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chemistry hl > s2.2 covalent model > Flashcards

s2.2 covalent model Flashcards

1
Q

what is a covalent bond?

A

a covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. the negatively charged electrons are attracted to the positive charges of both nuclei. the attraction overcomes the repulsion between the two positively charged nuclei

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2
Q

what is the octet rule?

A

the octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons
boron and beryllium are exceptions to the octet rule

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3
Q

what is single covalent bonding?

A

a single bond occurs when atoms are bonded by one shared pair of electrons

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4
Q

what is a lone pair?

A

a lone pair is an outer shell pair of electrons that is not involved in chemical bonding

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5
Q

what is double covalent bonding?

A

a double bond occurs when atoms are bonded by two shared pairs of electrons

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6
Q

what is triple covalent bonding?

A

a triple bond occurs when atoms are bonded by three shared pairs of electrons

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7
Q

what is the trend in bond strength?

A

the trend in bond strength (a measure of energy required to break the bonds) is the more bonds, the stronger they are

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8
Q

what is the trend in bond length?

A

the trend in bond length (a measure of distance between the two bonded nuclei) is the more bonds, the smaller the length (because of the increase in attractive force between the two nuclei)

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9
Q

what are lewis structures?

A

lewis structures show all the valence electrons in a molecule. this includes the bond pairs and the lone pairs

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10
Q

what is a dative covalent bond?

A

in a dative covalent bond (or coordinate bond), one of the atoms supplies both the shared electrons

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11
Q

what is a complex ion?

A

a complex ion is a central metal ion bonded to one or more ligands by coordinate bonds (dative covalent)

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12
Q

what is a ligand?

A

a ligand is a molecule or ion that can donate a pair of electrons to the central metal ion to form a coordinate bond

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13
Q

what is the coordination number?

A

the coordination number is the total number of coordinate bonds formed between a central metal ion and its ligands

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14
Q

what is VSEPR theory?

A

valence shell electron pair repulsion theory can be used to deduce the shapes of molecules. pairs of electrons (electron domains) in the valence shell of an atom repel each other and therefore will take up space to minimise these repulsions

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15
Q

what is electron domain geometry?

A

EDG is the arrangement of electron domains surrounding the central atom of a molecule or ion
(includes the lone pair as a domain)

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16
Q

what is molecular geometry?

A

MG is the 3d shape that a molecule occupies in space

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17
Q

2 bond pairs, 0 lone pairs

A

linear, 180°

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18
Q

3 bond pairs, 0 lone pairs

A

trigonal planar, 120°

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19
Q

2 bond pairs, 1 lone pair

A

bent, 118°

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20
Q

4 bond pairs, 0 lone pairs

A

tetrahedral, 109.5°

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21
Q

3 bond pairs, 1 lone pair

A

trigonal pyramidal, 107°

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22
Q

2 bond pairs, 2 lone pairs

A

bent, 104.5°

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23
Q

5 bond pairs, 0 lone pairs

A

trigonal bipyramidal, 120°, 90°

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24
Q

4 bond pairs, 1 lone pair

A

see saw, 119°, 89°

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25
Q

3 bond pairs, 2 lone pairs

A

t shape, 89°

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26
Q

2 bond pairs, 3 lone pairs

A

linear, 180°

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27
Q

6 bond pairs, 0 lone pairs

A

octahedral, 90°

28
Q

5 bond pairs, 1 lone pair

A

square pyramidal, 89°

29
Q

4 bond pairs, 2 lone pairs

A

square planar, 90°

30
Q

3 bond pairs, 3 lone pairs

A

t shape, <90°

31
Q

2 bond pairs, 4 lone pairs

A

linear, 180°

32
Q

what are the factors that affect electronegativity?

A
  1. number of protons/nuclear charge (negative bonding pair electrons are attracted to the positive nucleus. the more protons, the stronger the attraction)
  2. atomic radius (the distance from the nucleus to the outer electrons - the smaller the atomic radii, the stronger the attraction)
  3. shielding (the more shells, the weaker the attraction)
33
Q

what is the bond polarity in a non-polar covalent bond?

A

Cl2 - similar atoms have the same electronegativity therefore the electrons are equally shared

34
Q

what is the bond polarity in a polar covalent bond?

A

HCl - different atoms have different electronegativities. the more electronegative atom will have a greater share of electrons and will be slightly delta negative. bonds that are polar have a permanent dipole (bond dipole moment)

35
Q

what affects the polarity of a molecule?

