CHEMISTRY COMMUNITY

kateraelDis1L

Lavelle sent out an email that if you make really helpful posts (I think maybe not social posts) then you can make up these points you lost for last week. He extended the deadline to Tuesday morning 9 AM! So get some good posts in.

kateraelDis1L

well when we use Q we know the reaction is not at equilibrium, maybe this helps you! So when calculating cell potential when reaction has not reached equilibrium use Q in Nernst.

kateraelDis1L

I think of like an insulated thermos. No energy escaping or put in the form of heat and no matter escaping or added.

kateraelDis1L

I think it it more accurate because it's the energy associated with the formation of one mole of a compound from its basic elements in standard states. These values are less of an approximation.

kateraelDis1L

if you look up a specific topic like "integrated rate law" add organic chemistry tutor at the end you will find his video on this topic and then a bunch of his videos on related content. spend like an hour watching a few of his videos and I bet you will feel better about your understanding.

kateraelDis1L

so if we were going from products to reactants the activation energy will still be lowered from the catalyst even after it already lowered the forward direction?

kateraelDis1L

Hi everyone! I am wondering if students who stayed home this year are planning on moving into the dorms when they open up at less capacity in the Fall? Ucla is committed to vaccinating all students so what is everyone's plans? ;))

kateraelDis1L

this is such a cute post. my motivation is lacking lately but I have to remember that I'm paying for my education so might as well try my hardest. In general I am feeling excited for the future. With the announcement that dorms are opening at 75% capacity in the fall I m excited to hopefully move in...

kateraelDis1L

I haven't seen re- used test questions so far so I would bet on no.

kateraelDis1L

salt increases water's ability to carry a current at the surface of a metal which enhances the rate of electrochemical corrosion and rusting.

kateraelDis1L

I remember it as the idea that we study initial rates because at the beginning of the reactions products are being produced faster than towards the end when the reaction is reaching equilibrium. This slowing down can mess with our rate numbers so we study initial (aka where the rate are at maximum) ...

kateraelDis1L

I remember last quarter it was said that a majority of the questions would be on content we spent more time on. I'd say knowing this exam is cumulative, it will be a similar case.

kateraelDis1L

as said above, you don't have to include C because anything to the zero power is 1. This might be why you were getting the k value wrong. plug in your numbers given for rate and the concentrations of [A] and [B]^2 to solve for k.

kateraelDis1L

Theres a difference between solving in a basic and acidic solution. But what works for me is solving it as if it always a basic solution and then adding H+ to react with the OH- to produce water. When this happens on both sides you can cancel out water molecules on both sides. But sometimes all of t...

kateraelDis1L

it means carbon is in its graphite form which means it is carbon as a solid. so yes it is significant.

kateraelDis1L

I think because they are in the same phase you could switch them and it won't make a difference. if one of them was a solid that would make a difference.

kateraelDis1L

I am still a little confused on where to put Pt when there's no conducting metal. I think I've seen it on the right and left.

kateraelDis1L

it is because pressure and conc are in standard conditions! Like the previous post, many things are able to cancel because of the few words "standard conditions".

kateraelDis1L

Hi all. After today lecture I am feeling a bit overwhelmed with all of the equations for rate laws for zero, first, and second order along with half-life. Does anyone mind first explaining to me what these orders mean so I can put a purpose behind the equations? Thanks.

kateraelDis1L

if someone could post the balanced reaction for this exact problem that would be awesome. I've tried it so many times.

kateraelDis1L

it has to do with the oxidation numbers. Lavelle didn't really go over finding oxidation states in lecture, I recommend watching a quick YouTube videos on how to determine oxidation numbers and then look in the chats above for how to solve and you'll be good!

kateraelDis1L

As stated above, Le Chatelier didn't go away after the midterms! He is still very much here. We know from him that if concentration goes up there's gonna be a shift in the reaction and this is also directly correlated to E in Volts of electrochem! Its all relevant.

kateraelDis1L

He was just talking about how if the cathode and anode both has aqueous solutions we have to add a solid (Pt).

kateraelDis1L

Think of it as the amount of electrons added in terms of moles. This number is the electrons transferred to the reduced side (because remember reduction is gaining electrons).

kateraelDis1L

I struggle with other responsibilities outside of school as well. I've learned that it helps to look way in advanced at your schedule to plan ahead. It sounds like you are doing a good job at that. Once you've got your schedule planned out take it one day at a time. Create to do lists for each day i...

