Aromic Structure

Question 1

Why do isotopes have the same chemical properties?

Answer 1

Why do isotopes have the same chemical properties?

The chemical properties of an element are decided by its electron configuration
Isotopes have the same electron configuration, so the same chemical properties

Question 2

Explain why isotopes can have different physical properties?

Answer 2

Explain why isotopes can have different physical properties?

Physical properties depend on the mass of the atom
Isotopes have different masses so can have different physical properties

Question 3

One of the methods of ionising samples in time of flight mass spectrometry is by electron impact. How is this ionisation done?
[4 marks]

Answer 3

One of the methods of ionising samples in time of flight mass spectrometry is by electron impact. How is this ionisation done?
[4 marks]

high energy electrons
from electron gun
fired at sample
knocks off one electron

Question 4

A second method of ionising samples in time of flight mass spectrometry is by electrospray ionisation. How is this ionisation done?
[4 marks]

Answer 4

A second method of ionising samples in time of flight mass spectrometry is by electrospray ionisation. How is this ionisation done?
[4 marks]

sample dissolved in volatile solvent
injected through a fine hypodermic needle giving a fine mist
tip of needle has high voltage
each gains a proton as it leaves the needle

Question 5

Which method is most likely to lead to the break up of the ions into fragments?
[1 mark]

Answer 5

Which method is most likely to lead to the break up of the ions into fragments?
[1 mark]

Electron impact

Question 6

Ionisation by electron impact causes removal of one electron from each particle. Write an equation for the ionisation of each of the following species by loss of one electron.

Ca
C2H6

[2 marks]

Answer 6

 Ca(g) —> Ca+(g) + e–

 C2H2(g) —> C2H2+(g) + e–

Question 7

Electrospray ionisation creates an ion by protonation of a molecule.

Write an equation for the formation of a positive ion by electrospray ionisation of lactic acid (C3H6O2).
[1 mark]

Answer 7

Electrospray ionisation creates an ion by protonation of a molecule.

Write an equation for the formation of a positive ion by electrospray ionisation of lactic acid (C3H6O2).
[1 mark]

C3H6O2 + H+ —> C3H7O2+

Question 8

A molecule of NHF2 reacts with a molecule of the BF3

NHF2 + BF3 —> F2HNBF3

State the type of bond formed by the N atom and the B atom in F2HNBF3

Answer 8

R

Question 9

Why are there peaks with much lower m/z ratios?
[1 mark]

Why is there a peak / peaks with higher m/z ratios?
[1 mark]

Answer 9

Why are there peaks with much lower m/z ratios?
[1 mark]

Fragmentation

Why is there a peak / peaks with higher m/z ratios?
[1 mark]

due to some molecules containing 2H or 13C

Question 10

Calculate the mass, in kg, of a single 52Cr+ ion

Answer 10

Calculate the mass, in kg, of a single 52Cr+ ion

52/6.022x10^23
=8.635x10^-25

8.635x10^-25/1000
=8.635x10^-26kg

Question 11

Carbonate ions: 
Nitrate ions: 
Hydroxide ions: 
Sulfate ions: 
Ammonium ions:

Answer 11

Carbonate ions: CO32-
Nitrate ions: NO3-
Hydroxide ions: OH-
Sulfate ions: SO42-
Ammonium ions: NH4+

Question 12

Ionic compounds have certain specific properties:

Answer 12

Ionic compounds have certain specific properties:

They dissolve in water - this is because water is polar.
They conduct electricity when molten or dissolved, this is because the ions can move and carry charge when molten
They have high melting points, this is because the ionic bonds are very strong.

Question 13

Ionic compounds have what is called a giant ionic lattice structure. This means that the structure is made of …

Answer 13

Ionic compounds have what is called a giant ionic lattice structure. This means that the structure is made of repeating units of identical structure.

Question 14

Covalent bonding happens between ___

Answer 14

Covalent bonding happens between non-metals

Question 15

Molecules have covalent bonds holding the atoms together.

