CHEMISTRY COMMUNITY

Crystal Hsueh 2L

I was confused about this question too but the way I solved it was by looking at all the substances and finding the commonality. For example: one of the elements they are asking might have reduction potentials for 1, 2, and 3 electron transfers, another one has reduction potentials for 1 and 2 elect...

Crystal Hsueh 2L

'A' is the frequency factor. This means the fraction of collisions in the reaction that have enough energy to overcome the activation barrier. It is also called the pre-exponential factor. In terms of calculations, the A is either given to you and they'll ask you to solve for activation energy or vi...

Crystal Hsueh 2L

I don't have access to the link but I believe if you search up "Endgame" in chem community you can find a review on electrochemistry and kinetics that was made by the same UA that did marshmallow.

Crystal Hsueh 2L

Hi! The reason you need Pt(s), or any other inert electrode, in this is because I2(s), while a solid, is not a metal. So it would not be able to generate that electric current. When you have a cell notation with a solid in the redox half reactions, make sure to check whether or not that solid is a m...

Crystal Hsueh 2L

From what I have seen, k' is just used to differentiate between the rate constant at a certain time and the initial rate constant. So at time you might see (k'/k) or (k2/k1).

Crystal Hsueh 2L

I'm currently on the pre-med track and because I'm a freshman and a STEM major I haven't really been noticing anything different from my non pre-med peers who are taking the same classes as me, considering all life science majors take the same lower division classes and the classes overlap with pre-...

Crystal Hsueh 2L

Hi! So for this question you kind of have to look at the appendix in order to get the correct half reactions because doing it the normal way wouldn't have the correct balances. So the best bet is to look at the appendix and flip the one that will still yield a positive E cell

Crystal Hsueh 2L

How do we determine the order of the half reactions? Can you possibly point out which lecture/ textbook section goes over this ? I'm having trouble finding it :/ So I'm not really sure what you mean about the order? Do you think you can expand on the question more? If you mean the order of writing ...

Crystal Hsueh 2L

Hi Emily! So basically for the cell diagrams, the order of the species in the notation doesn't really have to do with the order of the species in the half reaction. Life you said, for the cathode in this problem would be Cl2 + 2e- —> 2Cl- But in the cell notion is it presented as Cl-|Cl2 The reason...

Crystal Hsueh 2L

Hi Emily! So basically for the cell diagrams, the order of the species in the notation doesn't really have to do with the order of the species in the half reaction. Life you said, for the cathode in this problem would be Cl2 + 2e- —> 2Cl- But in the cell notion is it presented as Cl-|Cl2 The reason ...

Crystal Hsueh 2L

The easiest way, in my opinion, on balancing redox reactions in basic solutions is going about it the same way that you would if it were an acidic solution. Just with a few extra steps at the end. Here's what I mean: - Once you solve it as if it were an acidic solution, you will have H+ on one side ...

Crystal Hsueh 2L

Hi there! For 6L.3 it is not necessary to look at the appendix to figure out the anode and cathode. The problem is presented in cell notation format which means every species on the left side of the salt bridge (the ||) is the anode and every species on the right of it is the cathode For example, fo...

Crystal Hsueh 2L

For textbook problem 6M.7 it asks to rank metals in order of increasing strength as reducing agents for species in aqueous solutions. However, as I was looking through Appendix 2B, I saw there are multiple standard potentials depending on how many electrons are being gained. For example: For Cu+ —> ...

Crystal Hsueh 2L

Why were you able to solve Q that way? Seems like the units don't match? Hi! Sorry this is such a delayed response but here's what Lavelle said in response to another question similar to this one: "Since both pressure (1 atm approximately equal to 1 bar) and concentration are referenced (stand...

