CHEMISTRY COMMUNITY

Zaid Bustami 1B

Hey Jake, Based on high school chem I think it's definitely possible to have to deal with a cubic equation when you have a large K value, but like was said earlier maybe not in this class since Prof. Lavelle never went over it. Just thought it could be helpful to explain how it works. For this scene...

Zaid Bustami 1B

Hey Siwa! Wanted to mention real quick that the K c value the table lists at 298K is incorrect, and it should be 6.1 x 10 -3 rather than 6.1 x 10 23 . Check out https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=49&t=70852&sid=00c68c80630373633d338553da5437c3 for more info about that. Also ...

Zaid Bustami 1B

Hey Minahil! Just to add to Samantha's awesome explanation, for any problems like this there's a rule that states that if you multiply all stoichiometric coefficients by a certain number, the K for that equation gets raised to that number. If you're given this equation: 1 / 3 N 2 + H 2 ⇌ 2 / 3 NH 3 ...

Zaid Bustami 1B

Hey Minjoo! I would say from looking at the table that the numbers on the left are K p values and then obviously the ones on the right are K c values. What the book does I think is they omit the "p" subscript on the K for the first column because using K by itself implies that it correspon...

Zaid Bustami 1B

Hey Bella, This could be an error in the textbook itself actually. There's a pattern in the K values in the table where they increase as temperature is increased, and so I'd expect the same pattern in the K c values listed. It would make more sense if the K c for the reaction N 2 O 4 <->2NO 2 at 298...

Zaid Bustami 1B

Hey Elizabeth, Correct me if I'm overlooking something but I think this could be the textbook's fault. The reaction they list is 2HD + H 2 <-> D 2 , which is doesn't make sense because there are no regular hydrogens present on the right side of the reaction (I say regular because D stands for deuter...

Zaid Bustami 1B

Hi Dana! HF is not a strong acid because fluorine is a very small atom. Although the electronegativity difference in this molecule is larger than for HBr and HI, which are strong acids, the small length of the bond makes for a very strong H-F bond. The short length of the bond overcomes any extra we...

Zaid Bustami 1B

I think I answered something similar in another topic right here, might be helpful to look at too https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=57&t=69400&sid=f39e3f22a505137e8fd07240f943ce1c . Basically the idea is that for things that completely dissolve in water, writing either both...

Zaid Bustami 1B

Hey Shannon! De Broglie's hypothesis tells us that everything with mass (matter) has a dual particle-like and wave-like property. Based on that, look at each answer choice: a. This experiment revealed how atoms are largely made of empty space between the nucleus and the orbiting electrons. It tells ...

Zaid Bustami 1B

Yea to add on all atoms with 2 or more regions of electron density around them need to be hybridized in order to properly distribute their valence electrons about themselves in a way that each of them can be used to form a bond/lone pair and in a way that allows the proper bond angles. Any species w...

Zaid Bustami 1B

Hi Eileen, So I think what you're referring to is whether or not an acid will deprotonate depending on its conditions. In the lecture, Prof. Lavelle said that an acid will deprotonate if the pH of its environment (i.e. the solution it's in) is greater than its pKa. In other words, an acid that is mo...

Zaid Bustami 1B

Hey Aina! Just like for carbon, the orbitals in N from NH3 have to be hybridized so that the NH3 molecule can have the proper shape. Although NH3 is a molecule in which the N is only bonded to three atoms, which is theoretically possible without hybridization because nitrogen has 3 p orbitals with u...

Zaid Bustami 1B

Hey Hana! Keep in mind that in order to calculate pH/pOH you need the concentration of the strong acid/base that you're dealing with. So, to solve these problems this is the value that you're trying to get to before you do the logarithmic calculation for pH/pOH. (d) For this one you're dealing with ...

Zaid Bustami 1B

Hi Brett, Thought it would also be helpful to give some examples of structures that would be impacted by pH. So organic structures often contain atoms that allow them to act as acids or bases. A lot of organic molecules often contain a nitrogen with a lone pair on it, and under normal pH conditions ...

