CHEMISTRY COMMUNITY

sabrina ghalambor 2J

commenting to boost :) thank you again to Dr. Lavelle, the TA's, UA's and also my classmates for being so active on CC and always being incredibly helpful <3 i'll miss u all!

sabrina ghalambor 2J

since this equation comes from inserting the temp at 298K, we shouldn't use this when the rxn is not occurring at standard conditions

sabrina ghalambor 2J

i know that increasing A and T will increase K, but i don't think we'd actually to worry about that for A since in most problems its omitted(like a difference problem)/given to us as a constant

sabrina ghalambor 2J

the question should indicate whether the rxn occurs in basic or acidic solution!

sabrina ghalambor 2J

How do we know C is the zero order reactant? We know because in the context of the problem, there are two examples where the concentrations of [A] and [B] stay the same and [C] changes, yet their rates do not change. Since the rates do not change, that means that the concentration of [C] cannot aff...

sabrina ghalambor 2J

the ° symbol doesn't imply that the rxn is at 298 K, but rather that this is a value calculated from the reaction at equilibrium (i believe). so yes, we can calculate these values at different temperatures. i would assume that H and S will be adjusted to different temperatures but we're normally giv...

sabrina ghalambor 2J

but would an overall rxn order of (for example) 5 mean anything without the context of the problem? because that could involve multiple first order reactants or one first order and 2 second, with infinite possibility for zero-order reactants?

sabrina ghalambor 2J

the overall order of a reaction is found by calculating the sum of the orders of the individual reactants

sabrina ghalambor 2J

i believe we'll be going over this topic in the last 2 lectures! the last ~8 problems on sapling are all from the new content it seems

sabrina ghalambor 2J

this is a chart kate showed us in her workshop last week that might help you!

sabrina ghalambor 2J

your set ups are correct so maybe just try it again in the calculator? also maybe theres an issue in how you entered it or because you moved the ln[A] to be before the -kt

sabrina ghalambor 2J

a linear plot of ln[A] vs time means the rxn is first order,
a linear plot of 1/[A] vs time means the rxn is second order,
a linear plot of [A] vs time means the rxn is zero order.

hope this helps :)

sabrina ghalambor 2J

hi! i'm a bit confused on what the overall order of a reaction means and why we can just add those numbers together? i know how to find values for m,n, and l experimentally but i don't understand why for the overall order of the reaction we just add those values together? thank you in advance!

sabrina ghalambor 2J

i had the exact same question! since removing the parentheses was the solution, i presume that this is because the iron oxide trihydrate acts as a single compound, so adding a coefficient of 2 in front of the whole thing will still multiply the entire compound by 2. we don't need to multiply the wat...

sabrina ghalambor 2J

so your reaction on the right side is correct, but the order is the issue. what the feedback is telling you is that the cathode needs to be on the right, which is referring to the solid Ag. the Cl- ions should both be closest to the salt bridge, so the final answer is what they listed above ^^ hope ...

sabrina ghalambor 2J

ok thank you that makes sense! i just remembered kate went over that in her workshop too lol

sabrina ghalambor 2J

You balanced it right! Don't put the parenthesis... I did that too and Sapling kept marking me wrong :) It should be: 4Fe+3O2+6H2O⟶2Fe2O3∙3H2O Fe2O3•H2O indicates that it contains water. Any chemical compound that has the dot followed by water indicates that it is a hydrate, which means that it's a...

sabrina ghalambor 2J

hi, i'm fairly certain that i've balanced this equation correctly but i'm still getting errors and i'm not sure whats wrong with it. I'm also not totally certain what the product/multiplication of Fe2O3•H2O means so if anyone could also explain that I would really appreciate it. thank you! Screen Sh...

sabrina ghalambor 2J

hi! i'm a bit confused on #11 in sapling because i thought that we should be changing the signs for the standard reduction potentials if the reaction is being oxidized. by this logic, all of my E˙ cell potentials ended up being negative when i did cathode - anode. can anyone explain what i'm doing w...

