CHEMISTRY COMMUNITY

Yixin Angela Wang 2H

I don't know if people are still posting these, but I want to thank Dr. Lavelle for being an encouraging and dynamic professor; his courses really helped me to gain confidence and proficiency in chemistry. I can now firmly say that I like chemistry more than I do physics. Before, they were sort of t...

Yixin Angela Wang 2H

I asked my TA and he said we only needed to be able to identify the functional groups that Dr. Lavelle went over in class.

Yixin Angela Wang 2H

Hi! I totally understand your rustiness. I took a basic chem class in my sophomore year of high school and then never again. I feel that even though AP Chem would have better prepared me for a lot of 14A and B, I was able to understand the concepts by paying attention in class and going to review se...

Yixin Angela Wang 2H

The complex refers to the thing formed as the intermediate between reaction and product. It's also used when referring to the form that enzymes take when they bind with substrates.

Yixin Angela Wang 2H

I think in class today, Dr. Lavelle used k and k' to denote the rate constants for the forward and backwards reactions of a reaction at equilibrium.

Yixin Angela Wang 2H

Dr. Lavelle gave a baking example today. If it takes 30 minutes for all the other steps of cookie-making but 45 minutes to bake the cookies, then no matter how much dough you prepare you can only make cookies as fast as the oven will bake them.

Yixin Angela Wang 2H

Yes, I believe that's correct. Molecularity is the number of reactants in a particular step, so for example a reaction that requires 2 molecules of one compound and 1 molecule of another would be 2A+B--->product, and it should be termolecular.

Yixin Angela Wang 2H

I don't think we will need to do anything too difficult in regards to derivations. Maybe we will need to manipulate some equations or combine them, like we have done on tests or midterms, but I don't think we will need to completely derive anything.

Yixin Angela Wang 2H

Yes. Two molecules of a +2 molecule would have a total +4 effect, so you would need to evaluate the amount of negative charge needed accordingly.

Yixin Angela Wang 2H

I think the H+ is the part that matters if its taking place in an acidic solution, but H30 is the technical substance that is being formed.

Yixin Angela Wang 2H

I think I've only ever seen hydroxide used to balance the charges, hydrogens, and oxygens.

Yixin Angela Wang 2H

Also looking at a graph of the data will give you an idea of what order a reaction is. For example, if the data looks linear, its first order.

Yixin Angela Wang 2H

So we don't ever have to assume a default of either acidic or basic?

Yixin Angela Wang 2H

They are opposites of each other! The oxidizing agent is the one DOING the oxidizing, which means that it is oxidizing the other agent, so the oxidizing agent itself is reduced.

Yixin Angela Wang 2H

I feel like acidic seems to be the standard, based on the problems I've done. I feel like whether the solution is acidic or basic should be given in the problem or the skeletal equation though.

Yixin Angela Wang 2H

Is it because the ΔS in the Gibbs free energy equation is only for the system, and not the total?

Yixin Angela Wang 2H

Also, as per the third law of thermodynamics, a perfectly organized crystal will have 0 entropy. Otherwise, I would look at the state of the substance in question, because there is a big difference between liquid and gas, but not between solid and liquid. I am not sure, however, if a complex molecul...

Yixin Angela Wang 2H

I think the per mole part can be omitted, at least that's what I recall Dr. Lavelle saying. At least for this class.

Yixin Angela Wang 2H

I'm piggybacking onto this post, but I was just wondering if the substance is solid, does it extend into both containers? Because the way that Professor Lavelle described it in lecture, I thought that if one side had a solid reactant and the other didn't, the side without solid reactant would use pl...

Yixin Angela Wang 2H

Yeah, I don't think it makes sense logically if we try to calculate the potential when it equals zero because wouldn't that mean that there was no difference? Haha, I don't know if I'm expressing myself clearly.

Yixin Angela Wang 2H

Is there sort of a "hierarchy" regarding which trait we should consider first when determining molar entropy?

Yixin Angela Wang 2H

I agree. Also, since the specific heat is given per gram, then that means the more grams of substance there are, the more C there must be. Thus, multiplying by grams makes sense.

Yixin Angela Wang 2H

I don't think you can really compress or expand a liquid's volume, but you can increase its entropy by mixing it with another substance or increasing its temperature.

Yixin Angela Wang 2H

Dr. Lavelle gave the example of a piston weighed down by sand. The reversible reaction would occur if the weight was removed, grain by grain, slowly. This way, there wouldn't be energy lost as heat.

Yixin Angela Wang 2H

yeah, i think a lot of the problems I saw in the hw deal with closed systems.

Yixin Angela Wang 2H

Then how would you be able to add energy to an open system?

Yixin Angela Wang 2H

Which gas constant R do we use, and how do we choose?

Yixin Angela Wang 2H

enthalpy of formation is related to the formation of the compound at hand. H2 making H2 doesn't require any extra heat because hydrogen just prefers to stay in H2 form naturally without any other factors influencing it.

