CHEMISTRY COMMUNITY

Emily Ding 1J

Oh wait, is there supposed to be an implicit 2H2 on the right to balance the number of H's? If so, why isn't it included in the equation?

Emily Ding 1J

Hi, this feels like a really stupid question but I just can't figure out why for 5J.5 (D) neither the products or reactants are favored if the system is compressed. 2HD(g) + H2(g) --> D2(g) (the arrow is supposed to be the equilibrium one, sorry) Aren't there more moles on the left so if pressure in...

Emily Ding 1J

I remember it like Ka = strength, so as Ka increases so does the strength of the acid. And because we know that pKa is just the -log of Ka, it's the inverse: as pKa decreases, acid strength increases. I hope this helps!

Emily Ding 1J

Hi, I'm confused when we need to write out the net ionic equation for the equilibrium constant and when we just disregard solids from Kc. Any clarification is appreciated!

Emily Ding 1J

Amphiprotic means it can donate and accept protons. It's just more specific. All amphiprotic compounds are amphoteric but not all amphoteric compounds are amphiprotic.

Emily Ding 1J

pKa and pH are definitely related: pH = pKa + log ([conjugate base]/[weak acid]). In 14A we discussed what happens when one is greater than or less than the other (it will be deprotonated or neutral), but I think a more basic idea is that pKa is constant for a specific acid whereas pH is for the sol...

Emily Ding 1J

We approximate when Kc is really small because that means that x must also be really small, so subtracting/adding it to a larger number is insignificant. This does not apply when Kc is large because then it would be significant. Also, Professor Lavelle mentioned that we don't have to solve cubic equ...

Emily Ding 1J

I was super confused by that question too! I don't think we've learned it in class yet and my explanation is a bit crude but you're supposed to "write" the equation whose K you're solving for with the given equations. It's like systems of equations that you do in algebra, putting equations...

Emily Ding 1J

Adding on to that, stability is directly related to lower energy and we know that what likes to form is usually lower energy. So if K is large, that means there's a higher concentration of products and therefore they must be more stable because they tend to form more. Vice versa for if K is small. I...

Emily Ding 1J

Adding on to that, I think acids and bases are probably the most directly applicable because we're going to do a whole unit this quarter about them, but definitely all the fundamentals are important for this class.

Emily Ding 1J

If it is solid or liquid/solvent, don't include it!

Emily Ding 1J

To add on to the previous replies, I also kept getting this question wrong but realized at the end that I had forgotten to substitute back in the value I got for x to find equilibrium! It's really easy to forget so I just wanted to emphasize that.

Emily Ding 1J

Yes, I believe they're very connected, especially with the acid/base topic because Chem 14B includes equilibria and we go into more depth.

Emily Ding 1J

Yes, we approximate that both strong acids and bases have been 100% ionized for the sake of simple calculations.

Emily Ding 1J

The way I like to do questions like these is to start off with an arbitrary concentration amount, calculate the pH, and then just take 12% of whatever value you decided to use for the concentration and recalculate the pH with this value to find the difference. I hope this makes sense! I believe the ...

Emily Ding 1J

I don't think we have to write them. Like Dr. Lavelle said, you can include the spectator ions like Na or Cl if you'd like because it's a more complete equation anyways, but if you happen to know that an ion doesn't affect the pH you can just leave it out for simplicity. I don't think we directly le...

Emily Ding 1J

I believe if the salt has an anion and cation from both a strong acid and strong base, it will not affect the pH. One of the Sapling questions for this week explained this in depth in the solution, so I would recommend looking for that!!

Emily Ding 1J

Hi, I was confused on how you determine the hybridization/bonding scheme of a pi bond. I understand how to find it for a sigma bond, but I'm unsure about pi bonds!

Thanks!

Emily Ding 1J

Thank you for your response! I understand the basics of adding and subtracting H, but I'm mainly confused about how you know to even write the equation with the Al bonded to H2O. It's just not very intuitive for me to go from AlCl3 to all of a sudden Al(H2O)6^+3. In the other parts of this question,...

Emily Ding 1J

I think en, dien, and edta are the only complex ones required for us to memorize. The naming coordinate compounds pdf on Lavelle's website provides a full list which is really helpful even for simpler ligands like oxalate or carbonate.

Emily Ding 1J

Hi, I'm confused about this question from the textbook which asks if AlCl3 is acidic/neutral/basic and if it is not neutral, to write the equation. I understand that AlCl3 is acidic, but the answer key's equation was this: CodeCogsEqn.gif How do you know to add water to Al like that? How do you get ...

