CHEMISTRY COMMUNITY

Bethany Yang 2E

Why is this true? I think I was thinking large equilibrium would mean products are favored, but would this be a little different because it is talking about the equilibrium CONSTANT? "For a reaction with a very large equilibrium constant, the rate constant of the forward reaction is much larger...

Bethany Yang 2E

Hi! Does anyone have any good videos or articles that explain what we've been learning pretty well? Just looking for some good summaries that review everything. Thanks :)

Bethany Yang 2E

Question relating to balancing charge... does that mean after you add the H+ and OH- parts, then you balance the charges at the end? Or do you guys balance the charge before adding H+ and OH- and then check the balance again after?

Bethany Yang 2E

Hey! 1.) So, we can start first by determining the order for C in the rate law. Since we don't see C remain constant in two rxns, we can infer that it does not affect the rate of the rxn. For C, l=0. 2.) Then, we can look at A. Dr. Lavelle explained the sure-fire way to determine the order of speci...

Bethany Yang 2E

For the reaction 2A(g)+2B(g)+C(g)⟶3G(g)+4F(g) the initial rate data in the table was collected, where [A]0 , [B]0 , and [C]0 are the initial concentrations of A , B , and C , respectively. Experiment [A]0 (mmol⋅L−1) [B]0 (mmol⋅L−1) [C]0 (mmol⋅L−1) Initial rate (mmol⋅L−1⋅s−1) 1 19.0 125.0 280.0 5.94 ...

Bethany Yang 2E

Determine the average rate of change of B from t=0 s to t=292 s.
A⟶2B

Time (s) Concentration of A (M)
0 0.790
146 0.485
292 0.180

rate B= ? M/s

how would you approach solving this problem?

Bethany Yang 2E

Thank you to all the people who planned it, participants, judges, etc! I know a lot of time and effort went into it, so I hope you know it did not go unappreciated :)

Bethany Yang 2E

Does anyone have any recommended review sessions? I've only been to a few of the same peoples and I was wondering if anyone stood out to you guys specifically in terms of covering the most content? Thank you everyone! Also, everyone is so great in terms of covering material, but I would recommend J...

Bethany Yang 2E

Hello! I just wanted to clarify. On the week 10 review session sheet, are the times bolded in dark blue the TA's review session and then the non bolded are office hours?
I know there was some confusion last quarter, so I just wanted to double check. Thanks :)

Bethany Yang 2E

Does anyone know that if after this quarter we don't have Lavelle anymore, do we still have access to our accounts on Chem Community? I found some really helpful threads on advice for pre-med students, etc, and just feel like this has been an encouraging place for people to talk. Would we still be a...

Bethany Yang 2E

I think it should also be ok if you take 14BL over the summer because although it is condensed into shorter weeks, it still is online. I've heard that with it being online, you don't have to spend as much time doing labs as you would in person. Goodluck!

Bethany Yang 2E

Given the context of the problem, we need to use the Nernst equation, E=E^0-RT/nF*ln(Q) . First we need to write out the half equations for both the anode and the cathode. Anode: Zn(s) ---> Zn2+(aq) + 2e- Cathode: Sn2+(aq) + 2e- ---> Sn(s) Using this, we can solve the rest of the missing re...

Bethany Yang 2E

I just logged out and then it went away from me!

Bethany Yang 2E

For the first one: the phases of each species present should correspond to their phases as originally specified in the equation given in the question: MnO4- is (aq) S2- is (aq) H2O is (l) MnO2 is (s) OH- is (aq) S is (s) For the second one: First begin by writing out the half-reactions. This is in ...

Bethany Yang 2E

Can someone explain to me how to balance this? For some reason I keep getting the wrong answer...

ClO2- + Ag + -> ClO4- + Ag

Bethany Yang 2E

On CCLE, is your guys' saying you do not have the full 100% in Chem Community because we're only on week 8? I'm not sure if I should be concerned or just wait till the end of the quarter and hopefully it will update to 100% with all the points...