kateraelDis1L

I remember this example from a book question that asked. Which substance has the higher molar entropy? 1.0 mol Ar(g) at 1.0 atm or 1.0 mol Ar(g) at 2.0 atm. The answer is at 1.0 atm because it will occupy larger volume than that at 2 atm. Hope this helps.

kateraelDis1L

if you're looking for a very easy ge I recommend Arts&Arc 10. Everyone needs a break sometimes.

kateraelDis1L

Larger elements/compounds have greater entropy because all of the different positions it could be in (more disorder). To add on, a molecule with a linear orientation with different elements has greater entropy that a molecule in a pentagon or octagon shape because the linear one can bend and is less...

kateraelDis1L

Just think of "equilibrium constant" when you see a K. As states above, sometimes the reaction is not at equilibrium and a K value is given. In this circumstance, you would need to calculate Q and compare it with K.

kateraelDis1L

We use the boiling point because the rate at which water turns into gas is same as gas into water, hence these reactions are at equilibrium for that boiling point temperature.

kateraelDis1L

I think there's a possibility for residual entropy because it can have more than one state

kateraelDis1L

midterm 2 is outline 3 and 4 correct? nothing on outline 5?

kateraelDis1L

hi guys. I've noticed that I struggle with quickly thinking of equations I need to use to solve these questions. recently my brain has had an overload of equations and its hard to differentiate when to use what. Does anyone have any advice for figuring out which equations to use? I tend to get confu...

kateraelDis1L

the equation you used is correct and the deltaH looks good. its so annoying when the answer key leaves out things.

kateraelDis1L

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 ;)

kateraelDis1L

I feel you! literally the only way I can get a homework question done is if iv'e seen something like it before. I've been going to UA step ups and workshops and we go over questions that I can look back on and re trace my steps for the homework problems. it's an extremely slow and tedious way of lea...

kateraelDis1L

internal energy is like the energy contained within a thermodynamic system. it is the energy necessary to create of prepare the system in any given internal state.

kateraelDis1L

could someone remind me what infinitesimally means again and why it is important ?

kateraelDis1L

What seems to be helping me is actually writing out my units so I can visually see what needs to cancel out in order to get what I want in the end. Especially when we use the R constant, if you use the wrong one it could mess up your units.

kateraelDis1L

so would entropy be decreasing when molecules in chemical reactions are forming ?

kateraelDis1L

I was worried about this too but this post definitely eased my stress so thank you! Now I know not to freak out when a question about hydrolysis comes up.

kateraelDis1L

I agree with the above comments. shifting to the right means rxn proceeds forward. this happens because the conc of reactants is less than the conc of products than at equilibrium. in other words when K>Q. K doesn't chance, we just compare it to Q.

kateraelDis1L

saying my numbers out loud while I am typing into the calculator helps me. your brain catches it if you have to remember how to say the number you're typing in.

kateraelDis1L

I agree^ it's really hard to force yourself to slow down especially on midterm day but this is a great example of how a simple calculation throws off an entire answer. check your work! you're doing awesome.

kateraelDis1L

what's the best way to type the values into the calculator? that's where I'm messing up and its the easiest part.

kateraelDis1L

Lavelle usually sends an email telling us the cut off with a cute little smily face emoji at the end. Always makes me smile.

kateraelDis1L

So lowercase q means heat transferred = enthalpy??

kateraelDis1L

Im not entirely sure this is what you're asking but I have written down in my notes that the rule to estimate Ph of polyprotic acids is use only the first deprotonation equilibrium and assume that further deprotonation is insignificant. (with the exception of sulfuric acid which is a strong acid in ...

kateraelDis1L

So when pressure increases (volume decreases) the reaction shifts to the side with less mols gas and K > Q so shifts right. When pressure decreases (volume increases) the reaction shifts to the side with more mols gas and K< Q shift left.

kateraelDis1L

To answer your question about steam burning more than water at the same temp, steam releases a lot of heat just through the phase change back into water (horizontal part of graph) and then continues to burn you more as the water cools down. In other words, there is a large enthalpy of vaporization.

kateraelDis1L

Up all night studying for chem?
You need some Cobalt and iron buddy,
CoFe2

kateraelDis1L

I learned a great trick in a workshop. So if you pretend your hands are a scale, right hand products, left hand reactant, when you add heat to your right hand it will drop because it is "heavier" so you have to move left to go back to equilibrium. It works the opposite way too. I use this ...

