CHEMISTRY COMMUNITY

Adrienne Chan 1G

Hey! So kinetics vs thermodynamically controlled depends on temperature. Kinetics controlled reactions are at lower temperatures, and thermodynamically controlled ones are at higher temperatures. One example Lavelle gave during class was diamond turning into graphite. Normally, we don't see diamond ...

Adrienne Chan 1G

For steam, there's a heat of vaporization that we have to factor in, beyond the actual energy due to heating. They may both be at the same temperature but the steam will have much more energy.

Adrienne Chan 1G

If there's no metal to serve as an electrode, we use Pt to as the electrode.

Adrienne Chan 1G

Catalysts merely increase the rate of a reaction by lowering the activation energy. This does not affect the overall heat of reaction, as the products and reactants are still the same in the end. You can think of it like an enzyme if that helps!

Adrienne Chan 1G

Not taking any chemistry for spring, but I'll definitely take a chem class in fall and maybe this summer as well!

Adrienne Chan 1G

Thank you this response was helpful! Where else would 14A material show up, besides in 14C?

Adrienne Chan 1G

Denaturing unravels the enzyme, and poison makes it so that's inactivated, so the poison would take the place of the substrate.

Adrienne Chan 1G

Unfortunately 14C and BL filled up for me, but I'll be taking HC 50, LS 7C, 23L, and Psych 10. If anyone else is taking these, feel free to let me know!

Adrienne Chan 1G

Big thanks to Andre and co for all the organizing! I had such a fun time and I loved seeing everyone else's performances!

Adrienne Chan 1G

Follow up question to this, what if another step is very close in speed to the slow step, but just a little bit faster? In this case, I feel like the speed would be dependent on both of these.

Adrienne Chan 1G

Everyone has great answers so far, I just want to add that for first order (ln[A] vs time) being linear, means that [A] vs time will be exponential for this!

Adrienne Chan 1G

I wonder if Dr.Lavelle will be blessing us with some vocals?

I'm wondering this too! Would love a response from chem mod :)

Adrienne Chan 1G

I actually do both of these in very similar ways, which simplifies the process a lot. I first balance both equations as if they were acidic, using H+ and H2O. Then, after I've balanced the electrons and added the two half-reactions together, I add enough moles of OH- to both sides to turn all the H+...

Adrienne Chan 1G

Hi! So my TA last quarter said that there was a bot that detects the time we've made posts, so I think we do need to do five per week even once we've hit 50. Personally, I would do it to be safe. Hope this helps!

Adrienne Chan 1G

Yes, that's actually a great way of remembering the oxidizing agent, I like to think of the two being the opposite of each other.

Adrienne Chan 1G

The anode is traditionally written on the left and the cathode is on the right.

Adrienne Chan 1G

So oxidizing agents are going to be reduced, so we can look at the standard reduction potential for the atoms/molecules listed. We look at the table for those and determine and add that reduction potential to the oxidation of the equations given. One of the answer options will make E naught over 0 f...

Adrienne Chan 1G

Thank you so much for organizing this! I've just filled out the form

Adrienne Chan 1G

Hi, I'm not really sure which solution you're talking about? I checked the solutions and they all seemed to contain the Faraday constant, could you please specify which one?

Adrienne Chan 1G

Hi Cristian, are you talking about the permanganate and Fe 2+ in water equation? Lavelle breaks down the redox equation into its two components: the oxidation and reduction components. In oxidation: 5 Fe 2+ -> 5 Fe 3+ + 5e- we see that each Fe atom is long an electron, for a total loss of five elect...

Adrienne Chan 1G

Hey Kiana (and everyone else), I definitely feel your struggle! Most people I know are busy due to clubs, but not too many people I know are working, so it feels really nice to find other people who are. I'm lucky that I'm just tutoring, so the families are often very flexible. I do my best to let t...

Adrienne Chan 1G

LEO is simply a pneumonic device to remember how redox reactions influence oxidation numbers. You can think of it as Losing Electrons (occurs during) Oxidation. In high school, I learned it as OIL RIG (Oxidation is Losing, Reduction is Gaining). Whichever way works best for you!

Adrienne Chan 1G

According to the 4 year plan: https://www.chemistry.ucla.edu/sites/default/files/undergraduate/Biochemistry_Major_2019-2020.pdf it says that you can take 30A after 14B in the actual plan itself. Either 14A/B or 20A/B appears to be fine for the major, so they would seem odd if they didn't let you do ...

Adrienne Chan 1G

@Ellison I believe delta G and delta G naught can have the same units! Usually it's kJ/mol

Adrienne Chan 1G

You may be familiar with q = mCdelta T from high school, and that one uses mass (m), and C is the specific heat per gram. This one uses moles (n), and the C is specific heat per mole.

