CHEMISTRY COMMUNITY

Joshua Hughes 1L

Thank you, Dr. Lavelle, for being such a great professor for the last two quarters. I'm really going to miss you in 14C and 14D, your smile is always so contagious in lecture. Also, the amount you care for your students is really evident based on all the outside hours and resources you made availabl...

Joshua Hughes 1L

these two problems from the UA Final Review session were both False Can someone please help explain why for both of these, please? 1D) True/False: Ecell will increase if the size of the anode metal is increased. 1E) True/False: Ecell will increase if a substance is added to the cathode side that pre...

Joshua Hughes 1L

by standard state do you mean its standard phase state (ie liquid solid etc)? If so, if you are told that the temperature is RM Temp or 25 Celcius than it should be in its standard state. Two elements are liquid in their standard state: mercury and bromine. everything else you can kind of guess if i...

Joshua Hughes 1L

I believe it meant to say that it is equal to .59 and not a minus .59. when you calculate it out Ln(k'/k) is .5937

Joshua Hughes 1L

Cooper has a Csp of .3846, Aluminum has .9025 and Iron has .4495 So the Specific heat for the different pots is Cu<Fe<Al Also, you want to use Csp (specific heat) and not Cm(molar heat capacity) because Sp takes into account the grams or weight of the material and the question states for Identical w...

Joshua Hughes 1L

It would be more efficient to use a copper pot because the specific heat capacity is the lowest of the three (lower than that of iron and aluminum) meaning that it would take less heat to 'heat' up a copper pot and therefore less heat in total to bring the temperature of the water up

Joshua Hughes 1L

usually when there is a large volume change due to a difference or change in pressure(ie the external pressure). Large and/or fast reactions would likely be irreversible reversible reactions are smaller, continuous little fluctuations/changes and often (in this class) are isothermal. When doing this...

Joshua Hughes 1L

for this problem, it really shouldn't matter so long as you use a specific heat that has Kelvin in the units so they can cancel out properly. The only time you really should worry about converting to Kelvin is when it is in the natural log of T2/T1. make sure T2 and T1 Kelvin in those cases I think

Joshua Hughes 1L

remember the bottleneck effect thing? its like if the slow step only allows a certain rate for that step to be complete then it doesn't matter how much faster the other steps are because the overall reaction is limited by the slow step EDIT: OOPs didn't see someone else commented, their explanation ...

Joshua Hughes 1L

Wait I am confused, someone said mechanisms shouldn't be on the test and then also the class website says "Test 3 covers Kinetics Chapter 15.1 to end of 15.6"... why is everything fair game? Catalysts and a lot of the other topics weren't really covered in class were they (yet)? Im sure we...

Joshua Hughes 1L

Im just going to go over all the problems that would likely be relevant to this weeks test knowing that it only covers up section 15.6. For chapter 15 the assigned problems 3-39 im pretty sure are all included but was wondering about some of the integrated exercises / which of those would be most re...

Joshua Hughes 1L

has he taught us catalysts yet? It's in section 15.14 I think so it shouldn't be on our test right?

Joshua Hughes 1L

so if it says average rate of reaction or just rate of reaction I should know it should be positive. Rate of formations should also be positive. I would only have a negative rate if it specifically was asking for the rate of consumption or something like that of a certain reactant? What are other ca...

Joshua Hughes 1L

ln [At]/[Ao] = -kt solve for k so it k=(ln([At]/[Ao]))/(-t) or k= ln([Ao]/[At])/t then plug in the given values ([Ao] being the .67 mol/L)

Joshua Hughes 1L

For part, c use the equation ln [At]/[Ao] = -kt and rearrange to set equal to K. Then plug in given values of [Ao] and the time(115s). You also need to plug in [At] which you get by subtracting the rise in [B](multiplied by mole conversion of 2) from [Ao]

Joshua Hughes 1L

For part A of your question, you can use the half-life for first-order reaction equation to solve k. k = .693/1000 second = about 6.93 x 10^-4 /s

Joshua Hughes 1L

So for question 15.3 part A. they did final minus initial concentration of NO2 which was negative since it was decreasing. The solutions manual added on another negative to make the rate positive. Why? if it is the reaction of NO2 isn't it like asking for the consumption or rate of degradation of NO...

