CHEMISTRY COMMUNITY

Grecia Velasco 1G

What are your favorite YouTubers/channels for reviewing chemistry? Thought making a thread like this would be very helpful for everyone studying for finals :)) Some of my favorites are CrashCourse and Professor Dave Explains! Hands down, organic chemistry tutor. He is a UCLA alumni too sooooo <3 Ju...

Grecia Velasco 1G

Algernon Jackson 2l wrote:How are you guys coping with the anxiety that this class causes?

WE FINISHED!! i hope the class didn't hurt you guys too much <3 Good job this quarter !

Grecia Velasco 1G

JTieu_1L wrote:Are chem community scores additive, meaning the scores are based on how many posts total?

I think its additive! Bc by week 7 I already had 50/50 chem points on CCLE :))

Grecia Velasco 1G

Hey guys, I'm trying to understand [still] when intermediates and catalyst are present in elementary step rate laws and overall rate laws. So you can include intermediates and catalysts in elementary step rate laws. But you can not include catalyst and intermediates in overall rate law unless they a...

Grecia Velasco 1G

Hi! I'm very confused on whether catalysts are actually consumed in a reaction or not. In lecture and in a post on Chemistry Community, Dr. Lavelle said that catalysts are consumed first in a reaction, and then are produced. However, there was a question in the textbook that said catalysts are neve...

Grecia Velasco 1G

I didn't do last weeks chemistry community posts, does anyone know how many points I will be losing and if there is any way to make them up? Thank you You should be losing 5 pts, however, I would do 10 this week just in case overall is counter instead of per week. I think this is the case bc I post...

Grecia Velasco 1G

Do not include catalyst in overall rate. You can include intermediates in elementary steps but not in the overall rate, I'm pretty sure!

Grecia Velasco 1G

Hey guys, I know that intermediates can be used to elementary rate laws and I thought we weren't supposed to involve them in the overall rate law, but in a video I was watching (https://www.youtube.com/watch?v=4czm7qIbUjA) the guy involved the intermediate in the overall rate law. Can the intermedia...

Grecia Velasco 1G

How do you identify reducing and oxidizing agents from a redox reaction/half reactions? I'm having trouble knowing which species in the anode/cathode reaction it is. Thanks! The species in the half rxn that is oxidized, is the reducing agent Ex: Sn^2+ -> Sn^4+ +2e- In this case, the Sn^2+ is the re...

Grecia Velasco 1G

: This is the problem :)

Hey guys can someone explain to me why we exclude H+ in both the HNO3 and the HAuCl4 in problem 4 in week 7-8 sapling? I understand the rest, but I was just confused on why we exclude the H+, thank you.

Grecia Velasco 1G

This quarter has been pretty tough and I have been in burnout for the majority of it. How do you guys deal with burnout and how do you regain your motivation to do work? Just try your best. I just tell myself "Come on, Grace. You just have to pass to graduate come onnnnn". But if you're p...

Grecia Velasco 1G

connie ma 3G wrote:are we gonna have lectures all throughout week 10 (on MWF) or are we ending early? just wondering cuz im currently super behind on watching lectures haha.

I'm pretty sure lectures are ending this Wednesday!! You should catch up asap :))

Grecia Velasco 1G

The higher (more +) the reduction potential of a species, the better oxidizing agent it is.

Grecia Velasco 1G

KMcFarland_2L wrote:How do you determine the overall order of a reaction?

You can't tell from just looking at the rxn equation, so you have to take the sum of the order of the individual reactants and that will be the order of the products.

Grecia Velasco 1G

Fdonovan 3D wrote:How do you calculate the half life of a zero order rxn?

Dr. Lavelle gave us the equation in class. It is t1/2=[A]0/2k for the half life of a zero order rxn.

Grecia Velasco 1G

Michael Iter 2F wrote:Hi, can someone explain to me the purpose of the salt bridge? What happens if we don't have it?

It neutralized both the cathode/where reduction is occurring and anode/where oxidation is occurring.

Grecia Velasco 1G

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... Hmmm I already posted more than 50 times this quarte...