Bethany Yang 2E

Oh man, sorry to hear about the internet. If you are in the chem groupme, there is a drive with the UA worksheets that people post in! I find it helpful when I can't make it to a session to do it after on my own. People often post the answers too so it helps!

Bethany Yang 2E

I was doing textbook problem 4J.5 and was wondering how do you find ΔH°, ΔS°, and ΔG° from the equation of formation reaction?

Bethany Yang 2E

"Nanotechnologists have found ways to create and manipulate structures containing only a few molecules. However, orienting the molecules in specific ways to assemble such structures can be difficult. Calculate the entropy of a solid nanostructure made of 64 molecules in which the molecules (a) ...

Bethany Yang 2E

Wait can someone explain again why/when sometimes we use q=nCdetlaT or q=mCdetlaT or just without the moles and mass, and it is just q=CdeltaT

Bethany Yang 2E

Why do we use the equation qp = nCpdeltaT and qv= nCvdeltaT? Or how would we go about solving this problem? "Calculate the final temperature and the change in enthalpy when 765 J of energy is transferred as heat to 0.820molKr(g) at 298 K and 1.00 atm (a) at constant pressure; (b) at constant vo...

Bethany Yang 2E

In a combustion chamber, the total internal energy change produced from the burning of a fuel is -2573kJ.The cooling system that surrounds the chamber absorbs 947 kJ as heat. How much work can be done by the fuel in the chamber? in the solutions manual, it says that the q would be -947kJ. why would ...

Bethany Yang 2E

can someone help me with this? this is the last part i need for sapling and i keep getting it wrong. i thought the answer would be 863.20. The final step in the industrial production of nitric acid. 3NO2(g)+H2O(l)↽−−⇀2HNO3(aq)+NO(g) no2's delta G is 86.55kJ; h2o is -237.13kJ; hno3 is -111.25kJ; no i...

Bethany Yang 2E

Given the information

A+BC⟶2D ΔH∘=−709.3 kJ ΔS∘=359.0 J/K
C⟶D ΔH∘= 480.0 kJ ΔS∘=−156.0 J/K

calculate ΔG∘ at 298 K for the reaction

A+B⟶2C

can someone help me with this please? i keep getting the wrong answer

Bethany Yang 2E

Consider the following reaction at 298 K.

C(graphite)+2H2(g)⟶CH4(g) ΔH∘=−74.6 kJ

Calculate the following quantities. ( ΔS system ; ΔS surroundings ; ΔS universe). Refer to the standard entropy values as needed.

Can someone help me with this please?

Bethany Yang 2E

Justin does really good review sessions. For midterm one, he did a review session on Thursday (the day before the exam), and it helped a lot!

Bethany Yang 2E

My TA said that if we can do up to 4J by Monday, we will be on the right track with timing for the midterm.

Bethany Yang 2E

someone made this doc, and this might help with trying to recall equations!
https://docs.google.com/document/d/1A-G ... sp=sharing

Bethany Yang 2E

Can someone help me with this? I keep getting it wrong :(. 0.461 mol sample of NO2(g), initially at 298 K and 1.00 atm, is held at constant pressure while enough heat is applied to raise the temperature of the gas by 14.1 K. Calculate the amount of heat q required to bring about this temperature cha...

Bethany Yang 2E

Can someone help me with this problem? A 0.461 mol sample of SO2(g) initially at 298 K and 1.00 atm is held at constant volume while enough heat is applied to raise the temperature of the gas by 14.9 K. Assuming ideal gas behavior, calculate the amount of heat (q) in joules required to affect this t...

Bethany Yang 2E

Hi! For this problem would the initial temperature be 45 degrees C? I know I am solving for the final temp, but on both sides of the equation would I use 45 as the initial temperature to solve? I think I am just a little confused. "An ice cube with a mass of 52.0 g at 0.0 ∘C is added to a glass...

