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chemistry > shapes of molecules and intermolecular forces > Flashcards

shapes of molecules and intermolecular forces Flashcards

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1
Q

what is electron pair repulsion theory

A

an electron has a negative charge so electron pairs repel one another so that they are arranged as far apart as possible

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2
Q

why is electron pair repulsion theory useful

A

its a model used in chemistry for explaining and predicting the shapes of molecules and polyatomic ions

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3
Q

why do lone pairs repel more than bonding pair charge clouds

A

lone pair charge clouds are slightly closer to the central atom of a molecule and occupy more space than a bonding pair. additionally a lone pair is slightly more electron dense than a bonded pair.

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4
Q

which has a stronger repulsion

lone pair/lone pair or lone pair/bonding pair

A

lone pair/lone pair

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5
Q

which has a stronger repulsion
lone pair/bonding pair or bonding pair/bonding pair

A

lone pair/bonding pair

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6
Q

what does a normal line in 3D structures show
____________

A

a bond in the plane of the paper

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7
Q

what does a bold wedge show in 3D structures

A

a bond coming out off the plane of paper towards you

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8
Q

what does a dotted wedge show in 3D structures

A

a bond going out of the plane of paper away from you

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9
Q

what happens to the size of the angle when a lone pair is added

A

the bond angle reduces by about 2.5 degrees

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10
Q

what would the name , shape and angle size be for 0 lone pairs and 2 pairs of electrons

A

linear 180 B–A–B

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11
Q

what would the name, shape and angle size be for 0 lone pairs and 3 pairs of electrons

A

trigonal planar 120
B
|
A
/ \
B B

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12
Q

what would the name , shape and angle size be for 1 lone pair and 3 pairs of electrons

A

non linear 117.5
..
A
/ \
B B

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13
Q

What would the name and angle be for 4 electron pairs and 0 lone pairs

A

Tetrahedral 109.5

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14
Q

What would the name and angle be for 1 lone pair and 4 electron pairs

A

Pyramidal 107

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15
Q

What would the name and angle be for 2 lone pairs and 4 electron pairs

A

Non linear 104.5

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16
Q

What would the name and angle be for 2 lone pairs and 5 electron pairs

A

Trigonal planar 120

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17
Q

What would the name and angle be for 5 electron pairs and 3 lone pairs

A

Linear 180

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18
Q

What would the name and angle be for 0 lone pairs and 6 electron pairs

A

Octahedral 90

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19
Q

What would the name and angle be for 4 lone pairs and 6 electron pairs

A

Linear 180

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20
Q

What happens when we have a double bond

A

Each double bond is treated as a bonded region , in the same way as a bonded pair

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21
Q

Why are atoms held together

A

Because their nuclei are both pulling on the same shared pair of electrons

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22
Q

What effects electronegativity

A

Nuclear charge , shielding and atomic size

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23
Q

What is the definition for shielding

A

Sheilding is the repulsion between electrons in different inner shells , shielding reduces the net attractive force between the positive nucleus and the outer shell electrons

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24
Q

What is the definition for electronegativity

A

Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

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25
Q

What is the definition for polarity

A

Polarity is a separation of electric charge leading to a molecule or it’s chemical groups having an electric dipole (a separation of charges)

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26
Q

How is electronegativity measured

A

The Pauling scale , numbers run from 0-4

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27
Q

How does the difference in electronegativity affect the dipole

A

The greater the difference in electronegativity between bonded atoms the greater the permanent dipole

28
Q

What are the trends and reasons in electronegativity across periods

A

Electronegativity increases this is because nuclear charge is increasing and atomic radius decreases ( the smaller the atomic radius the closer the bonding electrons will be to the nucleus ) and no extra shielding as outer electrons are in the same shell

29
Q

What’s the trend and reason for electronegativity as we go (down)the group

A

Electronegativity decreases because Sheilding increases , outer electrons are further away , atomic radius increases.

30
Q

If there is a electronegativity difference of 0 what is the bond type

A

Covalent 100%

31
Q

If there is an electronegativity difference of 0-1.8 what is the bond type

A

Polar covalent

32
Q

If there is a electronegativity difference of more than 1.8 what is the bond type

A

Ionic

33
Q

Is a molecule of hydrogen polar or non polar and why

A

Non polar because there is no difference in electronegativity because it is 2 identical bonding atoms which means there is an equal attraction for the bonding pair of electrons. 100% covalent

34
Q

If an atom is more electronegative what happens with the force

A

It exerts a greater force of attraction on the bonding pair of electrons and the bonding pair will be closer to that atom

35
Q

What is a permanent dipole

A

Is a small charge difference across a bond that results from a difference in electronegativities of the bonded atoms . A polar covalent bond has a permanent dipole

