CHEMISTRY COMMUNITY

Nan_Guan_1L

Hi! I can help with this one. so remember that the rate of the overall reaction is governed by the rate of the slowest step. so first you would need to identify the sow step with is step #2. then, write out the rate law for step #2: rate=k2[HClO][I^-] however, comparing step 1 and step 2 we know tha...

Nan_Guan_1L

first of all, [X] vs t should be shown as a line with negative slope in both cases since reactants decrease in concentration; [Y] vs t should be shown as a line with a positive slope since products accumulate in the process. for first or second order reactions, rate=k[X] or rate=k[X]^2. which means ...

Nan_Guan_1L

I also tried multiple times trying to figure out this one. I did not notice that M^2/s should stay in the answer box since it is neither one of them. Ultimately I figured out this question by writing out the formulas for each order reaction and then figure out the expression of k and thus the proper...

Nan_Guan_1L

big THANKYOU to Dr. Lavelle. Had a great time learning Chem 14A & 14B with you. I love how you approached every concept in detail and explained everything so thoroughly! To be honest I didn't really need to read the textbook because you explained the concepts so well! I can only imagine how diff...

Nan_Guan_1L

you would use an inert conductor such as Pt to transfer electrons if the half reaction has no conducting solid. for example, if both the reactant and products are in solution, then you would need pt.

Nan_Guan_1L

I would really suggest going over the textbook questions Dr. Lavelle assigned. Personally, I've found them very helpful in terms of preparing for exams.

Nan_Guan_1L

Dr. Lavelle talked about many aspects that you can use to tell whether a reaction is zero, first, second order. for one you can look at the graphs if given; you can also look at the values of half lives; you can also do a ratio of two rates of the same reaction, and see what the exponential is, whic...

Nan_Guan_1L

we generally use Ea for activation energy.

Nan_Guan_1L

Hey I was just wondering why electrodes are usually represented as only on one side of the cell diagram? For example, Cu(s) | Cu 2+ (aq) || Fe 3+(aq), Fe 2+(aq) | Pt(s). I thought Pt is in contact with both sides of the cell? Thank you!

Nan_Guan_1L

you know how LS 7A provides you with a weekly study guide, you can check Dr. Lavelle's website for something similar to that which helps you check your understanding of the big concepts and ideas. Other than that, I would really suggest go over the textbook questions Dr. Lavelle has listed out on th...

Nan_Guan_1L

i think kelvin is a safer choice, since all the formulas were created using kelvin basically. using celsius would cause an issue with the units not matching up, and you would have to switch all other components that are related to temperature from kelvin to celsius. that sounds like a lot of extra w...

Nan_Guan_1L

from the equation that connects delta G, delta H and delta S, in case of when delta H and delta S are both positive, it really comes down to the temperature to determine whether the reaction is spontaneous or not.

Nan_Guan_1L

Yes these are state functions. Remember the diagram Dr. Lavelle drew in class-- if it doesn't matter how you reached from A to B, then this would be a state function.

Nan_Guan_1L

the determinant really is delta G. if Delta G is negative, then the reaction is spontaneous. But you would also need to consider factors that influence the delay G, ie delta H, temperature, and delta S. remember the formula that puts these three together: delta G=delta H -T delta S

Nan_Guan_1L

I had the same issue understanding the concept. I think for irreversible reactions, the process happens rapidly and the change is significant and relatively larger. But for reversible processes, the changes took multiple steps to happen and each step is not as significant and thus reversible. This i...

Nan_Guan_1L

In order to get the correct ICE table, both factors would matter since the coefficient of reactants and products determines the coefficient of x, and whether it’s used up or being produced determines the sign in front of x

Nan_Guan_1L

We would use the ice table cuz weak acid / bases don’t ionize completely, whereas compared with strong acid / base. The C in ice allows us to consider the change in concentration.

Nan_Guan_1L

I believe based on what we’ve learned so far, temperature is the only factor that affects the Equilibrium constant. Pressure and Volume doesn’t affect it at all.

Nan_Guan_1L

I think the best solution to that is by being clear of the list of strong acid/ bases. You can find the list in sapling solutions and also in the textbook. Basically everything beside those would be considered a weak acid / base.

Nan_Guan_1L

Basically what matters is the unit of R. So you would want to take a look at the units you have got and the unit you want to end up with. Usually R could fix the difference between the units and you would just select the R value with the proper units.

