CHEMISTRY COMMUNITY

Angela Prince 1J

Khan Academy is definitely helpful for me, the organic chemistry tutor I heard is also a good one to check out!

Angela Prince 1J

did lyndon already put up the worksheet?

Angela Prince 1J

I was curious about this too and whether or not we simply needed 100 posts by the end of this quarter. I believe I had more than 50 last quarter but was not sure if I would still need 50 more posts this quarter or just the remainder to 100. I would definitely assume that it resets this quarter so t...

Angela Prince 1J

ashwathinair wrote:Sometimes I post multiple times on the same thread - is this counted as separate posts or as just one?

if you post multiple times on the same thread it will still count as separate posts.

Angela Prince 1J

So the syllabus says that a 50% is required o pass the class with at least a C- but at the same time, "there's no curve" is what I've been hearing from most students so if I have a 75% in the class it'll remain the same but if I get a 50% it'll jump up to 70%? The "curve" is bas...

Angela Prince 1J

He hasn't sent an email yet, but I'm assuming the review sessions are cancelled, right?

Angela Prince 1J

So just to clarify, if we use the E^o= E^o(Cathode)- E^o(Anode) method, we don't switch the signs but if we use the adding method, we do have to switch the signs?

Angela Prince 1J

how is the Cu(s)|Cu2+ the anode if it is on the right of the cell diagram?

Angela Prince 1J

for part a) why could we not just split up the half-reactions into Ag and Br separately?

Angela Prince 1J

Also, why do we not use the balanced redox reaction (and each molecule's stoichiometric coefficients) when writing out the cell diagram?

Angela Prince 1J

When you have a nonmetal in the reaction, how do you know when to use platinum and when to use graphite as an inert conductor?

Angela Prince 1J

Can someone please tell me what is being covered on test 2 again?

Angela Prince 1J

Leslie Almaraz 4G wrote:what is the purpose of identifying the agents?

I think the main reason is just to understand that redox reactions work of each other. A reaction that oxidizes one molecule has to be paired to one that reduces another, and the electrons that are taken from one are given to another.

Angela Prince 1J

Why are the species that are oxidized called reducing agents and vice versa? something that is oxidized is a reducing agent because in becoming oxidized, it reduces another molecule. the same goes for the other way around: a molecule that was reduced had to have oxidized another for it to reach tha...

Angela Prince 1J

That makes sense but when I tried it for 6K.3 on the hw, I got confused again. How would you write the half reactions for part a?

Cl2 + S2O3^2- goes to Cl^- + SO4^2-

Angela Prince 1J

it makes sense to do it that way, but would it necessarily be wrong to write it the other way around?

Angela Prince 1J

You only reverse one of the reactions and change the sign of one of the standard potentials with the first method. If you try reversing and changing the signs with the second method, you get a different value entirely. Conceptually, why can you not reverse the cell potential value of the reversed r...

Angela Prince 1J

Why is platinum used above graphite and other inert conductors?

Angela Prince 1J

The basic structure of a cell diagram is (left) anode || (right) cathode, where || represents a salt bridge. You want to put the substance being oxidized on the left and and the substance being reduced on the right so that the voltage is positive. Within the left or right side, you want to write ou...

Angela Prince 1J

the purpose of the salt bridge is to neutralize the two reactions.

Angela Prince 1J

Orrin Zhong 4G wrote:Galvanic cells use the energy from spontaneous redox reactions. As the electrons go from the anode to the cathode along a wire, the flow of electrons can be used to do work.

Is it possible for the electrons to flow the opposite way?

Angela Prince 1J

Leila_4G wrote:We are never given T so we can't solve the last step for w and therefore q.

You can use pV=nRT to solve for T and then solve for work.

Angela Prince 1J

It is because you are considering delta U and delta S over the ENTIRE process, and because they are state functions, their pathway does not matter. In the problem, the final volume and pressure are the same so the change in either is 0.

Angela Prince 1J

Why is heat capacity a state function?

Angela Prince 1J

"Suppose that 50.0g of water at 20.0 C is mixed with 65.0g of water at 50.0 C at constant atmospheric pressure in a thermally insulated vessel. Calculate delta S and delta S total for the process."

How would I go about solving this problem?

Angela Prince 1J

I don't know how to calculate this with the information given, but I think it has to do with the nCLn(T2/T1) formula, right? "Calculate the standard entropy of vaporization of water at 85 C, given that its standard entropy of vaporization at 100 C is 109.0 J/Kmol and the molar heat capacities a...