Crystal Hsueh 2L

In order to solve this problem you will need the Nernst equation:

E = Enaught − (RT/nF)ln(Q) or Enaught − (0.0592 V/n)log(Q)

The trickiest part of the question is solving for Q, which you will do by using:

Q = (Ph2)/[H+]^2

You should be able to get your answer now by plugging everything else in

Crystal Hsueh 2L

Just like the above replies said, state properties only care about where the process starts and where it ends. There is a very helpful image/diagram in the textbook that analogizes state and path properties as climbing a mountain that you should check out if you need a visual

Crystal Hsueh 2L

I may be wrong but I believe E naught is always positive for galvanic, spontaneous cells. But other than that, it is not always certain that Ecell is positive.

Crystal Hsueh 2L

Hi! I was wondering if someone can help me with this homework problem: One of the most recognizable corrosion reactions is the rusting of iron. Rust is caused by iron reacting with oxygen gas in the presence of water to create an oxide layer. Iron can form several different oxides, each having its o...

Crystal Hsueh 2L

Hi! I'm sure there are practice worksheets somewhere with easier redox reactions to get you started but in terms of video resources, I highly recommend watching Tyler DeWitt on Youtube. He's made like 3 videos just on balancing redox reactions and he goes into a lot of depth, as well as provides a l...

Crystal Hsueh 2L

The way to find degeneracy (W) is basically:

the number of possible positions ^(# of molecules)

For example, if you had 2 of the same molecules with 4 possible positions, it'd be 4^2.

Crystal Hsueh 2L

I may be wrong but I think what the textbook means is that the building of RNA and DNA from the complex amino and nucleic acids lessens disorder because it is going from single compounds randomly produced into the structured DNA and RNA molecules. If you think about it like a puzzle, the individual ...

Crystal Hsueh 2L

For part b, because it is asking for the melting of ethanol, you would use the deltaH fusion, which from the chart is 4.60 kj/mol. You'll need to convert it to joules before plugging it into the entropy equation but that values are there. Just remember that melting and fusion are synonymous. They bo...

Crystal Hsueh 2L

At constant volume, the change in enthalpy is equal to the heat absorbed at constant volume (q) and the change in internal energy. You would use delta H = nRdeltaT + delta U In this question the deltaU is the value given in the problem (765). Just plug everything into that equation and you will get ...

Crystal Hsueh 2L

This basically goes in hand with entropy being a state function. So delta S only cares about where the system starts and ends up, hence the "delta" part. Reversible reactions indicate that a process can reverse without leaving any affect on the surroundings. So if the system ends up at the...

Crystal Hsueh 2L

Lavelle stated in his email that the 8 point questions will have 4 point partial credit and there are more partial credit questions than there were on the last exam. He also mentioned two bonus questions. Here is the link to the pdf he made with all the info: https://lavelle.chem.ucla.edu/wp-content...

Crystal Hsueh 2L

Hi, I was working on textbook problem 4D.9: The enthalpy of formation of trinitrotoluene (TNT) is −67kJ⋅mol−1, and the density of TNT is 1.65g⋅cm−3. In principle, it could be used as a rocket fuel, with the gases resulting from its decomposition streaming out of the rocket to give the required thrus...

Crystal Hsueh 2L

One of the bullet points for outline 4 says "Know how to derive and use the van't Hoff equation..." so I think we'll be expected to know it conceptually as well as know how to use it to solve equations. I don't think he expects us to strictly use that on the midterm if there is another way...

Crystal Hsueh 2L

Hi! I had different equations in my Sapling for that problem so I can't compare them but I think the reason you might be confused on why it does work is because of the phases of the substances. Chances are either 2A or B is solid, which does not contribute to deltaV or delta N. This problem is only ...

Crystal Hsueh 2L

Yes phases matter in Hess's law. If you remember for some of the Sapling questions when the delta H index list was provided for us, The delta H values for the same compound was different with each phase it is in. For example, delta H of H2O(l) is -285.8 while delta H of H2O(g) is -241.8. So this is ...

Crystal Hsueh 2L

Like what the person above said, it depends on what the answer is looking for. If they're asking for molar enthalpy then that means you would use m(mass) instead of n(moles). The units will give a big indication as well and with questions like that you can be asked to find q, the initial or final te...