Grecia Velasco 1G

Kristine_Aramyan_3K wrote:what's your fave food/snack/dessert? i'm personally craving banana bread rn

I really like mac n cheese especially after an exam or something stressful ehehehe butttt my fav snack is probably fresh mangooo is big yummyyy

Grecia Velasco 1G

Sean Phen wrote:What are some good rules to remember which is the oxidizing agent and which is the reducing agent?

https://www.youtube.com/watch?v=dF5lB7gRtcA

This video is very helpful to understanding redox rxns if you're a visual learner! TOCT is very good at explaining alot of CHEM 14B topics!

Grecia Velasco 1G

Does anyone know if we'll do what we did last quarter in 14a with points where it won't be counted per week but as long as you have 50 total? I think so! I already posted by 60 times or something and I already have 50/50 in that category on Ccle! Just in case though, you should still do your 5 post...

Grecia Velasco 1G

I think either tonight [wed.] or tomorrow [thurs.]! Just like last time if I had to guess.

Grecia Velasco 1G

Also, if you're a visual learner, this video is very good [like most of TOCT's vids] at explaining how to calculate oxidation numbers.
https://www.youtube.com/watch?v=j0hI-a6EWWo

Grecia Velasco 1G

Wasila Sun 2I wrote:Under what circumstances can we use the delta U=delta H+ w equation? Like what allows delta H to be interchangeable with q?

Must be in constant pressure and a open or closed system.

Grecia Velasco 1G

I'm debating if I want to take Chem 14C and Chem 14BL at the same time, because I also have two other classes that I want to take (LS 7B and one more that I'm undecided on). I might end up taking Chem 14BL fall quarter though. Not sure what is the best choice Heyyyyy! I'm taking chem14bl and chem14...

Grecia Velasco 1G

The units are kj or J! If you solve it in L.atm, convert it into J by multiplying it by 101.325. Once it's in J, divide by 1000 to convert it into kj. Hope this helps :)

Grecia Velasco 1G

Q is the quotient reactant that we learned for and before Midterm 1! It is [P]/[R] like K but NOT at equilibrium. To compare Q and K use le chatelier's princple: Q<K; will shift to products so -DeltaG and spontaneous Q=K; equilibrium Q>K; will shift to reactants so +DeltaG and spontaneous in the rev...

Grecia Velasco 1G

Yes! No heat/energy or matter can enter or exit the system in the mug so it is an isolated system :) unless you open it! Then it would be an open system.

Grecia Velasco 1G

When a gas is doing work, it is a -w. When the surroundings are doing work on a system, it is +w because the system is gaining work.

Grecia Velasco 1G

I finished it at 12 pm today!! Afterwards I played videogames till now so likkee 3 hours ;w; , treat yourself afterwards!! you deserve it! @everyone reading this <3

Grecia Velasco 1G

Hey guys, In lecture Lavelle said that deltaU= 0 and therefore, q = -w can apply to both isothermic reversible and irreversible pathways. In what scenerios is this true (like constant pressure, volume, etc) ? Or is it always true? I understand why deltaU=0 for isothermic rev. rxns but not for irreve...

Grecia Velasco 1G

Hey guys, can someone explain the relationship between stability [in respect to elements] with a + or - deltaG? I don't understand how a -DeltaG is stable with respect to elements and a +DeltaG is not.

Thank you!

Grecia Velasco 1G

Hello, fellas
Can someone explain why DeltaS total = 0 for reversible reactions and DeltaS total > 0 for irreversible reactions? I can't seem to conceptually understand this :(.
Is it because in a reversible rxn, the Pex and Pint are in equilibrium?
Thank you!

Grecia Velasco 1G

Are all reversible rxns isothermic? If the temperature stays constant (a aspect of a reversible rxn), would all reversible rxns be isothermic? If no, can you explain a scenario?

Thank you :>

Grecia Velasco 1G

Hey guys, I just wanted to know if deltaU=q+w is only applicable to closed systems or also open systems? In a class problem, he used this relationship with a open system but the definition says for closed system so I just want to make sure.