Joshua Hughes 1L

Oh, same question applies to the bolded equations for half life which are farther down in the document

Joshua Hughes 1L

On the class website, it has a link to some of rate laws https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Kinetics_Integrated_Rate_Laws_Examples.pdf I was wondering if it would be recommended to know and have a basic understanding of the differential and integrated rate laws for 1st...

Joshua Hughes 1L

Are unique average rates always positive when it is about the products and negative when talking about the consumption of the reactants? In a general sense, I am a little confused as to what the unique average rate is/its use conceptually

Joshua Hughes 1L

I really don't understand why one of the half-reactions seems flipped for this problem. I looked at a recent post and still didn't understand it. I thought the half reactions would be slit so that Au+(aq) goes to Au3+ and Au respectively but for some reason one of the half-reactions is Au goes to Au...

Joshua Hughes 1L

You could also look up other youtube vids or find some Khan academy ones that help too
I just looked this up and found an hour khan academy vid all about Chemical Kinetics Rate Laws
https://www.youtube.com/watch?v=irJ4ANrUXPI

Joshua Hughes 1L

For any chemistry or life science videos, i just go to youtube and look up to see if Bozeman Science has any videos on the topic. Examples: https://www.youtube.com/watch?v=6mAqX31RRJU&list=PLllVwaZQkS2op2kDuFifhStNsS49LAxkZ&index=35 https://www.youtube.com/watch?v=WDXzVI8SmfE&index=36&am...

Joshua Hughes 1L

oops sorry didn't see someone else posted

Joshua Hughes 1L

the half reactions were balanced first (parts c and b) and then they were added together. the whole thing was rebalanced for number of atoms and then things were canceled out. the half reaction from part b was multiplied by 3. For cancelling out the 14H+ had (3)2H+ subtracted to get in part d having...

Joshua Hughes 1L

Yep. its the same thing except its the concentrations of some reaction that isn't at equilibrium and we use Q to compare to K to see how the reaction will shift or change as it heads towards equilibrium

Joshua Hughes 1L

it should apply both to reversible and irreversible reactions so long as it is isothermal. often times though it seems like our class focuses on reversible isothermal examples (like on the midterm)

Joshua Hughes 1L

So we only need to know up to 11.3 right with Gibbs free energy and stuff right? We don't need to know redox reactions or any Electrochem right?
Are there any topics covered in Ch 11 (section 1 to 3) that aren't going to be on the test?

Joshua Hughes 1L

actually wait. I looked up the sig fig rules on the chem 14b website and basically, it says for logs you do it to the number of decimals. so for part C of that problem, it is lnK=-52.6 (according to answer key) which is e^-52.6. This is one sig fig because it has one decimal in the exponent. Same wi...

Joshua Hughes 1L

Hmm... Honestly I can't think of any reason and i think it could just be a mistake. Usually coefficients are used for sig figs since they considered pure numbers (i forget the real term but they basically numbers not used to determine sig figs

Joshua Hughes 1L

In today's review session a calculation was done that used the formula Delta U =(3/2)nRdeltaT=0 and it was said to be for isothermal reactions (this is for problem 4). Im confused because for an isothermal reaction wouldn't Delta U = w. Q=MCDeltaT and since it isothermal Delta T would be zero and th...

Joshua Hughes 1L

the problem was: True/False: One cannot calculate the entropy of vaporization for water at room temperature because water has a boiling point of 373K. The answer was False. Can someone please help explain this to me because I didn't understand the bit in the session. He started setting up a calculat...

Joshua Hughes 1L

its actually supposed to be 3/2 time R. (3/2)R is Cv and (5/2)R is Cp. The problem in the solutions manual had some issues and you should check the Errors pdf on the website because 9.35 was on there

Joshua Hughes 1L

No, I really doubt you will need to draw out the orientations as long as you can know the number there really are. Drawing them is just helpful but like if you had some molecule in which there were many many possible orientations it could get pretty annoying. So only do it if it helps you or if the ...

