CHEMISTRY COMMUNITY

Melody Wu 2L

Vienna by Billy Joel is my comfort song :) (Ben Platt does a really good cover of it too!)

Melody Wu 2L

Reactants bind to catalysts. So when a catalyst is saturated, it means they're all already bound to a reactant and thus no more reactant will bind to it (there is excess reactant and not enough catalyst). You can think of it as like a "buddy system" and all the catalysts have already found...

Melody Wu 2L

Watching Top Chef gets me through the week :')

Melody Wu 2L

I'm part of the triathlon club, APHC (Asian Pacific Health Corps), and SCOPE Patient Health Advocates! :D How was Foundations online? I heard amazing things about it and might try it out in the fall if we're in person!

Melody Wu 2L

I usually work through all of them and check my answers after each one that I do (or each section if the questions are short). If I get it wrong, I'll first look through and see if I made any calculation errors and then check the solution manual to make sure I did it right. Also, if I get a question...

Melody Wu 2L

Definitely focus most of your time on textbook questions and really understanding every single question + then studying the concepts you aren't too comfortable with. Beyond that, attend as many of the final review sessions by TAs/UAs during week 10 to get familiar with solving all types of questions...

Melody Wu 2L

Atomic Habits by James Clear is amazing for learning how your brain works during habit formation and the science behind motivating yourself. Definitely a really quotable book. Also, Thinking Fast and Slow by Daniel Kahneman is a fascinating book on psychology research but it's very digestible for th...

Melody Wu 2L

I think it's normal to feel nervous - just shows that you care about the work that you're doing which is good! It's inevitable and for me it doesn't really fully go away until the scores come out. It's one of those things that's out of your hands after you turn it in, and the lack of control over th...

Melody Wu 2L

I second Google Calendar - I write down every single event I have right after I get the announcement so I don't forget and basically have it open on my desktop the whole day. On Sunday, I write down my schedule for the upcoming week in hour/2 hr chunks depending on what events I have + then schedule...

Melody Wu 2L

Last quarter I definitely wished I had gone through all the textbook problems one more time. A couple questions on the 14A final that I missed, I would have gotten right if I had reviewed the textbook problems again! Make sure you understand each and every one of them, especially the more difficult ...

Melody Wu 2L

You can never go wrong with Khan Academy & The Organic Chemistry Tutor :) Beyond that, taking the break before spring quarter to relax your brain for a bit is the best preparation IMO! Tests are highly dependent on what the professor specifically teaches rather than just the general material, an...

Melody Wu 2L

TOP CHEF !! Binging all 17 seasons this year haha. Besides that, my comfort TV shows are The Office, Friends, New Girl, Psych, and The Good Place! Really fun and lighthearted shows that make me laugh amidst the exhausting midterm season :')

Melody Wu 2L

I'm craving some apple pie rn :'))

Melody Wu 2L

^^I think that's right. Also Lavelle often rounds people up if they're close at the end of the quarter!

Melody Wu 2L

This quarter I took on a pretty heavy workload so I definitely understand the burnout tendency. Last quarter since I only had 3 classes, it was easy to just study a bit for like 2-3 days before the test but this quarter that did not work out very well. I think the most important thing is to be reall...

Melody Wu 2L

I'm planning on taking Chem 14C, Chem 14BL, LS7B, and a class for my minor :)

Melody Wu 2L

Where can I find UA worksheets if I can't make it ? Some of them post it on chemistry community! If you search up "Matthew's Workshop," he has his worksheet posted for every week up until now with the answer key as well. If you search "Brooke's Workshop," I believe she posted he...

Melody Wu 2L

I second UA sessions. Though I noticed the test questions look a little different on our online midterms, the UA sessions really help you understand the concepts on a deeper level so you're essentially equipped to tackle any type of question that comes your way. Also, if you know every textbook ques...

Melody Wu 2L

I've been feeling a bit nervous too because my schedule has been really hectic lately. I suggest going to UA sessions to watch how they solve the problems step by step! They are also really good about answering literally every question you have, and I've learned a lot about how to approach problems ...