Bethany Yang 2E

I keep getting stuck on this part of the problem. Can someone help me?

4C(s)+S8(s)⟶4CS2(l) ΔH=+358.8 kJ
Calculate the amount of CS2 produced when 401 kJ of heat is absorbed. and then it asks for the mass

Bethany Yang 2E

I was also thinking about waiting to take labs in person, but I think I'm just going to take BL spring with C, and then maybe take CL over the summer or in person. Although I've heard that in person labs seem useful, I have also heard that you can still get lab experience through research, and they ...

Bethany Yang 2E

Is anyone planning to take CL over the summer?

Bethany Yang 2E

Consider these reactions, where M represents a generic metal. 1. 2M(s)+6HCl(aq)⟶2MCl3(aq)+3H2(g) ΔH1=−802.0 kJ 2. HCl(g)⟶HCl(aq) ΔH2=−74.8 kJ 3.H2(g)+Cl2(g)⟶2HCl(g) ΔH3=−1845.0 kJ 4.MCl3(s)⟶MCl3(aq) ΔH4=−210.0 kJ Use the given information to determine the enthalpy of the reaction: 2M(s)+3Cl2(g)⟶2MCl...

Bethany Yang 2E

Can someone remind me again what the difference between the first ionization and second ionization is? I'm looking at problem 6E.1 and I'm having a hard time understanding what it is saying/how to go about the question since I think if you look at the table, it only gives Ka2 & pKa2 "Calcul...

Bethany Yang 2E

Wait for part A do you just disregard the Cl part of the NH4Cl because in the solution manual it just has NH4+. Or how would go approaching this problem?

Bethany Yang 2E

What are the equations for finding the percent ionization and percent protonation? And are there any tricks you guys have for memorizing what ionization vs protonation does? Thanks!

Bethany Yang 2E

This will be so helpful! Thank you so much for sharing :). Goodluck on the midterm!!

Bethany Yang 2E

Can someone explain why the change for NH3 would be +x instead of -x? I thought it would be 0.200+ x, but it seems both Nh3 and H2S will change with +x When solid NH4HS and 0.400 mol NH3(g)were placed in a vessel of volume 2.0 L at 24 °C, the equilibrium NH4HS(s)⇌NH3(g)+H2S(g), for which Kc=1.6×10−4...

Bethany Yang 2E

Hi! Has anyone made or found a study guide for this upcoming midterm that was really useful for them that they would be willing to share/recommend? I feel like I don't have a super concreate foundation on what we've been learning, and it would mean a lot to me if someone could share a resource :). T...

Bethany Yang 2E

The Kb for an amine is 3.227×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.786? Assume that all OH− came from the reaction of B with H2O. I keep getting 52.82% as my answer but I could be completely off ahah. Can someone please tell me what they got as th...

Bethany Yang 2E

Can someone help me with this problem? For some reason, I am not sure where to start: Muscles produce lactic acid, CH3CH(OH)COOH(aq) , during exercise. Calculate the percent ionization (deprotonation), pH, and pOH of a 0.1173 M solution of lactic acid. The acid‑dissociation (or ionization) constant,...

Bethany Yang 2E

You are going to want to start with the ICE table and use the initial bar pressure of PCl5 in the beginning, with the other two at 0. Then at equilibrium you would set the equation equal to the Kp that was given. You should be able to do the quadratic formula and solve for x and then plug it back in...

Bethany Yang 2E

I was wondering how you guys approach doing the outlines? I know the textbook problems are very useful and so I want to do them, but do you guys complete them as we are learning them or at the end of the weeks? I think I'm just feeling a little overwhelmed since we're definitely moving a lot faster ...

Bethany Yang 2E

Hi! What I did was since you are given the equilibrium values, you can immediately find the Kc by plugging the the products/reactants. Then to set up the ICE table, you would put the other two's values that did not change as the I and the I for the changed molecule as the new change valued. Then you...