sabrina ghalambor 2J

i'm not 100% certain on the definition of the heat capacity of the calorimeter, but google says it the quantity of heat absorbed by the calorimeter for each temperature increase of 1 ˙C, which makes sense with the definition of C we've established with other compounds like liquid water or ice. you w...

sabrina ghalambor 2J

a rxn is thermodynamically favorable if the ∆G˙ is negative, and unfavorable if ∆G˙ is positive!

this same rule applies to molecules as well, meaning that a compound will be thermodynamically stable if the ∆G˙of formation is negative, and unstable if ∆G˙ of formation is positive

sabrina ghalambor 2J

work done on a system is positive, work done by the system is negative!

sabrina ghalambor 2J

hi! i'm a bit confused on part c of this problem, which is asking for "changes in entropy of the surroundings and of the system for the freezing of 1 mol C2H5OH liquid" this part of the answer is confusing me because they wrote ∆Hfus as a negative value despite the equation for ∆Ssurr = -...

sabrina ghalambor 2J

this question is "why are so many exothermic reactions spontaneous?" the answer says: Screen Shot 2021-02-17 at 3.14.36 PM.png is another suitable answer for this that the ∆H for an exothermic reaction will typically be large? i didn't understand why the entropy is greater if heat is relea...

sabrina ghalambor 2J

hi! i'm a bit confused on part c of this problem, which is asking for "changes in entropy of the surroundings and of the system for the freezing of 1 mol C2H5OH liquid" this part of the answer is confusing me because they wrote ∆Hfus as a negative value despite the equation for ∆Ssurr = -∆...

sabrina ghalambor 2J

i'm not sure where you got that first equation for ∆U but i would assume that equation is some derivation/comes from the equations for q and w. i believe that ∆U does depend on q w and T just given that it represents internal energy. you can't change the internal energy without some type of heat or...

sabrina ghalambor 2J

as said above, flipping T1 and T2 will just change the sign of the final calculation, but the answer for this one was a positive value (at least thats what mine said was correct lol)

sabrina ghalambor 2J

yes you're correct! if work is done on the system, its sign is positive. if work is done by the system, its sign is negative

sabrina ghalambor 2J

i'm not sure where you got that first equation for ∆U but i would assume that equation is some derivation/comes from the equations for q and w. i believe that ∆U does depend on q w and T just given that it represents internal energy. you can't change the internal energy without some type of heat or ...

sabrina ghalambor 2J

a closed system is just that, closed, but heat can still be transferred in and out of the system. it's like a closed tupperware! we can still heat the food using the microwave, but the matter inside is still the same

sabrina ghalambor 2J

for #6, i thought we would have to include a constant (5/2) to be multiplied by R because neon is a monoatomic ideal gas. i understand how to do the problem (separating the change in volume then adding to the change in temperature) and i tried using this constant during the first step where i calcul...

sabrina ghalambor 2J

Yeprem wrote:I don't know why I was doubting myself thinking there had to be something in the middle columns too. Thank you!

could you explain why nothing goes in the middle columns? is it impossible for both dH to be negative and dS to be positive (or vice versa)? thank you!

sabrina ghalambor 2J

so i'm watching lecture 13 from 2/5 and i'm a bit confused on the deltaS= q/T equation. since we said that q=-w, are we also allowed to consider deltaS= -w/T another equation? if that's wrong could someone please explain? thank you in advance!

sabrina ghalambor 2J

For path A, we need to know the number of moles (n). We find this by using PV=nRT Where P is given (2.54 atm), V is given (3.98L), T is given (298K) and R is the constant 0.0821 Latm/Kmol After solving for n, you would plug it into w=-nRT x ln(Vf/Vi) We use this specific equation because it is a re...

sabrina ghalambor 2J

hi! if you set the q of water = -q iron, it should give you the right answer because they both have the same Tfinal since no heat is lost to the surroundings.