Nan_Guan_1L

Cp=Cv + R. Cp refers to the conditions under constant pressure, Cv refers to constant volume. If you take a look at the textbook problems you’ll find abundant examples

Nan_Guan_1L

In a closed system, you would need to consider both work and heat as the two aspects are different in nature. The total amount of energy change doesn’t really tell you much, since you would also want to consider the nuance changes going on inside the closed system.

Nan_Guan_1L

hey guys, i was wondering how does the units match up for the formula w= -P delta V? cuz P is usually in atm or barr, and V is usually in mL or L. how does multiplying these two together get the unit for work which should be J or KJ? don't we always have to multiply the result by R to convert it to ...

Nan_Guan_1L

Dr. lavelle said in class about reversible expansion in isothermal conditions. I was wondering what does it mean by expansion is reversible? I think Dr. Lavelle said since the change is very small each time, it is reversible. just checking my understanding.

Nan_Guan_1L

it's pretty much similar to ΔH and ΔH⁰, ΔS and ΔS⁰. the ⁰ simply means the calculations are done under standard state

Nan_Guan_1L

you can use the moles of NaN3 to calculate the moles of N2 since according to the chemical reaction their mole ratio is 2:3. you can find the moles of NaN3 by dividing its mass by its molar mass

Nan_Guan_1L

that's what i did for my sapling questions, and the final answer seems to be the same. As long as you make sure the units cancel out, I don't think there would be any problems.

Nan_Guan_1L

i think both ways would work and would give you the same results. because delta G, delta H and delta S are all state functions and so it shouldn't matter how you got your results. for this specific question, I simply added the delta G together since it was provided. so you would reverse the second e...

Nan_Guan_1L

just to add on to what's being said above,
bond enthalpies can reflect bond strength: the higher the bond enthalpy, the more energy is needed to break the bond and thus the stronger the bond

Nan_Guan_1L

I think we would need to consider the phase changes especially if the reactants / products aren't in their most stable form there was a sapling question that required similar calculations. There was also an example given in the lecture where Dr. Lavelle mentioned if the product water isn't in liquid...

Nan_Guan_1L

to be exact, NH3 and NH4+ are a conjugate acid and base pair, but you can use Kb of NH3 to calculate Ka of NH4Cl because Cl- would mainly be inert in water.

Nan_Guan_1L

Chem_Mod wrote:NH3 and NH4+ are indeed a conjugate acid-base pair. NH4Cl is simply a salt of NH4+, the Cl- will be completely inert in water. You can use Ka x Kb= Kw to find Ka of NH4+ from Kb of NH3

okay got it! thanks so much :-)))

Nan_Guan_1L

for question a, how did you guys find the Ka for NH4Cl since it wasn't given in the charts provided. I'm confused about whether we can use the Kb for NH3 to calculate the Ka for NH4Cl, since the two aren't exactly conjugate acid and base pairs are they?

Nan_Guan_1L

ellenulitsky Dis 1I wrote:Since you need to know the concentration which is mol/liters, it will be .0025mol/.1L which is 0.025M. as the concentration of [Ba(OH2)] and [Ba+], the concentration of [OH-] is 2*.025M since there are two [OH-]. Hope this helps!

ooops thank you I didn't read the whole question lmao!!

Nan_Guan_1L

just to add on to the discussion above, when I was solving this question myself, I think it's helpful to note that concentration x volume = moles. Also, it's helpful to keep in mind of what stays the same and what changes throughout the questions. For example, the concentration of the 5ml extracted ...

Nan_Guan_1L

can anyone explain how they got their answers for 6A.23 textbook question? Somehow my answer is off by 10^-1. I know that since Ba(OH)2 is a strong base we don't need to do the ICE table. so I thought [Ba(OH)2] = [Ba2+] = 0.43g x 1 / 171.34(molar mass of Ba(OH)2) = 2.509 x 10^-3? the textbook answer...

Nan_Guan_1L

can anyone explain why the answer for this question is -312 instead of 312 kj / mol? I got 312 by doing H of products minus H of reactants.

Nan_Guan_1L

I also faced some difficulties while answering this question. What should we consider as included in the solution in NaClO? I'm confused by the wording of this question.

