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A-level Chemistry > 2.5 - Shapes of molecules and intermolecular forces > Flashcards

2.5 - Shapes of molecules and intermolecular forces Flashcards

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1
Q

What are the two types of electron pairs molecules and ions posses ?

A
  • Bonding pairs ( the two shared electrons in a covalent bond )
  • Lone pairs ( two electrons in a pair not involved in bonding = non-bonding pairs )
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2
Q

What is the shape of molecules and ions determined by ?

A

The electron pairs surrounding the central atom of a molecule/ ion determine the shape of its molecule/ ions

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3
Q

What is ‘repulsion theory‘ ?

A

Repulsion theory is a model used in chemistry used for explaining and predicting shapes of molecules and poly atomic ions

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4
Q

Explain how electron pairs surrounding the central atom determine the shape of a molecule/ ion ?

A
  • Electron pairs repel each other as they have alike charges
  • This means electron pairs are arranged as far as possible
  • Arrangement of electron pairs minimises repulsion and therefore holds bonded atoms in a definite shape
  • Different number of electron pairs result in different shapes
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5
Q

Why do lone pairs repel more than bonding pairs ?

A

Lone pairs are more compact than bonding pairs and therefore lone pairs repel more than bonding pairs

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6
Q

What is the effect of lone pairs being more compact than bonding pairs on the bond angle ?

A

Since Lone pairs are more compact than bonding pairs this reduces the bond angle to a small extent

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7
Q

What is the name, shape and bond angle of molecules with two electron pairs ?

A
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8
Q

What is the name, shape and bond angle of molecules with three electron pairs ?

A
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9
Q

What is the name, shape and bond angle of molecules with four electron pairs ?

A
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10
Q

What is the name, shape and bond angle of molecules with five electron pairs ?

A
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11
Q

What is the name, shape and bond angle of molecules with six electron pairs ?

A
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12
Q

What is the name, shape and bond angle of molecules with three bonding pairs and one lone pair ?

A

pyramidal

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13
Q

What is the name, shape and bond angle of molecules with two bonding pairs and two lone pairs ?

A
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14
Q

What is the name, shape and bond angle of molecules with four bonding pairs and two lone pairs ?

A
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15
Q

What is the name, shape and bond angle of molecules with two double bonds ?

A

linear, 180o

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16
Q

What is the name, shape and bond angle of molecules with one triple and one single bond ?

A

linear, 180o

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17
Q

What is the name, shape and bond angle of molecules with one double bond and four single bonds ?

A

planar, 118o

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18
Q

What is the method to figure out the shape of molecules/ ions ?

A
  • Work out how many outer shell electrons are on the central atom (accounting for any charge on the species)
  • Work out how many atoms are bonded to the central atom (= the number of bonding pairs)
  • Work out how many lone pairs there are (which is half the difference between (a) and (b))
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19
Q

Do you know how to explain the shape of molecules and ions such as linear, non-linear, trigonal planar, pyramidal, tetrahedral and octahedral using electron pair repulsion ?

A

Yes

eg. It is … shape because there are … bonding pairs and … lone pairs ( and lone pairs repel more than bonding pairs )

20
Q

What are the bonding angles for the following : CHv4, NHv3 and Hv2O ?

A

CHv4 = 109.5o
NHv3 = 107o
Hv2O = 104.5o

21
Q

What is a polar bond ?

A

A bond formed when there is a difference in the electronegativity of two covalently bonded atoms ( the bond will have a dipole )

22
Q

What is a permanent dipole ?

A

Partial charges

eg. H ( δ+ ) - Cl ( δ- )

Cl is more electronegative than H, so has a greater share of bonding electrons

23
Q

What is a polar molecule ?

A

A molecule where the dipoles of the molecule do not cancel out resulting in the molecule being polar. These dipoles are permanent.

24
Q

What is a non-polar molecule ?

A

A molecule where the dipoles of the molecule cancel out meaning it is non-polar. Therefore these dipole are not permanent.

25
Q

When are molecules non-polar ?

A

When the molecule is symmetrical

26
Q

What are the three types of intermolecular forces ?