sabrina ghalambor 2J

depending on the unites, we will use a different value of R. all are listed on the constants sheet. if you have a problem that has L.atm, you would use .08206. if the problem is with joules, you use 8.314

sabrina ghalambor 2J

thank you! whatever i do though i still can't get the correct calculation for this problem and i think it might have to do with the moles. are we using delta n or just n, and which value of the pressure do we use to calculate n? thanks

sabrina ghalambor 2J

can anyone explain why we can't use the original w= -P dV equation instead? i'm comfortable using the other but i don't understand why my answer using w= -P dV is wrong.

sabrina ghalambor 2J

volume has very very small changes so it's written as dv!

sabrina ghalambor 2J

yes! steam causes burns instead of just 100 C water because of the extra amount of energy used to convert liquid water into water vapor. all that extra energy hits the skin and converts steam back to a liquid, causing the burn

sabrina ghalambor 2J

can anyone explain why we have to rewrite the equation as N2+ (5/2)O2 >>> N2O5 ? the question explain why we need to do that or how it changes the outcome of the problem. in general, if anyone could explain the steps to this problem i would really appreciate it!!

sabrina ghalambor 2J

can anyone explain why they calculated the moles for the NH4(NH2CO)2 ? if this counts as a pure solid, then it should be irrelevant right? it's not used anywhere else in the actual calculation so is there a reason for adding this or no? thank you!!

sabrina ghalambor 2J

we can assume x is not significant if the percent protonation is less than 5% and if the Ka is less than 10^-4. i normally check both rules though to make sure the approximation is safe to make

sabrina ghalambor 2J

i think the reason why its telling you to add them in the solution is because its assuming you didn't leave [B] in the denominator as x- (1.29*10^-5). since 1.29*10^-5 is equal to [BH+], that's the extra bit its telling you to add. basically since the Kb was less than 10^-3, you probably left out th...

sabrina ghalambor 2J

we can assume x is not significant if the percent protonation is less than 5% and if the Ka is less than 10^-3. however, i normally check first to see if the 5% rule works even if the questions doesn't ask for percent protonation because sometimes the Ka<10^-3 but the percent is still above 5%. it m...

sabrina ghalambor 2J

this depends on whether the cation or anion is often found in strong bases or strong acids. If the cation of a strong base pairs with an anion of a weak acid, it would be a basic salt. If both are strong or both are weak, the salt is neutral. If the cation of a weak base pairs with an anion of a str...

sabrina ghalambor 2J

commenting here so i don't forget to check back hehe i really need to go this week!

sabrina ghalambor 2J

does anyone know why in this question we don't have to account for ignoring the Na in the .058M solution? I initially tried to calculate what the molarity would be without the Na and I used that value in my calculations instead. can we just ignore that since we ignore Na as a product and as a reacta...

sabrina ghalambor 2J

yes! i personally think graphing is a faster method of finding the x values but you can also use the quadratic formula to solve :)

sabrina ghalambor 2J

normally there will be a negative answer that would be extraneous if it ends up being a quadratic equation. but if thats not the case, i'm not sure how to differentiate. anyone else know what to do?

sabrina ghalambor 2J

total pressure is the sum of all the equilibrium partial pressures

sabrina ghalambor 2J

i agree with the comment above because unfortunately you dint ask any specific question. that said, you would solve this equation as if the molarities given were the equilibrium molarities and use those values to find Q. Q would equal [NH3]^2/([N2]*[H2]^3)

sabrina ghalambor 2J

shifting to the left favors reactant production, and shifting to the right favors product production!

sabrina ghalambor 2J

i just redid this problem after the comments here said to use the inverse Kp, but that kept giving me the wrong answer even though ever other part of my problem was set up similarly to everyone's in the comments. i only got the correct answer once i used 391 instead of 1/391, can anyone explain why ...

sabrina ghalambor 2J

Why is the gas constant a constant? To me that part is a bit confusing because I would have thought the gas constant differs for each gas. my understanding is that the gas constant has more to do with the state of the matter rather than its content if that makes sense? like it doesn't matter if its...