Adrienne Chan 1G

Internal energy is just the energy of the system, so we can see how work and heat are increasing/decreasing. That's because U = q + w, so work and heat are what we pay attention to.

Adrienne Chan 1G

State functions do not depend on the path taken from the beginning to the end state. Work and heat are not state functions, as they do depend on the pathway taken.

Adrienne Chan 1G

Thank you so much this is really helpful! @silvi I think delta S being 0 is for reversible reactions, I don't think it's for an irreversible reaction.

Adrienne Chan 1G

In an isothermal reaction, the heat stays the same when taken at the start to end. But the actual temperature cannot stay the same throughout the entire reaction, but we still say that the temperature is the same. An adibatic reaction is isolated, there is no heat or mass transfer with the surroundi...

Adrienne Chan 1G

q is just heat in general, but qrev is heat of a reversible reaction

Adrienne Chan 1G

Like everyone else, Kelvin or Celsius would be good. But for PV=nRT I believe we use Kelvin for the temperature, hope this helps!

Adrienne Chan 1G

Reactants have their bonds break and products have bonds forming.

Adrienne Chan 1G

Closed systems can have energy flow in and out, whereas in isolated systems, nothing can interact with the surroundings.

Adrienne Chan 1G

A reversible reaction is when the pressures inside and outside are the same. When the external pressure is slowly decreasing and there's an small volume change, the equation is w = - (integral) P dV. As for an irreversible reaction, the pressures are different, but the external pressure is constant....

Adrienne Chan 1G

Yes, phase does matter in Hess's law. It takes energy to go from one state to another, so you need to take that into account for Hess's law.

Adrienne Chan 1G

Water has a high heat of vaporization due to the hydrogen bonding. When transitioning from liquid to vapor form, these hydrogen bonds are being broken, thereby forming a vapor that has molecules that are much farther apart.

Adrienne Chan 1G

I thought all of the problems except for one were quite straightforward. The one that was difficult was extremely confusing, I interpreted it as not having enough information to solve and later couldn't figure out how to solve it.

Adrienne Chan 1G

V1 is the initial volume and V2 is the final. For all equations I've seen so far, variables with a subscript 1 represent the initial value and a subscript 2 represents the final one.

Adrienne Chan 1G

Water is going from gas -> liquid phase, so you can think of the equation like H2O (g) -> H2O (l) + heat. Exothermic reactions release heat, making this reaction exothermic.

Adrienne Chan 1G

Monoatomic gases are gases that are noble gases like halogen and other substances.

Almost! Monoatomic gases are gases with only one atom, so it would just be noble gases. Halogens are diatomic gases.

Adrienne Chan 1G

This seems true for monoprotic acids!

Adrienne Chan 1G

Because Na+ is part of a strong base (NaOH), it does not affect the pH and can be ignored.

Adrienne Chan 1G

It depends what phase it's in. If it's a liquid we ignore it, but if it's aqueous then we do count it.

Adrienne Chan 1G

Kandyce, yes for this problem the two are interchangeable.

Adrienne Chan 1G

I don't really listen to music when studying, but sometimes I'll listen to rain noises or the like. As for music I enjoy, it runs the gauntlet from punk rock to French pop.

Adrienne Chan 1G

Even though they both might be positive, Lavelle said only one of them will make sense when they're plugged into the equilibrium values. I just plug them into all of the values in the "E" row of the ICE table and it's always given me the right answer.

Adrienne Chan 1G

We don't have to use mol/L, any unit for concentration will work.

Adrienne Chan 1G

We don't always multiply/divide the equilibrium constant, that's only if we're adding the coefficients.

Adrienne Chan 1G

I don't really know what you mean by this question, could you please specify? The time it takes for any system to reach equilibrium will depend on the concentration of products and reactants, I don't think it has much to do with the fact if it is forward or reverse. In addition, the heat can have an...

Adrienne Chan 1G

Yes, that's correct! When Q and K are different, we can tell that the concentrations are not at equilibrium.

Adrienne Chan 1G

In the second scenario you mention, for [P] being greater than [R], this would mean that the condition Q<K is no longer true, and the reverse reaction would be favored. Hope this helps!

Adrienne Chan 1G

I don't think Lavelle said anything about this, but either way, it seems to be proportional to the amount of work we've had in 14A so far.

Adrienne Chan 1G

Yep! It's in the name itself. Q is the reaction quotient and K is the equilibrium constant.

Adrienne Chan 1G

We usually use atm for units as it is part of the SI system. Bar is not, and so we usually convert bar to atm unless the problem asks us to use bar specifically.