Covalent bonds are strong because …

Answer 15

Molecules have covalent bonds holding the atoms together.

Covalent bonds are strong because the shared electrons are attracted to the nucleus of both atoms.

Question 16

Which is the best description of a covalent bond?

Answer 16

Which is the best description of a covalent bond?

Shared pair of electrons between two atoms

Question 17

What best describes diamonds?

Answer 17

What best describes diamonds?

Hard but doesn’t conduct electricity

Contains covalent bonds and high mpt

Question 18

properties of graphite?

Answer 18

properties of graphite?

Conducts electricity

Contains covalent bonds

Soft

Question 19

In graphite, the carbon atoms form layers of ____ rings, with each layer held together by …

Answer 19

In graphite, the carbon atoms form layers of hexagonal rings, with each layer held together by weak van der waals

Question 20

Metals are good electrical conductors because …

Answer 20

Metals are good electrical conductors because the free electrons can move and carry current.

Question 21

The melting point of a metal is related to its group - a group 2 metal will donate two electrons to the sea of free electrons.

This means that the positive ion cores carry a …

The 2+ ion has a stronger attraction to …

This means the melting point is higher.

For example, Na melts at 98oC, but Mg melts at 650oC!

Answer 21

The melting point of a metal is related to its group - a group 2 metal will donate two electrons to the sea of free electrons.

This means that the positive ion cores carry a 2+ charge.

The 2+ ion has a stronger attraction to the free electrons.

This means the melting point is higher.

For example, Na melts at 98oC, but Mg melts at 650oC!

Question 22

Types of solid structure (examples)

-

-

Answer 22

Types of solid structure

Ionic
Sodium chloride

Giant covalent
Diamond
Graphite

Metallic
Magnesium

Molecular solid
Ice
Iodine

Question 23

What is the shape of ammonia and why does it form this shape?
1
Draw out the dot and cross diagram of the molecule:
Make sure to draw a simple dot-and-cross diagram to see how many electrons are involved in bonding.
2
State the number of bonding and lone pairs of electrons:
Ammonia has 3 bonding pairs and 1 lone pair.
3
State what electron pairs do:
Electron pairs repel each other. Electron pairs try to get to a position of maximum separation OR minimum repulsion (this is a different way of saying the same thing - when pairs are far away they repel each other less).
4
State what difference lone pairs make:
Lone pairs repel more strongly than bonded pairs and this causes the bond angle between bonding pairs to decrease.
5
Finally state the shape that is formed and the bond angle:
3 bonding pairs and 1 lone pair form a trigonal pyramidal shape. This has a bond angle of 107

Answer 23

What is the shape of ammonia and why does it form this shape?
1
Draw out the dot and cross diagram of the molecule:
Make sure to draw a simple dot-and-cross diagram to see how many electrons are involved in bonding.
2
State the number of bonding and lone pairs of electrons:
Ammonia has 3 bonding pairs and 1 lone pair.
3
State what electron pairs do:
Electron pairs repel each other. Electron pairs try to get to a position of maximum separation OR minimum repulsion (this is a different way of saying the same thing - when pairs are far away they repel each other less).
4
State what difference lone pairs make:
Lone pairs repel more strongly than bonded pairs and this causes the bond angle between bonding pairs to decrease.
5
Finally state the shape that is formed and the bond angle:
3 bonding pairs and 1 lone pair form a trigonal pyramidal shape. This has a bond angle of 107o.

Question 24

Why does H2O have a much higher boiling point than H2S? The same applies for HF vs HCl and for NH3 vs PH3.