A
  1. the presence of a polar covalent bond in the molecule
  2. the geometry of the molecule
    e.g. there is a difference in electronegativity between C and Cl in each covalent bond causing a permanent dipole on each C-Cl bond. however, CCl4 is a symmetrical molecule, so the permanent dipole bonds act in different directions and cancel each other out. symmetrical molecules are non-polar even if they contain polar bonds
36
Q

what are the properties of simple covalent molecules?

A
  1. low melting and boiling points (because little energy is needed to break the weak intermolecular forces)
  2. doesn’t conduct electricity (because they do not have any free electrons or an overall electric charge in any state of matter
37
Q

what are the properties of diamond?

A

diamond is an allotrope of carbon in which each atom is tetrahedrally covalently bonded to four other carbon atoms. it has a giant covalent network structure with sp3 hybridisation (bond angle is 109.5)
1. diamond is very hard and insoluble
2. diamond has a very high melting and boiling point (lots of energy required to break strong covalent bonds)
3. diamond cannot conduct electricity (no delocalised electrons)

38
Q

what are the properties of graphite?

A

graphite is composed of layered sheets (held together by london forces) in a hexagonal arrangement. each carbon atom is bonded to three other carbon atoms (geometry is trigonal planar) creating a giant covalent network structure (sp2 hybridised, bond angle 120)
1. graphite is soft and slippery (layers can easily slide over each other because the weak london forces are easily broken)
2. graphite can conduct electricity (due to delocalised pi electron)
3. graphite is insoluble

39
Q

what are the properties of graphene?

A

graphene is a giant covalent network structure, consisting of a single planar sheet of carbon atoms (1 atom thick.)
each carbon atom is covalently bonded to three other carbon atoms (geometry is trigonal planar, sp2 hybridised, bond angle 120)
graphene is an excellent thermal and electrical conductor (due to delocalised electrons) but is insoluble in water
they have a high tensile strength so have been used in car design

40
Q

what are the properties of buckminsterfullerene?

A

buckminsterfullerene (C60) is an allotrope of carbon, however it is a simple covalent molecular structure
C60 is composed of strong covalent bonds, however there are weak london forces between each molecule
C60 is a polyhedron cage, 20 hexagonal surfaces and 12 pentagonal surfaces. each carbon atom is bonded to three other carbon atoms (geometry is trigonal planar, sp2 hybridised, bond angle 120). however it does not conduct electricity as it is a simple covalent molecule

41
Q

what are the properties of silicon and silicon dioxide?

A

silicon forms a 3D giant covalent network structure where each Si atom is bonded to four other silicon atoms.
silicon dioxide, SiO2, is a giant covalent network structure, similar in shape to diamond.
each oxygen is bonded to two silicon atoms. due to two lone pairs on each oxygen atom, the basic shape about each Si-O-Si unit is bent.
Si and SiO2 have a high melting point and boiling point due to strong covalent bonds. they are insoluble in water

42
Q

what are intramolecular forces?

A

intramolecular forces are forces within a molecule (ie ionic bonds, covalent bonds and metallic bonds)

43
Q

what are intermolecular forces?

A

intermolecular forces are forces between molecules. only intermolecular forces are broken when simple covalent molecules are melted or boiled - covalent bonds are not broken

44
Q

what are london forces?

A

london forces exist between all non-polar covalent atoms or molecules.
london forces are weak attractive forces that occur between induced dipoles in neighbouring molecules.
london forces are instantaneous dipole-induced dipole interactions

45
Q

what causes london forces?

A

because electrons move quickly in orbitals, their position is constantly changing: at any given instant, they could be anywhere in an atom. the possibility will exist that one side will have more electrons than the other, which will give rise to an instantaneous dipole.
this instantaneous dipole will induce dipole on nearby atoms/molecules. molecules are now attracted to each other by a weak force

46
Q

how does branch length affect the strength of london forces?

A

as the length (and so the relative molecular mass) of a hydrocarbon chain increases, so do the boiling points. this is because the london forces between the molecules are stronger as there are more atoms and therefore more electrons

47
Q

what are dipole-dipole forces?

A

polar covalent molecules have weak attractive forces between neighbouring molecules known commonly as dipole-dipole forces. they are stronger than london forces
nb. london forces are also present between polar covalent molecules, therefore intermolecular forces are stronger in polar covalent compounds compared to non-polar

48
Q

what are hydrogen bonds?

A

a hydrogen bond is a stronger dipole-dipole force between an electron deficient hydrogen atom and a lone pair of electrons on a highly electronegative atom on a different molecule (N, O, F)

49
Q

how does solubility relate to intermolecular forces?