Thank you :>

Grecia Velasco 1G

Hi, I was wondering if anyone knows if there is any way that we can review our midterm once the midterm scores are out? I would like to know what questions I got correct and incorrect to help me prepare for the cumulative final. I'm sure if you ask your TA during their office hours they will go ove...

Grecia Velasco 1G

kateraelDis1L wrote:Hi, I am wondering how many points are needed (or how many we can miss) in 14B in order to end with a B? B-? C+? C? and C- ? Idk how to do the calculations so if someone has figured it out I'd love to know.
Thanks in advance ;)

You only need a 50% (200/400) to pass the class with a C- :>

Grecia Velasco 1G

Has chem community been updated since grades were last put in mid week 5? Just wondering because mine still doesn't reflect an accurate count and I'm wondering if I should reach out to my TA at this point or continue to wait it out. I think Lavelle does them in intervals so he'll fix it next time h...

Grecia Velasco 1G

I am confused on when to use which value for the gas constant: R = 8.3145 J/mol·K. and R= 0.08204 liter·atm/mol·K. Can anyone help? It depends on what you're calculating. Best way to find out what you have to use is looking at the units and seeing if they cancel out with whats given to what you wan...

Grecia Velasco 1G

Melissa Solis 1H wrote:Can someone explain what a state function is again and why free energy is a state function?

For a state function, the path you took to get the your values does not matter so you can only take into consideration your final and initial values.

Grecia Velasco 1G

Hi guys, can anyone explain the textbook problem 5.55 explanation for part a ? I understand how to find K but I'm having a hard time understanding the explanation for part a. The equation: A reaction used in the production of gaseous fuels from coal, which is mainly carbon, is C(s)+H2O(g)⇌CO(g)+H2(g...

Grecia Velasco 1G

Clean work area, try your best to not look at the clock on respondus [force yourself to focus], I like to write the components I need to solve the problem on my scratch work and write very organized [makes me feel like Ik what im doing], MAKE SURE YOU EAT WELL BEFORE EXAM, use the bathroom before th...

Grecia Velasco 1G

Hi guys im panicking to the point of depression. I did so so bad on the exam. When I saw that the class average was so high, I almost died because I did so bad. For those who are successful this quarter, how are you guys staying successful? I was successful in 14A by doing the textbook practice pro...

Grecia Velasco 1G

I'm sure if you ask your TA they will let you see what you got wrong and you may be able to review/ask them!

Grecia Velasco 1G

Melissa Solis 1H wrote:During lecture today (2/3) prof. Lavelle was explaining that U is at equilibrium/ equal to zero when it's at the minimum of the reaction and for entropy, it's at equilibrium when it's at the maximum, right?

Yes! You are correct. Entropy [S] and Degeneracy [M] are maximum while U is minimum [0].

Grecia Velasco 1G

Hi, Right now, I have a 21/25 for Chemistry Community posts. However, shouldn't we have a minimum of 20 posts since we have not completed week 5 yet? I am just making sure I didn't miss anything. They are most likely grading on how many posts/responses you have atm! It will most likely change later...

Grecia Velasco 1G

hi, I would just like some clarification as to what the unit should be for specific heat (more specifically regarding the temperature unit)? I noticed in lecture Professor Lavelle mentioned both Celsius and Kelvin, and it seemed interchangeable. J/gºC or J/g K. I think both are fine but I would use...

Grecia Velasco 1G

Whenever I'm watching a lecture or video, I usually like to have subtitles on as well to help me follow along. Whenever I try to turn them on through the button on the video they don't actually show up. Has it worked for anyone else? I don't have CC either and it makes lecture following a long a li...

Grecia Velasco 1G

When solving for the work of expansion, the equation uses the integral from V1 to V2. Are these values the initial volume of the container and then the final volume of the container after expansion? if you want to read more on this it's on 4A.3: Expansion work in the textbook. It explains V1 (initi...

Grecia Velasco 1G

Just to clarify, will we be going over outlines 3 and 4 during this week? I kind of got confused because of last week's midterm, but I'm sure last week we went over outlines 3 and this week is beginning to enter week 4.