Emily Ding 1J

To Evie's question, I don't think we need to know that for this class. Dr. Lavelle said in lecture that we wouldn't be going that in depth.

Emily Ding 1J

Yes, that's a good idea. Many textbook and probably final questions ask about shape/bond angles.

Emily Ding 1J

Hemoglobin is in the blood while myoglobin is in the tissue! Also, hemoglobin contains four myoglobin like molecules so it can transport much more oxygen.

Emily Ding 1J

They have biological significance as explained in today's lecture. Hemoglobin, myoglobin, vitamin B12, insulin, enzymes, etc are all examples!

Emily Ding 1J

There is no difference! All molecules have LDF/van der Waals forces.

Emily Ding 1J

I doubt it, but even if we did need to know, anions would just have really low polarizing power and vice versa right?

Emily Ding 1J

I think all we need to know about the Schrodinger equation is that it gave us 3/4 quantum numbers, which in turn give the shell, subshell, orbital, and even the exact electron if you add the 4th quantum number (which is important of course, but it just wasn't derived from the Schrodinger equation).

Emily Ding 1J

Just know that F is the most electronegative, as well as N, O, and Cl which are right next to it. That way, you can easily remember that electronegativity increases across periods and decreases down groups. Don't worry about memorizing exact values!

Emily Ding 1J

I think one important application of the uncertainty principle was to prove that the atom must be larger than originally thought and that there must be a physical limit to the minimum size of atoms based on physical parameters (because the range of velocity was greater than the speed of light). I'm ...

Emily Ding 1J

You want to put the negative formal charge on the more electronegative element, and vice versa!

Emily Ding 1J

Also, flourine has a higher effective nuclear charge as it's to the right of oxygen, so it's harder to remove the 2p electrons of F than O, resulting in a higher ionization energy.

Emily Ding 1J

There would only be weak London Dispersion forces rather than dipole-dipole or H bonding. I think you're mixing up intermolecular forces and intramolecular. I hope this helps!

Emily Ding 1J

Effective nuclear charge is just the nuclear charge felt by different shells, and it decreases as you move out towards greater energy shells because the inner shells shield the outer shells from receiving the full charge from the nucleus. I hope this makes sense!

Emily Ding 1J

Carbonate has a -2 charge while Li has a +1 charge. I didn't immediately catch it but once you think about it or have to draw its Lewis structure for example it'd become pretty clear.

Emily Ding 1J

Hi, I'm confused about the shape of SO3^-2. I understand that drawing different resonance usually shouldn't matter because regions of electron density remain the same but in this case doesn't adding a lone pair on the sulfur instead of a double bond on the oxygen change the shape from trigonal plana...

Emily Ding 1J

As long as you have the correct central atom, I think for now it's okay because we haven't formally learned shape. Also, I've noticed a lot of the textbook questions tell you in the question what the central atom or bond is, especially if it's a larger or more confusing molecule. Hope this helped!

Emily Ding 1J

Hi, I'm confused about how "far" you can break the octet, if that makes sense. I thought that if you broke the octet, the only possible numbers of electrons that atom can have would be 10 or 14 because that's how many a d or f orbital could hold; however, I was doing 2C.3 C)., which asks f...

Emily Ding 1J

In this question, it gives you the general bond length for all four bonds, then the specific lengths of a single and double bond. The value of the double bond is closer to the general bond length, which is why the Lewis structures with more double bonds are more plausible. I hope this made sense!

Emily Ding 1J

Hi, I'm confused about what exactly dispersion forces are. In lecture, they were said to be -2 kJ/mol, so does this mean that dipole-dipole and dipole-induced dipole are within or a type of dispersion forces? If anyone could help clarify the relationship between ion-ion, ion-dipole, dipole-dipole, d...

Emily Ding 1J

Hi, this is a very simple clarification but I felt really lost towards the end of today's lecture. I'd just like to clarify that a dipole moment is when there's a charge difference, and specifically a dipole moment in a covalent bond is because the atoms aren't the same so the electrons aren't share...

Emily Ding 1J

Yep, sulfur is also in the third period so n=3 and l could be equal to 0,1,2. l=2 corresponds with the d orbital, which can hold more electrons and therefore it can "violate" the octet guideline.

Emily Ding 1J

The single bonded oxygens had two electrons from the bond with nitrogen (the line), and then 6 total unpaired electrons drawn as dots. The double bonded oxygen had four electrons from the bonds, and 4 unpaired. If you add up all of the electrons, it matches the expected amount of valence electrons, ...