Joshua Hughes 1L

honest question here, does posting in the chem jokes section count as a weekly post that we can get credit for?

Joshua Hughes 1L

The book splits this problem into two steps to find the entropy: Step 1: Reversible isothermal compression at the initial temperature from the initial volume to the final volume (and use Eq. 3a). Step 2: An increase in temperature of the gas at constant f final volume to the final temperature (and u...

Joshua Hughes 1L

Isn't just because the problem says assume at constant pressure. It gives you some of the molar heat capacities assuming constant pressure (ie Cp)
I don't' have to solutions book with me to see what they did but that's what I would guess from the wording of the problem.

Joshua Hughes 1L

Can someone go over with me the different meanings of the different symbols, please. I think I know the one with the degree symbol in the corner and the enthalpy of formation ΔHf and ΔHfus and the like but there a few others I was kind of unfamiliar with:
ΔHc
ΔHL
ΔHb

Joshua Hughes 1L

I remember a question that was like why would there be a difference in the enthalpy calculated from bond enthalpies and the enthalpy of formations. Why is there a difference and which is more accurate? Would the bond enthalpies be more inaccurate because its just an average and can vary a lot due to...

Joshua Hughes 1L

I understand how to do the calculations, I got what the Solutions Manual got but conceptually I don't get what the values I got mean and how they prove that benzene with resonance is more stable then if it had 3 single bonds and 3 double bonds. I understood it when we learned it in Chem 14B but how ...

Joshua Hughes 1L

I don't think we will need to know the derivation, its good to know how to get and rearrange some of the equations but as far as the integral goes it doesn't really appear in the homework so it probably won't be included in our upcoming test.

Joshua Hughes 1L

"Thermal energy not in the process of being transferred; it is not in transit," You can think of this as the average kinetic of all the atoms of something/a system at a moment, there isn't any transfer of energy. heat is the transfer/transit of energy Often times those words are used kind ...

Joshua Hughes 1L

According to the textbook "When energy leaves the system as work, the internal energy of the system decreases and w is negative"(in other words when the system is doing work). if the air expanded and release energy the system is doing work and releasing energy (thus the negative sign) When...

Joshua Hughes 1L

Δ U= q + w
In this example it said Δ U = 0, meaning there was no change of internal energy of the system.
So long as Δ U is 0 with no change to the internal energy, q and w should be the same but with opposite signs I think. q=w- if Δ U=0

Joshua Hughes 1L

The equations be used I think is Q = mcΔT q (heat) =(mass in grams)(Specific heat capacity)( Tfinal-Tinitial) They had to convert the mass from grams to moles because the specific heat they used wasn't in terms of grams (they got the specific heat from one of the tables in the chapter) The reason th...

Joshua Hughes 1L

The equations for the heat of the water of the ice and the heat of the surrounding water should be set equal to each other because the two temperatures will approach an equilibrium (which should be 31 degrees C), ice warming up and melting and the surrounding water will cool down because of the ice.

Joshua Hughes 1L

OOps sorry, I didnt see someone else posted.
You can also look up some websites that explain stuff a bit more in detail or provide more examples than the book.
Try this web page:
http://www.brighthubengineering.com/the ... processes/

Joshua Hughes 1L

Are you just looking for how they differ in their definitions? otherwise I'm not sure I entirely understand. A reversible process is one whose direction can be "reversed" ( the system can be restored to the initial state from the final state) by inducing infinitesimal changes to some prope...

Joshua Hughes 1L

It probably meant like vespr with molecular shape (not counting lone pair areas of electron density)

Joshua Hughes 1L

Can someone explain to me the HCl and NaOH pH question please. Isn't this some kind of salt thing which I thought wouldn't be on the test. How does one find pH for that problem because I wasn't sure/ I wanna know if what I did was right Also... If we aren't supposed to ask questions about the final ...