I just want to know so I can catch up on reading :) ty

Grecia Velasco 1G

I took a 3 hour nap after!! Self care <3 I hope you and anyone reading this enjoyed the rest of friday after the exam! <3 keep trying u can do!

Grecia Velasco 1G

Nina Fukui 2J wrote:What are the differences between the two???
percent protonation/deprotonation

% deprotonation = used for acids bc acids deprotonate [give off H+]
% protonation = used for bases bc bases protonate [take H+]

Calculated a similar way, but describes either acids or bases.

Grecia Velasco 1G

samanthaywu wrote:Will we need to have memorized any common bond enthalpies and standard enthalpies of formation or will we always be provided a table when this information is needed?

Nope! They should be given on exams. :)

Grecia Velasco 1G

Hey guys! I hope the midterm treated you well and I hope this is the right place to ask this, but I just wanted to know when we can expect midterm grades? It was multiple choice so I was expecting to see the results right after lol.

Thank you!

Grecia Velasco 1G

Hey guys,
One TA told me buffer solutions would be on tomorrow's midterm and another one told me it won't be on it. Im a little confused. Does anyone know?

Grecia Velasco 1G

Hey guys,
Can someone explain why Cl2 is more stable than F2? The answer book says because it has a larger K, so because it's [P]/[R] is larger? Is it because there's more products than reactants?

Thank you!

Grecia Velasco 1G

Should we memorize any standard states for Friday's midterm?
So far, I only know that carbon's standard state is graphite and water's is liquid.

I'm not sure if we should know others, so I was wondering what you guys thought. Thanks ! :)

Grecia Velasco 1G

What I heard from my TA was that we will join our discussion sections on our phone and then will take the exam using respondus. In terms of allowed materials, just print out the periodic table, equation/constants sheet from the course website and bring a calculator. Do you know if it will be respon...

Grecia Velasco 1G

IreneSeo3F wrote:Can someone explain why the concentration of the reactants and products remain the same when inert gas is added?

Inert gases do not affect the rxn [non-reactive gas] so they will not affect the K value.

Grecia Velasco 1G

I’ve been seeing that there are two R values (0.0821 atm•L/mol•K and 8.3145 J⋅K−1⋅mol−1) and I’m just wondering how we know when to use which one? Thanks! It depends what you are given in the problem; bars or atm. In lavelle's equation and constant sheet, both forms are given so just choose to mat...

Grecia Velasco 1G

Hi, so I understand that there are three methods to calculating enthalpy changes: Hess's Law, Bond Enthalpy, and Standard Enthalpy, and that bond enthalpy is usually the most inaccurate, but is there a specific situation where we should use Hess's law or standard enthalpy? Is one more accurate than...

Grecia Velasco 1G

Hey guys, I just wanted to make sure I'm understanding how to find standard enthalpies of formation correctly. The problem is: Calculate the standard enthalpy of formation of dinitrogen pentoxide from the following data: 2NO(g)+O2(g)→2NO2(g); ΔHo=-114.1 kJ 4NO2(g)+O2(g)→2N2O5(g); ΔHo=−110.2kJ and fr...

Grecia Velasco 1G

you add [BH+] to [B] because when you solved for x, that x didn't include the concentration that was protonated. The whole thing would be your initial concentration. Oh, I see. So does this only occur when calculating the % protonated for a base? I know % ionized/dissociated for an acid, you only c...

Grecia Velasco 1G

Hi guys, I was wondering if someone could explain why, in week 2's #5 in Sapling, the formal value of [B] includes the sum of [B] and [BH+]? The formal concentration of B is the sum of the concentrations of B and BH+. % amine protonated=[BH+]/[BH+]+[B]×100=4.95×10−5 M4.95×10−5 M+0.000119 M×100%=29.5...

Grecia Velasco 1G

Hey guys, I'm having a difficult time understanding how to identify acids, bases, and if a salt is acidic, basic or neutral (kind of hard when you mistake a base for an acid, ahaha). Do you guys have any tips in trying to distinguish a acid from a base without being given the equation? For example, ...

