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Covalent Bonds and Molecules
• --you don’t mind a trick question, do you?
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
• Carbon monoxide—colorless, odorless gas. Toxic. Careful, you can’t afford any more brain damage.
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
??!?!!
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
The oxygen has a full octet, while the carbon is not yet full. Does this mean carbon
monoxide can’t form?
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
No. This is a demonstration of why we chemists don’t circle electrons anymore.
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
The correct bonding diagram.
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
We notice that the atoms have (4 + 6=) 10 valence electrons.
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
Two octets require 16 electrons.
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
The 6 electrons short represent three bonds.
OC
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• Show the formation of a covalent bond between a carbon and oxygen atom by sharing electrons
Each atom needs an unshared pair of electrons for an octet
OC
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• This is called a Lewis diagram (after Gilbert Norton Lewis)
• The bars represent covalent bond
( 2 shared electrons)
• Unshared pairs fill out the octets.
• Double and triple bars represent double and triple bonds.
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Lewis diagramsStep 1: Count the total valence electrons available
--use the columns of the periodic chart--negative ions have extra electrons, --positive ions are missing electrons
Step 2: Count the total valence electrons needed--duet rule for hydrogen, or the --octet rule for everything else
Step 3: Number of bonds = (electrons needed-electrons available) / 2 electrons per bond
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Lewis diagrams
Step 4: Choose the central atom (almost always the odd one), surround it with the others.
Step 5: Connect with one bond to each outer atom. (PS Recheck your formula!)
Step 6: Fill in enough multiple bonds to satisfy step 3
Step 7: Draw in unshared pairs to fill valence levels.
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Don’t…
…try to figure out whose electrons are whose. Electrons are identical.
…string the atoms along. Put one atom in the center, unless you have 6 or more atoms.
…EVER put two bonds or an unshared pair on H.
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Lewis diagrams
• Draw a Lewis diagram of hydrogen cyanide, HCN
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Lewis diagrams
• Draw a Lewis diagram of hydrogen cyanide, HCN
• Step 1: Total valence electrons available =1 (from H) + 4 (from C) + 5 (from N)= 10
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Lewis diagrams
• Draw a Lewis diagram of hydrogen cyanide, HCN
• Step 2: Total valence electrons needed = 2 (for H) + 8 (for C) + 8 (for N) = 18
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Lewis diagrams
• Draw a Lewis diagram of hydrogen cyanide, HCN
• Step 3: Number of bonds = (18-10) / 2 =8/2= 4 bonds
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Lewis diagrams
• Draw a Lewis diagram of hydrogen cyanide, HCN
H C N• Step 4: The central atom is the carbon,
since it’s written that way; HCN
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Lewis diagrams
• Draw a Lewis diagram of hydrogen cyanide, HCN
H C N• Step 5:
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Lewis diagrams
• Draw a Lewis diagram of hydrogen cyanide, HCN
H C N• Step 6: Since hydrogen can't make more
than one bond, the 3rd and 4th bonds have to be between the C and N
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Lewis diagrams
• Draw a Lewis diagram of hydrogen cyanide, HCN
H C N• Step 7
Unshared pair!
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Try an ion.
• Draw a Lewis diagram for the nitrite, NO2- ,
ion
Anions have extra electrons!
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Lewis diagrams
• Draw a Lewis diagram for the nitrite, NO2- ,
ion
[ O N O ]-
Use [brackets] around an ion
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Resonance
• Which one is preferable?
[ O N O ]-
[ O N O ]-
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Resonance
• Each is valid. The multiple bond exists in both locations. This is called resonance.
[ O N O ]-
[ O N O ]-
(the double-headed arrow signifies resonance)
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Resonance
• Draw three resonance structures for carbon dioxide.
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Coordinate covalent bonds
• How did this
become this?
OC
OC
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Coordinate covalent bonds
• How did this
become this?
Carbon monoxide really does have the third bond. The oxygen donates both electrons to share. This is a coordinate covalent bond
OC
OC
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Coordinate covalent bonds
• Draw a Lewis diagram of the ozone (O3) molecule.
