Week 0
Q: On the Homepage, there are 3 discussion sessions {Thu. 12 - 1 pm (2F), Fri. 12 - 1 pm (3H), Fri. 2 - 3 pm (3I)}, should we come to all of them?
A: No. You should have been registered into 1 specific discussion session per week, please check back on your CCLE page to make sure which session you are in, 2F, 3H, or 3I.
Q: There is no lecture Zoom meeting links, what are the lectures functioned?
A: All lectures have been pre-recorded and uploaded by the professor, you can access them via CCLE as many times as you like at any time. This is to accommodate students who are studying in different time zones. However, the discussion sessions, professor and TAs' office hours are real-time Zoom meetings.
Week 1
Q: When are the discussion worksheets due?
A: Discussion worksheets are due by the date and time indicated on Gradescope.
Q: Are discussion worksheets graded for accuracy or completion?
A: Discussion worksheets are graded solely for completion. However, please make an attempt to try each question even if you're not sure of your answer. The worksheets are good practice for the midterm as the problems given will be somewhat similar.
Q: When will the study materials be uploaded on the website?
A: The discussion review materials or any additional notes will be uploaded on each Saturday. The solution to the discussion practice problems will be posted online on the following Wednesday.
Week 2
Example: Balancing Chemical Equations
Q: How to balance a complicated reaction, such as HCNO + NO2 ------ N2 + CO2 + H2O?
A: 1) Find if any UNIQUE elements, i.e., elements present only in one compound on both sides, set a reference, and balance them firstly! We do have TWO in this reaction, do you see H and C? Set H2O to be 1 mole as a reference, this immediately tells us that we have 2 moles of HCNO. Why?
2) Reaction is now: 2HCNO + NO2 ------ N2 + CO2 +1H2O; as 2HCNO is determined, how many moles of C there? Yes, 2! This tells us that 2 moles of CO2 we must get on the righthand side, right, right?
3) Reaction is now: 2HCNO + NO2 ------ N2 + 2CO2 + 1H2O; anything that goes to a single atom or homoatomic compound should be considered lastly! Because it is relatively "free" on the right-hand side, do you see why? So, instead of considering N, we care O now, we have 2*2+1 = 5 moles of O on RHS, for the LHS, we have 2 moles from 2HCNO, need 3 moles more, thus, we need 3/2 moles of NO2, correct?
4) Reaction is now: 2HCNO + 3/2NO2 ------ N2 + 2CO2 + 1H2O; now, let's deal with N! On LHS, we have 2+3/2 = 7/2 moles of N, so on the RHS, we need 7/4 moles of N2 to balance it, don't mess up here by putting 7/2!
5) Reaction is now: 2HCNO + 3/2NO2 ------ 7/4N2 + 2CO2 + 1H2O; we don't want to see any fraction coefficients, we finally make them all integers by multiplying both sides by a factor of 4 to get rid of denominators. Thus the answer turns to be: 8HCNO + 6NO2 ------> 7N2 + 8CO2 + 4H2O.
Have fun with such a balancing process !!
Week 3
Potential Energy Curve, Full Explanation (Laurie)
Q: Where to find the solution to the weekly discussion problem set?
A: To be consistent with the whole class, we will start to use the solution posted on CCLE from Week 2's problem set. If you have any questions or comments about the problems/solutions, please email the TA or LAs, we will respond through emails. If we see any representative problem(s), we will explain it(them) in the next discussion session. :-)
Q: It is urgent that I found my Zoom passcode did not work, or other technical issue, what might be the best way to resolve?
A: The best way is actually to email one of the LAs, if the discussion is about starting or has been in progress. The TA may need to concentrate on the first 20-min lecturing, may have a hard time to take care of emails. Thanks for your understanding!
Week 4
More on Coulomb's Force/Energy
Q: For VSEPR Geometries, how do we determine the correct molecular geometry based on the steric number?
A: When determining molecular geometry, it is always important to account for nonbonding electrons. The nonbonding electrons will occupy more space compared to the bonding electrons and will slightly change the molecule's geometry. For a summary sheet of VSEPR Geometries, click here!
Week 5
Q: For this week, our task is to finish Week 5 practice set, what about that one for Week 4?
A: We still need to finish the problem set for Week 4, the deadline is on Gradescope. It is mainly about Lewis Structures, Formal Charges, VSEPR, and Geometries. Please feel free to work in groups to solve problems. If you need help on the problem set, please email TA/LAs, or come to the TA's office hour!