Bethany Yang 2E

The equilibrium constant would be Kc=[HI]^2/[H2][I2]. When you express Kc in terms of x it would be 53.3=(2x)^2/(0.800-x)^2, which equals 7.30=2x/(0.800-x). If you solve the equation x equals 0.628M. Since [HI] is 2x, 2(0.628M) will result in 1.256M. Oh I totally forgot to also square root the 53.3...

Bethany Yang 2E

Hi! Would someone be able to help me with this problem? For some reason I keep getting 1.542 as the answer, but I guess it isn't right. At a certain temperature, the equilibrium constant, Kc, for this reaction is 53.3. H2(g)+I2(g)↽−−⇀2HI(g)Kc=53.3 At this temperature, 0.800 mol H2 and 0.800 mol I2 w...

Bethany Yang 2E

Hi :) Is each step up session different? Like do they go over different problems in each session? Yes! Each UA puts together their own worksheet, so the content of each session will be based on what they want to do. I haven't found any of the sessions to have the same practice problems, so it's bee...

Bethany Yang 2E

Hi! I'm not completely sure, but I would assume that we would just be tested on 14B content that we learn in these coming weeks since we are taking this class, and not 14A. If there would be anything from 14A, it would probably be the last part of what we learned last quarter, as it overlaps a bit.

Bethany Yang 2E

Yes! Midterms will be given on a Friday during your lecture time, instead of discussion times like 14A. I believe one of the UAs said that this was probably so that everyone takes it on the same day instead of it being scattered around. The final should be on a Sunday though.

Bethany Yang 2E

Hi! Can someone clarify the difference between an equilibrium constant vs initial concentration? Specifically I was looking at the modules posted on the website and for part 1A, #17 says "for a balanced chemical equation with all stoichiometric coefficients equal to one, what is the simplest ex...

Bethany Yang 2E

I'm still a bit confused on what to look at to tell if an acid is stronger or not (besides memorizing the strong acids). When would we look at the bond strength and when would we look at electronegativity? And within electronegativity, why is that the polar it is then the stronger acidity it is?

Bethany Yang 2E

I think he said yesterday in the session that you had to email him and ask for it.

Bethany Yang 2E

Hi! I am a little confused on the difference between molecular geometry and electron density, and when to use them. I noticed in the textbook it says to "find the shape" but does not specify which one. What is the difference between the two and which one do we normally find in regards to l...

Bethany Yang 2E

I would suggest memorizing the strong acids/bases, so that in case there is a question about it (i can forsee him throwing one or two), then it would be easier to use process of elimination.

Bethany Yang 2E

For this question, how come the solution manual says that it would be trigonal planar? Because wouldn't the shape be bent? Unless the solution manual is referring to the electron geometry... but are we usually asked for electron geometry?

Bethany Yang 2E

I was also confused on both times, but I just found out that the time in the parenthesis is the normal office hour time, and the other time is when the review session is.

Bethany Yang 2E

Hi ! Can someone help me classify which compounds are strong acids, weak acids, strong bases, weak bases, and others? I've tried a couple times, but can't seem to get the right answer.

HNO3, HI, H2CO3, HCN,CH3COOH, NaOH, Ba(OH)2, NH3, (CH3)2NH, NaCl

Bethany Yang 2E

I was also wondering the same thing! I think though at least for his Wednesday one, it will probably be at 5-6pm PST because that's when his normal discussion section is.

Bethany Yang 2E

What would the name of [Co(NH3)3Cl3] be? And how did you come to that conclusion?

Bethany Yang 2E

this video also helped me a lot if you want to check it out!
https://youtu.be/Q9-JjyAEqnU

Bethany Yang 2E

For me, if you look on youtube, the Organic Chemistry Teacher has been really helpful for a wide range of topics! Also Khan Academy haha.

Bethany Yang 2E

When naming, within the brackets does it matter the order of which one goes first, second,etc?

