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Chemistry HL IB 2020-22 > 14.1 Further aspects of covalent bonding (HL) > Flashcards

14.1 Further aspects of covalent bonding (HL) Flashcards

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1
Q

Sigma bonds

A

Formed by direct head-on/axial overlap of atomic orbitals

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2
Q

Which covalent bonds have sigma bonds?

A

All covalent bonds have atleast one sigma bond

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3
Q

Electron density in a sigma bond

A

It is concentrated in region directly between nuclei

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4
Q

What type of symmetry does sigma bonds have?

A

Cylindrical symmetry along bond axis

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5
Q

Which orbitals can sigma bonds be formed between?

A

Can be formed between s and s orbitals or s and p orbitals

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6
Q

Pi bond

A

Formed by sideways overlap of two unhybridized p orbitals

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7
Q

If a carbon has formed double bonds like in alkenes, which bonds does it consist of?

A

One sigma and one pi bond

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8
Q

What bonds does a carbon triple bond consist of?

A

One sigma and 2 pi bonds

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9
Q

How are single bonds different from double?

A

There is no free rotation around double bonds like single bonds and doing so would require bond breaking.

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10
Q

Where is electron density concentrated in pi bonds?

A

Above and below plane of nuclei of bonding atoms

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11
Q

What does the strength of C-C bond (346 kJmol-1) and C=C bond (614 kJmol-1) tell us about strength of sigma and pi bonds?

A

It tells us sigma is stronger than pi. If sigma bond is 346 kJmol-1, that means pi bond in C=C bond is only 268kJmol-1

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12
Q

Why does sigma bond have more strength than pi bond?

A

Extra strength is due to greater overlap of orbitals whereas in pi bonds, they cannot overlap as much

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13
Q

What are non-equivalent Lewis structures and give example

A

It is when one molecule can have different number of multiple bonds eg. CO2 can have 2 double bonds or 1 single and 1 triple bond

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14
Q

Formal charge

A

It determines which Lewis structure is preferred

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15
Q

What does formal charge ignore?

A

Ignores difference in electronegativity between atoms and assumes 100% covalency

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16
Q

What is the equation for formal charge?

A

No. of valence electrons - 1/2(no. of bonding electrons) - no. of non-bonding electrons

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17
Q

Which formal charge is preferred?

A

The one closest to 0

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18
Q

Formal charge for both structures of CO2 and which is preferred

A
  1. Double bonds:
    - C= 4-4-0=0
    - Both oxygens= 6-2-4=0
  2. 1 single and 1 triple bond
    - C=4-4-0=0
    - Single O= 6-1-6= 1-
    - Triple O= 6-3-2= 1+

So DOUBLE bond CO2 is preferred

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19
Q

What if two structures have same formal charge?

A

Most electronegative atom with negative formal charge is preferred

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20
Q

Why can elements in Period 3 and beyond have expanded octets?

A

They have d orbitals which can be used for bonding

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21
Q

What does expanded octets result in?

A

5 or 6 electron domains instead of 2-4

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22
Q

What are molecules with 6 domains?

A

Octahedral

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23
Q

What are molecules with 5 domains?

A

Trigonal bipyramidal

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24
Q

Molecular geometry of octahedral molecule without lone pairs + bond angle

A
  • Octahedral
  • 4 will be on plane and 1 domain will be on top and bottom each
  • Bond angle in plane =90
  • Bond angle from top-bottom= 180
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25
Q

Molecular geometry of octahedral molecule with 1 lone pair + bond angle

A
  • Square pyramidal
  • Lone pair will be on top or bottom as 180>90
  • Bond angle is less than 90 on plane due to extra repulsion
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26
Q

Molecular geometry of octahedral molecule with 2 lone pairs + bond angle

A
  • Square planar

- 90 due to equal repulsion from top and bottom and 180 from top to bottom

27
Q

Molecular geometry of trigonal bipyramidal molecule with 0 lone pairs + bond angle

A
  • Trigonal bipyramidal
  • 2 domains on top bottom and 3 on the plane
  • Bond angle is 90 between top and one on plane
  • Bond angle is 120 between those on plane
  • Bond angle is 180 between top and bottom
28
Q

Molecular geometry of trigonal bipyramidal molecule with 1 lone pair + bond angle

A
  • See saw
  • Bond angle will be 90 and the ones on the plane will have slightly less than 120 due to repulsion and 180 from top to bottom
29
Q

Molecular geometry of trigonal bipyramidal molecule with 2 lone pairs + bond angle

A
  • T-shaped
  • 2 domains on plane so bond angle is slightly less than 90 on plane + 90 bond between top and ones on the plane + 180 between top and bottom
30
Q

Molecular geometry of trigonal bipyramidal molecule with 3 lone pairs + bond angle

A
  • Linear

- 3 domains on plane so there will be a 180 bond between top and bottom and 90 on the plane

31
Q

Delocalized pi electrons

A

Electrons shared between more than 2 nuclei due to overlap of pi bonds

32
Q

Where are delocalized pi electrons located?

