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chemistry module 2 foundations in chemistry > Chapter 6 Shapes Of Molecules And Intermolecular Forces > Flashcards

Chapter 6 Shapes Of Molecules And Intermolecular Forces Flashcards

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1
Q

What does Electron pair repulsion theory state?

A

-Electron pairs repel
-Bond pairs repel equally
-Lone pairs repel more
-The shape of a molecule is determined by reducing the repulsions between electrons

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2
Q

State the name of the molecule with two electron pairs?

A

Linear

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3
Q

State the name of the shape of a molecule with three electron pairs?

A

Trigonal Planar

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4
Q

State the name of the shape of the molecule with four electron pairs?

A

Tetrahedral

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5
Q

State the name of the shape of the molecule with six electron pairs?

A

Octahedral

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6
Q

What is the bond angle of a linear molecule?

A

180 degrees

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7
Q

What is the bond angle of a Trigonal Planar molecule?

A

120 degrees

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8
Q

What is the bond angle of a Tetrahedral molecule?

A

109.5 degrees

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9
Q

What is the bond angle of an Octahedral molecule?

A

90 degrees

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10
Q

How many lone pairs do regular shapes have?

A

0

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11
Q

What are the four regular shapes?

A

Linear
Trigonal Planar
Tetrahedral
Octahedral

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12
Q

What are irregular shapes?

A

Contain lone pairs of electrons

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13
Q

State the name of the shape of the molecule with four electron pairs, three bonding pairs and one lone pair of electrons?

A

Pyramidal

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14
Q

State the name of the shape of the molecule with four electron pairs, two bonding pairs and two lone pairs of electrons?

A

Non- linear

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15
Q

What is the bond angle of a pyramidal molecule?

A

107 degrees

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16
Q

What is the bond angle of a Non- linear molecule?

A

104.5 degrees

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17
Q

When drawing a 3D diagram, what does a solid line represent?

A

A bond that is in the plane of the paper

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18
Q

When drawing a 3D diagram, what does a wedged line represent?

A

A bond that comes out of the plane of the paper

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19
Q

When drawing a 3D diagram, what does a dotted line represent?

A

A bond that goes into the plane of the paper.

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20
Q

How do lone pairs change the bond angle in molecules?

A

Lone pairs provide an additional repulsive force which therefore changes the bond angle, for every lone pair the bond angle between covalent bonds is reduced by 2.5 degrees.

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21
Q

When explaining the shape of a molecule what should you include?

A

Identify the number of bond pairs and if any lone pairs
Then state the electron repulsion theory- electron pairs repel, bond pairs repel equally and lone pairs repel more
Finally states the associated name and bond angle of the identified shape.

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22
Q

What is important to remember when identifying the shape of a molecule in an exam?

A

It is important to remember that the shape of a molecule is determined by the number of bond or lone pairs that surround the central atom.
For example, Predict the O—B—O bond angles:
This therefore means to identify the number of bonding or lone pairs surrounding Boron to find the bond angle

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23
Q

The F—B—F bond angle in BF3 is different from the F—B—F bond angle in H3NBF3
Predict the bond angles of F—B—F in BF3 and in H3NBF3

A

BF3= 120 degrees (Trigonal planar)
H3NBF3= 109.5 degrees (Tetrahedral)
This is because the dative bond acts as the extra bonded pair

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24
Q

What is electronegativity?

A

The ability of an atom to attract a pair of electrons in a covalent bond

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25
Q

What is the scale used to measure electronegativity called?

A

Pauling scale

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26
Q

What factors affect electronegativity?

A

Nuclear charge
Atomic radius
Shielding

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27
Q

How does nuclear charge affect electronegativity?

A

The more protons there are the stronger the attraction between the nucleus and and bonding pair of electrons there is.

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28
Q

How does the atomic radius affect electronegativity?

A

The smaller the distance between the nucleus and outer electrons, the stronger the attraction between the nucleus and the bonding pair of electrons.

29
Q

How does shielding affect electronegativity?

A

The amount of shielding from the inner electron shells, which can shield the attraction between the nucleus and the bonding pair of electrons. Therefore, the less shells there are, the less shielding which will occur (less repulsion), the stronger the attraction there is.

30
Q

What happens to electronegativity in a group?