Angela Prince 1J

I know that Lyndon always posts a worksheet before his big review session, but I don't think he has uploaded one yet (hopefully soon!)

Angela Prince 1J

if anyone knows more about this may they please update us, I have this similar question. As long as you receive half the points in the class, you will pass with at least a C-, but the actual letter grades will vary each quarter depending on class averages (which is why he waits until the end to dec...

Angela Prince 1J

So I'm assuming that everything covered up through today's lecture (Fri 02/07) is going to be on the midterm, right?

Angela Prince 1J

i find that the tests are very straightforward and math based questions, but the midterm questions are very implicit and challenging (in my opinion). you really need to know and understand the concepts to be able to solve the questions on the midterms and the final.

Angela Prince 1J

there's no official study guide, but there are plenty of review sessions. the UA's also put together a big review session going over all the topics, and they upload a worksheet for it (which is very helpful).

Angela Prince 1J

For question 4C.3, it asks "Calculate the final temperature and the change in enthalpy when 765 J of energy is transferred as heat to 0.820 mol Kr(g) at 298 K and 1.00 atm (a) at constant pressure; (b) at constant vol- ume. Treat the gas as ideal." I understand how to calculate the final ...

Angela Prince 1J

I am confused about how finding the standard enthalpy using the standard enthalpies of formation works. How and why does it work? Are the values of the standard enthalpy of formation negative because the molecule is losing energy in creating a bond?

Angela Prince 1J

Delta H is equal to q only when the gas is at a constant pressure. By definition, enthalpy (deltaH) is the amount of heat released/absorbed at a CONSTANT PRESSURE, so the heat required to change temp of gas at a constant pressure is the same as enthalpy.

Angela Prince 1J

I am kinda confused on the differences between the molar heat capacity of a gas at constant volume versus constant pressure. Why does it matter in terms of molar heat (is it applicable for specific heat as well)? Can you use the q=nCdeltaT equation for either situation?

Angela Prince 1J

c) is an isolated system because the calorimeter does not allow exchange of matter or eneergy with the surroundings, and d) is an open system because gasoline in an automobile can change in tempertaure (energy) and in matter (which is why we need to refill our tank).

Angela Prince 1J

Yes, a pH lower than 7 is acidic, and a pH higher than 7 is basic.

Angela Prince 1J

solids do not have concentration or partial pressure.

Angela Prince 1J

connie 2C wrote:when a chemical equation is written and all the reactants and products are gases, do you use Kp rather than Kc? in what cases do you use Kc rather than Kp?

You can use whatever is convenient when they're all gases. You cannot use KP if it's not a gas.

Angela Prince 1J

you can either do acids/bases or thermochemistry.

Angela Prince 1J

test 1 only covers chemical equilibrium & acids/bases.

Angela Prince 1J

If the temperature increases, the reaction will favor the products in an endothermic reaction because it requires heat to form the product. If you add heat, the reaction is going to want to use up that heat. Therefore, in an endothermic reaction, the reaction will continue forward, towards the produ...

Angela Prince 1J

Le Chatelier's principle just emphasizes the fact that reactions like to maintain equilibrium, so if anything disrupts that equilibrium (i.e. change in conc., change in volume/pressure), the reaction will shift in order to get back to that equilibrium state. We just use this knowledge to recognize t...

Angela Prince 1J

the inert gas will not affect the concentration or partial pressures of the products or the reactants, they will remain constant so the reaction does not shift.

Angela Prince 1J

I know Prof. Lavelle mentioned this in lecture briefly, but why are neither the products nor the reactants favored when the volume decreases and the number of moles are equal on both sides?

Angela Prince 1J

N2 is more stable in that it has a triple bond. energy is always required to break bonds and released to make bonds. since the N2 bonds must be broken, it is an endothermic reaction.

Angela Prince 1J

The coefficients go with the molecule it is next to in the chemical reaction. The coefficients will be the exponent the concentration of the respective molecule is raised to.

Angela Prince 1J

Ok I see, but there won't be any problems that give both concentration and pressure for different elements right? manipulating the ideal gas law equation gives us a way to convert either partial pressure to concentration or vice versa. so, even if a problem provides values both ways, we should be a...

Angela Prince 1J

Q is the reaction quotient, which is the ratio of the reaction at any given time, not just when the system is at equilibrium.

Angela Prince 1J

R is the ideal gas constant, which will probably be given to us on a test/exam.