Nan_Guan_1L

if you don't want to do the 5% test, you can check if Ka or Kb given is less than 10-4. usually we can use approximations if they are less than 10-4.

Nan_Guan_1L

After reading this thread I am a little confused. What does it mean to approximate x as 0? Could you do this on any of the homework problems? I solved all of them with the quadratic equation. How would you do it the other way? when you list out the ICE table, you can assume that the reactant change...

Nan_Guan_1L

just to add on to the discussion, when i did my calculations, i remember double checking to see if we can use approximation. but since Ka of this acid is 8.40×10−4, I remember I had to solve the quadratic equation to get the accurate answer. This might be the reason you didn't get the right answer. ...

Nan_Guan_1L

Hello! to simplify things I usually just remember them as one since basically the calculations are the same. but the only difference to remember is that K is for when the reaction is at equilibrium and Q is anything beside that.

Nan_Guan_1L

saw this on instagram lmao

Nan_Guan_1L

just to add on to the discussion, you can refer to the professor's lecture 2 for guidance. he gave a very similar example during class i remember.

Nan_Guan_1L

can anyone recall why isn't gas considered a pure substance but solid and liquid are? I think Dr. lavelle mentioned in lecture but I can't quite recall. Thank you!

Nan_Guan_1L

besides the discussion above, you can also think of it similar to the density formula: density = mass / volume. In this case, mass would be similar to n and concentration would be like density, if that helps understand the equation better.

Nan_Guan_1L

just to add on to the discussion above,
it also depends on what is given to you. if the question gave you conditions in barr or atm, then go for Kp. if the question gave you concentration, then go for Kc.

Nan_Guan_1L

P = Pressure (in atm)
V = Volume (in L)
n = # of moles
R = gas constant
T = Temperature (in K)

remember to use SI units!

Nan_Guan_1L

i don't think there is a preference as long as your value is correct. but usually we use atm or bars out of convenience.

Nan_Guan_1L

you would only need to consider the aq formed components. gases and solids would not be considered!

Nan_Guan_1L

partial pressure is the gas pressure when this gas alone occupies the container at the same temperature. if it helps the total pressure of an ideal gas is the sum of all partial pressure of the gasses included.

Nan_Guan_1L

[OH-] is the concentration of OH- ions in the solution. pOH represents how alkaline a solution is. Mathematically, we can get pOH by calculating -log [OH-].

Nan_Guan_1L

I'm confused about how to characterize the bond types and count the # of bonds for a resonance structure. for example, how many pai bonds does the resonance for SO2 have? This resonance contains two possibilities and each has 1 pai bond. how do we characterize the bonds in the resonance as a whole i...

Nan_Guan_1L

in textbook problem 2E 25, the answer said that SF4 is a non polar molecule. however when I drew out the VSEPR, the molecule has a lone pair on S atom and shouldn't it be polar? thanks!

Nan_Guan_1L

seesaw has 1 lone pair and 4 bonding pairs, that's a total of 5 areas of electron density.
trigonal pyramidal has 1 lone pair and 3 bonding pairs, that's a total of 6 electron density. hope that helps!

Nan_Guan_1L

In class, Dr. Lavelle gave an example of cisplatin. I was just wondering why does Guanine replaces the Cl on cisplatin and not the NH3?

Nan_Guan_1L

quick question, do we need to use respondus lockdown browser for the final? because I didn't see the link where you could check if your respondus is all set up under the FINAL tab on CCLE.

Nan_Guan_1L

Dr. Lavelle in class said whenever we have a hybridized orbital it is always going to be 90 degrees to the unhybridized orbital. I'm confused as to why it that? can someone please explain. thank you!!

Nan_Guan_1L

I understand how hybridization works, but I'm confused about what's the driving factor for hybridization to occur. for example, Dr. Lavelle gave an example of CH4 in class and said Carbon atom would be hybridized to sp3. So what's driving this hybridization process? how did the electrons just move t...

Nan_Guan_1L

Looking back at my screenshots from Dr. Lavelle's class, I found THIS!! this must be the cutest H2O I've ever seen lmaooooo!! looks just like Mickey Mouse :))))))

Nan_Guan_1L

quick question, does it mean that the chemical bonds are more stable if the bond angle is larger?

