CHEMISTRY COMMUNITY

Xinyu Li 1C

I feel a little better about the final than the midterms. The questions were more simple and straightforward.

Xinyu Li 1C

I'm definitely dorming next fall. I need human interactions and I can't study at all when I'm at home.

Xinyu Li 1C

Salt bridge is represented by double vertical lines (||) and porous disk is represented by a single vertical line (|).

Xinyu Li 1C

I think Professor Lavelle only teaches 14A and B

Xinyu Li 1C

I think Professor Lavelle said 10^-4 just to be safe. You can also double check with the 5% rule.

Xinyu Li 1C

E naught is under standard condition and does NOT depend on how many times the reaction has occurred.

Xinyu Li 1C

I handwrite my notes on my iPad using Goodnotes. It helps me retain more information. It's also a lot easier to draw out diagrams and figures.

Xinyu Li 1C

Chemical equilibrium

Xinyu Li 1C

I have to admit that it's been a pretty tough quarter. I'm a premed so I like to watch medical dramas or read doctor stories to deal with burnouts. It's like rediscovering why I wanted to be a doctor and how much this means to me. I also like to exercise. It resets my mood and helps me find the moti...

Xinyu Li 1C

You would need to write the full molecule Cr2O7.

Xinyu Li 1C

Go to review sessions! Last quarter, Professor Lavelle, TAs, and UAs all hosted review sessions in week 10. They were super helpful. You can also go over some homework problems and past exams.

Xinyu Li 1C

In this case, n represents the number of e- being transferred. You can find this by balancing the half-reactions.

Xinyu Li 1C

I'm currently reading "To Live" by Yu Hua, highly recommend!

Xinyu Li 1C

Time may vary depending on your lecture but it will be on Sunday, March 14th.

Xinyu Li 1C

It usually comes out within a week following the midterm.

Xinyu Li 1C

Thank you for sharing this!

Xinyu Li 1C

I love bite sized nutter butter cookies!
(definitely not a healthy choice tho)

Xinyu Li 1C

To calibrate the calorimeter, all you need to do is divide heat gained by the calorimeter (or heat lost by the reaction) by the change in temperature.

Ccal=qcal/delta T

Xinyu Li 1C

From my understanding, the 14 series is designed for life science majors. You probably need to take the 30 series to fulfill your major prereqs. Best of luck.

Xinyu Li 1C

I don't think we'll be expected to memorize any of these. Everything we need to solve the problem should be provided on the test or the constants and equation sheet.

Xinyu Li 1C

I think work done on the system is when energy is being applied, like compression. Whereas, work done by the system would be expansion since the system is using energy to push against external pressure.

Xinyu Li 1C

From my understanding, residual entropy ignores thermal motion and can be calculated by using the Boltzmann equation (S = Kb * ln W).

Xinyu Li 1C

I think people usually take it with 14C. I might take it next quarter if it doesn't fill up by second pass.

Xinyu Li 1C

Thank you so much!

Xinyu Li 1C

I think you can review questions you missed in your TA's office hours. Just shoot them an email.

Xinyu Li 1C

I don't think Professor Lavelle posted an outline for midterm 2 yet.

Xinyu Li 1C

Use the number you see when you click on "Your posts".

Xinyu Li 1C

Delta U is the change in internal energy. You can calculate delta U by using the following formula:

Delta U = n * Cv * delta T

Xinyu Li 1C

Thank you so much for sharing this!

Xinyu Li 1C

Like everyone said, you'd do 10^(-pKa) to go from pKa to Ka.

Xinyu Li 1C

I didn't feel as confident about this midterm. I felt it was more difficult than 14A midterms.

Xinyu Li 1C

Professor Lavelle said the only factor that affect K is temperature.

Xinyu Li 1C

Usually I just watch the lectures and do a UA worksheet. So around 6 hours per week.

Xinyu Li 1C

I'm planning to take 14BL next quarter!