A
  • Hydrogen bonding
  • Permanent dipole-dipole
  • London force/ Induced dipole-dipole
27
Q

What is the order of strength of the different intermolecular forces ?

A

1 ) Hydrogen bonding
2 ) Permanent dipole-dipole
3 ) London force/ Induced dipole-dipole

28
Q

Explain what is London force and how it comes about ?

A
  • London force is the weakest type of intermolecular force
  • This occurs due to constant movement of electrons which causes uneven distribution of charge which causes temporary dipoles
  • This induces a temporary dipole in the adjacent molecule which produces an induced dipole-dipole attraction
29
Q

What is an induced dipole-dipole interaction also known as ?

A

London forces

30
Q

What factors affect London force ?

A
  • Number of electrons
  • Shape of molecule
31
Q

Explain how the number of electrons affects London force ?

A
  • The greater the number of electrons , the greater the size of the temporary and induced dipoles
  • This means the London force is stronger between molecules
32
Q

Explain how the shape of the molecule affects London force ?

A
  • Unbranched molecules have a greater surface area and greater surface contact between molecules than branched molecules
  • This means the London force is greater between molecules
33
Q

Explain what is permanent dipole-dipole interaction and how it comes about ?

A
  • Permanent dipole-dipole interaction only happens between polar molecules
  • It is the attraction between the permanent dipoles of adjacent polar molecules
34
Q

What factors affect permanent dipole-dipole interaction ?

A

Magnitude of electronegativity difference

35
Q

Explain how the magnitude of the difference in electronegativity affects permanent dipole-dipole interaction ?

A
  • The greater the difference in electronegativity between two bonded atoms, the greater the permanent dipole
  • The greater the permanent dipole on a molecule, the stronger the permanent dipole-dipole interaction between molecules
36
Q

What is a hydrogen bond ?

A
  • A special type of permanent dipole-dipole interaction
  • It acts between a hydrogen atom in a molecule and a lone pair of electrons on an electronegative atom
37
Q

Where does a hydrogen bond occur ?

A

It occurs between a very electronegative atom ( ie. oxygen, nitrogen or fluorine ) with a lone pair of electrons and a hydrogen atom bonded to a very electronegative atom

38
Q

What should you include the drawing hydrogen bonds ?

A
  • Dipoles
  • Lone pairs
  • Hydrogen bonds
39
Q

Do you know how to draw hydrogen bonds ?

A

Yes

40
Q

What are the anomalous characteristics of water due to hydrogen bonding ?

A
  • Ice is less dense than water
  • Water has a relatively high melting and boiling point
41
Q

Explain why ice is less dense than water ?

A
  • Hydrogen bonds hold water molecules apart in an open lattice structure
  • Therefore the water molecules are farther part in ice than in water and it floats
42
Q

Explain why water has a relatively high melting and boiling point ?

A
  • Water contains London forces between molecules
  • Water contains hydrogen bonds between water molecules
  • This means that more energy is required to break hydrogen bonds between water molecules causing water to have a higher melting and boiling point
43
Q

Explain the structure of simple molecules ?

A
  • Atoms are chemically joined together by strong covalent bonds
  • Molecules are joined together by weak intermolecular forces
  • This forms a simple molecular lattice structure
44
Q

What are the features of the simple molecular lattice structure ?

A
  • High melting / boiling point
  • Cannot conduct electricity
  • They are insoluble in polar solvents
45
Q

Explain why simple molecules have a low melting/ boiling point ?

A
  • They have strong covalent bonds between atoms but weak intermolecular forces between molecules
  • Therefore, not a lot of energy is required to break weak intermolecular forces between molecules and simple molecules have a low melting / boiling point
46
Q

Explain why simple molecules cannot conduct electricity ?

A
  • They have no mobile ions and therefore cannot conduct electricity
47
Q

Explain why simple molecules are insoluble in polar solvents ?

A
  • They are typically non-polar molecules
  • This means they are not soluble and are not dissolved by a polar solvent

A-level Chemistry (29 decks)

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