Yixin Angela Wang 2H

I think the answer is saying that with the expansion, the molecules have more room to move around and thus are less likely to collide with each other or the container. Thus their kinetic energy decreases. Collisions with the container create the pressure exerted on the container, so with less collis...

Yixin Angela Wang 2H

Another way of thinking about it is by using the mountain climbing metaphor Dr. Lavelle went over in class. The change in altitude would be a state function but the distance traveled by foot of the hiker would not be, because the hiker's distance could vary based on the nature of the path he took. I...

Yixin Angela Wang 2H

Thank you for the worksheets and practice exams! Is there any way I could get the answers to the first practice midterm?

Yixin Angela Wang 2H

so we would list all possible oxidation states, even if some are more or less likely than others? Also, would it be possible to do something like this for the transition metals (like in coordination complexes) without having to refer to the charges of the other elements and molecules involved?

Yixin Angela Wang 2H

Could someone explain the notation for writing out and describing sigma and pi bonds, and how we know which orbitals are involved?

Yixin Angela Wang 2H

So basically, the first bond formed is always a sigma bond, and the second or third bond, if there is one, is always a pi bond?

Yixin Angela Wang 2H

I think in general, yes you always subtract from reactant and add to product. The thing to keep and eye out for is the relative amount to add/subtract. I always double check my equation to make sure that it's balanced, and I try to pick a reactant with a stoichiometric coefficient of one to set as m...

Yixin Angela Wang 2H

Hey! Sorry I don't have an answer to your question, but I was wondering if you could walk me through how you got the 0.17 number. I was a little confused about that. Thanks!

Yixin Angela Wang 2H

In the case of adding more reactant or product, would we then be able to say that the equilibrium has "shifted"? What effect does that have on K?

Yixin Angela Wang 2H

Self-test 11.2A asks for the equation for the equilibrium constant for the reaction between atmospheric oxygen and oxygen dissolved in water. The solution given is K = [O2 ]/PO. Can someone explain to me how this is ok? I thought the entire equation had to be written either in terms of concentration...

Yixin Angela Wang 2H

Basically, when the amount of diatomic molecules stays the same between flasks that's when equilibrium has been reached. I have another question regarding 11.7. For the part that asks you to calculate K but only gives you the partial pressure of the diatomic molecules, how do you figure out the pres...

Yixin Angela Wang 2H

To add on to what is being discussed, a UA in a midterm review today said that the maximum amount of electrons an expanded valence shell can have is 12.

Yixin Angela Wang 2H

In response to Shannon's question, I think that in general, the noble gases should have an electron affinity of basically 0 because they already have a full valence shell and thus would not tend to try to gain or lose electrons. Halogens would have a high electron affinity because they really want t...

Yixin Angela Wang 2H

For trickier problems, I like to write out the neutral atom's electron configuration first, putting all the orbitals in their proper order by the amount of energy they have. Then, to form ions, specifically anions, I would remove the specified number starting from the outermost, highest-energy elect...

Yixin Angela Wang 2H

Cations and anions have positive and negative charges respectively, but why do they have those charges? Imagine a neutral atom like Silver. In its zero-charge state it has 79 electrons and thus it must also have 79 protons to cancel those charges out so we end up with a net zero. If one electron is ...

Yixin Angela Wang 2H

As you move down the periods, electrons start to occupy higher-level orbitals, so the shielding effect Dr. Lavelle mentioned goes into effect. The outermost electrons are shielded from the full attractive force of the positive nucleus, so there isn't as strong of a pull on the outer layers of electr...

Yixin Angela Wang 2H

One trick that makes figuring out unpaired electrons easier is if you write your configuration starting from the last noble gas above the period with the element you are working on. Since noble gases have a full valence electron shell, we know that their orbitals are all full. Thus, you only have to...

Yixin Angela Wang 2H

Basically, since electrons are in constant, random motion, you cannot pinpoint where a single electron is at any given moment. However, since electrons have different levels of energy and are all attracted to the positive charges in the nucleus, you know that the electrons must occupy a certain area...

Yixin Angela Wang 2H

I took it today and can confirm: the test is just 1.1-1.5.
For those of you taking it on Friday, good luck!

Yixin Angela Wang 2H

If I need a precise answer, I use as many significant digits as I am given. However, if I am solving something for a ratio or something where rounding does not change the final outcome, I will use a more general number and save some time.

Yixin Angela Wang 2H

I'll just add some things that Dr. Lavelle covered in lecture today. The photoelectric experiments showed that light can behave like a wave, and the properties of light, specifically the effects of increasing its intensity, change when light is thought of as particles. In wave form, light's energy i...

Yixin Angela Wang 2H

Molarity is moles of solute/volume of solution. The Mi*Vi = Mf*Vf is dilution, which is not what this particular problem is asking. To solve this problem, you first convert grams of KCl to moles by dividing the mass by the molar mass. Then, you divide the number of moles by the volume in liters, whi...

Yixin Angela Wang 2H

Additionally, a lot of formulas are given in specific, usually SI units. For example, temperature is usually in Kelvins in formulas, but it is in Celsius when you measure it. In that case, to use the formula correctly, you have to convert Celsius to Kelvin.

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