Xinyu Li 1C

Add water (H20) to the reactant and hydronium ion (H3O+) if it's an acid or hydroxide ion (OH-) if it's a base. The balance everything out.

Ex: HClO + H2O = H3O+ + ClO-

Xinyu Li 1C

Rosa told me that there's a helpful review midterm sheet from last year.

Link: viewtopic.php?f=160&t=58508&p=224708&hilit=pizza+rolls&sid=297c8e7a86a4e1bca675da3c35540d64#p224708
*if the link doesn't work, search pizza rolls

Xinyu Li 1C

https://lavelle.chem.ucla.edu/wp-conten ... PL_ALL.pdf

Xinyu Li 1C

Here's the link.

https://lavelle.chem.ucla.edu/wp-conten ... PL_ALL.pdf

Xinyu Li 1C

UA sessions are peer learning sessions hosted by undergraduate assistants (UA). These are small group discussions and Q&A.

Xinyu Li 1C

Professor Lavelle mentioned in lecture that some textbook uses 10^-3 but he prefers us go by 10^-4 just to be on the safe side. You can always check by dividing X by initial, if X is less than 5% of initial then it's okay to approximate.

Xinyu Li 1C

You can use it for all problems involving pH I believe.

Xinyu Li 1C

You can! Just make sure you convert it into molar concentration before you calculate K.

Xinyu Li 1C

There is a list of strong acids and bases in the textbook. But generally, strong acids will contain halogens (Cl, Br, I) and strong bases will contain elements from group 1 & 2 (Li, Na, K, Rb, Cs, Ca, Sr, Ba).

https://sites.google.com/site/chempendi ... #h.p_ID_32

Xinyu Li 1C

The change in reactants is always negative and the change in products is always positive because reactants are being used up to form products.

Xinyu Li 1C

K would also change when the coefficients of the species in a reaction are multiplied by a number. In that case, K would be raised to the power of that number.

Xinyu Li 1C

Yes, you will. Oftentimes there are less people posting questions so this is bound to happen.

Xinyu Li 1C

Like the Josh said, technically it would be favoring one side or the other unless K=0, just not "strongly favored."

Xinyu Li 1C

Sunday 11:59 PM PST is the official deadline for your weekly posts but most TAs will allow you to make up for missed points in the following weeks.

Xinyu Li 1C

I think Professor Lavelle said that solids don't have a concentration (since it's not in liquid) and changes in solvent concentration is insignificant so it is excluded from calculations.

Xinyu Li 1C

You should use Kelvin.

Xinyu Li 1C

You are really close. The coordination number for this compound is actually 6 because there are 6 coordinate covalent bond here, 1 from SO4 and 5 from (NH3)5. 1+5=6

Note: each NH3 contributes 1 coordinate covalent bond

Xinyu Li 1C

I'd assume it's the same TA link as our last midterm but I'm not 100% sure.

Xinyu Li 1C

Coordination number = number of coordinate covalent bond

There are 6 coordinate covalent bond here, 1 from each ligand (1 SO4 + 5 NH3). Hope this helps.

Xinyu Li 1C

I'm a very worried about making silly mistakes and not doing well on the final. Especially since there is no partial credit and grade is heavily dependent on exam scores :<

Xinyu Li 1C

Chelating complexes are complexes containing a ligand that forms a ring of atoms that includes the central metal atom. In order to form a chelate, you need a ligand with the ideal geometry, which is (atom w/ lone pair) + (spacer) + (spacer) + (atom w/ lone pair) and the bonds need to be able to rota...

Xinyu Li 1C

In order to form chelating complexes, you need bonds that can rotate (single bonds) and the the ideal geometry, which is (atom w/ lone pair) + (spacer) + (spacer) + (atom w/ lone pair)

In this case, the answer would be b) because a) and c) have too many spacers between atoms w/ lone pairs.