kateraelDis1L

I have this problem too and its kind of annoying sometimes. I just have to trust that I got the right answer. Look at it as good practice for a real test when an answer key isn't given lol.

kateraelDis1L

Hi, this one confused me also. I guess even after the addition you don't have to use the reverse reaction, all you have to do it account for the change in your ice table and solve for x like normal. Iv'e been looking for more practice like this one.

kateraelDis1L

Ice tables help you figure out what you know and don't know. If the problem gives you reactants you know you need to put a +x for product but if you need to solve for x in reactants you need to put -x. More practice will help you become more familiar with when and where the x goes.

kateraelDis1L

So if asked to find concentration we'd have to convert what we found for K into Kc ??

kateraelDis1L

Pertaining to a comment above, how does one go about using the reverse reaction to solve. Have we learned that in lecture and when do you know that it is okay to do this?

kateraelDis1L

Nope! No ICE tables yet. It's really easy to feel like you are getting behind in this class because so much content is being thrown at you at different times. Try to remember that if you are up to date on the lectures you are doing a great job! Don't feel like you constantly have to feel ahead of th...

kateraelDis1L

This is such a great topic to chat about and I just finished writing out all my goals to improve my study habits in this class. I wrote down that I am going to try the self-explanation method of studying where you legit talk to yourself out loud to teach yourself and hear the concept. Even better, t...

kateraelDis1L

Hi all, I'm having some trouble answering a sapling question. reactants : H2(g) + I2 (g) product: 2HI (g) Kc= 53.3 Given: 0.700 mol H2 and 0.700 mol I2 placed in 1.00 L container Question: what concentration of HI present at equilibrium? I have my ICE table correct but I think I am getting stuck wit...

kateraelDis1L

No partial credit. each question was worth 6 points.

kateraelDis1L

If anyone is in the 14 A group me rn ( which I'm hearing is going to be 14 B next quarter) please send the link!

kateraelDis1L

I think of "lone pair, spacer, spacer, lone pair"

kateraelDis1L

I have been writing down a list of a bunch of fun activities that I can spend some leisure time on after I get everything done for school. Once I finallyyy get through my finals I wanna have an intense gingerbread house decorating contest with my family, go shopping for Christmas gifts, and actually...

kateraelDis1L

I remember doing a problem that asked for bond lengths and we got to the answer by adding up two given bonds lengths to make the whole coordinate covalent bond. if you're referring to the question on the final, I think we had to decide whether the bonded atoms were held together with either single o...

kateraelDis1L

Acid rain is the result of sulfur dioxide and nitrogen oxides getting into the air after being released as pollutants from factories and vehicles. These pollutants mix with water vapor and O2 to form acid rain usually with a pH of 4.

kateraelDis1L

I have seen a pattern when asked to identify this type of question. That is that N bonded to 3 other things has the lone pair. So it is a site of attachment of a H+. So it can accept a proton. If given a huge lewis structure, ask yourself if there are lone pairs on the Nitrogens that you can "d...

kateraelDis1L

This happens to me sometimes too and I freaked out the first time. It should load soon, check back tomorrow and if nothing changed contact a TA.

kateraelDis1L

Yes! More specifically, they are acids that have more than one ionizable H+ atom per molecule. You can look at the anion of the acid to see whether it is polyphonic or not.

kateraelDis1L

I know it's generic but creating a list of things to do whether topics to go over, or chores around the house really help make you feel like you're getting something done. I for one have been very overwhelmed with the amount of hours of chem review that I could go to so I feel like there's just too ...

kateraelDis1L

Hi! this confused me too. yes you do take into account the Cl outside the brackets when calculating oxidation number!

kateraelDis1L

I don't think this has been covered in lecture yet. Has it? So don't worry about now knowing how to do that rn.

kateraelDis1L

I'm thinking on the Final, Lavelle will ask the question more like "describe" or "identify how many sigma and pi bonds" like the book does. Just understand that its all tasing the same thing. Multiple choice questions will make that clear also.

kateraelDis1L

so the Cl2 is outside of the brackets yes, why do we name it after everything else and not before?

kateraelDis1L

For this kind of question, whenever I see that the answer could be tetrahedral, I automatically think "or square planar". We aren't expected to know which one it is between those two, but square plane is only one plane while tetrahedral comes out into a different plane. So just have that m...

kateraelDis1L

^ I agree. Resonance forms with low formal charges are favored. I think that's the best way to tell.

kateraelDis1L

Could someone better explain the rule that Dr. L said during lecture, "regions of e- density = number of hybrid orbitals. I wanna make sure it's clear because he stressed the importance during lecture. Thanks.