Adrienne Chan 1G

We get the inverse of K, this is calculated as 1/K.

Adrienne Chan 1G

Cl- has 17 protons, whereas K+ has 19 protons. However, they have the same number of electrons. This means that K+ has a greater effective nuclear charge and will pull on the electrons more tightly than Cl-, making its atomic radius smaller. Hope this helps!

Adrienne Chan 1G

Thank you, this was super helpful! I definitely had trouble telling when a salt would be basic or acidic and this video was really helpful in helping me distinguish between the two.

Adrienne Chan 1G

Like a lot of other people here, very, very tired, even after getting more than 8 hours of sleep each night. Even though it seems to be rough, for many of us, it's been our very first quarter in college, and I think we're doing a great job of doing our best!

Adrienne Chan 1G

I don't think size matters in this case, I remember Lavelle saying that it's the resonance and the delocalization of electrons (influenced by the electronegativity of various atoms) that influences the stability of the anion. Unfortunately, I still don't know how to answer your question and was wond...

Adrienne Chan 1G

My guess is that since fluorine is so electronegative, it doesn't let go of the electron as easily as other halogens might. This would make it harder for HF to dissociate in water, and so weaken the acid, thus making it not a strong acid.

Adrienne Chan 1G

It would be helpful if you could provide a little more info to specify what you're confused about, but hopefully this helps as a starter. When written out in a coordination compound, ligands will be the part inside of the brackets. Physically, ligands are part of the coordination sphere and attach t...

Adrienne Chan 1G

In this class, c will always be the speed of light. Although this is technically the speed of light in a vacuum and light will travel slower through other substances, in this class we will just use this optimal maximum value.

Adrienne Chan 1G

If you know which compounds form strong bases/acids, everything else that isn't one of those few will be a weak acid/base. Weak bases will have OH but not be a strong base, and weak acids will be the other remaining ones. Strong bases will have the highest pH and strong acids will have the lowest.

Adrienne Chan 1G

Yes, I think you got it right! We don't need to specify the numbers for the anions outside of the brackets, as that will be given to us based on the other parts of the compound that we figured out before.

Adrienne Chan 1G

VSEPR just tells us the shape of the molecule, not the number of bonds or bond strengths. It tells us the number of regions of electron density in relation to the central atom.

Adrienne Chan 1G

We're focusing on the number of regions of electron density, not just the number of bonds in hybridization. It seems like in both examples you mentioned, there are four bonds attaching the same central atom to four other atoms. So, there would be four regions of electron density in both examples, an...

Adrienne Chan 1G

Yeah I think Lavelle said there wasn't a curve, at least from what I can remember!

Adrienne Chan 1G

An sp3d orbital would look like the trigonal bypiramidal shape! It looks like a fidget spinner along the equator, and then two axial orbitals with one going up and the other going down.

Adrienne Chan 1G

Lavelle gave an explanation in lecture about why double bonds had to be one sigma and one pi, but I still didn't understand. Could someone please explain it in a different manner?

Adrienne Chan 1G

Can the bonds in a molecule be polar even if they all bond to the same element? I'm not sure entirely what you mean, if the central atom is different from the other ones, then yes I definitely think it's true! The other people above have some great responses. But if all the atoms in the molecule ar...

Adrienne Chan 1G

An unpaired electron would still be considered an area of electron density and we would still put E as usual, no 1/2 subscript needed.

Adrienne Chan 1G

I'm pretty sure Lavelle said we didn't need to memorize the bond angles for specific molecules, but it's probably a good idea to know what it is for the AX_n version, and just know that ones with lone pairs will have a smaller bond angle.

Adrienne Chan 1G

How many bonds does aluminum form? I keep getting confused on this and can never figure it out. Thanks!

Adrienne Chan 1G

I know that NO₂ is polar, but now that I think about it I'm not entirely sure why. Could someone please explain why it's polar? Thanks!

Adrienne Chan 1G

I think the person above forgot to include covalent bonds! This would go between ion-ion and hydrogen bond. I agree with everything else they said, I hope this helps.

Adrienne Chan 1G

Not an answer per se, but I had the exact same question. Fluorine is even more electronegative than oxygen, so I thought, if anything, the hydrogen bonding between HF molecules would be stronger. Curious to see if anyone understands Lavelle's thought process!

Adrienne Chan 1G

Do these atoms (N, O, F) also have to be attached to another atom that is significantly different in electronegativity? No they don't! The only requirement is that they must have at least one lone pair. This lone pair has the potential to form hydrogen bonds with the hydrogen on another molecule. a...