36
Q

What charge does the more electronegative atom have

A

Delta -

37
Q

What charge does the less electronegative charge have

A

Delta positive

38
Q

What can bond polarity influence

A

Boiling point , solubility , structure and chemical reactivity

39
Q

What does a polar molecule have

A

A polar molecule has a overall dipole

40
Q

What is a non symmetrical molecule

A

Charge difference exists around the whole molecule because dipoles do not cancel out

41
Q

What happens when the molecule is symmetrical

A

The dipoles cancel out , it becomes a non polar molecule with polar bonds

42
Q

Why are no Pauling electronegativity values given to the Noble gases

A

Atoms don’t form covalent bonds because they have no affinity for electrons due to their electron structure

43
Q

What are the three types of intermolecular forces

A

Induced dipole -dipole interactions (London forces) , permanent dipole-dipole interactions and hydrogen bonding

44
Q

What is the bond enthalpy of covalent bonds

A

150-500

45
Q

What is the bond enthalpy of London forces

A

1-10

46
Q

What is the bond enthalpy of hydrogen bonds

A

10-40

47
Q

What is the bond enthalpy of permanent dipole - dipole interactions

A

3-25

48
Q

Describe London forces (induced dipole - dipole forces )

A

They are the weakest of all intermolecular forces and they exist between all molecules , polar and non polar

49
Q

How does an instantaneous dipole occur and induce one in neighbouring molecules

A

Movement of electrons produses a changing dipole in a molecule. At any instant an instantaneous dipole exists but it’s position is constantly changing. This induces a dipole in neighbouring molecules

50
Q

How does the strength of a London force depend on the number of electrons in the molecule

A

The more electrons
The larger the instantaneous and induced dipoles
The greater the induced dipole -dipole interactions
The stronger the London forces between the molecules

51
Q

How does London force strength affect bp

A

The stronger the London force means more energy is needed to overcome the intermolecular forces (London)

52
Q

How does being branched or straight effect London forces

A

Straight molecules can approach more closely than branched molecules therefore packs closer and more points of contact than branched molecules . So London forces are stronger in straight molecules than branched

53
Q

What are permanent dipole - dipole interactions

A

Only occur in polar molecules , the permanent dipole interaction is between the positive dipole of one atom and the negative dipole of the atom from another molecule

54
Q

What do simple molecular substances exist as as solids

A

A simple molecular lattice which the molecules are held together with weak intermolecular forces but atoms within molecules are bonded together with strong covalent bonds

55
Q

What bps do simple molecular substances have and why

A

Low because the weak intermolecular forces holding the simple molecular lattice together need little energy to be overcome

56
Q

What is hydrogen bonding

A

A special time of permanent dipole dipole interactions found between molecules containing : an electronegative atom with a lone pair of electrons eg O,N OR F
and a hydrogen atom attached to an electronegative atom eg H-O or H-N or H-F

57
Q

How does hydrogen bonding result in a strong permanent dipole

A

There is a large difference in electronegativity between H and N,O,F which results in a strong permanent dipole

58
Q

What’s the strongest intermolecular force

A

Hydrogen bonding

59
Q

Why is solid water (ice) less dense than liquid

A

In ice the hydrogen bonds hold water molecules apart in an open lattice structure . The hydrogen bonds are slightly longer and hold the water molecules further apart than in liquid water
The holes in the lattice decrease the density of water on freezing . The lattice collapses when melting and the molecules move closer - increasing density

60
Q

Why does water have relatively high melting and boiling points

A

Extra hydrogen bonds add strength and more energy is needed to overcome the forces of attraction . Melting causes breaking in the rigid arrangement of hydrogen bonds in ice and boiling breaks all hydrogen bonds between water molecules

61
Q

Why is the boiling point of hydrides so high

A

Increasing number of electrons in each molecule , larger instantaneous and induced dipoles , greater dipole dipole interactions, stronger attractive forces , higher bp

62
Q

Why do non polar substances dissolve in non polar solvents

A

Intermolecular forces form between the molecules and the solvent. The interactions weaken the intermolecular forces in the simple molecular lattice and the intermolecular forces break , and the compound dissolves

63
Q

Why might polar substances dissolve in polar solvents

A

They can attract each other , but the solubility depends on the strength of the dipole and can be hard to predict

64
Q

Why is hydrophilic polar

A

Contains electronegative atoms that can interact with water

65
Q

Why is hydrophobic non polar

A

Comprised of a carbon chain

66
Q

Why do simple molecular structures not conduct electricity

A

There are no mobile charged particles

67
Q

Polar bond definition

A

Uneven distribution of charge caused by colvalently bonded atoms with different electronegativities

chemistry (19 decks)

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