sabrina ghalambor 2J

P is pressure, V is volume in liters, n is moles, R is a constant (that changes depending on the unit of pressure), and T is temperature (in Kelvin)

sabrina ghalambor 2J

n/V is rearranging the variables to represent concentration because n=mols of x and V is liters!

sabrina ghalambor 2J

Q is P/R at any time during the reaction and we can use it to determine which reaction is favored.

sabrina ghalambor 2J

For a balanced chemical equation with all stoichiometric coefficients equal to one, what is the simplest expression for the equilibrium constant? A. The ratio of the equilibrium concentrations of reactants to the equilibrium concentrations of products. B. The ratio of the equilibrium concentrations ...

sabrina ghalambor 2J

Does anyone know why the [OH-] is different from the pOH. Why is it a very small negative number? Thanks in advance! the [OH-] is different from pOH in the same way [H+] is different from pH! [OH-] is the concentration of OH- ions in the water and the pOH is used instead of just the concentration b...

sabrina ghalambor 2J

i'm fairly certain that any d-block element in groups 15,16,17, and 18 can have an expanded octet, but to different extents

sabrina ghalambor 2J

i don't think i got the full 1.5 hours because it took a while to check everyone's papers but towards the end i had to guess on easy fundamental problems bc i didn't have enough time to do the full calculation :( definitely not enough time

sabrina ghalambor 2J

the overall compound is an ion, so you add an "ate" at the end

sabrina ghalambor 2J

yes! a lower pH indicates a strong acid :)

sabrina ghalambor 2J

Hi, this was very helpful and wanted to add in my compound for others to see a different example. Compound: [Co)NH 3 ) 5 Cl]Cl 2 Practically the same steps as Idella, Since NH3 is my ligand and there are five, I said Penta ammine. Next is the one chlorine, so I did not put a prefix just "chlor...

sabrina ghalambor 2J

gonna add a comment in here so i don't forget to come back and check it hahaha

sabrina ghalambor 2J

hi! use the original name first, the prefix is irrelevant for the alphabetical naming order.

sabrina ghalambor 2J

hi! en is shorthand for NH2CH2CH2NH2, which is ethylenediamine, a bidentate ligand.

sabrina ghalambor 2J

thank you both!! these were really helpful

sabrina ghalambor 2J

i've watched the lecture and a few youtube videos about ligands by now and maybe something just hasn't clicked yet but is there a difference between ligands and polyatomic ions? i understand that the more complex ions are created through reactions but i previously learned about CN- as just a polyato...

sabrina ghalambor 2J

someone else explained it here very well!! https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=30&t=68848&p=278161&hilit=coplanar&sid=8a3b656ed31d8fef121069bf63826893&sid=8a3b656ed31d8fef121069bf63826893#p278161 basically it has to do with pi-bonds and that they should be placed ...

sabrina ghalambor 2J

tetrahedral is correct! if it's not cooperating when you write that in maybe its a spelling error? otherwise i'm not sure how to help :(

sabrina ghalambor 2J

there's no favorable orientation here because none of these are resonance structures! the molecule will have the same charges no matter the shape so any orientation will work

sabrina ghalambor 2J

yes, every bond or lone pair is considered a region of electron density!

sabrina ghalambor 2J

^ i think you're right! 4 regions of electron density translates to sp3, 5 is d1sp3, 6 is d^2 sp^3, etc.

sabrina ghalambor 2J

more ionic vs more covalent character in a bond will relate to the polarizability/polarizing power of an atom, the size of an atom, and it's electronegativity. periodic trends are very important for this!

sabrina ghalambor 2J

does anyone here know what the limit is on an expanded octet? i understand why/how they exist but how do we know that an atom can only hold (for example) a max of 10 electrons and not say 16 electrons?