Joshua Hughes 1L

oops sorry, we posted almost at the same time and i didnt see that someone else posted

Joshua Hughes 1L

nm is nanometers which is 10^-9 meters and you can use this to convert between the two. so 2nm is 2 x 10^-9 meters.

Joshua Hughes 1L

yeah 2.60 is normally three and 2.6 is normally 2 sig figs except i thought for pH you don't count the numbers to the left of the decimal point in sig figs. So in order to have 3 sig figs you would need 3 numbers after the decimal ie 10.111 and not 10.1 would be three sig figs for pH.
I

Joshua Hughes 1L

I'm confused about rounding with pH. I thought that you only count sigfigs after the decimal point, so two sig figs for pH would actually be like 2.60 or 3.55. The solutions manual is confusing me because I can't seem to follow how they are rounding for some of the problems. Problem 12.29 part a get...

Joshua Hughes 1L

I really want to get back my test 4 early before my final so I can have time to review it because I know I missed questions on it and I am not very strong with that material. We were going to get them this Thursday at discussion but all Thursday classes are canceled according to UCLA.edu and bruin a...

Joshua Hughes 1L

Does this mean the material that was going to be covered today(something about salts right) wont be included on the final or will we just learn it Friday in place of the review for Sunday's final? Also, does this mean the review sessions later today at like 7 will be canceled or rescheduled?

Joshua Hughes 1L

Just googled this question after posting to understand it a bit more Because of this large electronegativity difference, and the fact that Fluorine is a smaller atom, it doesn't let go of the Hydrogen and donate an H+ very easily because that would require something to break the really strong bond, ...

Joshua Hughes 1L

I was wondering why the ionic equation wouldn't be (H+)+(F-)+(Na+)+(OH-) > (Na+)+(F-)+H2O instead of HF+(Na+)+(OH-) > (Na+)+(F-)+H2O I think it is because HF doesn't disassociate in water. Fluorine is like the most electronegative and there is a big electronegativity difference between Hydrogen and ...

Joshua Hughes 1L

Oops sorry didn't see someone else responded before me

Joshua Hughes 1L

what's the difference between an amphoteric compound and an amphiprotic compound? Amphoteric compounds are compounds which can act as either an acid or a base. (either by the Lewis or Bronstead and Lowry acids and bases definition, by the Lewis definition it is by the donation/acceptance of lone pai...

Joshua Hughes 1L

The easiest way is to look at how many lone pairs the central atom will have. AX3E2 is T-shaped AX4E is Seesaw shaped. There are 5 regions of electron density, meaning it has is a trigonal bipyramidal shape in regions of electron density, but two of those 5 are lone pairs so it becomes a molecular s...

Joshua Hughes 1L

Here is a link to a website that kind of explains the reasoning for the bond angles for H2O, OF2, and OCl2. https://www.dartmouth.edu/~genchem/0102/spring/6winn/VSEPR2.html OCl2 is kind of an exception because the bond angle is actually greater than the expected 109.5 because for some reason the rep...

Joshua Hughes 1L

Why is it that molecules with the same general VSEPR formula can still differ in angle degrees? Do you mean how like H2O and say Cl2O or OF2 would have slightly different bond angle even though they should all have the configuration of AX2E2 giving them a bent molecular shape (tetrahedral in regions...

Joshua Hughes 1L

First draw out the Lewis structure with Carbon at the center, single bonds between C and the Hydrogens and a double bond between C and O. Seeing that there are three atoms that carbon is bound to we know this is trigonal planar and that the bond angles are all expected to be 120 degrees. Then figure...

Joshua Hughes 1L

Does CH2 require hybridization? It seems like the two hydrogen atoms would bond with the unpaired electrons in the 2p orbital of carbon without the need for hybridization. If this is so, why doesn't CH2 form instead of CH4? In the brief moments that CH2 would exist by itself, I don't think it would ...

Joshua Hughes 1L

Actually, scratch that, H20 wouldn't be a good example of what I was describing. The oxygen would have 2 lone pairs that would cause the shape to act more like a bent tetrahedral. Oops, my mistake.