kateraelDis1L

^^ the structures in my question showed no resonance either! Kinda a weirdly worded question if you ask me.

kateraelDis1L

I think the answer to your question may have to do with pi bonds. There is a pi bond in double bonds (one is a sigma one is a pi) and pi bonds do not allow bound atoms to rotate or else they break. Maybe this is what he meant by locking in??

kateraelDis1L

this is correct^^ single, double, and triple bonds are considered one region of electron density. ;)

kateraelDis1L

LDF strength has to do with size and shape. When an atom is large it is highly polarizable because electrons aren't feeling a strong pull of the nucleus This causes it to have a larger dipole and a stronger London Force. But also, if a molecule's Lewis structure is more linear (larger surface area) ...

kateraelDis1L

I think it has to do with the fact that hydrogen bonds only have to do with molecules that are polar and the bonds are loacted between a pair of other atoms having a high affinity fro electrons. I wouldn't questions Dr. Lavelle, just memorize what he tells you is true with regards to strength.

kateraelDis1L

Yes! they can be expanded further than 10.

kateraelDis1L

^^Yes! so to clarify, electronegativity increases from left to right across periodic table and also increases from bottom up. I liked to think of it as if the element I was looking fro is super close to Flourine, then its super electronegative.

kateraelDis1L

I agree with them^^ to further help, I just have it memorized that highly polarizable means strong London. It is better to differentiate in your head inter and intra molecular forces so you don't get confused.

kateraelDis1L

I feel so relieved after finally being done with that midterm. I think for people who have a more math oriented brain, the content for midterm 1 is a little easier. But for me, I like the conceptual stuff, patterns, and explanations of the content for midterm 2.

kateraelDis1L

In lecture when talking about London Forces, we learned that molecule's forms can differ with respect to the amount of electrons. For example, Pentane is C5H12 and that is a liquid, while Octadecane is C18H38 and is s solid. This is because more electrons means higher negative value, means higher po...

kateraelDis1L

The bonds are polar because of the electronegativity difference. The difference is small (0.38), but differences greater than 0.2 are usually considered polar. We haven't talked much about molecule shape yet, but H2S has a bent shape (four electron domains, two of which are bonded and two of which ...

kateraelDis1L

You are correct in that nickel wants to be in the 3d orbital (n=3) but the configuration of nickel is actually 3d84s2. So you still add the s when writing the electronic configurations for all the elements in this "weird" n=3 type block beginning with Sc and ending with Zn.

kateraelDis1L

What does make use of the d-orbital mean in in this situation ? ^^

kateraelDis1L

So the order goes like this (strongest at top weakest at bottom)
Ion-ion (ionic bond)
Hydrogen bond
dipole-dipole
dipole-induced dipole
London Dispersion Force

(Hydrogen-LDF are all covalent bonds)

kateraelDis1L

Does anyone know why (for example) H2S is a polar molecule? I have learned that the two hydrogen atoms each have partial positive charge and the sulfur atom has partial negative charge. How do we know this though?

kateraelDis1L

Ionic bonds are attraction of opposite charges so like Na+Cl-. Covalent bonds are sharing electrons, have the octet rule, and formal charge so like PO4^3-. Just some background.

kateraelDis1L

As nuclear charge increases there is greater attraction. Keep it simple ;)

kateraelDis1L

So if less points per question does that mean less partial credit available ?

kateraelDis1L

I've heard rumors about the class being curved a little? Does anyone know how that would work with the point system?

kateraelDis1L

I think I need to first understand the octet rule, can anyone help with that?

kateraelDis1L

How many photons with a wavelength of 819 nm are needed to melt 423 g of ice?

I know that it takes 334 j to melt 1 gram of ice, but I can't seem to find where to add this info in to solve the problem???
Thanks!

kateraelDis1L

Hey Kate! Since you know the number of spectral lines is 5, you have to use the equation En= -hr/n^2 where n = 5. You should get an energy of -8.7195x10^-20J. Then you should use the equation Ef-Ei, where Ef = the energy of n=6, and Ei = the energy of n=5. You should get 2.6643 x 10^-20J. Afterward...

kateraelDis1L

The electron in a hydrogen atom is excited to the n=6 shell and emits electromagnetic radiation when returning to lower energy levels. Determine the number of spectral lines that could appear when this electron returns to the lower energy levels, as well as the wavelength range in nanometers. I got ...

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