Crystal Hsueh 2L

I think it's usually a mix of trial and error and having a certain way about going at it. I tend to first check what is on the reactant and product side of the desired equation and then flipping the ones I need accordingly. Then I tackle the coefficients. It's really easy for everything to get messy...

Crystal Hsueh 2L

Hi! I think in the case of the question you provided it would be monoatomic because it is just krypton (Kr). I believe all noble gases are monoatomic and all others are diatomic. Usually for other problems you would have to draw the Lewis structure of the substance that is given to see if it is line...

Crystal Hsueh 2L

Hi! I've been having trouble with this question: A sample of an ideal gas in a cylinder of volume 4.44 L at 298 K and 2.41 atm expands to 7.73 L by two different pathways. Path A is an isothermal, reversible expansion. Path B has two steps. In the first step, the gas is cooled at constant volume to ...

Crystal Hsueh 2L

Both equations solve for delta G, I believe it is just a matter of what information and values are given to you in the question for you to figure out which of the two equations to use.

Crystal Hsueh 2L

Last quarter it never took longer than a week. I think we should have our grades back by Friday unless there were some problems with any of the text questions

Crystal Hsueh 2L

I agree with what Samantha said! If you don't know how to start the textbook problems (which is very helpful for midterm studying), you should attend Step-Up sessions or your TA's office hours. Reading the textbook may help some people but as this course is very calculation-based, I feel like workin...

Crystal Hsueh 2L

When delta H is negative, that means it is exothermic and heat is one the products side of the reaction. When delta H is positive, that means it is endothermic and heat is on the reactant side of the reaction. When temperature is increased, depending on which side heat is on, it will shift away from...

Crystal Hsueh 2L

He said in a recent email that he covers everything in order of the lectures. I think he'll either send an email of what outline we should be on or he'll mention it in an upcoming lecture

Crystal Hsueh 2L

I believe bond enthalpies are the least accurate because they are based on averages of other values. Only bond enthalpies of diatomic molecules are exact.

Crystal Hsueh 2L

Hi guys, I've been stuck on this question for a few days and was wondering if someone could help me out: An ice cube with a mass of 45.6 g at 0.0 ∘C is added to a glass containing 444 g of water at 45.0 ∘C . Determine the final temperature of the system at equilibrium. The specific heat capacity of ...

Crystal Hsueh 2L

No, I don't think catalysts affect equilibrium position. Catalysts just help the system reach equilibrium faster, speeding the forward and backward reaction to the same extent.

Crystal Hsueh 2L

For part b) the carbon would be negligible because it is solid. In cases of pressure, you would only be considering gasses. For part e), assuming that it is increasing the total pressure, the reaction will shift toward side of the equation with the lower pressure. Increasing pressure condenses the v...

Crystal Hsueh 2L

I think it relates to the equation that we use to calculate q. Because what matters is the change in temperature, it does not matter whether or not you use Kelvin or Celsius because it will still be the same change value. For example, a temp change from 50 degrees C to 37 degrees C is 13. If you con...

Crystal Hsueh 2L

In Lecture #9 Dr. Lavelle uses the Qsys + Qsurr equation to determine the sign of the qp value. How would that same calculation look for an exergonic rxn? What is the difference between an exothermic and an exergonic reaction? Wasn't the example shown in the lecture an exothermic reaction? Exotherm...

Crystal Hsueh 2L

I remember learning this in AP Chem in relation to osmosis with concentration. But I assume the basis is the same. With osmosis, water or solvent molecules flow across a semipermeable membrane from low to high solute concentration. To prevent pure solvent from passing through the membrane, pressure ...

Crystal Hsueh 2L

Yes! You're allowed a calculator, the constants and equations sheet, and as much blank scratch paper you think you need

Crystal Hsueh 2L

If all reactants and products are gases and it doesn't specify any units or values (atm vs conc.) then I would use Kp. Luckily, both have the same equation formatting so it's just a matter of semantics

Crystal Hsueh 2L

As long as you complete all of the problems correctly, without clicking "give up" on any of the questions, it doesn't matter how many tries it takes. I once took over 20 attempts on a Sapling problem in 14A and, while it was embarrassing to look at, it didn't affect my points in the slight...