sabrina ghalambor 2J

as others have said, bond angles are experimentally determined so we don't need to calculate them. however, as long as you can remember the shape of a molecule and how the atoms are arranged, you can probably determine an angle by dividing the number of atoms around the central atom by 360

sabrina ghalambor 2J

yes, LDF are found in all molecules, but they are the weakest bonds!

sabrina ghalambor 2J

a lewis acid would be an electron acceptor and a lewis base would be an electron donor.
lewis acids can also be: a molecule w electron-deficient central atoms, polar double bonds, and cations.
lewis bases can be any atom/molecule with extra electrons or molecules with a lone pair

sabrina ghalambor 2J

everyone has stated why its HF above, but I think it's also important to remember that the bond lengths between H and other molecules in group 17 increase as you go down!

sabrina ghalambor 2J

1 picometer = 1 * 10^-12 meters, but i think it might be more important to remember angstroms!! ( 1 * 10^-10 meters) both of these are on our formula sheet though

sabrina ghalambor 2J

we won't need to know anything exact, but know that the order from longest to shortest goes single>double>triple

sabrina ghalambor 2J

i saw a post where someone explained it, ill add the link!!

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=31&t=67475&sid=46008056ab505b72841f0ac3d7c559c6

sabrina ghalambor 2J

knowing that the overall charge of the ion should be 3-, i drew single bonds between the P and each O so that every oxygen would have a negative formal charge. phosphorus will have one lone pair to satisfy its formal charge and 5 valence e-. hope this helps!!

sabrina ghalambor 2J

Okay, I've gotten to the last part of this question where there is a table with bond lengths. I'm not sure what ample & overwhelming bond character mean. Hi Jacquelyn! so, in this question, ample just means sufficient or enough to be deemed to have this bond character while overwhelming would b...

sabrina ghalambor 2J

i'm also wondering how do we know which contributes the most/least to the structure of an ion?

sabrina ghalambor 2J

look at the outlines on his website!! he'll probably tell us which exact outlines to know but i'm assuming we should know the rest of outline 2 and outline 3

sabrina ghalambor 2J

i recommend the drop in sessions and workshops because they answer and explain all the questions so well!! i also use the worksheets from other discussion sections and i think those problems are the most helpful for practice and you can go to any TA for help with them.

sabrina ghalambor 2J

i'm worried about grades and if i'll have enough time to prepare for our next exam but we have plenty of review options so i feel confident we can study to all do well!!

sabrina ghalambor 2J

to the person above, i'm pretty sure most of them are all or none, but i know at least one of the multiple choice questions offers partial credit if you had the right answer but the wrong explanation or something like that!

sabrina ghalambor 2J

i normally use the other equation, freq= Rydberg constant*((1/n^2)-1/n^2)) where the order is n1-n2, for problems like this. i find it easiest to calculate and does the two electron levels in one step! hope this helps

sabrina ghalambor 2J

yes, c is a constant always equal to 3 x 10^8 m/s

sabrina ghalambor 2J

i think you might've combined 2 equations or something like that because the speed of light shouldn't be involved as far as i know! it could also be you trying to convert lambda to frequency but put the c in the wrong place? i'm not sure why you have that written but just know that its wavelength= h...

sabrina ghalambor 2J

happy halloween!! i worked at the polls today and will be everyday until election day so halloween was catching up on hw for me :( still had some reese's tho!!

sabrina ghalambor 2J

yes it should end in n=1! Lyman corresponds to UV light so it has a higher energy level

sabrina ghalambor 2J

i'm confused what this question means when it says wavelengths plural and how you knew to divide 1 by that. could you explain how you got there? and if you did solve the rest of the problem i would also rly appreciate if you could explain it! thanks

sabrina ghalambor 2J

we don't need to do the week 4 problems before the midterm! only 1-19 will be tested on and the rest are for week 4 which we can do afterwards :)

sabrina ghalambor 2J

it's back up now! but i've also saved sapling as a bookmark on my computer and it worked for me!

sabrina ghalambor 2J

similar to calculus or when computing a rate of change, you always do final-inital. that way it should be positive since it ends on a lower n level than it starts!

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