A

Electronegativity will decrease going down a group.
This is because the atomic radius increases and more shielding occurs as the number of shells increases. These two factors outweigh the increase in nuclear charge, therefore there is less attraction between the nucleus and the bonding pair of electrons.

31
Q

What happens to electronegativity in a period?

A

Electronegativity increases going across the period.
This is because the nuclear charge increases as an extra proton is added each time you move across the period. Additionally, because there is the same number of shells, shielding remains the same, however this causes the atomic radius to decrease because the greater nuclear charge causes a larger attraction between the outer electrons, therefore pulling them in slightly so distance is slightly reduced. Therefore, there is a stronger attraction between the nucleus and the bonding pair of electrons.

32
Q

What group is not electronegative? Why?

A

Noble Gases, this is because they have a full outer shell and therefore don’t need to attract electrons.

33
Q

When is a molecule considered non-polar?

A

When it is symmetrical
When the dipoles cancel each other out.

34
Q

What type of bonding has a large difference in electronegativity?

A

Ionic bonding

35
Q

What type of bonding has a small difference in electronegativity?

A

Polar covalent bond

36
Q

What type of bonding has no difference in electronegativity?

A

Non polar covalent bonding

37
Q

What are the three most electronegative atoms?

A

Fluorine
Oxygen
Nitrogen

38
Q

What is a simple way to distinguish whether a molecule is polar or not?

A

Consider
Has the molecule got different atoms around the central atom?
Has the central atom got any lone pair?

If the answer is yes to either, then the molecule is polar.
HOWEVER, DON’T WRITE THIS IN THE EXAM
A molecule is polar if is not symmetrical or the dipoles don’t cancel each other out

39
Q

What are induced dipole- dipole interactions (London Forces) caused by? {3 marks}

A

Random movements of electrons in a molecule
At any instant this causes an instantaneous dipole in a molecule
This induces a dipole in a neighbouring molecule

40
Q

What are Induced dipole- dipole interactions?

A

Weakest intermolecular force in between non- polar molecules.

41
Q

How do induced dipole- dipole interactions get stronger?

A

As the number of electron increase. Atoms with a greater number of electrons will have a higher boiling point because the London forces are stronger.

42
Q

What are Permanent dipole- Permanent dipole intermolecular forces?

A

Between molecules which have a permanent dipole.
For example,

H—Cl | | | | | H—Cl
Chlorine is more electronegative than hydrogen, so it has a slightly negative charge while hydrogen has a slightly positive charge. This creates permanent dipole: permanent dipole interactions between molecules.
|||- permanent dipole : permanent dipole

43
Q

What is Hydrogen Bonding?

A

Strong permanent dipole- dipole interaction found between molecules containing:
- An electronegative atom with a lone pair of electrons ( such as: O, N, F)
-A hydrogen atom attached to the electronegative atom ( eg. O—H, N—H, F—H )

44
Q

What are the anomalous properties of water?

A

Ice is less dense than water
Water has a relatively high melting and boiling point
High Surface Tension

45
Q

Why is ice less dense than water?

A

Hydrogen bond hold water molecules apart in an open lattice
The water molecules in the ice are further apart than in water
This therefore allows ice to float

46
Q

Why does water have a relatively high melting and boiling point?

A

A large quantity of energy is needed to break the hydrogen bonds

47
Q

Why does water have a high surface tension?

A

Because it is able to form hydrogen bonds

48
Q

Explain the different boiling points of NH3, F2 and Br2.
NH3: -33
F2: -188
Br2: 59

A

Both F2 and Br2 have induced dipole forces, while NH3 has hydrogen bonding between molecules. The induced dipole interactions in Br2 are stronger than the induced dipole interactions between F2, this is because Bromine has more electrons than Fluorine, therefore it will have stronger bonds and a higher boiling point. Additionally, the London forces between molecules in Br2 are stronger than the hydrogen bonds within NH3, however the hydrogen bonds in ammonia are stronger than the induced dipole interactions in F2.