Nan_Guan_1L

Just to add to the discussion above: I usually think that Lewis acid and base as a bigger, more general group and it includes the traditional acid and base concept we’ve been taught in high school. Some of the compounds might not be traditionally considered as an acid but it may be considered as a L...

Nan_Guan_1L

Just to add on to the discussion above:
I find this on google and I think it explained the concept well: https://www.shmoop.com/study-guides/che ... d-strength
Hope this helps!!!

Nan_Guan_1L

To find the conjugate base, you remove a proton, which would give you SO4-. To find the conjugate acid, you add a proton, which would give you H2SO4. Hope that helps!

Nan_Guan_1L

Yes you are definitely right that Lewis acid is an electron acceptor. I think for the second part about proton production, you are right as well. Dr. Lavelle gave an example about HCl in class. He said that HCl only exhibits Lewis acid characteristics in aqueous form. So i guess this also depends on...

Nan_Guan_1L

if you are talking about a covalent compound, then the last part is correct because it would provide a more stable structure. As for ionic compounds, I don't think the formal charges apply to that, or is it just we haven't covered yet. I think what we discussed in class about formal charge only appl...

Nan_Guan_1L

from my understanding, yes higher electronegativity generally means the element is more reactive. Take F for example, the element with the highest electronegativity, it has 7 valence electrons and only need 1 more to achieve an Octet, and thus it is very reactive.

Nan_Guan_1L

excluding the d block, you can just look what column is the element in. elements in the first column has 1 valence electron and etc. for the d block, you can write out its electron configuration and see how many valence electrons it have.

Nan_Guan_1L

I think you can still draw the structure horizontally it doesn't really matter. as long as you represent the bond angle is roughly 180 degree I think it works either way you draw it.

Nan_Guan_1L

the equation is meant to describe the wave properties of matter. It is also used to describe the mathematical relationship between the velocity, wavelength and momentum of a particle. this equations applies to both electrons and photons and contributes to the discussion of the dual property of both.

Nan_Guan_1L

for question 2c1 on the textbook. the answer said that only a and b are radicals. but what about c? OH has 7 electrons in total, doesn't that make it a radical, with an unpaired electron on O?

Nan_Guan_1L

hi! for this question, I understand the part about carbon nitrogen bond, since the bond length is closer in value to the carbon-nitrogen double bond. But for carbon oxygen bond, the value is approximately the average of carbon oxygen double and single bond. So what kind of ample bond character does ...

Nan_Guan_1L

besides what listed above, it's also favorable to have negative formal charges on more electronegative atoms.

Nan_Guan_1L

Just to add on to the discussion above, I think it's atoms in period 3 or higher have d-orbitals in their valence shells, so they can accommodate additional electrons.

Nan_Guan_1L

Just to add on to the discussion above: I found this on a chem learning website that says there are three general exceptions to the Octet rule: 1. Molecules, such as NO, with an odd number of electrons; 2. Molecules in which one or more atoms possess more than eight electrons, such as SF6; and 3. Mo...

Nan_Guan_1L

Hi! just to add on to the discussion above. I think what separates LDF from other intermolecular forces is that it exists in all kinds of molecules, no matter what other intermolecular bonds are there. while they are the weakest, some molecules are held together only/mainly by LDF, such as the halog...

Nan_Guan_1L

Hello! I've been a little troubled by the term "delocalized" when describing electrons in resonance structures. I understand how resonance structures are formed, but I don't see in what sense are these electrons delocalized? can someone please explain! thank you!

Nan_Guan_1L

Just to add on to the discussion above, I think there always is a limiting reactant. so if O2 is the excess reactant, you'll have to calculate which is the limiting reactant since it's not always the case where you only have two reactants. But I agree that there is always a limiting reactant in ques...

Nan_Guan_1L

yeah I agree that it should be referring to electrons, especially in this scenario you are describing. And from what we've learned in class, I think most of the times when we say "energy" we would be referring to electrons, since they are the ones moving around and generally exhibiting ene...

Nan_Guan_1L

yeah it's 10 not 5. I googled this question and found a 2017 post on chemistry community where Chem_Adm confirmed this. There's also a section on Dr.Lavelle's website that includes all the textbook corrections. You can also refer to that! Hope this helps!

Nan_Guan_1L

I think both can be used to calculate the energy of an electron, it just depends on the information you're giving in the question. From my experience, I think the equation we need to be careful with is E=mc squared.

