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a-level chemistry > Chapter 6 Shapes and intermolecular forces > Flashcards

Chapter 6 Shapes and intermolecular forces Flashcards

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1
Q

Electron-pair repulsion theory

A
  • Electron pairs surrounding a central atom determine the shape of the molecule
  • The electron pairs repel one another so that they are arranged as far apart from each other as possible
  • The arrangement of electron pairs minimises repulsion -> holds the bonds in a definite shape
  • Different numbers of electrons result in different shapes
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2
Q

Wedges

A
  • Solid line
  • Solid wedge
  • Dotted wedge
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3
Q

Solid line

A

Bond in the plane of the paper

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4
Q

Solid wedge

A

Bond comes out of the plane of the paper

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5
Q

Dotted wedge

A

Bond goes into the plane of the paper

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6
Q

Bonding pair and lone pair repulsion comparison

A
  • Lone pairs repel more than bonding pairs
  • for each lone pair, bonding angle decreases by 2.5 degrees
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7
Q

Tetrahedral

A
  • 4 bonding pairs
  • 0 lone pairs
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8
Q

Pyramidal

A
  • 3 bonding pairs
  • 1 lone pair
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9
Q

Non linear

A
  • 2 bonding pairs
  • 2 lone pairs
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10
Q

4 electron pair shapes

A
  • Tetrahedral
  • Pyramidal
  • Non-linear
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11
Q

3 electron pairs shapes

A
  • Trigonal planar
  • Bent
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12
Q

Trigonal planar

A
  • 3 bonding pairs
  • 0 lone pairs
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13
Q

Bent

A
  • 2 bonding pairs
  • 1 lone pair
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14
Q

2 electron pair shapes

A
  • linear
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15
Q

linear

A

2 bonding pairs + 0 lone pairs

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16
Q

octahedral

A
  • 6 bonding pairs
  • 0 lone pairs
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17
Q

Shapes of ions

A
  • Ammonium (NH4+)
  • Sulfate (SO4 2-)
  • Carbonate (CO3 2-)
  • Nitrate (NO3 - )
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18
Q

Ammonium ion

A
  • 4 bonding pairs + 0 lone pairs
  • Tetrahedral
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19
Q

Sulfate ion

A

Tetrahedral

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20
Q

Carbonate ion

A

Trigonal planar

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21
Q

Nitrate ion

A

Trigonal planar

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22
Q

Electronegativity

A

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

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23
Q

Pauling electronegativity scale

A

In periodic table left to right:
* Nuclear charge increases
* Atomic radius decreases

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24
Q

What happens to electronegativity as Pauling value increases?

A

Electronegativity of the atoms of that element increases

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25
Q

The most electronegative atoms

A

N
O
F
Cl

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26
Q

Least electronegative atoms

A

Group 1

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27
Q

What happens when electronegativity difference of atoms is large

A

The more electronegative bonding atoms will gain control of the electrons, and the bond becomes ionic (not covalent)

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28
Q

Diatomic molecules

A
  • Atoms are of the same element
  • Bonded electron pair is shared evenly
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29
Q

When bonded atoms are different:

A
  • Atoms are different sizes
  • Nuclear charges are different
  • Shared pair of electrons maybe closer to one nucleus than the other
30
Q

Non-polar bonds definition

A

Bonded electron pair is shared equally between the bonded atoms

31
Q

Characteristics of non-polar bond

A
  • Pure covalent bond
32
Q

Example of non-polar bond

A

Hydrocarbon liquids (e.g. hexane)

33
Q

When do non-polar bonds occur

A
  • Bonded atoms are the same OR
  • Bonded atoms have same or similar electronegativity
34
Q

Shape of non-polar molecules

A

Symmetrical

35
Q

Polar bonds

A

bonded electron pair is shared unequally between the bonded atoms

36
Q

Characteristics of polar bonds

A

Polar covalent bond

37
Q

Example of polar bond

A

e.g. hydrogen chloride
* Cl is more electronegative than H so Cl has ,ore attraction to bonded pair of electrons = polar covalent bond

38
Q

δ signs

A
  • δ+ : Atoms with smaller electronegativity
  • δ- : Atoms with larger electronegativity
39
Q

Dipole

A

Seperation of opposite charges

40
Q

When do polar bonds occur

A
  • Bonded atoms are different OR
  • Bonded atoms have different electronegativity
41
Q

Shape of polar bonds

A

Assymetrical

42
Q

Solubility of ionic compounds

A
  • Positive ions are attracted to oxygen (δ-) in water molecules
  • Negative ions are attracted to hydrogen (δ+) in water molecules
43
Q

Intermolecular forces

A

Weak attraction between dipoles of different molecules

44
Q

What do intermolecular forces determine?