Adrienne Chan 1G

You're right the O has two lone pairs! The C=O bond should be a double bond, with the C as the central atom.

Adrienne Chan 1G

How do we know that nitrite has a double and single bond instead of two double bonds? I'm pretty sure the formal charge for both of these is the same, it's just that the formal negative charge is on the nitrogen versus the single-bonded oxygen.

Adrienne Chan 1G

The more bonds a bond has, the shorter it will be. So the bond length order would go single > double > triple.

Adrienne Chan 1G

p orbitals can have three quantum numbers, l = -1, 0, 1. Px, Py, and Pz are their specific names.

Adrienne Chan 1G

The speed of light will always be represented by c (so far as I know!). However, the actual speed of light changes depending on what material it travels through. The constant c is the speed of light in a vacuum, but it will change if it's going through a different material.

Adrienne Chan 1G

So what helped me the most was drawing different structures and calculating the formal charges for each atom in each configuration. It takes me a little while to do it but it's the most reliable way I've found!

Adrienne Chan 1G

There's a problem with formal charge in this case! Although it might technically follow the rules, the individual atoms don't have the optimal formal charge.

Adrienne Chan 1G

Carbon can have non-bonding orbitals. If carbon had 4 bonds, the amount of energy gained by forming the bond would be offset by the electron repulsion.

Adrienne Chan 1G

An ionic bond has one atom completely "stealing" the electron of the other, there is a complete transfer. Covalent bonds still share electrons between the atoms, but one atom might steal the electron more often (resulting in a partial negative/positive charge on the respective atoms).

Adrienne Chan 1G

Bent and angular mean the same thing! You can use either one, I've personally seen bent the most frequently.

Adrienne Chan 1G

Brian_Wu_3G wrote:Are expanded octets more unstable than regular octets?

Yes, they generally are. One example is the free radical like the methyl radical and CFCs.

Adrienne Chan 1G

Brian_Wu_3G wrote:Are expanded octets more unstable than regular octets?

Yes, they generally are. One example is the free radical like the methyl radical and CFCs.

Adrienne Chan 1G

The only requirements for hydrogen bonds are that the hydrogen in one molecule must have a slight positive charge and F, O, or N must have a slight negative charge. Hydrogen bonds are not "true bonds" as they form between different molecules.

Adrienne Chan 1G

An atomic orbital is the mathematical model of where we think electrons are most likely to be. How do we calculate these probabilities? From the Schrodinger wave function! Each model, or orbital, can be described using three quantum numbers: n, m, and m l . The s, p, d, and f orbitals are when l=0,1...

Adrienne Chan 1G

A bit simpler of an explanation (I'm assuming we're talking about one n and l in this case): 1. Fill up each orbital with one "up" spin electron until all orbitals are half-filled. 2. Put a "down" spin electron in each of the orbitals until they are filled. That's it! You can loo...

Adrienne Chan 1G

@Neel Bonthala 3E Not sure if you get notifications for this, but thanks for a great explanation! What does it mean by the word "relieved"? Does it mean that since its ionized, the oxygen now experiences electron-electron repulsion?

Adrienne Chan 1G

Just to add on, electrons have wave like properties so to model them on a graph, we use a sine wave. The Schrodinger wave function shows the height of the wave at x, y, and z in 3D space, but in order to use it, we need to make it have physical sense, as electrons cannot have negative probability of...

Adrienne Chan 1G

In today's lecture, Lavelle gave an example in the slideshow that said when n=2, l=1, and m_l=-1, the electron is in the 2_p_x state. But then on the whiteboard he wrote the same thing but said that m_l=+1. Could someone please explain which one it is?

Adrienne Chan 1G

When the electron is freed, its energy is at its maximum and its given as 0. And then for the energy level, the closer the electron is to the atom, the more it will be bound. So when the electron is free, the energy level can be said to be infinite because it's not bound to the atom at all.

Adrienne Chan 1G

Just to add on, the Balmer series is in the visible region so we can see the photon emitted when an electron falls down to n=2.

Adrienne Chan 1G

Orbitals past f do not exist (so far as we know!) so l will always be less than or equal to three. It would be great if someone could help give me an explanation.

Adrienne Chan 1G

Hey Sami, thanks for your reply and the article, it's a good one to come back to. I actually rewatched Lavelle's lecture and he said that wavelengths that weren't absorbed actually pass through the atom. Still trying to see how this information works with the basics physics info that materials abso...

Adrienne Chan 1G

Hey Sami, thanks for your reply and the article, it's a good one to come back to. I actually rewatched Lavelle's lecture and he said that wavelengths that weren't absorbed actually pass through the atom. Still trying to see how this information works with the basics physics info that materials absor...

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