Melody Wu 2L

I think it depends on what you're looking to get out of it. If you really value the in-person lab experience, then it's perfectly acceptable to wait till we're in person. A lot of my friends waited until they were juniors/seniors to take it. I heard labs online are much less time-consuming so if you...

Melody Wu 2L

Don't worry I think a lot of us are in the same boat. Last quarter's final I made some stupid mistakes and I felt really awful, but at the end of the day it turned out fine with the extra 10 points and the tiny bit of rounding at the end of the quarter. We still have another midterm and a final to g...

Melody Wu 2L

If I look at it and don't really know how to solve it, I'll skip it and come back later like others have said. I found that subconsciously the problem kinda ruminates in the back of my head and I have a better idea on how to solve it on the way back. It's also much less stressful this way to have mo...

Melody Wu 2L

I'm taking BL next quarter and was considering doing CL over the summer since I heard labs in person take up a lot of time.

Melody Wu 2L

I haven't taken it yet, but next quarter I'm planning on taking chem 14BL, chem 14C, ls7b, and potentially a class for my minor. I heard labs are a lot easier online since you don't need to do the experiment yourself, so I think you're fine taking multiple STEM courses. :)

Melody Wu 2L

I believe it will mostly be the rest of thermochemistry and thermodynamics, so I'd focus on those textbook sections. Lavelle will prob send out details later.

Melody Wu 2L

I'm not sure, but since all discussion sections are cancelled I wouldn't count on it. It looks like some TAs hold office hours on friday though, so you could email them to double check. I'd go either today or tomorrow to ask all your questions just in case!

Melody Wu 2L

I think you can take labs whenever :) I heard some people push them out pretty far for whatever reason too. As long as you take BL before CL you're good!

Melody Wu 2L

No need to memorize! Just make sure you know how to convert between Ka/Kb/pKa/pKb and I think you'll be good :)

Melody Wu 2L

On the midterm, they'll give you all the information you need. I doubt there will be a table like in the textbook; it's much more likely that it'll just explicitly state what Ka/Kb/pKa/pKb is. I would just know how to convert between pKa and Ka and know how to use all the equations we learned so far...

Melody Wu 2L

I think it just depends on what is given in the problem (match the same units in the question). If nothing is given, I don't think it matters which unit you use. The good thing is that tests are multiple choice so it won't matter too much on test day :)

Melody Wu 2L

Victor's explanation is really great. If you want more explicit examples, check out table 6D.1 and 2 in the textbook! They show the different ions that contribute to making a salt solution acidic or basic. Acidic solution: - Conjugate acids of weak bases (ex. NH4+) - Small, highly charged mental cat...

Melody Wu 2L

UA workshops are great for practice questions to solidify your ability to apply the concepts. In terms of questions that replicate the test the most, last quarter textbook problems were the most similar to the test. If you know how to do all of those and explain them to someone, I think you're in a ...

Melody Wu 2L

I believe you used the approximation when you were finding x, which is causing the error. Since Ka is larger than 10^-4, you can't use the approximation and have to use the quadratic formula.
Thus you would do 0.00743 = x^2/(0.143-x) to get x and I think the rest of your steps are right :)

Melody Wu 2L

I also recommend doing as many problems as possible--textbook problems, UA workshops, etc. Keep track of the ones you got wrong and really take the time to understand why you got them wrong, whether it's a careless mistake, solving for the wrong thing, knowledge gap, etc. I find it really easy to ma...

Melody Wu 2L

Just to add on, if the percentage ends up being > 5% then you need to go back and redo the calculation without the approximation (add back the -x and use the quadratic formula)!

Melody Wu 2L

Since last quarter was the first quarter of college, I found that I had a lot more energy and was much more readily involved in things. This quarter is a little tougher for motivation. The main thing for me was learning to prioritize--going to less UA sessions in order to finish other more time sens...