Crystal Hsueh 2L

Hi! I'm a bit confused on this too but from what I read after doing some googling, it seems like work has to do with the difference between constant pressure and constant volume. Work involves a change in volume so at a constant volume, the system is not able to do work, so all the heat you put into...

Crystal Hsueh 2L

Hi! So the reason you can't find anything for NH4Cl on the tables in the textbook because it is presented as the conjugate acid of NH3, which the Kb of should be given on the bases tables. Looking at the table, the Kb is 1.8E-5 but because you're working with the conjugate acid, you want to convert ...

Crystal Hsueh 2L

I think you are right! If there is an increase in A, the reaction shifts to the right. To think about it more generally, when there is an increase in one substance, the reaction shifts to the opposite side and when there is a decrease in that substance, the reaction shifts toward the side the changi...

Crystal Hsueh 2L

Because Kc and Kp are both found using the same equation format, I wouldn't worry about it. You can usually tell the difference between using Kc or Kp when given the units. Kc will be given in molarity and Kp will be given in bars or atm or other units of pressure.

Crystal Hsueh 2L

Just to be safe I would try to keep up with posting 5 a week but last quarter Lavelle was pretty gracious and wanted to ensure that none of the points being lost were from Sapling or Chem Community, especially because all of the other points are coming from exams. I would email your TA to be sure be...

Crystal Hsueh 2L

Hi! Assuming that the exam format is the exact same as it was last quarter in 14A (with a few changes that are noted on the syllabus) here's what you mainly need to know: - Midterms will be on the Friday of Week 4 and Week 7. On the syllabus it says during class so I assume we will start when lectur...

Crystal Hsueh 2L

The difference between partial pressure and pressure is essentially that partial pressures add up to equal pressure. If there are 3 different types of gas in one container, the pressures of each individual gas would be their partial pressure, but the pressure of the whole system is the (total) press...

Crystal Hsueh 2L

There are two reasons why H2O is left out of the equilibrium expression: The first is that H2O, which acts as a solvent in a lot of these reactions, is in excess. This means that the concentration doesn't change enough (barely changes at all) to need to be included in the equilibrium equation. The s...

Crystal Hsueh 2L

I like to watch "coming of age" movies for comfort--so usually like, The Perks of Being a Wallflower, The Edge of Seventeen, Lady Bird, etc. I don't really know why, but films like this align my chakra lol. Especially on a cloudy day. Same here! My favorite movie and book is The Perks of ...

Crystal Hsueh 2L

Hi! In some problems the concentration will be given to you. It will normally have the wording "at equilibrium, the concentration of ___ is ____." Those are the most straightforward problems. In other problems where you will have to use an ICE box to calculate the equilibrium concentration...

Crystal Hsueh 2L

It depends on you and your schedule! If you are understanding the content pretty well and don't feel like you need to actually attend the workshops for extra help, you can always access the worksheets to practice on your own time (this is what I do). Heads up though, not all UA's give access to thei...

Crystal Hsueh 2L

In addition to what was said above, I think the only time you would swap the products and reactants to create a reverse reaction would be dealing with the factors of Le Chatelier's principle. And this would be if the reaction was already at equilibrium and it was altered in some way. For example, if...

Crystal Hsueh 2L

Hi! When converting from bars to atm you would multiply the bar value by 0.986923. When converting from atm to bars you would multiply the atm value by 1.01325. Hope this helps :)

Crystal Hsueh 2L

Hi! I don't remember the exact answer for this question but I believe it is supposed to be set up this way:

[0.14]/([1.2E-2][0.054])

I don't have a calculator nearby so I can't calculate it but this should give you the correct answer. It will just be [P]/[R]

Crystal Hsueh 2L

Hi! I think we might be given reactants and products in moles along with the volume of the container. For example, we can be given 0.5 moles in a 2L container and we would have to know to use n/v to convert it into concentration. I feel like for the sake of time on exams, we would not be given anyth...

