14.1 Further aspects of covalent bonding (HL) Flashcards
Sigma bonds
Formed by direct head-on/axial overlap of atomic orbitals
Which covalent bonds have sigma bonds?
All covalent bonds have atleast one sigma bond
Electron density in a sigma bond
It is concentrated in region directly between nuclei
What type of symmetry does sigma bonds have?
Cylindrical symmetry along bond axis
Which orbitals can sigma bonds be formed between?
Can be formed between s and s orbitals or s and p orbitals
Pi bond
Formed by sideways overlap of two unhybridized p orbitals
If a carbon has formed double bonds like in alkenes, which bonds does it consist of?
One sigma and one pi bond
What bonds does a carbon triple bond consist of?
One sigma and 2 pi bonds
How are single bonds different from double?
There is no free rotation around double bonds like single bonds and doing so would require bond breaking.
Where is electron density concentrated in pi bonds?
Above and below plane of nuclei of bonding atoms
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?
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
Why does sigma bond have more strength than pi bond?
Extra strength is due to greater overlap of orbitals whereas in pi bonds, they cannot overlap as much
What are non-equivalent Lewis structures and give example
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
Formal charge
It determines which Lewis structure is preferred
What does formal charge ignore?
Ignores difference in electronegativity between atoms and assumes 100% covalency
What is the equation for formal charge?
No. of valence electrons - 1/2(no. of bonding electrons) - no. of non-bonding electrons
Which formal charge is preferred?
The one closest to 0
Formal charge for both structures of CO2 and which is preferred
- Double bonds:
- C= 4-4-0=0
- Both oxygens= 6-2-4=0 - 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
What if two structures have same formal charge?
Most electronegative atom with negative formal charge is preferred
Why can elements in Period 3 and beyond have expanded octets?
They have d orbitals which can be used for bonding
What does expanded octets result in?
5 or 6 electron domains instead of 2-4
What are molecules with 6 domains?
Octahedral
What are molecules with 5 domains?
Trigonal bipyramidal
Molecular geometry of octahedral molecule without lone pairs + bond angle
- 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
Molecular geometry of octahedral molecule with 1 lone pair + bond angle
- 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
Molecular geometry of octahedral molecule with 2 lone pairs + bond angle
- Square planar
- 90 due to equal repulsion from top and bottom and 180 from top to bottom
Molecular geometry of trigonal bipyramidal molecule with 0 lone pairs + bond angle
- 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
Molecular geometry of trigonal bipyramidal molecule with 1 lone pair + bond angle
- 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
Molecular geometry of trigonal bipyramidal molecule with 2 lone pairs + bond angle
- 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
Molecular geometry of trigonal bipyramidal molecule with 3 lone pairs + bond angle
- Linear
- 3 domains on plane so there will be a 180 bond between top and bottom and 90 on the plane
Delocalized pi electrons
Electrons shared between more than 2 nuclei due to overlap of pi bonds
Where are delocalized pi electrons located?
In molecules and polyatomic ions with resonance structures.
Describe structure of CO3 2- ion and bonds
- 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
Where do remaining electrons in p orbitals of CO3 2- go?
They can overlap with p orbital
What do delocalized pi electrons do?
They lower energy of molecule
Where is delocalized pi electrons found in CH3COO- ion and why?
They are only found in COO- region as double bonds can be on either C-O bond
Where is delocalized pi system in benzene (C6H6)
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
How does resonance structure contribute to hybrid structure and which contributes the most?
It contributes depending on its energy and the one with the least energy contributes the most
If there’s symmetry in a molecule, what does that say about resonance structure and energy?
It means there is equal energy and hence equivalent resonance structures
Resonance energy
Difference in energy between resonance hybrid and most stable resonance
What does presence of dashed lines in a diagram mean?
It means bonds are identical in terms of strength and length
Why doesn’t resonance involve equilibrium between Lewis structures?
Because Lewis structures are not shifting between the different forms
Axial and equatorial bonds
Axial are usually the top to bottom while equatorial are the ones on the plane
Why do lone pairs prefer equatorial bonds in trigonal bipyramidal?
Equatorial are repelled by 2 axial while axial are repelled by 3 equatorial
In trigonal bipyramidal or octahedral, what’s an easy way to know the number of bonding electrons and give examples
The formula usually tells you:
- PCl5= 5 bonding electrons
- SF4= 4 bonding and one lone etc.
What does ozone form?
It reacts with chemicals at ground level to form smog, damage materials and respiratory systems
Why is ozone essential for life?
- It absorbs harmful UV radiation.
- Exothermic reactions cause a temperature inversion, warm layer prevents convection, keeping layers of atmosphere stable
What is O2 + UV (Wavelength <242nm)?
2 gaseous O radicals
O3 + UV (Wavelength < 330nm)
Gaseous O2 + O radical
Bond angle in ozone
<120, around 117 degrees
Why are radicals reactive?
They are missing an electron, so they are easily reactive
Calculation of wavelength required to dissociate oxygen and ozone
- Divide bond enthalpy by Avogadro constant to get energy
- Use wavelength = hc/E to obtain wavelength
- Convert to nm
If O3 has bond enthalpy of 364m calculate wavelength required to dissociate
- 364/ 6.0210^-23= 6.04710^-22kJ= 6.047*10^-19J
- Wavelength = 6.6310^-34 x 310^8/6.04710^-19 —> 3.2810^-7m
- 3.28^10-7= 328nm
What pollutants lead to ozone depletion?
CFC’s and nitrogen oxides
Chlorofluorocarbons
Highly stable compound which remains intact in lower atomsphere
CFCl3 + UV
CFCl2 radical + Cl radical
What do Cl radicals act as?
Catalysts
How do Cl radicals react with ozone and oxygen? How do these 2 reactions interact?
- Cl radical + O3 = ClO radical + O2
- ClO radical + O radical = Cl radical + O2
1+2: O3 + O radical= 2O2
How many ozone molecules can a single Cl radical destroy?
About 1 million ozone molecules
Why do nitrogen oxides cause depletion?
They have unpaired electrons and are therefore free radicals
What is N2 + O2
2NO radicals
How does nitrogen monoxide react with oxygen and ozone? What do the two reactions form?
- NO radical + O3 = NO2 radical and O2
- NO2 radical + O radical = NO radical + O2
1+2: O radical + O3= 2O2
What type of reactions do CFC and nitrogen oxides create?
Chain reactions. Constantly make radicals to destroy ozone
What does O3 + O radical = 2O2 tell us
Ozone is constantly being broken down into oxygen hence the depletion of the ozone layer