Nan_Guan_1L

we usually consider the first electron to spin up (+1/2) and the second electron to spin down (-1/2). but I think Dr. Lavelle mentioned in class that these are just math models that scientists employed to represent the opposite spin of electrons in the same sub shell and they don't really exist.

Nan_Guan_1L

Is there a way to see what question we missed on midterm 1? Thank you!

Nan_Guan_1L

I know Dr.Lavelle went through this in lectures, but I don't think I really understand it: so for electrons in the same shell, why does s- electrons have a lower energy than p-electrons in the same shell? if the outer electrons feel a reduced electrostatic attraction, then why is the order of energy...

Nan_Guan_1L

Hi guys, I have a question regarding diffraction. I understand that as Dr. Lavelle puts it, if light acts as particles, there would be two patterns shown at the spot of the openings, but instead we get this diffraction pattern. I just don't understand why this diffraction pattern shown in the diagra...

Nan_Guan_1L

just to add on to the discussion above, I find the textbook's summary of this very helpful: the textbook mentioned the wave-particle duality of electromagnetic radiation, and said the photoelectric effect means that electromagnetic radiation has particle like properties, and that diffraction means e...

Nan_Guan_1L

by diatomic structure, this question is referring to H2. so in order to find the mass of one H2 molecule, you just take the molar mass of H2, which is 2.014, and divide that by Na (Avagadro's constant), because this constant is referring to the number of molecules in one mole of substance. divide th...

Nan_Guan_1L

In an experiment, 4.14 g of phosphorus combined with chlorine to produce 27.8 g of a white solid compound. (a) What is the empirical formula of the compound? (b) Assuming that the empirical and molecular formulas of the compound are the same, what is its name? I had trouble getting the right answer ...

Nan_Guan_1L

Just want to share: for part c, I was really confused whether you could use the 2.50 x 10^6 hz from part b because that was not given in the main question. but it turned out that the problem can't be solved just using what's given in the main question. Also, when I plugged in 9.11 x 10^-31 for mass ...

Nan_Guan_1L

In the initial question, wouldn't you need to convert the velocity of the electron 3.6x10^3km.s^-1 into m.s^-1 so it would be 3.6m.s^-1 to be in SI units or am I mistaken? for part a of this question, yes you would need to convert that to SI units, but it's 3.6x10^6 m/s. the value should be getting...

Nan_Guan_1L

for most of the questions I've seen, they usually give you the actual yield, mass/molar mass, and some clues to find the actual chemical equations. with these, you could calculate the theoretical yield and there you get your yield percentage.

Nan_Guan_1L

Hello, I also would like to pose a question in terms of understanding with limiting reactant questions; if we ignore the stoich coefficients, we would be focusing on just the mass, correct? So while the moles of a compound do indicate the amount of an element in terms of mass, we still need to keep...

Nan_Guan_1L

I think we do need to include the state of matter at the bottom right of each substance. Most of the questions would give you the state of matter, if not I think we can rely on past experience.

Nan_Guan_1L

the definition for theoretical yield says "amount", so I think we should automatically assume the answer should be in grams if the question didn't say otherwise. Prof.Lavelle also said in class that the unit should be grams I remember.

Nan_Guan_1L

attached is a screenshot from lecture 8 when discussing the circular standing wave model. I understand that waves at the same phase could be connected and can become a stable model. But I don't understand how the circular lightwaves are formed here. there was a motion picture showing the circular wa...

Nan_Guan_1L

this molecular is nonpolar. because C H I have similar electronegativity and the structure of the molecule is tetrahedral. tetrahedral molecules can "cancel out" the bonds because of it being symmetrical.

Nan_Guan_1L

I think it's because H only has 2 protons. and also, Hydrogen is not a very typical atom. although it's placed in the upper right corner on the periodic table, it doesn't follow some of the trends that apply to most other atoms.

Nan_Guan_1L

can someone summarize the major trends that can be found in the periodic table? Thank you!

Nan_Guan_1L

to check whether a molecule has a resonance or not, draw the lewis structure and see if you can find multiple possibilities that satisfy the requirement. N2H2 doesn't have a resonance structure because there's only one way to meet all the requirements.

Nan_Guan_1L

NH3 can be considered as a special case because it has hydrogen bond which is relatively strong. NH3 and PH3 have similar molar mass.

Search found 106 matches