Melody Wu 2L

I answered your other question and it's a similar type of problem. Since K is very small (<10^-4), you can assume x = 0 when you plug the equilibrium concentrations into the K equation. Since K is so small, the change in concentration is negligible so we can ignore it. It's nice because it simplifie...

Melody Wu 2L

Since K is very small (<10^-4), you can assume x = 0 when you plug the equilibrium concentrations into the equation (aka plug in 0.522 instead of 0.522-2x and 0.633 instead of 0.633-x)
K = [SO^3]^2 / ([SO2]^2)([O2]) = (2x)^2/ (0.522)^2(0.633)
x = 4.94 x 10^-6

Hope this helps!

Melody Wu 2L

I got the same answers as you. The initial concentration for [Cl2] = 1 x 10^-3. Since the problem calls for two sig figs, if you round up the equilibrium concentration (that you calculated as 9.95 x 10^-4), you get 1 x 10^-3. I was confused at first too because it looked like I got a different answe...

Melody Wu 2L

I also think there might be a typo in the textbook and H2 should be on the right side of the reaction so it's balanced (all the other reactions in other parts appear to be balanced).

Melody Wu 2L

There may be a better way to go about this but how I think about it is in terms of Le Chatelier's Principle: the composition will change to minimize the increase in pressure. (Keep in mind that equilibrium shifts towards the direction that has lower concentration.) If they're on the same side of the...

Melody Wu 2L

I'm a psychobiology major and I think it'll be a fun major. I've always enjoyed reading psychology books/articles and loved my AP psychology class. I also wanted to be a STEM major though so psychobio was a perfect combo of the two areas I wanted to learn more about. My teachers and mentors always t...

Melody Wu 2L

I believe it should be okay since discussions technically aren't mandatory. I'd join and if you wanna be sure, maybe just PM the TA to double check if it's okay.

Melody Wu 2L

Stability is basically just a measure of whether there are more products or reactants at equilibrium (which side is favored). If you look at Cl2 vs. Cl, the amount of Cl that forms is pretty small in comparison to how much Cl2 you start off with (Cl2 is roughly 10^-3 and Cl is 10^-5). This means the...

Melody Wu 2L

If you can do all the textbook problems, you're pretty solid. If you get one wrong, make sure you take the time to really figure out WHY you got it wrong. See if it was just a computational error, or a gap in knowledge. Going to UA workshops also were really great for getting a solid handle on the c...

Melody Wu 2L

I think the most important thing to know is that if K is very small (<10^-4), then you can approximate x = 0 when you plug it into the K=[Products]/[Reactants] equation. It simplifies a lot of complex calculations and helps to prevent computational errors :)

Melody Wu 2L

To add on, K is a constant. It describes only the equilibrium concentrations/partial pressures. Q is used whenever a chemical reaction is not in equilibrium (aka if it doesn't match the given K value). Since both Q and K are calculated as [Products]/[Reactants], if Q is bigger than K, it means it ha...

Melody Wu 2L

The UAs are absolutely amazing! The workshops were the most helpful for me because it was all just practice questions. Michael Nguyen does really good mini recap lectures at the start of his workshops that I really like, and Justin/Matthew/Brooke/Kate are amazing at answering questions/providing pra...

Melody Wu 2L

I looked around and couldn't find it either. Dr. Lavelle will likely post it on CCLE sometime soon before his first office hours :)

Melody Wu 2L

For the equilibrium constant, you want to use the equilibrium concentrations because those are constant. The initial molar concentrations can be different, but will always adjust back to the equilibrium quantities. It's not guaranteed that the initial always equals the equilibrium, so it's safer to ...

Melody Wu 2L

They should be an hour I believe.

Melody Wu 2L

I actually don't think it's mentioned in the textbook--I did a quick search and nothing showed up. I recommend watching Lavelle's lecture again or stopping by a UA session/TA office hours for clarification. Here's basically what you need to know: Cisplatin is a chemotherapy drug that forms a coordin...