Week 6
More on Quantum Numbers (Laurie)
Q: Why psi(x) is called probability amplitude function, but we take |psi(x)|^2 to get the probability distribution function?
A: Consider psi(n=1,x) = a*sin(b*x), this is a sine function, ranged between -a to a. Can a probability distribution go to negative? No! Also, a full wavefunction is actually a complex function when the spherical harmonics involved. Can a probability distribution become imaginary? No! |psi(x)|^2 is a complex conjugate of psi(x) times psi(x), this will avoid imaginary value, and automatically avoid all the negative parts.
Q: Does a probability distribution (density) function have a unit?
A: Yes! If we are in 1-D space, p(x) = |psi(x)|^2 carries the unit of 1/[L], where [L] is the length unit (pm, angstrom, nm, m, ...). If we are in 3-D space, p(x,y,z) = |psi(x,y,z)|^2 carries the unit of 1/[V], where [V] is the volume unit (pm^3, angstrom^3, nm^3, m^3, ...). However, dose a probability have a unit? No! Recall what we did, we either integrate from x_1 to x_2 p(x)dx, or we approximate p(x_0)*Delta(x), do you see we canceled out the dimensional unit (1/[L]*[L])? Same thing in 3-D, as we integrate p(V)dV or approximate p(V)*Delta(V), we canceled out the dimensional unit (1/[V]*[V]). Also, now, we know why it is called "density."
Q: For a simple particle-in-a-box model, is the interval the same as the length of the box?
A: Great question! The short answer is: it depends on the problem. In some instances, the interval only describes part of the box rather than the entire length. For example, on #38 for the discussion worksheet, the center of the box is at x = 2.11 Angstroms with the interval being from x1 = 2.06 Angstroms and x2 = 2.16 Angstroms. In this case, the interval only covers 0.1 Angstroms of the box, so you'll need to determine the entire length of the box based on the information they give you. (Hint: the distance for the center of the box is extremely useful!)
Week 7
Q: What is the Lanthanide Contraction?
A: There are a few ways to think about lanthanide contraction: In terms of periodic trends, moving left to right across a row decreases size. At the same time, since 4f is so deep inside the electron shell (meaning we aren't adding to the outer layer of electrons, as starting with our first lanthanide, Cerium, we are in the 6s shell!) we are not adding anything to the outside of the shell which would be the general way that an element gets larger (think about creating an anion, does the radius get smaller or larger?). This in consequence makes the 3rd row transition metal elements almost the same size as the 2nd row elements, as even though they are containing far more electrons, not many more were added to the valence while our Z effective increased at a faster pace than the shielding of the electrons. (Gregory)
Q: Why Fe(2+) is created by removing 2 4s electrons, instead of 3d?
A: 4s would fill up first because it is closer to the nucleus than 3d. However, after filling up the 3d orbital, it is easier (higher probability) to find the 4s orbital further out, so it will get oxidized first. A radial distribution graph is shown here. (Gregory)
Week 8
Have a nice and safe Thanksgiving Holiday!!! :-)
Week 9
Example: Organic Molecule and Hybridization
Q: How to draw a molecular orbital (electron correlation) diagram for HF?
A: (1) F is more electronegative than H, 2p is lower energy than the H 1s;
(2) the 2s from F is non-bonding because the H 1s and F 2s orbitals are too far apart in energy to interact;
(3) the 2px and 2py from F are non-bonding because they do not have correct symmetry to interact with the H 1s orbital.
Week 10
Q: Where could I get some extra practice?
A: If you want more practice or a "quick" review of the material, I'd highly suggest either Khan Academy or Master Organic Chemistry, especially the latter as it typically outlines common misunderstandings students may have and will have cover more information than Khan Academy. As always, if you have any questions, feel free to email Barry or the LAs and we'll address them as soon as we can. Good luck on all your finals, everyone! We know you'll all do great (and, hopefully, we'll see you in Chem20B next quarter)! (Anson)
Finals Week
Good Luck on all of your Finals!
Don't forget the end-quarter Evaluations!
Stay Safe and Take Care as Always!
Merry Christmas and Happy New Year, in advance!
Thank you all the Learning Assistants,
and thank you all the students!
See you all Next Year!
Barry (Yangtao) Li
12/12/2020