49
Q

Which of the following bonds is the most polar?
The electronegativity values are: F- 3.0, O- 3.5, C- 2.5, Cl- 3.0, N- 3.0, Si-1.9

F—O in OF2, C—Cl in CCl4, Si—Cl in SiCl4, N==O in NO2

A

Si—Cl
This is because it had the greatest difference in electronegativity values meaning it was more polar

50
Q

Which of the following is not a general property of most small covalent molecules?
A)They are less dense as solids than as liquids
B) They are usually not electrically conductive
C) They are held together by intermolecular forces in a solid state
D) Polar compounds are more soluble in polar solvents

A

A) They are less dense as solids than as liquids

51
Q

Which substances experience induced dipole- dipole interactions?
C2H5OH
H2O
SiO2

A

C2H5OH and H2O
This is because all molecules have London forces as well as other intermolecular forces (such as Hydrogen bonding or Permanent dipole- dipole interactions)
It is only non-polar molecules which only have London forces in between its molecules.

52
Q

Explain why (CH3)2S has a low boiling point and is insoluble in water?

A

(CH3)2S has a low boiling point due to the induced dipole-dipole interactions which don’t require a lot of energy to overcome.
(CH3)2S is insoluble in water because it is unable to form hydrogen bonds to water

53
Q

Predict the name of the shape of, and H—N—H bond angle in NH4+ and NH2-

A

NH4+ Tetrahedral Bond angle- 109.5
NH2- Non-Linear Bond angle - 104.5

54
Q

Which element has induced dipole-dipole interactions in its solid lattice?
Boron, Magnesium, Silicon, Sulfur

A

Sulfur this is because Magnesium and Silicon will have metallic bonding within its solid structure, while sulfur is a non-metal.
Boron has a giant covalent lattice as a solid and Giant covalent structures do not have any intermolecular forces between the molecules as a solid.

55
Q

What is the shape of a Carbonate ion?

A

Trigonal Planar
There are three bonding regions

56
Q

What is the shape of a Nitrate ion?

A

Trigonal Planar
There are three bonding regions

57
Q

What is the shape of a sulfate ion?

A

Tetrahedral
There are four bonding regions

58
Q

What do simple molecular substances form in a solid state?

A

A simple molecular lattice
This is where the molecules are held in place by weak intermolecular forces, however the atoms within each molecule are bonded strongly together by covalent bonds.

59
Q

What are the properties of simple molecular substances?

A

Low Melting and Boiling Point
Non-polar simple molecular substances tend to be soluble in non-polar solvents however are insoluble in polar solvents
Polar simple molecular substances solubility depends on the strength of the dipole and therefore can be hard to predict.
Simple molecular substances do not conduct electricity

60
Q

Why do simple molecular substances have low melting an boiling points?

A

There are weak intermolecular forces of attraction between the simple molecules which dont require a lot of energy to overcome.

61
Q

Why do non-polar simple molecular substances tend to be soluble in non-polar solvents?

A

Intermolecular forces form between the molecules and the solvent, these interactions weaken the intermolecular forces in the simple molecular lattice. The intermolecular forces then break, so the compound can dissolve.

62
Q

Why are non-polar simple molecular substances insoluble in polar solvents?

A

There is little interaction between the molecules in the lattice and the solvents molecules. Therefore, the intermolecular bonding within the polar solvents is too strong to be broken.

63
Q

What is the solubility of polar simple molecular substances?

A

Polar simple molecular substances can dissolve in polar solvents because the polar solute molecules and the polar solvents molecules are able to attract each other. However, it’s solubility is dependent on the strength of the dipole so can be hard to predict.

64
Q

Why are simple molecular substances unable to conduct electricity?

A

Because there are no mobile charged carriers in the simple molecular structure.

65
Q

Which molecule is not planar?
C2H4, C2H6, H2CO2, HCN

A

C2H6
The bond angles of hydrogen are not in the same plane as each other

66
Q

Which molecule contains the largest bond angle?
C2H4, H2O, NH3, CH4

A

C2H4
This is because you find the bond angle around the central atom, which is carbon.
This forms a Trigonal Planar shape.

67
Q

What is the difference between non-polar and polar bonds?

A

Non-polar bonds have the same (or very similar) electronegativity values, while polar bonds have a greater difference in electronegativity values

68
Q

Ammonia, NH3, is a Group 5 Hydride. Predict with reasons whether ammonia will have a higher or lower boiling point than phosphine, PH3?

A

NH3 will have a higher boiling point because ammonia has hydrogen bonding between its neighbouring molecules ,which is stronger and requires more energy to overcome, than the permanent dipole-dipole interactions between PH3.

chemistry module 2 foundations in chemistry (5 decks)

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