A

In molecules and polyatomic ions with resonance structures.

33
Q

Describe structure of CO3 2- ion and bonds

A
  • Central C atom + 3 O atoms in trigonal planar structure
  • C-O bond is sigma
  • Pi bond is formed by sideways overlap of p orbitals on carbon and one oxygen atom
34
Q

Where do remaining electrons in p orbitals of CO3 2- go?

A

They can overlap with p orbital

35
Q

What do delocalized pi electrons do?

A

They lower energy of molecule

36
Q

Where is delocalized pi electrons found in CH3COO- ion and why?

A

They are only found in COO- region as double bonds can be on either C-O bond

37
Q

Where is delocalized pi system in benzene (C6H6)

A

There is one electron from each p orbital of carbon so 6 in the system. These p orbitals overlap to form a ring system both on bottom and top

38
Q

How does resonance structure contribute to hybrid structure and which contributes the most?

A

It contributes depending on its energy and the one with the least energy contributes the most

39
Q

If there’s symmetry in a molecule, what does that say about resonance structure and energy?

A

It means there is equal energy and hence equivalent resonance structures

40
Q

Resonance energy

A

Difference in energy between resonance hybrid and most stable resonance

41
Q

What does presence of dashed lines in a diagram mean?

A

It means bonds are identical in terms of strength and length

42
Q

Why doesn’t resonance involve equilibrium between Lewis structures?

A

Because Lewis structures are not shifting between the different forms

43
Q

Axial and equatorial bonds

A

Axial are usually the top to bottom while equatorial are the ones on the plane

44
Q

Why do lone pairs prefer equatorial bonds in trigonal bipyramidal?

A

Equatorial are repelled by 2 axial while axial are repelled by 3 equatorial

45
Q

In trigonal bipyramidal or octahedral, what’s an easy way to know the number of bonding electrons and give examples

A

The formula usually tells you:

  • PCl5= 5 bonding electrons
  • SF4= 4 bonding and one lone etc.
46
Q

What does ozone form?

A

It reacts with chemicals at ground level to form smog, damage materials and respiratory systems

47
Q

Why is ozone essential for life?

A
  • It absorbs harmful UV radiation.
  • Exothermic reactions cause a temperature inversion, warm layer prevents convection, keeping layers of atmosphere stable
48
Q

What is O2 + UV (Wavelength <242nm)?

A

2 gaseous O radicals

49
Q

O3 + UV (Wavelength < 330nm)

A

Gaseous O2 + O radical

50
Q

Bond angle in ozone

A

<120, around 117 degrees

51
Q

Why are radicals reactive?

A

They are missing an electron, so they are easily reactive

52
Q

Calculation of wavelength required to dissociate oxygen and ozone

A
  1. Divide bond enthalpy by Avogadro constant to get energy
  2. Use wavelength = hc/E to obtain wavelength
  3. Convert to nm
53
Q

If O3 has bond enthalpy of 364m calculate wavelength required to dissociate

A
  1. 364/ 6.0210^-23= 6.04710^-22kJ= 6.047*10^-19J
  2. Wavelength = 6.6310^-34 x 310^8/6.04710^-19 —> 3.2810^-7m
  3. 3.28^10-7= 328nm
54
Q

What pollutants lead to ozone depletion?

A

CFC’s and nitrogen oxides

55
Q

Chlorofluorocarbons

A

Highly stable compound which remains intact in lower atomsphere

56
Q

CFCl3 + UV

A

CFCl2 radical + Cl radical

57
Q

What do Cl radicals act as?

A

Catalysts

58
Q

How do Cl radicals react with ozone and oxygen? How do these 2 reactions interact?

A
  1. Cl radical + O3 = ClO radical + O2
  2. ClO radical + O radical = Cl radical + O2

1+2: O3 + O radical= 2O2

59
Q

How many ozone molecules can a single Cl radical destroy?

A

About 1 million ozone molecules

60
Q

Why do nitrogen oxides cause depletion?

A

They have unpaired electrons and are therefore free radicals

61
Q

What is N2 + O2

A

2NO radicals

62
Q

How does nitrogen monoxide react with oxygen and ozone? What do the two reactions form?

A
  1. NO radical + O3 = NO2 radical and O2
  2. NO2 radical + O radical = NO radical + O2

1+2: O radical + O3= 2O2

63
Q

What type of reactions do CFC and nitrogen oxides create?

A

Chain reactions. Constantly make radicals to destroy ozone

64
Q

What does O3 + O radical = 2O2 tell us

A

Ozone is constantly being broken down into oxygen hence the depletion of the ozone layer

Chemistry HL IB 2020-22 (19 decks)

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