen bond predominant

Answer 11

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

Question 12

what is the trend of hydrates as you go down groups 4, 5, 6, 7

Answer 12

as you go down the group the boiling point increases

Question 13

what is the anomaly in group 5

Answer 13

NH3

Question 14

what is the anomaly in group 6

Answer 14

H2O

Question 15

what is the anomaly in group 7

Answer 15

HF

Question 16

why does the boiling point of dipole-dipole increase as the number of electrons increase

Answer 16

because a greater dipole would form

Question 17

what is the reason for the anomalies of HF, water, and ammonia

Answer 17

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.

Question 18

what are the two reasons for H2O having a higher boiling point than HF even though the hydrogen bond strength in HF is greater

Answer 18

1. 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
2. not all of the hydrogen bonds in HF are broken on vaporization, that is because HF is substantially polarized even in the gas phase

Question 19

why does ammonia form the lowset boiling temperature out of H2O and HF

Answer 19

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

Question 20

another than water high boiling point what another anomloy does it have

Answer 20

Ice at 0 c is less dense than liquid water at 0 c

Question 21

why is ice less dense than liquid water

Answer 21

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

Question 22

There are two main types of solvent:

Answer 22

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.

Question 23

For a substance to dissolve, the following two conditions must be met:

Answer 23

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.

Question 24

why are polar molecules soluble in polar solvents

Answer 24

This is because polar molecules are able to interact with polar solvents and form intermolecular forces with them

Question 25

The energy required to separate the ions in the solid comes form

Answer 25

either completely or partially supplied by hydration of the ions.

Question 26

what is the process of hydration in NaCl

Answer 26

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.

Question 27

the energy released from hydration is known as

Answer 27

hydration energy

Question 28

what is the interaction between sodium and water molecules in hydration called

Answer 28

ion-dipole interactions

Question 29

do alcohol dissolve in water

Answer 29

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

Question 30

as the hydrocarbon chain increases an alcohol solubility…

Answer 30

Solubility of alcohols in water decreases with increasing hydrocarbon chain length as London forces predominate between the alcohol molecules.

Question 31

why cant non-polar molecules like alkane dissolve in water

Answer 31

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

Question 32

why cant some polar molecules like halogenoalkanes and ethoxyethane dissolve in water

Answer 32

1. 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

Question 33

halogenoalkanes are more soluble in

Answer 33

ethanol thats whys one of their reactions are carried out in aqueous ethanol

Question 34

Examples of non-polar solvents

Answer 34

alkanes

Question 35

Examples of simple non-polar molecules

Answer 35

H2, O2,N2,Cl2,F2,I2,Br2 and ethene

Question 36

Simple non-polar molecules are soluble in

Answer 36

non-polor solvents

Question 37

why is it more convenient that Br water is used for testing saturations

Answer 37

because alkanes are not soluble in water but soluble in Br water