Upload
others
View
9
Download
0
Embed Size (px)
Citation preview
12.9.11
• Warmup: Given a formula, how can you tell what kind of hybridiza?on the central molecule has?
• HW: page 189 ques?ons 1-‐5
• YGWBAT • Describe the rela?onship between bond order and bond energy
• Draw structures that include a “formal” charge
Prac?ce quiz – what kind of hybridiza?on?
• Review: – sp3 will have FOUR σ bonds! – sp2 will have three σ and one π
– sp will have two σ and two π
– Prac?ce (will do a REAL one next Monday) what is the hybridiza?on of the central atom?
CH4 N2 CO2 C2H4 C2H2 H2O
review
• If you don’t know hybridiza?on right away, then use SHAPE to help you:
• If shape of orbitals around central atom is tetrahedral, then sp3
• If shape of orbitals around central atom is trigonal planar, then sp2
• If shape or orbitals around central atom are linear, then sp
Warning: excep?ons to octet rule
• Groups 1A, 2A, and 3A do not form octets, so do not try to put four covalent bonds – or lone pairs – around, for example Li, Be or B
• 3rd period elements may use “expanded octets” – they may hybridize using d orbitals and seemingly have more than eight valence electrons! Example: SF4, SF6
Assigning a “formal” charge
• Look at the total number of electrons immediately around each atom – not the total number of “shared” electrons! Compare this to that atom’s atomic number
• Example: NO3-‐
More on drawing structures
• What if total number of electrons is more/less than expected or in some other way they are weird?
• Some atoms have a formal charge. If given charge, then #protons ≠ #electrons!
• Otherwise – the number of electrons drawn in your bonds and lone pairs cannot exceed the total # valence electrons of the member atoms
• Examples: • NH4
+ SO42-‐ NO3
-‐ N2O
Review: drawing atoms with charge
• We were working on N2O • Write out atoms in a line, draw Lewis structures for each individually first
• Then make your σ bonds between each atom (one between N and N, another between an N and O)
• Make your π bonds – BUT, you cannot leave electrons “unpaired”, meaning you can’t break up a lone pair!
• For each atom, look at number of total protons and electrons and assign charges if necessary
12.12
• What if you can draw more than one “good” Lewis structure for a molecule, which is the correct one?
• YGWBAT • Explain resonance in terms of bond length and energy
• Draw resonance structures • But first…
Quiz
• What is the hybridiza?on of the underlined molecule in each of the following:
NH3 CH2O
CO CCl2O
NO2 SiO2
Quiz -‐ answers
NH3 sp3 CH2O sp2
CO sp CCl2O sp2
NO2 sp2 (look at lone pairs!)
SiO2 Si acts just like C – this is sp!
Review: drawing molecules and stuff
• Change of plans, instead of learning forwards let’s learn backwards
• I will present structures, you come up with how we got there!
Acetamide – first, what’s missing?
O3 – anything missing? How is charge determined?
N2O
azide
12.13.11
• Con?nued torture – drawing molecules and why it doesn’t have to be that hard ;)
• Turn in HW (the page 189 stuff)
• YGNTBAT • Correctly draw structures and iden?fy lone pairs and charges so that we can please move on from this oh-‐so-‐fun topic
Some rules for common elements
Element # bonds when neutral
Alt # bonds
Then charge of
C 4 -‐-‐ -‐-‐
H 1 -‐-‐ -‐-‐
N 3 + lone pair
4 +1
O 2 + 2 lone pairs
1 -‐1
• For example, if you are using Nitrogen, give it three bonds – but if you have to give it four then it will be +
• But you must give it either three or four
• Also – you cannot have unpaired electrons, all electrons must either be part of a lone pair or a covalent bond
Draw structures – what is hybridiza?on?
NH3 CH2O
CO CCl2O
NO2 SiO2
Handout: lewis structures
12.16.11
Warmup: If NO3-‐ was a stool, could you sit on it?
(think about it for a second)
• Stem fair papers due Monday!