Emily Ding 1J

I was wondering if the same exception for Cu electron configuration applied to Ag and Au. Would they also have the d shell filled and the s with only 1 electron, instead of 9 in the d shell and 2 in the s?

Emily Ding 1J

Nope, there was a note at the top that said the rest of the lecture would be included on Wednesday!

Emily Ding 1J

Yes, electrons are shared equally. It's like what we talked about with electron density in orbitals. The electrons are not localized to a single bond or atom.

Emily Ding 1J

Hi, I was struggling with this question: "Magnesium ions are a known cofactor in over 300 enzymatic reactions. What are the possible angular momentum quantum number(s) of the highest energy electrons in these magnesium ions? Indicate the corresponding subshell of each angular momentum quantum n...

Emily Ding 1J

The subscripts of the molecular formula show you that there are 2 Na's and 1 CO3 in every 1 Na2CO3. I hope that makes sense. It's like the mole ratios of chemical equations, just inside of a molecule.

Emily Ding 1J

Adding on to that, the Bohr frequency condition just describes the idea that the energy of the incoming photon must be equal to the energy difference between levels in order to be absorbed. And vice versa: an electron that drops down from its excited state must emit light equal to the energy differe...

Emily Ding 1J

For the diffraction experiment, I think one crucial detail is that the incident light showed up directly behind the barrier, which could only be possible if the electrons had wave like properties. I think Dr. Lavelle described the electrons as forming a wavelet and engaging in constructive interfere...

Emily Ding 1J

I think the most important conceptual aspect of Schrodinger's equation is just how it led to the wave functions and orbitals that we use for electron configuration and other electronic structure. We don't need to know the complicated math stuff!

Emily Ding 1J

It's not necessary but some questions on the audio-visual focus topics also asked if your answer was plausible or helpful, so a similar question might show up on the midterm.

Emily Ding 1J

I also think de Broglie considered the fact that if waves could behave as particles, particles also could have wavelike properties.

Emily Ding 1J

They're the same, the speed of light was just substituted in for velocity.

Emily Ding 1J

Hi, I had trouble with one of the audio-visual focus topic post-assessment questions: You are caught in a radar trap and hope to show that the speed measured by the radar gun is in error due to the Heisenberg uncertainty principle. If you assume that the uncertainty in your position was +/- 5m when ...

Emily Ding 1J

Also it's important to remember that in classical mechanics intensity and amplitude are directly proportional, but in quantum mechanics increasing intensity is related to increasing the number of photons!

Emily Ding 1J

I'm pretty sure both equations are the same thing, where the one equal to 1/wavelength has R=1.097x10^7 m^-1, and the other equal to v (frequency) has R=3.29x10^15 s^-1. Both equations give you either the wavelength or frequency when energy is absorbed or emitted as electrons change energy levels in...

Emily Ding 1J

Another important difference is that classical mechanics usually deals with continuous functions and equations whereas most variables are quantized and discrete in quantum mechanics.

Emily Ding 1J

It all depends on what the question asks! Try to think of the steps given as tools that you get to customize to your benefit depending on what the question wants instead of rigid rules that you always have to follow.

Emily Ding 1J

We haven't directly gone over nomenclature so some more difficult ones may be provided, but it'd probably be good to review basic nomenclature just in case! Some of the textbook questions require you to name compounds, so as long as you can do questions at the same level, you should be fine.

Emily Ding 1J

I once heard someone describe quanta as pennies of the quantum world. If you want to transfer energy, quanta are the smallest units that you can do so in.

Emily Ding 1J

What I did was first convert the 10 mL to molecules, and then set up an exponential equation with that answer in order to find the number of doublings needed to achieve I molecule. Also if you think about it logically, without the equations or math, it makes sense that after such an obscurely large ...

Emily Ding 1J

Probably, but you should always check to be sure!

Emily Ding 1J

With the new Zoom update going into place on October 10, do you guys think it's possible for lectures to be synchronous? I've heard some people say the update could have a capacity of 900 students or something, but I'm not sure if that's true.

Emily Ding 1J

It's the relative ratio of atoms that a molecule has, and I'm pretty sure you can't find the molecular formula without first figuring out the empirical.

Emily Ding 1J

It's easiest to visualize mass percentage by using 100g (because percents are obviously out of 100). I don't think it's necessary every time to convert to mass percentage if you already have the total grams (when I did the problem, I just used the 8 grams), but some people have been saying that it's...

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