Angela Prince 1J

It's also important to note that for a homogeneous gas equilibrium, we can calculate Kp, but we cannot calculate Kp for a heterogeneous equilibrium that involves both gas and aqueous species. Thank you for pointing that out! We would have to convert everything into concentrations, not partial press...

Angela Prince 1J

I just try to go over as much as I can without overwhelming myself; when I start to get stressed I just stop and do something I enjoy to distract myself. sometimes I will come back to studying later if I feel I need it.

Angela Prince 1J

yea, bronsted def has to do with protons while lewis def does not

Angela Prince 1J

the test will prob consist more of calculation with a few concept questions thrown in, but you need to know all the concepts to answer the questions.

Angela Prince 1J

Elizabeth Harty 3A wrote:How do you know if there are sigma bonds available for rotation?

all sigma bonds can rotate, but pi bonds cannot

Angela Prince 1J

is there a difference between amphiprotic and amphoteric?

Angela Prince 1J

What is the difference between cyano and cyanido when naming coordination complexes?

Angela Prince 1J

KayleyW_3L wrote:In addition to wondering when the Test 1 grades will be posted, I am also curious whether there is partial credit on the exams?

definitely!! show as much work as possible, and if you show that know the concept, you will get at least a couple points.

Angela Prince 1J

If anything, it might be covered in the next few lectures.

Angela Prince 1J

I heard that the final is 8 questions and will cover all the topics, but there will be quite a bit on quantum.

Angela Prince 1J

AlyshaP_3G wrote:
205150314 wrote:Does anyone know if we will get some sort of study tools again?

I'm sure Lavelle will be coordinating review sessions like he did for the midterm.

yea my TA said they are going to be holding multiple review sessions covering all the topics we've learned.

Angela Prince 1J

AlyshaP_3G wrote:
205150314 wrote:Does anyone know if we will get some sort of study tools again?

I'm sure Lavelle will be coordinating review sessions like he did for the midterm.

yea my TA said they are going to be holding multiple review sessions covering all the topics we've learned.

Angela Prince 1J

Just in terms of the number of bonds, a molecule with two double bonds would have a higher dissociation energy than one with two single bonds, granted there are no lone pairs on either molecule?

Angela Prince 1J

Okay, so just to clarify, lewis acids are the electron donors that give up the electrons and lewis bases receive those electrons, right?

Angela Prince 1J

Do coordinate covalent bonds occur between atoms in one molecule or between two or more molecules?

Angela Prince 1J

In general I think pretty much all of the stuff we talk about in this class occurs in nature. You can think of bonds as attempts to lower the energy of the atom. If forming a coord. covalent bond brings the atom to a lower energy that it had with a lone pair, it will do it. Probably a dumb question...

Angela Prince 1J

Do coordinate covalent bonds happen only in transition metals?
And does it occur in all transition metals or are there exceptions?

Angela Prince 1J

honestly, it's just preference, but I would personally take the lab with the class just bc every topic covered will stay fresh in your mind as you do the lab for that specific topic.

Angela Prince 1J

Is it a good idea to review online modules before lectures so that I can be familiar with what is going to be lectured on? Or should I review after I have had the lecture? Thank you :) YES this is what helps me the most! I think you can definitely try it both ways and see which works best for you!,...

Angela Prince 1J

A dipole moment happens when there is a separation of charge. It can occur in both ionic bonds and covalent bonds and are due to difference in electronegativity. It describes two opposite charges separated by distance.Essentially, it is a measure of the polarity of the molecule. so a compound is po...

Angela Prince 1J

Basically, in really broad terms, a dipole moment is found between two atoms who are sharing an electron. Due to one's higher electron affinity, it might hold the electron closer to it than its partner. The increase in negativity as the electron is closer to an atom is what the dipole moment is, if...

Angela Prince 1J

so, just to clarify, test 2 covers dipoles and molecular shapes?

Angela Prince 1J · Re: Final Yes, it is on Sunday Dec 8 at 11:30 am

Yes, it is on Sunday Dec 8 at 11:30 am

Angela Prince 1J

Can someone explain what a dipole moment is exactly?

Angela Prince 1J

KayleyW_3L wrote:The good thing is he will never curve down, only up.

Wait, so is there a curve or not?

Angela Prince 1J

Can someone explain the first question about finding the empirical formula of that compound that was on the midterm?

Angela Prince 1J

Could someone help me with this problem?
it is asking to list the following molecules/ions in order of decreasing bond lengths and explain your reasoning:
a) the CO bond in CO, CO2, CO3 2-
b) the SO bond in SO2, SO3, SO3 2-
c) the CN bond in HCN, CH2NH, CH3NH2

Angela Prince 1J

ShreyaKannan1B wrote:Is there any problem where you would use Rydberg with another equation?

from my understanding, you would only use the Rydberg EQ when you are looking for the change in energy when an electron moves from one energy level to another.