Xinyu Li 1C

Two chemists meet for the first time at a symposium. One is American, one is British. The British chemists asks the American chemist, "So what do you do for research?" The American responds, "Oh, I work with arsoles." The Brit responds, "Yes, sometimes my colleagues get on m...

Xinyu Li 1C

I think Professor Lavelle said both are acceptable.

Xinyu Li 1C

Pi bond is the lateral or side-by-side overlap of 2 orbitals (4 orbital overlap). It do not allow bound atoms to rotate and have 1 nodal plane. It usually between p- and d- orbitals.

Xinyu Li 1C

(en) refers to NH2CH2CH2NH2, or ethylenediamine. It's a bidentate ligand (coordination number=2). And chlorine is a monodentate ligand (coordination number=1). So together, the coordination number for [Co(en)2(CO)2]Br is 2*2+2*1=6.

Xinyu Li 1C

For each metal complex, give the coordination number for the metal species.
[Pt(en)Cl2]

Are we expected to know the name and denticity for ligands like ethylenediamine(en) and 1,2-bis(diphenylphosphino)ethane(dppe) on the midterms?

Xinyu Li 1C

I think it's highly likely that we will have a lecture on Friday, but discussions should be cancelled. Happy Thanksgiving :3

Xinyu Li 1C

I think we will need to know the bond angles for electron pair geometries (without lone pair), like 180, 120, 109, 90; and that having lone pairs influences the bond angles between bonded atom. But I doubt we will be required to memorize specific bond angles for different molecules.

Xinyu Li 1C

Ionic
- Typically between metal and nonmetal
- Involve transfer of electrons

Covalent
- Typically between nonmentals
- Involve sharing of electrons

Xinyu Li 1C

Dipole-dipole is between two polar molecules whereas dipole-induced dipole is between a polar molecule and a nonpolar molecule.

Examples
dipole-dipole: HF-HF (HF is polar)
dipole-induced dipole: HCl-N2 (HCl is polar and N2 is nonpolar)

Xinyu Li 1C

Typically, you want to place the negative formal charge on the more electronegative atom and the positive formal charge on the less electronegative atom. Hope this helps.

Xinyu Li 1C

That would not be possible because the s orbital is spherical. You need at least two p orbitals to form a pi bond.

Xinyu Li 1C

Sigma bonds are stronger because it's a head-on overlap between the orbitals, which allows electron density to be concentrated to a much larger degree between the two nuclei.

Xinyu Li 1C

Yes, it will be cumulative.

Xinyu Li 1C

NO2- have 2 electron pairs and 1 lone pairs, making it AX2E, which is bent/angular.

Xinyu Li 1C

Pi is the greek version of the letter p. In this case, it's referring to the p orbital. Pi bonds are commonly formed between p orbitals, as shown in Dr. Lavelle's lecture. However, d orbitals can also engage in pi bonding.

Xinyu Li 1C

Yes, these are intermolecular forces since they are interactions between molecules rather than within molecules.

Xinyu Li 1C

I think this midterm will be more focused on bonding and Lewis structures. So just be ready to calculate formal charge.

Xinyu Li 1C

I'm on a pre-med track and am currently a pre-psychobio major. Is anyone in a similar boat to me who knows what the consequences of changing to P/NP will be? What are the pros or cons? I believe changing to P/NP gives you credit without having it affect your GPA in a negative way (if you at least p...

Xinyu Li 1C

I think people usually take 14BL with 14C.

Xinyu Li 1C

My TA said it's fine as long as you have 50 posts by the end of the quarter.

Xinyu Li 1C

I'm feeling very burnt out and my mental health is taking a hit. The quarter system makes me want to die i_i

Xinyu Li 1C

I'm taking LS7B and CHEM14A this quarter along with LS30A and another GE. The workload is not too bad but it's certainly not light. It's definitely doable if you are willing to put in the effort.

Xinyu Li 1C

Aside from what everyone mentioned, ionic bonds tend to be stronger than covalent bonds due to the coulombic attraction between ions of opposite charges. However, covalent bonds are stronger than ionic bonds in water.