Crystal Hsueh 2L

I believe it is the right thing and the 6 month access should work fine. Most students purchased the extended access just in case they were not able to take Chem 14A and 14B back to back, so it provides wiggle room for students who had to take other classes or wanted a break from chem but didn't wan...

Crystal Hsueh 2L

Hi! I did this problem earlier today and I was stuck on it for a really long time but hopefully I can help: So what you want to do is basically use Hess's Law to "create" the equation that they gave you. Hess's law typically deals with enthalpy and in this case we have K values but the ide...

Crystal Hsueh 2L

I believe HF is a weak acid because of how strong the bond is between H and F. It is such a strong, short bond that it will not completely dissociate, and strong acids are characterized by being able to completely dissociate.

Crystal Hsueh 2L

In the Sapling Homework there was a link to a list of strong acids and HClO3 was stated as one of them but there was a note that said "Some sources classify HClO3 as a weak acid. Check with your textbook or instructor to determine how to classify this acid." Will we be considering HClO3 a ...

Crystal Hsueh 2L

I may be wrong but I'm pretty sure the bond angle is in fact 180 degrees, not 180.5. The Atkins solution manual for 2.E.13d says so as well: "The structure for N2O is linear with a bond angle of 180 degrees. AX2."

Crystal Hsueh 2L

In addition to all the above replies you can see the relationship between pKa and pH with the Henderson-Hasselbalch equation:
pH = pKa + log ([conjugate base]/[weak acid])

Crystal Hsueh 2L

To make a conjugate acid you would add H+ and to make a conjugate base you would take away H+

So in this case:
The conjugate acid of NH3 is NH4+
and
The conjugate base of NH3 is NH2-

Crystal Hsueh 2L

I believe we only have to understand it conceptually, such as how the probability of of finding an electron at a node is zero. I got a question on my midterm about the relationship between the probability of finding an electron in respect to the wavefunction so I think that would be the extent of wh...

Crystal Hsueh 2L

Just as the previous replies stated, the strength of an acid can be determined by how stable the conjugate base would be. The more stable the base, the stronger the acid. Because higher electronegativity means that the acid is more willing to lose H+, it makes for a "cleaner" break and the...

Crystal Hsueh 2L

Professor Lavelle said in his email today that if you are an international student who is unable to take the test at the allotted time, the alternative time is 8pm-9:30pm on Sunday, December 13.

Crystal Hsueh 2L

The PCl3 lewis structure has 3 single bonds and one lone pair. Because hybridization can be found with the # of bonds plus the # of lone pairs, that makes 4 attachments, which makes it sp^3.

Crystal Hsueh 2L

I believe you're right! It is nonpolar

Crystal Hsueh 2L

I think there has been some confusion from students regarding whether or not we need 5 posts a week or just 50 by the end of week 10. If I were you I would email my TA because some of them have said different things so it might depend from TA to TA

Crystal Hsueh 2L

In Dr. Lavelle's lecture today he said that you can use either chlorido or chloro, but chloro is used more frequently. So I'm guessing Sapling is just using the most popular term. But you are still technically correct using chlorido.

Crystal Hsueh 2L

London Dispersion Forces and Van der Waal Forces mean the same thing. They are both induced-dipole induced-dipole and everything has these forces.

Crystal Hsueh 2L

I don't believe it is possible to have a smaller molecular formula than empirical formula because empirical formula means the lowest ratio of atoms

Crystal Hsueh 2L

Isoelectronic means having the same number of electrons. For example, Ne and O2- have the same number of electrons even though they have a different number of protons.

Crystal Hsueh 2L

I'm sure there are a plethora of ways to determine viscosity but in terms of bonds, the stronger the bonds of a liquid, the higher its viscosity.

Crystal Hsueh 2L

I was definitely in a better mood before checking my score :,)

Crystal Hsueh 2L

The electron will be taken from the furthest orbital. This is the reason why second ionization energy is always higher than the first. When you take the first electron off, the pull of the protons become a little stronger, so the second ionization energy of the atom would be higher

Crystal Hsueh 2L

Adding electrons to an atom makes the atomic radius bigger because there is more electron repulsion and the protons do not have as strong of a grasp on the electrons as there are more of them. On the other hand, when electrons are taken off of the atom, the atomic radius decreases because there are ...