Joshua Hughes 1L

Lone pairs can shape the molecule based on how they repel bonded e- pairs and other lone pairs. The repulsion strength goes in this order: Lone-lone pair>Lone-bonding PR, Bonding-Bonding Pair. meaning that lone pairs repel other lone pairs the most and often 'want' to be opposite each other on an at...

Joshua Hughes 1L

Wait I'm also am a bit confused on the solutions manual's explanation. I still don't understand why the book would say that "electronegativity and polarity arguments would predict C- F bonds to be the weakest"? To me, the reasoning provided above about F being the most electronegative and ...

Joshua Hughes 1L

Does anyone have any suggestions for good websites or videos that go over figuring out how to do Lewis structures for a more basic explanation, maybe for some of you what you would often go to in AP chemistry to try and learn or get more practice? There aren't any modules for this chapter and I am k...

Joshua Hughes 1L

I have an additional question that kind of follows up my previous post. If phosphorous or some other exception is filling up beyond an octet why would it not fill up the 4s orbital before going into the 3d orbital? I mean I know that 3d comes before 4s but at least when going by the periodic table a...

Joshua Hughes 1L

Yeah, I was a bit confused about that as well from one of the review sessions earlier today. I think by size she meant just as you go up in the shells (ie n=3 or 4) and the atom is bigger there are more orbitals for the electrons to fill, and specifically, the exceptions for the octet rule come into...

Joshua Hughes 1L

How do we know if something is an exception to a period trend? Like can someone explain why Be and O would be an exception to the ionization trend ( I may have messed that up, I think it was O and Be)? I think it was briefly explained in class but I didn't understand it. I think for the most part I ...

Joshua Hughes 1L

So resonance is basically just the different possible configurations of a compound right? Like the Double bonds can be in different spots and we don't know which chemical structure the atoms actually are arranged in so we draw multiple Lewis structures to account for the multiple possibilities. What...

Joshua Hughes 1L

Wait why don't we use subscripts for d? If the other shell uses them why wouldn't we do it for other shells? Is it because there are just a lot of subshells that we would need to note?

Joshua Hughes 1L

I apologize in advance if I missed a similar question to this in a previous post, I did look through ones that I believed to have similar subject lines. I am confused how when we are given an N how we know what l is? I remember writing in my notes that l=n-1 and that l is allowed to be values like 0...

Joshua Hughes 1L

I apologize in advance if I missed a similar question to this in a previous post, I did look through ones that I believed to have similar subject lines. I am confused how when we are given an N how we know what l is? I remember writing in my notes that l=n-1 and that l is allowed to be values like 0...

Joshua Hughes 1L

If you need to find the specific amount of energy that would be needed to eject an electron from a metal that's when you know to set it equal to zero. Or if you know the energy(or are given enough information to calculate it yourself) that it took to eject an electron but need to find the work funct...

Joshua Hughes 1L

Would this also apply to Planck's constant? Sometimes I see 6.626*10^-34 Js but other times it's shortened to 6.63*10^-34 Js. I would use 6.626*10^-34 Js, my TA said to use that and in general, it is just more accurate. When it comes to the test I think Planck's constant should be given on our sheet...

Joshua Hughes 1L

So I did the problem and got the final answer as 3. What I'm confused about is N1 and N2, I know one is the final and the other is initial energy level but which one is N1 and which is N2. I wrote part of the equation as ((1/Nf)-(1/Ni)) or is it supposed to be the other way around with the initial m...

Joshua Hughes 1L

Quick question about these net ionic equation thing. Do we need to write the charges for everything or is that just a little extra thing that the solutions manual does? Like Cu2+ (aq) + 2OH-(aq) ... Also in regular reactions/equations when would we need to write the charges if ever? Also in a reacti...

Joshua Hughes 1L

Honestly, I doubt it, if it doesn't say it in the review section of topics we should have learned then we shouldn't have to. (link to the review document that lists the topics we should have some understanding of: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/Chem14A_Outline1_Rev...

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