Zaid Bustami 1B

Hey Sofia! Yea this goes back to how reactions taking place in water (in an aqueous environment) are written. Keep in mind that all these acid/base reactions need to be taking place in an aqueous environment for them to even work. The equation you wrote is showing the correct reaction, but you have ...

Zaid Bustami 1B

Hey Ian, Yea it's a bit counterintuitive because like Kailani said there are two regions of overlap in a pi bond, one up above and one down below. Keep in mind though that a pi bond is only made up of two overlapping p orbitals. Since p orbitals have two lobes, it looks like you have two pairs of ov...

Zaid Bustami 1B

Hey Jeffrey! To the best of my knowledge pi bonding can only happen between two p orbitals overlapping side by side, and not hybrid orbitals. If you look closely at molecules with double or triple bonds you'll notice that all of them have at least one unhybridized p orbital left over which is used t...

Zaid Bustami 1B

I would say hybridization happens only in circumstances where there are two or more regions of electron density around an atom. Like you said, for H2 since each H has only one region of electron density around it (the H-H bond), then there really wouldn't be anything to hybridize because only 1 s or...

Zaid Bustami 1B

Hey Kayko! I think it's just the geometry of the carbonate ion that allows it to do that. Since the two oxygens with formal charge -1 are relatively close to one another, a TM can form either one or two coordinate covalent bonds with the carbonate. Even if the carbonate has enough lone pairs to form...

Zaid Bustami 1B

I think the way to think about it is to prioritize giving the most electronegative atom an octet. If you use this approach to problems I think it will work (at least it works both with the ClO and OH radicals), but I'm not sure as to when electronegativity stops becoming a factor or what happens whe...

Zaid Bustami 1B

Hey Stuti! Yea you're absolutely right. p orbitals in an atom are oriented 90 degrees from one another, so in molecules like ammonia or methane the orbitals have to hybridize in order for the 109.5 degree bond angles to be possible. I have a picture here that visualizes what happens when carbon orbi...

Zaid Bustami 1B

Hey Ben! Yea, I think a good example would be ethyne (C2H2). I think any molecule where there are two central atoms each with two regions of electron density around them would all have two sp hybridized orbitals. So this would make nitrogen gas (N2) a good example too, and any others you can think o...

Zaid Bustami 1B

Hey Alvin! This goes back to when we talked about formal charge. I've drawn here what ClO2+ would look like with either one double bond or two double bonds: ClO2+.jpg When looking at the atoms' formal charges, it is clear that having only one double bond is more unstable because there are more forma...

Zaid Bustami 1B

Hello! I agree I don't think knowing nomenclature is required for this class, but I think it would be helpful going forward to be familiar with them. You're right those prefixes need to be paid attention to - they tell you how many of each atom is present in each molecule. This is the nomenclature s...

Zaid Bustami 1B

Hey Jaylin! What I find helpful to remember all the shapes is to memorize the shapes that are formed when no lone pairs are present (i.e. linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral) since these tend to be a bit easier to remember and they simply involve spacing out at...

Zaid Bustami 1B

The seesaw bond angles would be <90 and <120 (as well as <180 if you're comparing the atoms at the top and the bottom). This is due to the fact that the remaining atoms experience greater repulsion from the lone pair than they would if the lone pair was an atom. Thus, the bond angles normally observ...

Zaid Bustami 1B

Just wanted to let you know that your answer is correct, b is nonpolar and d is polar. The answers from the textbook solution manual say that is the correct answer as well, so I don't know if whatever it is you're accessing for the correct answers has the right answers listed. Hope this helps :)

Zaid Bustami 1B

Hey David, Yea it's kind of weird because for the textbook problems, it seemed like formal charge wasn't what determined which atom has the unpaired electron. However, from what I've seen it seems like the most electronegative atom will want the full octet, so the less electronegative species will l...

Zaid Bustami 1B

Keep in mind that the name of the shape comes from the arrangement of the atoms themselves, and not the arrangement of the regions of electron density (i.e. bonds and lone pairs). So even though SO32- does have a "tetrahedral" arrangement of electron density regions, you would name its sha...