• YGNTBAT explain resonance and how it meshes with experimental measurements of molecules
Bond energy and bond length
• The energy required to break a covalent bond and form neutral atoms
• The more bonds between two atoms, the closer the nuclei are to each other
• The higher the bond order (number of bonds) between two atoms, the higher the bond energy
Bond lengths and energies – know rela8ve
Bond Bond length (pm)
Bond energy (kj/mol)
Bond Bond length (pm)
Bond energy (kj/mol)
C-‐C 154 346 C-‐O 143 358
C=C 134 612 C=O 120 732
CΞC 120 835 CΞO * 113 1072
C-‐N 147 305 N-‐N 145 163
C=N 132 615 N=N 125 418
CΞN 116 887 NΞN 110 945 **
* Very rare ** only two things in nature can break this: lightning and certain kinds of bacteria!
Resonance
• Nitrate – one double bond and two single bonds
• If you could take a ruler of some sort and measure the bonds, how would their lengths compare?
Resonance
• Concept developed by Linus Pauling (molecular orbitals, electronega?vity, and numerous protein structure solu?ons)
• A molecule has resonance if that molecule’s dimensions are intermediate between purely single-‐bond character and purely double-‐bond character
Conceptual clarifica?on
• Resonance is a way for a molecule to reduce its energy even further than bond forma;on allows – a molecule with resonance will be more stable (lower in energy) than a molecule with no resonance
Resonance
• Simpler defini?on: if more than one Lewis Structure may be drawn for the same molecule, it has resonance
In experiments, only structure D can be shown to exist!
Acetamide – resonance?
12.19
• In a general sense, what does resonance mean? How can you tell if a structure has resonance?
• YGRNTBAT • Determine appropriate resonance structures (if applicable) for a given molecular structure
The week ahead (2nd)
• Today: resonance review – toys! • Tomorrow: structures with toys
• Wednesday: lab on ionic vs. covalent
• Thursday: review • Friday: test
Things to look for
• Thought ques?on: can a structure have resonance if all molecules are sp3 hybridized?
• Can a structure have resonance if none of the atoms have lone pairs available or have double-‐bonds?
• Hint – look for giveaways like Nitrogen or Oxygen
review
O3 – resonance?
N2O – resonance?
Ethene – resonance?
More examples
Note that in all cases, shape, net charge, octets are all SAME. Only the pi electrons are different
diazomethane Sulfate ion
Cau?on!
• Resonance does NOT mean that the molecule is flipping back and forth between each structure.
• The molecule (imidazole shown below) IS all of the structures simultaneously
How to draw resonance structures
• Rules: 1. Do not violate the octet rule! 2. Do not break any single bonds 3. Charges in each structure must add up to same total charge (you can’t turn a neutral molecule into a charged one, and vice versa)
4. While it may be possible to place a posi?ve charge on an electronega?ve atom, this is unlikely!
Example – determining resonance structures
• H2O • CCl4 • H3PO4
• CH3CN
• NH2COCH3
• Last two are structural formulas, I’m showing you the central atom(s)
Showing resonance
• If a molecule has resonance, we indicate which bonds are effected
Another example
• First – where are the lone pairs?
• Can the electrons in a double bond be converted into a lone pair?
• Can electrons in a lone pair be converted into a π bond?
• Can I do this without breaking single bonds or viola?ng the octet rule?
• Charge conserved? Hybridiza?on conserved?
12.20.11
• What does the number of electric fields around a nucleus due to arrangement in space of those electric fields
• HW – see selec?on at end
• YGNTBATEW • Valence shell electron pair repulsion can predict spa?al structure
• How polarity of molecules relates to spa?al symmetry
VSEPR
• Developed by Linus Pauling • Means we can predict the geometry of atoms, because we can predict that orbitals (containing) electrons repel each other
• Hybridiza?on determines spa?al shape
• Can be used to predict structures of even very large molecules such as proteins (which contain thousands of atoms)
We’ve sort of done this…
• Every orbital containing a lone pair or molecular orbital contains electrons
• So every orbital repels every other orbital • The number of regions of high electron density determines the arrangement of these regions around the nucleus (see handout)
Some geometries Number of σ bonds Number of lone
pairs shape example
2 0 linear BeCl2
3 0 Trigonal planar BF3
2 1 Bent NO2-‐
2 2 Bent H2O
4 0 Tetrahedral NH4+
3 1 Trigonal pyramidal NH3
5 0 Trigonal bipyrimidal PF5
6 0 Octahedral SF6
12.21
• About the test – do you really want to take one this week? Do you think *I* really want to give you one? Let’s talk…
• How about a project?