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Exceptions to the octet rule
• Draw a Lewis diagram for the triiodide ion, I3
-
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Exceptions to the octet rule
• Draw a Lewis diagram for the triiodide ion, I3
-
• …Hey, Wait a minute!
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Exceptions to the octet rule
• Draw a Lewis diagram for the triiodide ion, I3
-
• When you try to find the number of bonds, (24-22)/2=1 bond. That’s not enough to tie the ion together.
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• When that happens—go old school. Circle your electrons
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Exceptions to the octet rule
• When that happens—go old school. Circle your electrons
[ I I I ]-
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Exceptions to the octet rule
• When that happens—go old school. Circle your electrons
[ I I I ]-
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Exceptions to the octet rule
• When that happens—go old school. Circle your electrons
[ I I I ]-
Two single bonds will satisfy the outer two iodine atoms, the inner one breaks the octet rule (with 10 electrons).
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Exceptions to the octet rule
• Draw a Lewis diagram for XeF4
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Exceptions to the octet rule
• Draw a Lewis diagram for XeF4
• (Don’t start with me. I know.)
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Exceptions to the octet rule
• Draw a Lewis diagram for XeF4
XeF
F
F
F
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…now that you’re big kids…
• We know electronegativity!
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• The type of bond is better described using electronegativity differences instead of a simple metal/nonmetal distinction
• Subtract the two electronegativities (table on page 405)
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• We usually claim that an electronegativity difference of
0-.4 is a nonpolar covalent bond
.5-1.8 is a polar covalent bond
1.9 and above is an ionic bond
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• What kind of bond forms between chlorine and phosphorus atoms?
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• What kind of bond forms between chlorine and phosphorus atoms?
Cl P
3.0 2.1
Look up the electronegativities
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• What kind of bond forms between chlorine and phosphorus atoms?
Cl P
3.0 - 2.1=.9
• Subtract
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• What kind of bond forms between chlorine and phosphorus atoms?
Cl P
3.0 - 2.1=.9
• This is a polar covalent bond
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• What kind of bond forms between chlorine and phosphorus atoms?
Cl P
3.0 - 2.1=.9
• This is a polar covalent bond
• (use an absolute value for the difference)
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Polar bonds
• We use a symbol to show a polar covalent bond.
• The arrow points toward the more electronegative atom, the (+) end is less electronegative
H
O
HH
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Polar bonds
• Or, mark the molecules (+) and (-) parts
• The is the small Greek delta, indicates a small change. In this case, a partial charge
H
O
HH
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Polar bonds
• The less electronegative end of a polar
bond: + -
H Cl• --is more positive
• --cannot attract the electrons as well
• --is farther from the shared pair of electrons
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What kind of bond forms between these atoms?
H and H H and C
C and F F and F
Na and O Mg and N
Mg and Mg N and O
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Molecular Shapes
• Most molecules have a central atom that satisfies the octet rule. This allows the following shapes.– Tetrahedral– Trigonal pyramid (trigonal=having three– Bent corners)– Linear and– Trigonal planar
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Molecular Shapes
• Four bonds in four directions makes a tetrahedral shape
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Molecular Shapes
• Three bonds and one lone pair in four directions makes a trigonal pyramid shape
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Molecular Shapes
• Two bonds and two lone pairs in four directions makes a bent shape
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Molecular Shapes
• A double bond holds two electron pairs in the same direction. With no lone pairs, this makes a trigonal planar molecule
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Molecular Shapes
• One lone pair, with a single and a double bond gives a bent molecule.
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Molecular Shapes
• Two double bonds, or a single and a triple makes a linear molecule
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Molecular Shapes
• Two atoms are always in a straight line, a linear molecule.
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If A=central atom, B=atoms bonded to it, E=e- pairs:
• AB4 — tetrahedral
• AB3E-trigonal pyramid
• AB2E2 — bent
• ABE3 — linear
• AB3 — trigonal planar
• AB2E — bent
• ABE2 — linear
• AB2 — linear
• ABE — linear
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Look for double bonds and unshared pairs
• AB4—tetrahedral• (no double bonds)• AB3E-trigonal pyramid• (no double bonds)• AB2E2 –bent• (no double bonds)• ABE3 –linear• (no double bonds)
• AB3—trigonal planar• (one double bond)• AB2E –bent• (one double bond)• ABE2—linear• (one double bond)• AB2—linear• (2 doubles or 1 triple)• ABE—linear (“)
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• Determine the shape of each molecule and ion on the lab that has a single central atom.