Xinyu Li 1C

Dr. Lavelle said that he'll go over some of the questions but we won't be able to see which question we missed.

Xinyu Li 1C

You can use the "Constants and Equations" sheet from Dr. Lavelle's website during the midterm.

Xinyu Li 1C

I think it's just different units. It can also be:

$2.180*10^{-18} J$
$3.29*10^{15} s^{-1}$
$1.097*10^{7} m^{-1}$

Xinyu Li 1C

luludaly2B wrote:Thank you! I did this and then multiplied by 1000, in order to convert the number into J from kJ. Is this correct?

Yes :)

Xinyu Li 1C

In part b, you are given the threshold frequency, which is the minimum frequency of radiation required to remove an electron from a metal surface. To solve this problem, just calculate the work function from threshold frequency using: \phi =hf_{o} \phi = work function h = Planck constant f_{o} = thr...

Xinyu Li 1C

The roman numerals indicate the charge and oxidation state of transition metal ions. Typically, you want the overall charge to be zero.

Ex: if sulfide have a charge of -2, you'd want the iron ion to have a charge of +2, written as iron (II) or $Fe^{+2}$

Xinyu Li 1C

Dr. Lavelle told us that the midterm covers everything up the materials discussed on Wednesday's lecture :)

Xinyu Li 1C

Increasing amplitude = increasing intensity. I don't think it's related to wavelength and/or frequency. But I could be wrong.

Xinyu Li 1C

I think they are both referring to frequency.

Xinyu Li 1C

I think it stands for reduced Planck constant, which is equal to $\frac{h}{2\pi }$ .

Edit: spelling

Xinyu Li 1C

I usually keep all of my digits and round at the very end, not sure if it's correct. But, I don't think it will be a big deal for midterm since it's multiple choice.

Xinyu Li 1C

The work function is basically the minimum E needed to remove an electron from a metal surface. In this case, it is already given to you in the unit kJ/mol. To find the energy required to remove an electron from ONE sodium atom, just divide 150.6 kJ/mol by Avogadro's number.

Xinyu Li 1C

2878.68 nm? I'm not too sure.

Xinyu Li 1C

Your approach to the question sounds right to me, maybe double check your calculations?

Xinyu Li 1C

I think it indicates that the reaction is to be heated.

Xinyu Li 1C

Electronegativity: how well an atom can attract electrons toward itself

Electron affinity: amount of energy released when an electron is added to a molecule or atom

Xinyu Li 1C · Re: Chem Community Questions They are due at 11:59 PM PST/PDT every Sunday :)

They are due at 11:59 PM PST/PDT every Sunday :)

Xinyu Li 1C

1. Calculate the energy of electron in ground state and 5th energy level. E_{n}=-\frac{hR}{n^{2}} h = 6.626*10^{-34} R = 3.29*10^{15} n = energy level 2. Calculate the energy emitted. \Delta E=E_{final}-E_{initial} \Delta E = E_{1}-E_{5} 3. Calculate the wavelength from \Delta E \lambda =\frac{hc}{E...

Xinyu Li 1C

The term "work function" refers to the minimum E required to remove an electron from a metal surface. Remember, longer wavelength = lower frequency = lower energy. So when the problem asks you to find the longest wavelength, you can just calculate the wavelength of the electromagnetic radi...

Xinyu Li 1C

The unit for wavelength in E= $\frac{hc}{\lambda }$ is meter (m), not nanometer (nm). Therefore, you'd need to convert 102.6 nm to m before plugging it into the equation.

$E=\frac{(3.00*10^8)(6.63*10^{-34})}{102.6*10^{-9}}=1.94*10^{-18}J$

Edit: formatting

Xinyu Li 1C

From my understanding, E=hv is only used for calculating energy of photons whereas the de Broglie's equation is used for calculating energy of any moving particles with momentum.

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