Bethany Yang 2E

No problem! However, on a side note, does anyone have trouble when logging in to the chemistry community web page? When I log in, it always says that my account isn't created, so I have to keep registering under the same alias. Does anyone else have the same problem? Hmmm that is weird. For me, I j...

Bethany Yang 2E

In the email Lavelle sent, on the attachment when it said that there will be problems that come from homework, is that referencing textbook problems or sapling problems?

Bethany Yang 2E

I'm having a hard time understanding this... could someone explain?

-Why do the hydrogen atoms of H2CCH2 and H2CCCCH2 lie in the same plane?
-Why is it true that if the X is an even number in the general form H2C(C)x CH2, then the hydrogens are coplanar?

Bethany Yang 2E

If any of the elements within the atom have a lone pair, could you automatically assume that it is a polar molecule?

Bethany Yang 2E

How would you be able to tell which is the axial vs which is the equatorial if you are able to rotate it?

Bethany Yang 2E

I believe in his recent email he said there could be tests/midterms during lecture times for Chem14B. But if he only does 2 midterms again during lecture times, you could technically just sacrifice your other class 2 times if it overlaps a little.

Bethany Yang 2E

The very simplistic way I think of it, is a sigma bond is a single bond. So when there is a double bond, or triple bond something else needs to fill it. So for a double bond, you can always start out with the sigma bond (single) and then however many more bonds you need will be the pi bonds. Therefo...

Bethany Yang 2E

Did you guys do better or worse compared to the first one??

Bethany Yang 2E

Hi Dr. Lavelle!

I just wanted to say thank you for taking the extra effort to make some of us freshmen feel like we are in the big lecture hall learning chemistry. I know it takes a lot of energy to also get tested and make sure it is a safe environment, so just wanted you to know we appreciate it!

Bethany Yang 2E

Oh that is a good point, I did not even think of counting 15... I just looked at it that the f subshell has 14 orbitals, so that would be the amount of space one could fill.

Bethany Yang 2E

Hi! Do you know if Lavelle will be going over each midterm problem in preparation for the final or if there is a way for us to see which ones we got wrong and right? I feel like individually knowing what I got wrong can help me fix these mistakes so I can do better for the final, as I didn't feel su...

Bethany Yang 2E

Also, someone mentioned above that Lavelle is dropping 10 points from the first midterm to make it out of 100 rather than out of 110, does anyone have more information to confirm/deny this? I think someone said in the GroupMe that they went to his office hour one time and he said that was false. Ho...

Bethany Yang 2E

I'm not exactly sure, I mostly try to estimate my grade based on the points I'm receiving (like getting points for chemistry community, the hw, etc). And then based on how my midterms go, I try to see what range I'm in (like whether I'm passing or not lol). I don't think Lavelle has specifically sa...

Bethany Yang 2E

Hmm I think you should be correct in terms of properties. I think it would probably be important as well to know the relationship between how many orbitals are in the s,p,d, and f to the number of electrons each can carry, such as if there are unpaired electrons, etc.

Bethany Yang 2E

Can someone explain to me the answer for this: "Ionization energies usually increase on going from left to right across the periodic table. The ionization energy for oxygen, however, is lower than that of either nitrogen or fluorine. Explain this anomaly." Also, is oxygen just an exception...

Bethany Yang 2E

Hi! Does anyone have any tips for how to determine the number of unpaired electrons quicker? Right now I am looking at Textbook Problem 1E #23, when it asks for Ga, Ge, As, Se, Br and I'm not sure how off the top of my head to think about how many unpaired electrons each would have.

Bethany Yang 2E

For the nitrite ion (NO2-), why is it that there is only one double bond from N to one of the Os? And how would you know before doing the formal charges?

Bethany Yang 2E

Hydrogen bonds are attractions between a hydrogen and an electronegative atom. This happens because of the difference in electronegativity. Whereas on the other hand, covalent bonds are more about the sharing of electrons, so the electronegativities are relatively the same. Hydrogen bonds are weak b...