Melody Wu 2L

Hi! You're basically writing a chemical equation where the given compound is the product. For example, in part (a) you are given potassium bromide (KBr) as the product. The acid and base reactants, are HBr and KOH respectively. Basically, add an H before the anion and add an OH after the cation (the...

Melody Wu 2L

Tables 6D.1 and 6D.2 in the textbook are really helpful -- they lay out which cations and anions are acidic/neutral/basic. Usually either the cation or the anion is neutral, and whatever the other is is what the salt is classified as. For example, in NaF, Na is neutral and F is basic, so the salt is...

Melody Wu 2L

The shape is bent because there's two bonding pairs and one lone pair. As for the bond angle, since there are three regions of electron density, the "base" shape is trigonal planar (120 degree bond angles). Since it's bent, one of the three bonding pairs turns into a lone pair, which pushe...

Melody Wu 2L

After the first midterm, I learned how to study better and did well on the second midterm. I'm feeling pretty good overall about the content but a little daunted for the amount of material I need to review for the final. I'm sure we'll all do great though. Good luck everyone :D

Melody Wu 2L

Hi! I suggest rereading section 2F.2 because it explains it really well and in-depth. tl;dr Consider carbon in CH4. Carbon has only 2 valence electrons, but forms four bonds (tetrahedral shape). Why? One of the paired electrons in the 2s orbital is "promoted" to the empty 2p orbital, creat...

Melody Wu 2L

en is shorthand for the ethylenediamine ligand, which is bidentate. Since Br has one bonding site and there are two of them, plus the two bonding sites on the ethylenediamine ligand, there's a total of 4 bonding sites. Therefore the coordination number would be 4.

Melody Wu 2L

S-character refers to the ratio of the s-orbitals in the hybrid orbitals, or the contribution of sigma bonds in the hybridization. For example, in an sp orbital, since there is one s-orbital and one p-orbital, the s-character is 1/2 or 50%. In an sp^2 orbital, the s-character is 1/3 or 33%. In an sp...

Melody Wu 2L

If you look at the diagram he put in the slides, the porphyrin ligand is that whole plane with the pentagonal shapes. The four nitrogen atoms are just the four binding sites on it (hence it's tetradentate) that bind onto the Fe cation, but the ligand is the whole massive structure surrounding the Fe...

Melody Wu 2L

Hi! I found UA sessions to be extremely helpful for understanding concepts and knowing what exactly you should know for the test. I go to about 2-5 per week (may not stay for the whole time but if I can make some of it I still try to go get the worksheet). Beyond that, definitely read the textbook, ...

Melody Wu 2L

This problem also took me a bunch of tries. For both molecules, the BONDS are POLAR because the two atoms on either side of the bond have different electronegativities. The only time a bond would be nonpolar is if the two atoms on either side are the same (or if the difference is very small at least...

Melody Wu 2L

Yes resonance always implies the delocalization of electrons. I think this lecture was just tying in how delocalization can also be explained using hybridization. All of it is the same, just different ways to look at it!

Melody Wu 2L

Yes, I believe they do count as a region of electron density when determining hybridization. Those single electrons still occupy an orbital, although the effects on the shape may be different since it's a single electron instead of two.

Melody Wu 2L

Make sure you're scrolling down, there's a total of 5 parts to that question :)

Melody Wu 2L

Remember that sp hybridized orbitals are present in molecules with a linear molecular shape (two outer atoms, no lone pairs + symmetric). Some good examples to look into are BeCl2 and CO2, since they only have two regions of electron density and no lone pairs :) Hope this helps!

Melody Wu 2L

Just a quick thing I wanted to add. I found that when I try to rush the problems that ask "which orbital will an electron be taken from" I accidentally take it from the d-orbital because it's usually what's "added last." So quick tip for these is to write out the electron configu...

Melody Wu 2L

You want to look at intramolecular and intermolecular forces. 1. Ionic < Covalent character (compare differences in electronegativity) 2. London Forces < Dipole-Dipole < H-Bonding (drawing out lewis structures can help) Also for two molecules with only London dispersion forces, see which has a large...