Zaid Bustami 1B

Hey Noelle! Just wanted to add to the explanations. It's difficult to know exactly which reactants will donate a lone pair without drawing their lewis structures first. So, the first step is to draw out their structures, which I've shown here: Lewis Acid-Base Rxn.jpg By definition, a Lewis acid is a...

Zaid Bustami 1B

I'm doing some course planning and I was just wondering about how 14B and 14BL were designed to be taken. I've heard from some that deciding when to take each class depends on how much of a workload you can take, whereas others have said that 14BL applies concepts from 14B and it might be better to ...

Zaid Bustami 1B

Hey Cecilia, I would say for this problem you're going to need to use electronegativity. You're right in saying that you can use electronegativity to compare covalent bonds and polarizability to compare ionic bonds. It's just that in this case you're actually looking at a set of compounds with mixed...

Zaid Bustami 1B

Hey Vivian! 1. I asked Prof. Lavelle about this too, and he said that determining exactly which electrons are valence electrons for d-block elements can be inconsistent. What he told me is that if you define valence electrons as the electrons occupying the outermost shell, then the number of valence...

Zaid Bustami 1B

Hey Jalaila, Adding on, I just wanted to say something about why resonance hybrids exist. It's more stable to have a more evenly distributed electron density than to have differences in electron density throughout a molecule. Think about a dam. Sure it's possible to maintain a gradient in the water ...

Zaid Bustami 1B

Hey Thomas! This comes down to the electronegativity differences, since we're dealing with covalently-bonded atoms. The greater the difference, the more ionic in character the bond is because an electron can be more easily transferred vs shared when the electronegativity difference is higher (the mo...

Zaid Bustami 1B

Hey guys, So for part c, the question is asking how many valence electrons manganese has. How is it that Mn has 7 valence electrons and not 2, since the 3d electrons are in a lower energy shell than the 4s electrons? If valence electrons are defined as electrons in the highest energy shell, why are ...

Zaid Bustami 1B

I know we're probably gonna be going over this soon but I thought it would be helpful to explain a bit. So the strength of a bond tells you how much energy you need to break it. This energy (which you might have come across while researching) is called the bond enthalpy, so bonds with greater bond e...

Zaid Bustami 1B

Also Jeffrey I just wanted to add, when you're writing the electron configurations, you follow the order as indicated by the periodic table until Calcium, which has 20 protons. In calcium, 4s is filled rather than 3d because an electron with a 4s state in Ca has lower energy than an electron with a ...

Zaid Bustami 1B

Wait correct me if I'm wrong but isn't affinity positive when energy is released from an atom gaining an electron (since the calculation for affinity is initial energy of the atom minus its final energy)? So Alexandra wouldn't electron affinities can be negative if energy is absorbed and positive wh...

Zaid Bustami 1B

Hey Jaden, Just wanted to add, electron affinity is a measure of how favorable it is for an atom to gain an electron. If you remember the terms endergonic and exergonic (probably from bio), these would also apply to electron affinity. A positive affinity indicates that energy is released when an ele...

Zaid Bustami 1B

Also the thing about Boron is that it has a pretty high ionization energy for its three valence electrons, so it'll typically share electrons rather than give three of its electrons up. Now the thing is it will form molecules where it has an oxidation state of 3+, but won't necessarily exist as an i...

Zaid Bustami 1B

Hey Anirudh, So the thing is, once you remove an electron from an atom, the amount of e- repulsion decreases, so the electrons are not as easy to remove (the atom becomes smaller). Even though this is the case that doesn't mean the degree of increase for the second ionization energy is the same for ...

Zaid Bustami 1B

Hi Rita, To the best of my knowledge, when chemists make solutions, there's a lot of room for error when adding dry solute. Some solute can get stuck to the scoopula you use to scoop up the solute, some can get stuck on the sides of the beaker, some can get spilled while transferring the solute. To ...

Zaid Bustami 1B

Hey Talia! A very important technique used to determine the structure of molecules (in this case the spike proteins on SARS-CoV-2, the coronavirus which causes COVID-19) is known as x-ray crystallography, which Dr. Lavelle mentioned in his lecture. It's exactly what it sounds like: a beam of x-rays ...