A

generally a substance will dissolve in a solvent if the intermolecular forces in the solute and solvent are similar.
polar covalent molecules with hydrogen bonding can dissolve in water e.g. ethanol in water
ionic compounds can also dissolve in water. ion-dipole interactions form between the water and the ions

50
Q

why does ice float in water?

A

ice is less dense than water. ice has an open lattice with hydrogen bonds holding the water molecules apart. when ice melts, the rigid hydrogen bonds collapse, allowing the water molecules to move closer together

51
Q

what is chromatography (relating to intermolecular forces)?

A

chromatography is a technique used to separate the components of a mixture based on their relative attractions involving intermolecular forces to mobile and stationary phases

52
Q

whats the relation between intermolecular forces and chromatography?

A

if the component in the mixture has a stronger affinity to the mobile phase (solvent) due to the intermolecular forces of attraction, the the component will move higher up the chromatogram

53
Q

whats the relationship between TLC and polarity?

A

TLC is a more expensive but more sensitive counterpart to paper chromatography. the stationary phase is highly polar as it contains many hydroxyl (-OH) groups so polar components in the mixture will move slowly. the mobile phases is a non-polar organic solvent so any non-polar components in the mixture will move fast

54
Q

what are the limitations of chromatography?

A
  1. similar compounds often have similar Rf values
  2. unknown compounds have no reference Rf values for comparison
  3. it may be difficult to find a solvent that separates all the components in the mixture
    -> if the components are very soluble in the solvent, they will just be washed up by the TLC plate with the solvent front. if the components have little solubility, they will hardly move. trial and error may be required before a suitable solvent is discovered
55
Q

what is delocalisation?

A

delocalisation involves the spread of pi electrons over more than two nuclei. the electrons are shared by all atoms in a molecule or ion as opposed to being localised between a pair of atoms.
the bonds in the delocalised structure have equal bond orders, bond length and bond strength giving the delocalised structure greater stability.

56
Q

what is bond order?

A

a single bond is said to have a bond order of 1, a double bond has bond order 2 and a triple bond has bond order 3.
bond order = total number of bonding pairs / total number of positions

57
Q

what is resonance?

A

resonance involves using two or more lewis structures to represent a particular molecule or ion. resonance structures occur when there is more than one position for a multiple bond in a molecule. all resonance structures have delocalised pi electrons.

58
Q

how is benzene formed?

A

each p orbital laterally overlaps with neighbouring p orbitals forming a delocalised ring of pi electrons. due to this, the structure of benzene is represented by a hexagon with a circle inside.
each carbon atom in benzene has:
- 3 sigma bonds and 1 pi bond
- sp2 hybridisation
- bond angle of 120

59
Q

how is carbon bond length evidence for the structure of benzene?

A

in benzene, the c-c bond length are all the same (140pm). this bond length value is between a c-c single bond length (154pm) and a c=c bond length (134pm).
this means that each c-c has a bond order of 1.5

60
Q

how are addition reactions evidence for the structure of benzene?

A

benzene does not undergo electrophilic addition reactions (e.g. does not decolourise bromine water, which alkenes can do). this is due to the delocalisation of the pi electrons making the structure of benzene very stable.

61
Q

how is hydrogenation evidence for the structure of benzene?

A

the enthalpy of hydrogenation of benzene is less exothermic (-205 kj mol-1) wheras for cyclohex-1,3,5-triene the value is -360 jk mol-1. therefore benzene is more stable than expected.

62
Q

why do we use formal charge?

A

a limitation of the model of covalent bonding is that when drawing lewis structures for molecules, it is sometimes possible to come up with more than one structure while still obeying the octet rule.
this leads to the problem of deciding which structure is appropriate and is consistent with other information such as spectroscopic data on bond lengths and electron density.
one approach to determining which is the preferred structure is to determine the formal charge of all the atoms present in the molecule.

63
Q

what is formal charge?

A

formal charge is described as the charge assigned to an atom in a molecule, assuming that all the electrons in the bonds are shared equally between atoms, regardless of differences in electronegativity.
FC = valence electrons - 1/2 bonding electrons - non bonding electrons

64
Q

how do we use formal charge to determine the most stable structure?

A

the lewis structure with the atoms having FC values closest to zero is preferred. look for:
- FC that is closest to zero
- the one with the negative charges located on the most electronegative atom

65
Q

what is a sigma bond?

A

a sigma bond is formed by the axial/head on overlap of atomic orbitals, resulting in electron density concentrated between the nuclei of the bonding atoms.
this can be between two s, two p or one s and one p orbitals overlapping on the same axis. this explains why there is variation in the length on sigma bonds in different molecules, as it depends on the size of the orbitals.

66
Q
A

chemistry hl (11 decks)

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