Grecia Velasco 1G

Hey guys! I was wondering if anyone could explain the conceptual difference between pH, pOH, Kb, and pKb? Thanks in advance! Ph = -log (concentration of H3O+) POH= -log(concentration of OH-) Kb = basicity constant or base ionization constant, solved same was as K = [P]/[R] pKb= -log Kb Strength of ...

Grecia Velasco 1G

Can someone explain this idea of the conjugate seesaw? I don't really remember learning this before, but I'm curious what it is because it sounds interesting! Here are the rules between Acids/Bases and Conjugate Acids/Bases or the conjugate seesaw: If strong acid, then weak conjugate base If weak a...

Grecia Velasco 1G

Hiya, I am slightly confused on the autoprotolysis reaction's equilibrium constant from lecture. Why is [H20] removed from Kc = \frac{[H_{3}O^{+}][OH^{+}]}{[H_{2}O]^{2}} ? Professor Lavelle mentioned that the concentration of [H2O] is the same on the left and the right of the reaction, which is why...

Grecia Velasco 1G

Hi, so from lecture, Dr. Lavelle talks about how we use the quadratic formula to solve for the x change value in ICE tables, and then proceeds to show how we omit the negative x value and use the positive one. What happens when we get two positive x values? Would we test both x values into the ICE ...

Grecia Velasco 1G

How do you know whether to write K for a gas as for example PCl2/PBr2 or [Cl2]/[Br2] It also depends what you are given, if you are given Moles (M) and Litres (L) then it is Kc or concentration. If you are given bars or atm, then it is Kp or partial pressure. I am sure Dr. Lavelle will specify in e...

Grecia Velasco 1G

Just to clarify, any stresses that result in a change in accordance with le Chatelier's rule do NOT affect the value of the equilibrium constant other than temperature right? So any change in pressure or concentration still cause the equilibrium constant to remain the same value despite the change ...

Grecia Velasco 1G

Okay, that definitely makes sense. I was a bit confused because I knew that the amount of reactants in general would change, but at the same time the actual value of K doesn't take solids into account. However, it makes sense that having more reactants would ultimately lead to increased products, e...

Grecia Velasco 1G

Does the ideal gas law still follow the general rule of reverse reactions wherein the equilibrium constant for an equilibrium written in one direction is the reciprocal of the equilibrium in the opposite direction since the ideal gas law is a bit more complex (accounting for the differences of stoi...

Grecia Velasco 1G

Sorry guys, I don't know where to post this question so I'm putting it here! I took Chem 14A fall '20 and a couple of things have changed since then. We used to turn in textbook problems to our TAs but I think that was substituted with Sapling homework. Is the only homework the one on Sapling due Su...

Grecia Velasco 1G

From my understanding:
Kc is for concentrations (mol.L^-1) and Kp is for pressure (bars). The thing is, gases can also be calculated with concentrations so it can use Kc and Kp it just depends what you're given.
If this is incorrect someone please tell me haha.

Grecia Velasco 1G

Hey guys, So I was trying to solve 5I.13 in the textbook and I understand A and B where you have to solve for X using the ICE box and find out the concentrations of the species. However, I am having a hard time in understanding part C. The part goes as following: " Use your parts from (a) and (...

Grecia Velasco 1G

Naren_Ramesh_4D wrote:On question 2b of the mini marshmallow, does the order of the ligand in the coordination complex matter?

I don't think so BUT its good to go in order just in case :)

Grecia Velasco 1G

Are we supposed to know how to hybridize lone pairs? Because during class, Lavelle said no but its in marshmallows so im not sure.

Grecia Velasco 1G

ZevMarx-Kahn3C wrote:
Hannah Romano 4D wrote:when are the answers for marshmallows being posted???

I'd also love to know!

Fellas, i gotta feeling it's either gonna be really late at night or tomorrow morning. Fluoride ion (F-) in the chemistry community thread.

Grecia Velasco 1G

Hydrogen bonding occurs when there is a hydrogen bonded to a N,O, or F atom and is attracted to another N,O, or F atom or vise versa. So, the number of hydrogen bonding sites consists of the number or lone pairs on the N,O or F molecules (because they must essentially share electrons with the H ato...