Melody Wu 2L

Dipole moments arise out of asymmetry, where there are partial positive and partial negative regions that pull on each other. If they cancel out, it means the molecule is symmetric and thus nonpolar, so I believe you would say dipole-dipole forces do not play a role.

Melody Wu 2L

I go on long bike rides and do yoga :D I love hiking too but haven't been in a long time b/c of COVID :(

Melody Wu 2L

I take a moment to take a few deep breaths and reassure myself that I did put in the work studying so I will be okay. It also helps me to skip past a question if I can't figure it out so I'm not stressing with the time constraint + I find that at the end when I go back I'm in a clearer headspace sin...

Melody Wu 2L

I'm saving 14B for second pass as well since the lectures are most likely going to be asynchronous again (or at least recorded) and I have other high demand classes like LS30B :)

Melody Wu 2L

One of the UAs said a good way to remember the 3 elements that can hold more than 8 electrons (P, S, Cl) is to think of them as the "pascal" elements ( P a S C a l . No tied meaning there but just a helpful mnemonic so to speak. For the ones that hold less than eight, Boron is the most com...

Melody Wu 2L

Yes, there are always exceptions to the trends. Fluorine has 7 electrons and needs 1 more to have an octet, so the electronegativity is really high (b/c it wants an electron really bad) vs. Helium has its 1s orbital filled with the 2 electrons so it's satisfied. For me, conceptualizing the elements ...

Melody Wu 2L

It is in covalent bonds I believe, because the premise of dipoles is that the overall atom is neutral but has regions of slightly positive and slightly negative charge. If it were an ionic bond, it would be formed from two ions with a clear positive charge and negative charge on then.

Melody Wu 2L

I'm feeling pretty good. Really busy and I understand why people kept emphasizing how fast the quarter goes by... glad we have Wednesday off though to get some much needed rest/catch up time :)

Melody Wu 2L

dana hu 1B wrote:Did Lavelle announce the date of the second midterm?

It is during your week 7 discussion section :) All the test dates are listed on his website https://lavelle.chem.ucla.edu/class-websites/chem14a/ under the "Test and Exam Schedule" hyperlink.

Melody Wu 2L

For 1, the formal charges of Lewis Structure A and B were less than Structure C, which made them more stable and more likely to contribute to what the atom actually looks like (since atoms want to be stable). For 2, the middle two options are incorrect because of the word "overwhelmingly."...

Melody Wu 2L

Uyenvy Nguyen 3C wrote:Do people usually take Chem 14B with Chem 14BL? I'm planning on doing that, but I'm not sure which one will fill up faster.

Chem 14B is a pre-req/co-req for Chem 14BL so either way you would have to enroll in 14B first.

Melody Wu 2L

It depends on what classes you're taking. I know Chem 14BL fills up fast, so if you wanted to take that with Chem 14B then I'd sign up for both first pass b/c 14B is a co-requisite for 14BL. I need to take LS30B though and that fills up faster so I'm using my first pass on it, but I'm not sure if I ...

Melody Wu 2L

Right now this unit is more focused on the positions and transitions of electrons within the atom, so "energy" should be related to the electron since it's the one that is typically changing.

Melody Wu 2L

I agree with the people above, but sometimes I get confused if I write down too much and end up forgetting a step like squaring a number. I usually just stop periodically as I'm solving to make sure I'm not missing something before moving on to the next step. Slowing down a little helps save time at...

Melody Wu 2L

The energy required to absorb radiation is the work function, which is given in units of J/mol. There are 6.022E23 photons in one mole (Avogadro's #) so all you need to do it divide by Avogadro's # to find J/photon.

Melody Wu 2L

Bike rides, yoga, guitar and watching the office :)

Melody Wu 2L

Lyman: UV and n=1
Balmer: Visible Light and n=2

Melody Wu 2L

You need to first find the energy of the photon based on your answers from part a and b. So plugging your values into E_{photon} = E_{k} + \phi \frac{hc}{\lambda } = \frac{1}{2}mv^2 + \phi \frac{hc}{\lambda }= \frac{1}{2}(9.11\cdot10^{-31}kg)(3.6\cdot 10^{6}m/s)^{2} + 1.66\cdot 10^{-...