Answer 24

Why does H2O have a much higher boiling point than H2S? The same applies for HF vs HCl and for NH3 vs PH3.
1
Describe the intermolecular forces between H2S:
H2S forms Van der Waals forces. These are weak intermolecular forces and are easy to break.
2
Describe the intermolecular forces between H2O:
H2O forms hydrogen bonds between molecules. These are much stronger than Van der Waals forces and are therefore much harder to break.
3
Explain why H2O can form H bonds and H2S cannot:
To form a hydrogen bond there must be a Hydrogen atom bound to a highly electronegative atom such as Oxygen, Nitrogen or Fluorine. This positively charged hydrogen must then bond to a lone pair of electrons on another Oxygen, Nitrogen or Fluorine atom.

This also explains the trend in boiling points for HF and NH3.

Question 25

Why is CCl4 not polar even though the C-Cl bond is polar?

Answer 25

Why is CCl4 not polar even though the C-Cl bond is polar?

1
Describe the shape of CCl4:
CCl4 is tetrahedral and therefore has rotational symmetry.
2
Describe the effect of symmetry:
Because all the polar bonds are facing in opposing directions, the overall molecule is not polar.

Question 26

Why is there a drop in the first ionisation energy from phosphorous to sulfur?

Answer 26

Why is there a drop in the first ionisation energy from phosphorous to sulfur?
1
Describe the electron configuration of each element:
Phosphorous has 5 electrons in the 3rd energy level - 3s2 3p3. Sulfur has 6 electrons in the 3rd energy level - 3s2 3p4.
2
Describe Hund’s rule:
There are 3 p orbitals in each energy level. Hund’s rule states that every orbital in a sublevel is occupied by a single electron before the electrons start to pair up. This means that in phosphorous, each 3p orbital is occupied by 1 electron while in sulphur 1 p orbital is occupied by 2 electrons.
3
Describe the effect of electron pairing:
Electrons are negatively charged, therefore, two electrons repel each other. When electrons are paired in a 3p orbital there is electron pair repulsion which causes the energy of the electrons to increase. Increased energy makes the electron easier to remove and therefore the ionisation energy of sulphur decreases slightly.

Question 27

Element Z has two isotopes: 72M and 75M. The relative atomic mass of element Z is 73.2.

What is the relative abundance of each isotope? Give your answer as a percentage.

Answer 27

Element Z has two isotopes: 72M and 75M. The relative atomic mass of element Z is 73.2.

What is the relative abundance of each isotope? Give your answer as a percentage.
1
Remember the equation for relative atomic mass (RAM):
RAM = sum of (isotopic mass x relative abundance) ÷ 100
2
Sub in the values that you are given:
73.2 = (72 x Y) + (75 x Z) ÷ 100
3
Percentages add up to 100:
Because the percentage relative abundances will all add up to 100, we can do the following.
Y + Z = 100
This means that Y = 100 - Z
4
Sub this conclusion back into the equation:
Now we can replace Y in the equation we made initially:
73.2 = (72 x (100 - Z) + (75 x Z) ÷ 100 
5
Now solve the new equation for Z:
73.2 = (7,200 - 72Z) + 75Z ÷ 100
7320 = 7,200 + 3Z
120 = 3Z 
Z = 40
6
Now solve for Y:
Y = 100 - Z 
Y = 100 - 40
Y = 60 
7
State your answer:
72M = 60% 
75M = 40%

Question 28

Why is there a decrease in the first ionisation energy from Magnesium to Aluminium despite the trend of increasing ionisation energy across the period?

Answer 28

Why is there a decrease in the first ionisation energy from Magnesium to Aluminium despite the trend of increasing ionisation energy across the period?
1
Describe the electronic configuration of both elements:
Mg has an electron configuration of 3s2. Al has an electron configuration of 3s2 3p1.
2
Describe what the difference between these orbitals is:
The electrons in the 3p subshell are slightly higher in energy and are partially shielded by the 3s electrons.
3
Describe what effect this has:
This makes the 3p electrons slightly easier to remove and therefore decreases the first ionisation energy

Question 29

Atom economy =

Answer 29

Atom economy = (Mr of desired products ÷ Mr of all reactants) x 100

Question 30

Concentration formula

Answer 30

Concentration = number of moles / volume