chem. 1b – 12/1 lecture. announcements i exam 3 results –average was 66.3% –distribution...

Chem. 1B – 12/1 Lecture

Announcements I

• Exam 3 Results– Average was 66.3%– Distribution (narrower than

other exams)– Problems where students did

poorly (electrolysis problem, corrosion problem, bonus problem, most of Chapter 24 questions)

– On the rest of the Electrochemistry part, students did reasonably well

– Key has been posted

Score N

100 1

90s 4

80s 19

70s 39

60s 30

50s 23

<50 16

Announcements II

• Class Average and Grading– Post Exam 3 Ave = 71% (excluding students not

taking Exam 3, but including true score – see last bullet)

– If we were finishing the semester, I would probably reduce cut-offs by 1 or 2% (but I need to wait on homework, rest of lab, and final exam scores before doing so)

– Blackboard score is mostly correct except:• doesn’t include in-class bonus points yet (scores could

go up by up to ~0.8%)• some have non-entered scores (instead of zeros) – then

blackboard’s score is too high• lecture only students (can email me to find true score)

Announcements III

• Mastering Questions– Chapter 24 included several problems we

hadn’t covered yet. I expect to treat these and problems with errors as bonuses, but will need to see scores from Mastering so grading is fair

• Lab– Done with Quizzes– Experiment 12 was due– Finishing Scheduled Lab work– Lab Final on Wed./Thurs. next week (only on

experiments covered in class)

Announcements IV

• Make-Up Quiz– In lecture on 12/8– Five Question – Multiple Choice – on

Chapter 24 (any topics possible)– Optional; will replace your lowest quiz

score (if higher)

• Today’s Lecture– Coordination Compounds (Chapter 24)

• Crystal Field Theory (we are skipping applications subsection)

– Organic Chemistry (Chapter 20)• Carbon – carbon bonds• Alkanes

Chapter 24 Transition Metals

• Coordination Complex – Bonding Theory – Review from last time

– In octahedral binding, because the ligands bring the electrons, lower energy results when the binding axes orbitals (dz2 and dx2-y2) are UNFILLED

– Or alternatively, the ligands cause a split in energy levels of d shell orbitals

E

Free atom Metal in octahedral complex

On axisOff axis



– How does d orbital splitting affect coordination complexes?

– Electrons go to low energy states first

– Example: [Cr(CN)6]3- has 4 – 1 = 3 d shell electrons – they should occupy the three off-axes orbitals

On axisOff axis



– When we add more than 3 electrons (e.g. 4 electrons), there are two possibilities:

• fill bottom orbitals first• or go to top orbitals

– Filling depends on gap (larger leads to “low spin” states – first shown, while smaller leads to “high spin” states – second shown)


• Coordination Complex – Bonding Theory – Role of Ligands

– Particular metals, such as Fe, can form complexes with different properties (e.g. colors or magnetic properties) depending on ligands

– Ligands affect size of gap– “Strong” ligands result in large gap, while

“weak” ligand results in smaller gap (with the idea that more tightly held electrons will overlap more with d shell electrons)


• Coordination Complex – Bonding Theory – Role of Ligands and Metal

– Ligand Strength (see text for full range)

– Metal Ion Strength (greater charge, Fe3+ vs. Fe2+, increases )

strongest

CN-

weakest

NH3 Cl- I-H2O

Weak Field Ligands – tend to give high spin states


• Coordination Complex – Magnetic and Light Absorbing Properties

– Magnetic Properties:• Compounds or atoms with unpaired electrons

are magnetic (since half filled shells will have electrons with the same spin)

• Example: Fe [Kr]4s23d6 will have 4 unpaired electrons and is magnetic

• Other metals, e.g. Zn (d10), are not magnetic

E

4s

3d


• Coordination Complex – Magnetic Properties – cont.

– Octahedral Complexes will have d electrons split into to energy states by ligand field

– Large gap complexes give rise to “low spin” states that are less magnetic vs. “high spin” states

– Examples: [Fe(CN)6]4- vs. [Fe(Br)6]4-large small


• Coordination Complex – Light Absorbing Properties

– Gap between on- and off-axes d orbitals can also lead to transitions between two states

– Example: [Cr(CN)6]3- • Absorption of light causes electronic

transition from low energy to high energy state:


• Coordination Complex – Light Absorbing Properties – cont.