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Polarity of molecules
• When polar bonds are not cancelled by symmetry, you get a polar molecule. A polar molecule has (+) and (-) parts.
• POLARITY is the first property to look for when analyzing a molecule !
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Polarity
• CH4 has no polar bonds. It is symmetric
• PH3 has no polar bonds It is not symmetric
• CO2 has polar bonds. It is symmetric
• H2O has polar bonds. It is not symmetric•
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Polarity
• CH4 has no polar bonds. It is symmetric• Not polar!
• PH3 has no polar bonds It is not symmetric• Not polar!
• CO2 has polar bonds. It is symmetric• Not polar!
• H2O has polar bonds. It is not symmetric• Polar!
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• Mark each molecule on the lab that is polar.
• For those that are not polar—why not?
• (PS—don’t even look at the ions. If it has a whole charge, ignore the partial charges)
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Hybridization
• Consider CH4:
C 1s22s22p2
H 1s1 H 1s1 H 1s1 H 1s1
Who shares where?
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Hybridization
C 1s22s22p2
H 1s1 H 1s1 H 1s1 H 1s1
If the first two hydrogen pair electrons with the 2p electrons of C and the last two enter an empty orbital, you would see…
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Hybridization
• Consider CH4
C
H H H H
If the first two hydrogens pair electrons with the 2p electrons of C and the last two enter an empty orbital, you would see…
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Hydridization
• Consider CH4
C
H H H H
If the first two hydrogens pair electrons with the 2p electrons of C and the last two enter an empty orbital, you would see…
![Page 74: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/74.jpg)
• …a difference in the bonds.
You don’t.
• All four bonds are identical.
What really happens is…
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Hybridization
• The first step is a hybridization of the valence level
C
H H H H
forms…
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Hybridization
• The first step is a hybridization of the valence level
C
H H H H
The s and p orbitals hybridize to form sp3 orbitals. The sp3 designation shows one s orbital and 3 p orbitals make the new ones
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Hybridization
• The first step is a hybridization of the valence level
C
H H H H
The number of orbitals is preserved
(4 in 4 out)
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Hybridization
C
H H H H
All four bonds are identical. Methane is a symmetrical molecule.
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sp2 Hybridization
• When one p orbital is left out of the hybridization, it is used to make a double bond
• C
• H H O
…forms….
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sp2 Hybridization
• When one p orbital is left out of the hybridization, it is used to make a double bond
• C
• H H O
sp2 orbitals
Unused p orbitals
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sp2 Hybridization
• When one p orbital is left out of the hybridization, it is used to make a double bond
• C
• H H O
Makes the double bond!
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sp2 Hybridization
H
C O::
H
Carbon shares electrons in sp2 orbitals
Carbon & oxygen share electrons in unused p orbitals
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sp Hybridization
• When two p orbitals are left out of the hybridization, it is used to make two double bonds, or a triple bond
• C
• O O
…forms….
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sp Hybridization
• When two p orbitals are left out of the hybridization, it is used to make two double bonds, or a triple bond
• C
• O O
sp orbitals
sp2 orbitals
Unused p orbitals
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sp Hybridization
• When two p orbitals are left out of the hybridization, it is used to make two double bonds, or a triple bond
• C
• O O
![Page 86: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/86.jpg)
sp Hybridization
::O C O::
Carbon shares electrons in sp orbitals
Carbon & oxygen share electrons in unused p orbitals
![Page 87: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/87.jpg)
What is the hybridization of the carbon atoms in…
• CCl4
• H2CO
• C2H6
• C2H4
• C2H2
• CO
• CH3OH
• HCOOH
![Page 88: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/88.jpg)
Molecular orbitals
• Overlapping s orbitals, or hybridized orbitals makes a (sigma) bond
• The electron density is on the SAME line as the nuclei
s s
![Page 89: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/89.jpg)
Molecular orbitals
• Overlapping p orbitals, makes a (pi) bond
• The electron density is on a PARALLEL line to the line of the nuclei
p p + +
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Molecular orbitals
• A single bond is a bond
• A double bond is a bond, and a bond above and below the
• A triple bond is a bond, with two bonds– above/below and front/back
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For every bonding molecular orbital ( or ) an antibonding orbital is formed (* or *)
A bond is formed when there are more bonding than antibonding electrons
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• Count the and bonds in the following molecule. Label each bond as or
C C CC
O
C
H
H
H H
H
H
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• Count the and bonds in the following molecule. Label each bond as or
C C CC
O
C
H
H
H H
H
H
![Page 94: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/94.jpg)
• Determine the hybridization of the carbons and the oxygen atom
C C CC
O
C
H
H
H H
H
H
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The molecular aufbau order
1s2*1s
22s2*2s
22px22py,z
42py,z42px
2….