Crystal Hsueh 2L

My studying strategy is to do all of the textbook problems and go to at least 2 UA workshops before the midterm. I also print out as many of the workshop worksheets I can and do them all a few days before the midterm. I wouldn't say I've done exceptionally well on the exams but I think it's a good s...

Crystal Hsueh 2L

In order to have a hydrogen bond, one of the molecules must have a hydrogen attached to either N,O, or F and the molecule it is bonding to must have a lone pair of electrons on a highly electronegative atom (N, O, or f). Two NO molecules cannot hydrogen bond because there is no hydrogen atom involved.

Crystal Hsueh 2L

Based on the textbook questions in that part of the outline I believe we just need to know how the equation works conceptually.

Crystal Hsueh 2L

Formal charge is assigned to atoms using the number of bonds it has and the number of nonbonded electrons. And it is under the assumption that the electrons are equally shared, emphasizing covalent character. Oxidation numbers are assigned to the more electronegative atom and emphasizes ionic charac...

Crystal Hsueh 2L

12 units minimum is required to be considered a full-time student!

Crystal Hsueh 2L

There are a lot of exceptions and ways to draw Lewis structures (hence resonance structures) but I think generally you should try to make bonds over leaving electrons as lone pairs. I believe this tends to make the structure more stable depending on the element and its formal charge. Most of the tim...

Crystal Hsueh 2L

UA step-up sessions and workshops are normally really helpful for midterm review. However I'm not sure whether or not the sessions this week will pertain to what is on the midterm or if they will cover the new information from the recent lectures.

Crystal Hsueh 2L

I was working on the textbook problems and noticed in 1E13. and 1E17. that some of the electron configurations were written differently than it traditionally should be. For example, for silver the electron configuration in the answer key is [Kr]4d^10 5s^1 and manganese is [Ar] 3d^5 4s^2. I was confu...

Crystal Hsueh 2L

Specific to that question on sapling, the answer is that using oxidation numbers to find the most plausible structure is not consistent with the other two methods. This is because when assigning oxidation numbers (which was found in the step before that question), the oxidation number exaggerates th...

Crystal Hsueh 2L

The least electronegative atom will usually be the central atom. More things to consider (can't guarantee this always works) is finding the least numerous element in the compound. For example, in H2O and CO2, oxygen and carbon are the central atoms. This will not always work as there are compounds t...

Crystal Hsueh 2L

Lewis acids are electron acceptors and Lewis bases are electron donors. I think a good way to look at it is to figure out if there are any unbonded electron pairs on the central atom. If so, then this acts as a Lewis base. If the valence shell of an atom is filled then it acts as a Lewis acid.

Crystal Hsueh 2L

I heard it's really hard to get both chem 14b and chem 14bl the same quarter. But regardless, I heard it's recommended to take 14bl after 14b because bl might have some topics that have not been covered in 14b if you are taking it at the same time. The curriculums could be moving at different paces....

Crystal Hsueh 2L

Can someone explain how to find the magnetic quantum number (ml)? I understand the relationship between the first two quantum numbers but I'm having trouble understanding how to find the third. Thanks so much :)

Crystal Hsueh 2L

The longer the bond length, the lower the number of bonds. And vice versa. Single bonds have the longest bond length and the weakest bond, while triple bonds have the shortest bond length and strongest bond. It is also import to keep in mind that atomic radii play a role in bond length as well. Larg...

Crystal Hsueh 2L

Ionization energy is the energy required to remove an electron from a neutral atom. Electronegativity is the tendency of an atom to attract a pair of electrons. These both have the same periodic trend, so I don't think it matters which one you follow because it will ultimately result in the same thi...

Crystal Hsueh 2L

I believe as of right now, it does not matter. But as we start to learn about molecular shape and structure it will become more pertinent.

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