– Many coordination complexes absorb visible light (green light ~ 525 nm or E = hc/ = 3.8 x 10-19 J)

– The larger the gap, the greater the E, and the smaller the value energy

– Visible colors go ROYGBIV (red, orange, yellow, green, blue, indigo, violet – from longer to shorter wavelength)


• Coordination Complex – Light Absorbing Properties – cont.

– Example: [Co(H2O)6]2+ (used for the terrible Drierite color demonstration)

– Color is pink/purple (but pink is red + white = seen color because complex absorbs other colors)

– Using color wheel (text) expected absorbance is in green (measured in Chem 31 as 510 nm)

– Color wheel used because we see reflected light

– E = ?

– If we switched to NH3 as a ligand (stronger), what shift would be expected?


• Coordination Complex – Other Geometries

– Besides octahedral geometries, tetrahedral and square planar geometries have different overlaps with d orbitals resulting in different d orbital splitting

– In tetrahedral complexes, the complex can be positioned (see Fig. 24.17) where ligand bonds interact with “off-axis” d orbitals (dxy, dxz, and dyz) making these orbitals higher in energy and on-axis d orbitals lower in energy (however with small values and high spin states)

Metal in tetrahedral complexOn axisOff axis


• Coordination Complex – Other Geometries

– In square planar geometry, overlap is most with dx^2 – y^2 (but is more complex as shown below)

– Square planar geometry is common for d8 ions in which dx2 – y2 orbitals are unoccupied (low spin)

Metal in square planar complex

dx2 – y2

dxy

on axis and off axis in xy plane

dZ2

dxz dyz


• Questions1. Which two d orbitals do octahedral

complexes overlap with the most?2. Which d orbital is there the greatest overlap

in square planar complexes?3. Give the number of unpaired electrons for

the following metals in octahedral complexes for low spin states/high spin states

a) Fe3+ - octahedral b) Co2+ – octahedralc) Cu2+ - tetrahedral d) Mn3+ - octahedral


• Questions – cont.4. Ti3+ is purple while Ti4+ is uncolored. Explain.5. For which of the following metals in octahedral

complexes does the ligand NOT play a role in the number of unpaired electrons?

a) Mn2+ b) Fe3+ c) Co2+ d) Ni2+

6. [Fe(en)3]3+ undergoes a ligand replacement reaction and forms [FeX6]3-. The new complex absorbs at shorter wavelengths. What do we know about the strength of X as a ligand?

Chapter 20 Organic Chemistry

• Introduction– Organic Chemistry is a major area of

study (we offer 7 organic chemistry classes at the undergraduate level)

– In ~1 week, we only have time to introduce basic principles of organic chemistry


• Overview– Nature of Carbon – Carbon Bonds– Hydrocarbons (structure, naming and

isomers)– Reactions– Aromatic Hydrocarbons– Functional Groups


• Nature of Carbon – Carbon Bonds– Carbon is one of the few elements that form

fairly stable bonds with itself– Most alkanes, while combustible in air (more

stable as CO2 + H2O), have negative Gfº

– Carbon “likes to” form 4 bonds ([He]2s22p2, but mostly forms sp to sp3 hybrid bonds)

– Simplest hydrocarbon is CH4, methane, in which sp3 hybridization occurs (tetrahedral geometry)


• Nature of Carbon – Carbon Bonds– As carbon – carbon bonds are common,

in alkanes, they also occur with sp3 hybridization (tetrahedral for each C atom)

– Example alkane is ethane: CH3CH3


• Nature of Carbon – Carbon Bonds– Hydrocarbons containing double bonds

are known as alkenes– Hybridization is sp2 (see ethene

structure below – drawn in 3D)

C

H

H

C

H

H

remaining p orbital forms bond


• Nature of Carbon – Carbon Bonds– Simplest alkene is ethene (also called

ethylene and structure is CH2=CH2)

– alkenes are hydrocarbons with one or more double bonds

chem. 1b – 12/1 lecture. announcements i exam 3 results –average was 66.3% –distribution...

Documents