Angela Prince 1J

sbeall_1C wrote:Hey! I am planning on doing all the questions listed on the syllabus. Should I turn in all of them or just 5 of them?

you can turn in just 5 or all of them, I don't think it matters. the TA will probably just grade the first 5 if you turn all of them in.

Angela Prince 1J

everything covered so far up to focus 2D will be on the midterm, but I wouldn't worry too much about the fundamentals because we already had a test covering that material. I would still look over it though :)

Angela Prince 1J

there are many review sessions covering the same topic, so if you can't make one, don't worry about it! for specifics, the schedule is posted on the website

Angela Prince 1J

all those tips are really helpful! in addition, i would go over all the problem sets again and see if you can solve them without too much help from the textbook and your notes

Angela Prince 1J

What is the difference between an atom's ionization energy and the e- affinity, and what are the trends on the periodic table for both?

Angela Prince 1J

Are the hw problems for week 5 from the quantum topic set still or from the chemical bonds set?

Angela Prince 1J

KTran 1I wrote:Are there a lot of topics in the book that are not covered during lecture that show up on tests?

I don't think topics not covered during lecture will ever show up on any exam.

Angela Prince 1J

i understand why the first equation prof. lavelle showed us is negative (-hR/n^2). when he derived the second equation (-R[1/n1^2-1/n2^2]) he kept the negative in front but my TA showed that equation as positive. Which is correct?

Angela Prince 1J

no need to worry about it! Prof Lavelle mentioned the tests would already provide the photon's momentum!

Angela Prince 1J

APatel_4A wrote:He said that it is 720 to 400 but that we can just remember 700 to 400 since it's easier!

Because it differs so much according to different tables, I think it will be fine if you just remember 700 to 400

Angela Prince 1J

Liza Hayrapetyan-3K wrote:I just want to be clear, so there is no way to get notifications if Professor Lavelle has posted anything?

I don't think you will get actual notifications, you just have to look on the class announcements page.

Angela Prince 1J

Also, do the workshops go over what we talk about in lecture?

Angela Prince 1J

I attended a step up session the other day, but the pace was way too slow for me? I'm thinking I should just attend the workshops. Does anyone know how the pace is for the workshops?

Angela Prince 1J

I can't find my test grades on ccle, where do I go?

Angela Prince 1J

For homework assignment week 3 would we be able to do week 2 homework problems as well or is it just going to be week 3 homework problems? Week 3 homework is only on Quantum Mechanics; the only reason Prof Lavelle allowed us to use the fundamentals for week 2 homework is because he was giving us th...

Angela Prince 1J

ktran Disc 1I wrote:Are there a lot of topics in the book that are not covered during lecture that show up on tests?

I don't think so. I think everything that will be on a test will be covered in lecture or otherwise noted in the outlines for each topic.

Angela Prince 1J

Does anyone know if we will be on the quantum topic for more than just one week? I just want to know if the homework set for quantum will be used for both weeks 3 and 4. Thanks!

Angela Prince 1J

Because each question is worth so many points, it would be wise to show at least the basic steps that you used to get to your solution.

Angela Prince 1J

What works for me is to study the concepts that I struggle with, just look over the ones I know just to make sure I have them down, but not to psych myself out studying until the last minute. I tend to overstudy, and I've learned over the years that if I take the time just before a test to just rela...

Angela Prince 1J

I would start by using the equation for molarity and go from there: Molarity = number of mols/volume for part a, we can find the numbers of mols of Na2CO3 by taking 2.111 grams and dividing it by the molar mass of Na2CO3, which is about 106. Hence, we get 2.111/106 = 0.01992 mol Na2CO3. To find the...

Angela Prince 1J

So this problem is asking us to determine the volume of the solution should be transferred into a flask in order to obtain 2.15 mmol of Na+ . Importantly, this is not a dilution problem. Therefore, we won't be using the M1V1=M2V2 equation. The molarity of the solution won't be changing when we tran...

Angela Prince 1J

Could someone please help me solve this problem? I got as far as converting 2.111g to moles (.020 mol) and finding the molarity (.8 mol.L-1). I don't know how to solve the rest. Thank you! (p. F58) G.5 A student prepared a solution of sodium carbonate by adding 2.111 g of the solid to a 250.0-mL vol...

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