Bethany Yang 2E

I remember Lavelle saying for this class (14A) that he let everyone off the waitlist, but i do not think he specifically mentioned anything for 14B. I would assume it would be the same way because on the class planner, if I remember correctly, Lavelle is only teaching 14B, not A. So, I would say tha...

Bethany Yang 2E

In order to see if it is a nonpolar molecule or a polar molecule, you would need to look at the electronegativities of each atom. If the two atoms have the same electronegativity, then the electrons are equally shared because they are pulled on the same. However, if one atom has a higher electronega...

Bethany Yang 2E

Like Jonathan said above, dissociation energy is not necessarily a trend, but rather you can come to the conclusion through trends. For example, a big factor would be the size of the atom because that would determine the bond lengths, and further, the dissociation energy. Thus you could take into co...

Bethany Yang 2E

How would you know when there is an induced dipole? Does an induced dipole always happen?

Bethany Yang 2E

Can someone explain in simple terms why double bonds are shorter than single bonds?

Bethany Yang 2E

If I remember correctly, usually the atom with the lowest ionization energy will be in the center. This is because if you think about it in other terms, low ionization energy means it would require the largest number of electrons to complete its octet. Thus, it would make sense for it to be in the m...

Bethany Yang 2E

I could be wrong, but I would assume that we do not have to memorize them because I feel like Lavelle emphasizes that you can come to the conclusion of something by using previous knowledge of characteristics of each element.

Bethany Yang 2E

I believe that inert pair effect is common in the 15-17th group elements and usually is it because the inertness of the inner electrons do not have good shielding. But, I am also wondering the same thing because I do not know much about this topic.

Bethany Yang 2E

The optimist sees the glass half full. The pessimist sees the glass half empty. The chemist sees the glass completely full, half with liquid and half with air.

If the Silver Surfer and Iron Man team up, they’d be alloys.

Bethany Yang 2E

If I remember correctly, Lavelle said that he accepts everyone off the waitlist. Do you guys think it would be smart to enroll in CHEM14B with the second pass then? Or how are you guys approaching enrollment?

Bethany Yang 2E

To check my grades I go through the UCLA website under the class tab because I think it updates faster than the CCLE app. Under the class tab, where are you able to check grades through UCLA's website? Because I only see things such as "change grade type", "change units", etc......

Bethany Yang 2E

It should be on Lavelle's chemistry website, and then you scroll down to Course Materials, and there will be different links for the different TAs. You can see links with "TA Office Hours & Discussion Sections Lecture 1", etc for each lecture, or there should be another link right unde...

Bethany Yang 2E

Do you know if Lavelle will continue posting the modules on his class website for the next units that we will be learning? I find them to have really helped with these past couple ones, especially being able to do more practice problems and seeing the correct answer. It also was good to try the prob...

Bethany Yang 2E

Going off what John said above, my friends who have taken BL highly suggest taking it along side C, or after you have taken B. It is manageable to take it with B, but I just heard that it is easier to get a grasp of what is going on in the lab because you already learned the material. Also, I believ...

Bethany Yang 2E

When finding the percent yield, is theoretical yield only the amount of the limiting reactant? For some reason I thought you would add the product's grams that came from both the reactants and that was the theoretical yield...

Bethany Yang 2E

For some reason I am having a difficult time understanding this. I know that the higher energy it is, the shorter wavelength, and the shorter energy, the longer the wavelength. But how do you apply this to when looking at energy levels. For example, on sapling it says "the range of wavelengths ...

Bethany Yang 2E

Can someone clarify what units wavelengths and frequency are usually measured in? Also do you think on the midterm if the question does not specifically say, for example, " state the wavelength in nanometers", could you leave it in the unit that is not commonly used, and get full points?

Bethany Yang 2E

Can someone walk me through their thinking process to how they would know what the event is based on the other given information? A college student recently had a busy day. Each of the student’s activities on that day (reading, getting a dental x-ray, making popcorn in a microwave oven, and acquirin...

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