A

physical properties

45
Q

What do covalent bonds determine?

A

identity + chemical reactions of molecules

46
Q

London forces

A

Induced dipole-dipole interactions

47
Q

Strength of intermolecular forces

A

Weakest -> strongest

  1. Induced dipole-dipole interactions
  2. Permanent dipole-dipole interactions
  3. Hydrogen bonding
  4. Single covalent bonds
48
Q

Process of formation of induced dipole-dipole interactions

A
  1. Movement of electrons produces a changing dipole in a molecule
  2. At any instant, an instantaneous dipole will exist, but its position is constantly shifting
  3. The instantaneous dipole induces a dipole on neighbouring molecules
  4. The induced dipole induces further dipoles on neighbouring molecules, which then attract one another
49
Q

State of induced dipole-dipole interactions

A
  • Temporary
  • Can disappear at any moment
50
Q

What happens as the number of electrons in each molecule increases?

A

1.The larger the instantaneous-induced dipole
2. The larger the induced-induced dipole
3. The stronger the attractive force between molecules (higher b.p.)

51
Q

What molecules do London forces occur in?

A

Both polar and non-polar molecules

52
Q

What molecules do permanent dipole-dipole interactions occur in?

A

polar molecules ONLY

53
Q

Simple molecular substance

A
  • Made up of simple molecules
  • Small units containing a definite number of atoms with a definite molecular formula
54
Q

Structure of simple molecular substances as solids

A

Regular structure of simple molecular lattice

55
Q

Simple molecular lattice

A
  • Weak intermolecular forces hold the molecules together
  • Atoms within each molecule are held together by strong covalent bonds
56
Q

Properties of simple molecular substances

A
  • Low m.p. and b.p. -> weak intermolecular forces are easily overcome by small amount of heat energy
  • The strong covalent bonds do not break
57
Q

Solubility of simple molecular substances

A

Different for
* Polar simple molecular substances
* Non-polar simple molecular substances

58
Q

Solubility of non-polar simple molecular substances: Non-polar molecule + non-polar solvent

A

SOLUBLE

intermolecular forces form between the molecules and the solvent

These interactions weaken the intermolecular forces in the simple molecular lattice -> and break

59
Q

Solubility of non-polar simple molecular substances: Non-polar molecule + polar solvent

A

INSOLUBLE

Little interaction between molecule and solvent

Intermolecular forces in solvent are too strong to be broken

60
Q

Solubility of polar simple molecular substances: Polar molecule + polar solvent

A

SOLUBLE

Attract one another (similar to dissolving of ionic compounds)

61
Q

Electrical conductivity of simple molecular substances

A

Do not conduct electricity due to:
* No mobile charged particles

62
Q

Hydrogen bond

A

Special type of permanent dipole-dipole interaction between molecules

63
Q

What does a hydrogen bond contain?

A
  • An electronegative atom with a lone pair of electrons (O, F, N)
  • A H atom attached to an electronegative atom (H-O, H-N, H-F)
64
Q

What is a hydrogen bond shown by?

A

Dashed line

65
Q

What gives water unique properties?

A

Hydrogen bonding

66
Q

Unusual properties of water

A
  • Ice (solid) is less dense than water (liquid)
  • Water has relatively high m.p. and b.p.
67
Q

Why is ice less dense than water

A
  • Hydrogen bonds hold water molecules apart in an open lattice structure
  • The water molecules in ice are further apart than in water
  • Solid ice is less dense than liquid water and floats
68
Q

Structure of ice

A
  • Water has 2 lone pairs so can bond to 4 hydrogen atoms
  • These hydrogen bonds extend outwards, forming an open tetrahedral lattice, full of holes
  • Bond angle is close to 180 degrees
69
Q

What happens when ice melts

A

The ice lattice collapses and the molecules move close together

70
Q

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

A
  • Hydrogen bonds are extra forces
  • Energy is needed to break the hydrogen bonds
  • When the ice lattice breaks, the rigid arrangement of bonds in ice is broken. When water boils, the hydrogen bonds break completely
71
Q

Ionic lattice

A

Repeated pattern of oppositely charged ions

72
Q

Covalent bond

A

shared pair of electrons

a-level chemistry (18 decks)

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