• For example:
• O2 has 16 electrons. Its electron configuration is:
O2 1s2*1s
22s2*2s
22px22py,z
42py,z2
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The molecular aufbau order
What is the electron configuration of…
N2
NO
Ne2
Remember: we couldn’t do a Lewis diagram!
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The molecular aufbau diagram
![Page 98: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/98.jpg)
VSEPR
• Valence Shell Electron Pair Repulsion
Theory
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VSEPR
• Valence Shell Electron Pair Repulsion
Theory
--pronounced “Vesper”
Electron pairs repel each other. Just as it says.
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• VSEPR is used to predict bond angles. The pairs will space themselves out as far as possible.
• A lone pair will take as much room as a bond AND MORE!
• Consider sp3 hybridization
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• AB4—like methane. Tetrahedral 109.5o
• AB3E—like ammonia. Pyramidal 107o
• AB2E2—like water. Bent 104.5o angles
• --the unshared pairs force the bonds closer together
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With sp2 hybridization:
• AB3—like carbonate. Trigonal planar: 120o
• AB2E—like nitrite. Bent: less than 120o
• ABE2—like O2(2 atoms, has to be linear)
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With sp hybridization:
• AB2—like carbon dioxide. Linear: 180o
• ABE—like carbon monoxide. Linear: 180o
• --but that’s just if you always follow the rules– like the octet rule.
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With dsp3 hybridization:
• AB5—trigonal bipyramid
• AB4E—seesaw
• AB3E2—t-shaped
• AB2E3—linear
• ABE4—linear
![Page 105: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/105.jpg)
With d2sp3 hybridization:
• AB6— octahedral
• AB5E—square pyramid
• AB4E2—square planar
• AB3E3—t-shaped
• AB2E4—linear
• ABE5—linear
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What is the shape of…
• All of the molecules and ions on the lab?
• I3-, SF6, XeF4, PCl5, IF5
-?
![Page 107: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/107.jpg)
Bond Energies
The energy it takes to break a bond is the amount of energy released as the bond is formed.
• --measured in kJ/mol
• --can be used to estimate Hrxn
• --can be absorbed or emitted as light.
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What is the Hf of NH3?
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What is the Hf of NH3?
• Write the reaction
N2 + 3H2 2NH3
![Page 110: Covalent Bonds and Molecules --you don’t mind a trick question, do you?](https://reader036.vdocument.in/reader036/viewer/2022062518/5697bf921a28abf838c8eb8e/html5/thumbnails/110.jpg)
What is the Hf of NH3?
• Count the bonds made and broken
N2 + 3H2 2NH3
1 NN triple bond, 3 HH single bonds broken
6 NH single bonds made
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What is the Hf of NH3?
• Look up bond energies, and find a total
N2 + 3H2 2NH3
1 molx941kJ/mol+3 molx436kJ/mol= 2249kJ used
6 molx393 kJ/mol=2358 kJ released
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What is the Hf of NH3?
• Find the difference, express as kJ/mol
N2 + 3H2 2NH3
2358 kJ-2249kJ= 109 kJ more is released, as 2 mol NH3 is produced,Hf=109kJ/2mol=-55kJ/mol
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• It’s an estimate.
• My book claims -46 kJ/mole.
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What is the heat of reaction for
2H2O2 2H2O +O2