Zaid Bustami 1B

Hey Alex, Yea I think you're right. The probability of finding an electron in a specific point is a specific value associated with one location. You can think of probability density as a gradient where darker regions represent a greater probability density (you're more likely to find electrons there...

Zaid Bustami 1B

Hi Anna! So if you think about Heisenberg uncertainty, you know that uncertainty in position is inversely proportional to uncertainty in momentum. If an electron were to stand still, its uncertainty in velocity would be so low that the uncertainty in position would be larger than the size of the box...

Zaid Bustami 1B

Hey Megan! Just wanted to give a more detailed explanation of the solution. So for part b, what you're looking at is a problem relating to the photoelectric effect, and how much energy in a photon is needed to eject an electron. The question says that no electrons are ejected until a frequency of 2....

Zaid Bustami 1B

Hi Jaylin! So the problem says to use a 1-dimensional box (which is basically a line) to represent the atom because this simplifies the calculation of the uncertainty in x ( \Delta x ). You know in real life atoms aren't lines, and instead you have to take into account the volume of space an electro...

Zaid Bustami 1B

Hey Brett! Adding on, from what Professor Lavelle said these transitions happen by chance. Some electrons may release the full amount of energy to return to the ground state, whereas others may not and only go down to n=2 or higher levels. Not entirely sure why this is true though. One explanation t...

Zaid Bustami 1B

Hey Jaden! Your answer's correct - the wavelength is 1.64x10^-38m. It's hard to give a precise answer as to what is the lowest measurement we can detect since it all depends on what kinds of technology we have. It was only in the last decade that scientists claimed to nail down an accurate measureme...

Zaid Bustami 1B

Hi Anika! These patterns really only apply to H since they were the result of an empirical analysis of its atomic spectrum. You'll see a similar trend (increasing wavelength with increasing lower energy level) in other atoms if they only had ONE electron, making them similar in structure to H. Howev...

Zaid Bustami 1B

Hey Quinton! Yeah it's pretty cool to think about how it is that scientists were able to depict a property of e- and atoms. Don't know if there's modern technology that's changed how spectroscopy is done, but at least from its origins I know that the fingerprint is simpler than you'd expect. The ide...

Zaid Bustami 1B

Hey Nicole, Just to reaffirm the energy of a photon has to be GREATER THAN or equal to the work function in order for it to eject the e-. If it was less than, the energy of the photon would not be sufficient to eject the e- (i.e. E photon < \phi ) and no matter how many photons are shone onto the e-...

Zaid Bustami 1B

Hey Karen! I actually did go to a workshop yesterday, basically what happens is UA brings in a prepared worksheet and as a class you work through every problem. The UA gives you time to solve each question, then after that they go over it and you can ask questions. The problems tend to be harder tha...

Zaid Bustami 1B

Hey Chris!
Yes there is a groupme for it, here's the link: https://groupme.com/join_group/62438638/rIKb1aOc

Zaid Bustami 1B

Hi Tiffany! So those ratios Professor Lavelle was talking about were stoichiometric ratios, more specifically the ratios between the reactants' stoichiometric coefficients (the numbers in front of the reactants). In order to know these you need the balanced equation for the chemical reaction, which ...

Zaid Bustami 1B

Hi Julianna! In order to to find empirical or molecular formulas, you need to convert any mass amounts into moles. Remember, the numbers in empirical and molecular formulas ALWAYS indicate the ratios of the amounts (moles) of each and NOT the ratio of masses. So for these types of problems your fina...

Zaid Bustami 1B

Hey Vivian! I'm going to answer this the best I can, I haven't exactly gotten to Topic 1 but I'll try my best with my knowledge from high school and some skimming of the book :) We know on a basic level that when electrons in an atom are excited (meaning energy is added to them), they move to a high...

Zaid Bustami 1B

Hey Megan! So once you've identified the limiting reactant (which is ClO 2 since it is present in an amount that is less than 3 times the amount of BrF 3 ), by definition the amount of ClO 2 will determine how much product is formed. So, you can forget about the amount of BrF 3 for now because it wi...

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