Grecia Velasco 1G

If something is a chelate what does that tell us and how would we identify it? Is there more to know other than just the definition of it? I think a good way to see if something is a chelate is to ask yourself "does this central atom have multiple polydentate ligands attached to it?" If t...

Grecia Velasco 1G

705081525 wrote:More protons means lower pH which means a stronger acid? Correct?

Strong acid= more hydronium ion = low pH
Weak acid= less hydronium ion = high pH

So like, low pH is more acidic and high pH is more basic.

Grecia Velasco 1G

It's the number of points at which ligands are attached to the central atom. Remember some ligands can connect to the central atom twice (polydentate) so it's not exactly the number of ligands. Do the bi/tri/polydentate is how many times a single ligand can bind to a central transitional metal? Tha...

Grecia Velasco 1G

Chanel D_4E wrote:Yes, you need at 250 in order to pass with a C-.

Thanks! I totally forgot about this until now.

Grecia Velasco 1G

PKa= -Log[Ka] (for weak acids, log of Ka)

Ka= [A-][H+]/[AH] (acid dissociation constant)

Smalled Ka = larger PKa = weaker acid

Grecia Velasco 1G

Yazmin Bocanegra 3L wrote:Is the final exam going to be everything from throughout the quarter, or is it going to be everything from the midterm and beyond?

it's all the material from the quarter

Grecia Velasco 1G

Trent Yamamoto 4I wrote:In regards to our class, should we use the Bronsted definition or Lewis definition?

Lavelle said any of the two will do :)

Grecia Velasco 1G

If you count the number of valence electrons, you can determine the atoms hybridization. For example if you have a Carbon atom, which has 2 electrons in the 2s and 2 electrons in the 2p orbitals, then these orbitals would combine to form a hybridization of sp3. This means that now the Carbon atom c...

Grecia Velasco 1G

Haley Dveirin 4B wrote:Is there a way to access the chemistry textbook online?

If you got the bundle from the UCLA store, you should have received a launchpad code for the online chem book.

Grecia Velasco 1G

BeylemZ-4A wrote:can someone explain the difference between clean coal and dirty coal? I remember it being explained in the lecture in terms of Carbon of Sulfur dominant, but I don't remember which was clean and which was dirty.

Clean coal has lower sulfur content but both clean and dirty produce CO2.

Grecia Velasco 1G

905174774 wrote:Would the final be cumulative or will we be tested on everything after the midterm?

It's probably cumulative because the midterm was.

Grecia Velasco 1G

what is the electronegativity of hydrogen? like if a question asks which has greater ionic character (ie, more EN difference) between HCl and HI or between CH4 and CF4? similarly, where can you imagine hydrogen on the periodic table in terms of where it fits in trends? Hydrogen has low electronegat...

Grecia Velasco 1G

Ligands surround the central atom. It's just a name for bonded elements/molecules to central atom.

Grecia Velasco 1G

I kind of think of it like sigma bonds are gears that help rotate atoms. Kind of like a corndog, you can rotate the corn dog on the stick because the stick allows it to rotate. Weird way to put it, I know, but studying does that to you.

Grecia Velasco 1G

It the bond angle is 180*, won't adding a lone pair decrease the 180* and make it non linear? Kind of confused that it says "may have lone pairs" in 2E.1 of homework. Thanks!

Grecia Velasco 1G

Multiple bonds are still considered one region of electron density.

Grecia Velasco 1G

They're kind of like vectors and cancel when they face the opposite direction.

Grecia Velasco 1G

Lone pairs increase bond angles because they repulse electrons more than bonded pairs.

Grecia Velasco 1G

Majority of names are based on geometric shapes but in the case of the few that aren't, you should rely on flashcards with examples of each.

Grecia Velasco 1G

Lone pairs take up more space than bonded pairs.

Grecia Velasco 1G

Juliet Stephenson 4E wrote:Hydrogen, Helium, Lithium, and Beryllium are exceptions because they only have s orbitals. Elements that are period 3+ can also potentially expand their octet.

So any element besides the first 10 elements can have expanded octets?

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