• TYGAGTLA: • A set of rules (yay!) for determining orbital geometry
• Symmetry and polarity
Hybridiza?on and geometry
VSEPR geometry
Number of orbitals (do not count π bonds)
Hybridiza8on Example
Linear 2 sp BeF2
Trigonal planar 3 sp2 BF3
Tetrahedral 4 sp3 NH4+
Trigonal bipyramidal
5 sp3d PF5
Octahedral 6 sp3d2 SF6
Polarity
• A molecule, that due to asymmetry of electron distribu?on has a dipole
• Why? • What is a dipole? • Examples
• H2O • CHCl3 • CH2O
• How about…. • CH4
• CH3F • CH3COCH3
• What is the rela?onship of shape to polarity?
• What is the direc?on of polarity?
Determine polarity
• Rebuild (if necessary) the structures from yesterday’s ac?vity
• Determine if there is an asymmetry to the molecule
• If so, determine the polarity of the molecule
• Determine the hybridiza8on of the central atom
12.22
• Lab today – get your notebooks
• Objec?ve: what are the physical proper?es of covalent/ionic compounds?
• Work for over break – see website (blog sec?on) for details
Part A
• In notebook – write out your procedures into your notebooks
• Assemble ring stands with can lids
• Arrange a small amount of crystals of various compounds around the outside of your lid – or at least arranged so that they are apart from each other
Lab con?nued
• Some will begin to melt quickly – so have at least one person ready to record before you start
• Part B – use the glass s?rring rod, dis?lled water, and again, a small amount of your subtstances
• Part C – use your materials from part B – be sure to clean the probe in dis?lled water before tes?ng a new well
1.02
• What is the difference between intra-‐ and inter-‐?
• HW – page 207 1-‐6 (yes, including #6)
• Test Wednesday
• MSAADWTUBSNTLA:
• How and why different molecules interact
Hydrogen Bonding
• Differences to covalent bonding: • Characteris?cs -‐ • Bond energies – • examples
Dipoles and induc?on
• A polar molecule has a dipole moment • Polar molecules may awract other polar molecules; this is called a dipole-‐dipole interac?on
• Examples: CHCl3, acetone
Induced dipoles
• A polar molecule may induce a dipole in a non-‐polar molecule
• Electrons are in constant mo?on, and repel each other
• If close enough, a dipole may induce a dipole in a non-‐polar molecule
Induced dipole-‐induced dipole
• The reason why Helium (shown below) can be a liquid at very low temps
• Proximity induces dipoles in two molecules/atoms
• Called London Dispersion Forces
Hydrophobic interac?on
• Water hates non-‐polar molecules (such as lipids) because it cannot form Hydrogen bonds to them
• So water forms cages called clathrates around them, essen?ally a surface tension exists around oil droplets in water
Hydrophobic interac?on
This violates entropy!
2nd law of thermodynamics is sa?sfied – entropy must
always increase!
Key thought
• LDF requires proximity • No water (and hydrophobic effect) – no proximity, so no water -‐ no LDF
Examples from biology
• Cell membranes – hydrophobic effect condenses the lipids into balls, LDF awrac?on between individual lipid molecules
• Proteins – hydrophobic effect causes them to condense into structures which are then stabilized by Hydrogen bonds and LDF
• DNA – hydrophobic interior with LDF holding nucleo?des within strands together and Hydrogen bonds stabilizing the double helix together