Melody Wu 2L

This is actually G25, I believe. So first, you want to find the molarity after the first dilution so 0.01mol/L x 0.01L = 0.001 mol X 0.001 mol X / 0.02 mL = 0.05M Then, since you are repeating this process 90 times, it'll be 0.1M (0.5)^90 = 8.078 x 10^-29 mol/L To find the number of molecules, you w...

Melody Wu 2L

Not sure but I'd assume after everyone is done so no answers can be discussed.

Melody Wu 2L

Plug your values into the equation \Delta E = E_{final}-E_{initial}=\frac{-hR}{n^{_{f}^2}}-\frac{-hR}{n^{_{i}^2}} where n=5 is your final and n=6 is your initial (since it's emission, it would be moving down energy levels). Then you plug the energy into \lambda = \frac{hc}{E} and you should get 7.47...

Melody Wu 2L

I think for the Rydberg Equation, the condition is that n2 > n1. Since the equation is solving for frequency and frequency has to be positive, you want the right hand side to be positive. The only way for that to happen is if n2 is greater than n1. I haven't had a lot of experience with Rydberg beca...

Melody Wu 2L

Sorry I saw this super late, but I believe it's negative because the electron is emitting the energy. Emit = negative Absorb = positive (longer explanation: Lavelle mentioned that the reference point is when the electron is unbounded or n=infinity and the energy is 0. Therefore as it drops down the ...

Melody Wu 2L

Atomic spectroscopy has to do with just the properties of atoms and molecular spectroscopy involves looking at the molecule as a whole. Atomic spectroscopy is used to determine the composition of an atom and the transitions of electrons between energy levels, etc. but molecular spectroscopy is more ...

Melody Wu 2L

At this stage I don't think we need to know how to determine it, just know that electrons in the same sub-shell must have opposite spins. Also know that an up arrow is +1/2 and a down arrow is -1/2 in an orbital diagram.

Melody Wu 2L

The q1q2/r equation (Coulomb's Law) basically just shows us that as the distance between two charges increases, the electrostatic potential energy decreases and as the charge increases, the electrostatic potential energy increases. So for example, Mg2+ would have more electrostatic potential energy ...

Melody Wu 2L

That makes sense but I'm not exactly sure if that's a factor as well. The p orbital is farther away from the nucleus than the s orbital, so it has more energy because energy is required to separate opposite charges. More distance = more energy to keep the atom together if that makes sense.

Melody Wu 2L

It depends. If the electron is emitting energy, then it is moving to a lower energy level so n1 would be the final. If the electron is absorbing energy, then it is excited to a higher energy level so n1 would be the initial.

Melody Wu 2L

$E_{k}=\frac{1}{2}m_{e}v^{2}$

^This is the equation, where the mass of the electron is a constant (9.109x10^-31 kg). I believe he will provide the mass and other constants for the exams so no need to memorize them.

Melody Wu 2L

We are working on the quantum world section all through week 3 as well, so as long as they are completed by next weekend I think you should be fine :)
The textbook problems aren't technically due so you can do them on your own time. It's good extra practice before the midterm during week 4!

Melody Wu 2L

I did the same thing I think as the other person but wrote it out using the equation editor so you could see it better :) E=h\nu =6.626\cdot 10^{-34}Js(1.14\cdot 10^{14}m/s)=7.55\cdot 10^{-20}J E_{4}=\frac{-hR}{n^{2}}=\frac{-6.626\cdot 10^{-34}Js (3.29\cdot 10^{15}Hz)}{16}=-1.36\cdot...

Melody Wu 2L

I'm a current student and the modules (audio visual focus topics) have been really helpful for understanding the material prior to the lectures, so I can use the lecture to solidify the info in my mind instead of learning it for the first time. Practice problems are the most important thing in any S...

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