CHEMISTRY COMMUNITY

Jocelyn Thorp 1A

this topic has an asterisk, which means it is no longer covered in the curriculum/not needed to know for the tests

Jocelyn Thorp 1A

My TA said that she has no idea what is going on the final, beyond what we are told as students.

Jocelyn Thorp 1A

I'm very interested to know this as well. I scheduled my work as if the final were to be on Sunday, so I'd really like to know as well, especially as the weekend approaches.

Jocelyn Thorp 1A

It's cumulative so everything! Though it is take home so you can refer to past notes.

Jocelyn Thorp 1A

I don't think that you can easily guess what an intermediate species is for a reaction, it has to be given to you if you were going to try to solve a problem using it.

Jocelyn Thorp 1A

with acidic conditions you're balancing H2O and H+, and with basic conditions you're balancing H2O and OH-

Jocelyn Thorp 1A

it's the voltage of the cell

Jocelyn Thorp 1A

Tai Metzger 3K wrote:let's say: rate = [A]^m * [B]^n

the order of reaction = m + n

I hope this helps!

thank you, I was confused as well!!

Jocelyn Thorp 1A

I understand that values above 10^3 are large and below 10^-3 are small. I'm asking what values are reasonable, as in what are the minimum and maximum k values that have been found experimentally.

Jocelyn Thorp 1A

does anyone know what the possible ranges for K are? Like what is the the minimum/maximum value you would find for K in the real world?

Jocelyn Thorp 1A

TimVintsDis4L wrote:One of the rules our TA taught us regarding this was that it's based off what type of reaction you have.

If it's acidic, balance O with water, then balance H with protons

If it's basic, balance O with OH-, and H with H20

this is a way simpler way to think about it than the worksheet, thanks!

Jocelyn Thorp 1A

so fixing the balancing error I did get the right answer.

also, we can do Cl2 -> 2 Cl- because there was a typo in the book and the equation was supposed to be
Cl2 -> HClO + Cl- where we had to balance the extra H+ and waters.

Jocelyn Thorp 1A

Maybe this isn't included in this particular subcategory because it's more related to organic chem, but reduction and oxidation are the key ways in which cells get energy (fully reduced having the most available energy and fully oxidized having none available to use). This can either be through aero...

Jocelyn Thorp 1A

Ok so I know that there's a typo for 6K3.D where the one of the products is meant to be 2Cl - rather than Cl 2 . For the oxidation reaction I have: Cl 2 + H 2 O -> HclO + H + + e - For the reduction reaction I have: Cl 2 + 2e - -> 2Cl - so when balancing the electrons you multiply the oxidation reac...

Jocelyn Thorp 1A

thanks i was absolutely losing my mind at how to do this problem... can't believe there was a typo

Jocelyn Thorp 1A

if you're ever concerned, just check out the constants and equations sheet on lavelle's site. It's been the same since the course started and contains all the equations that will be given to us

Jocelyn Thorp 1A

when you have a larger Q, you have more products and less reactants. this makes for a low potential difference mathematically because you're subtracting the log of Q. Thinking of it in terms of electrical potential, since your reaction is going from left to right, anode to cathode, it would make sen...

Jocelyn Thorp 1A

hi, another point is that it is the max potential difference for a reason. The potential difference is created by having a difference in charge, and this difference in charge is where the electromotive force comes from. When the cathode and anode are first connected, that's when there's a maximum po...

Jocelyn Thorp 1A

honestly biggest thanks to the UA's. make the material something that you can actually work through

Jocelyn Thorp 1A

805097738 wrote:what is the difference between Ecell and Eocell?

the E0 always refers to standard conditions, I believe at equilibrium as well.

Jocelyn Thorp 1A

I'd like to know as well, so I'm commenting to easily navigate back.

Jocelyn Thorp 1A

AysiaB1I wrote:Does anyone know how we got the temperature for question 6 on pizza rolls? I have 304.55 from the review session but i'm not sure how we got it.

solve for it with PV=nRT, everything except temp is given in the problem

Jocelyn Thorp 1A

Can someone please explain why the v2/v1 for He is 4 and and 4/3 for Kr for problem 5? Yeah so V2 is the sum of the volume in both chambers, where one is 3 times the size of the other. So in one chamer you have one liter, and the other you have 3. So the total (V2) is 4. He starts out in the 1L con...

Jocelyn Thorp 1A

Can someone explain how to solve #11 on the practice questions? What steps do you need to take? 1) since you're dealing with an acid, you want to convert the pKb to Ka 2) given the concentration of the acid, you can set up an ice table to help you write the equilibrium expression 3) after setting u...

Jocelyn Thorp 1A

Luc Zelissen 1K wrote:For Question 5, I have $\Delta S=moles*8.314*ln(v2/v1)$ for both gasses, but when I add them up I dont get the right answer. Is there another equation to find the change of Entropy of the 2 gasses combined, if so what is it?

don't forget to account for the temperature change!

Jocelyn Thorp 1A

Here are some of the important ones - deltaT = change in temperature deltaS = change in entropy deltaH = change in enthalpy deltaU = change in internal energy deltaV = change in volume deltaP = change in pressure W = work The other W = degeneracy, which helps you find entropy Csp = specific heat ca...

Jocelyn Thorp 1A

Leonardo Le Merle 1D wrote:Also adding onto this question if that’s okay, do adiabatic and isothermal mean the same thing?

they do not! adiabatic means there is no transfer of heat, so q=0.
isothermal means that the temperature remains constant, so deltaT=0.

Jocelyn Thorp 1A

the beaker not being insulated is important for its distinction from an isolated system, since as it is not insulated heat/energy can be exchanged.

Jocelyn Thorp 1A

For a closed system, heat can be exchanged but volume remains constant. An open system has constant pressure yet a changing volume. An isolated system has no energy exchange whatsoever (eg. bomb calorimeter). exactly this! This information can be used to help solve problems. For example, for open s...

Jocelyn Thorp 1A

though you can never truly have an isolated system, if a problem says that the system is insulated (especially if it mentions being well insulated) then it's probably best to consider it isolated

Jocelyn Thorp 1A

You use Hess's Law when you are giving the enthalpy change of the total reaction. You use bond enthalpies when they give you the enthalpy change based on bonds, for example C-H 413KJ/mol. And you use standard enthalpies of formation when they give you the enthalpy of a compound. Hess's Law is for t...

Jocelyn Thorp 1A

The three things Dr. Lavelle was talking about are heat capacity, molar heat capacity, and specific heat capacity (also called specific heat). Heat capacity is the heat required to raise the temperature of any object by 1 degree Celsius and is a more general term than the others. Molar heat capacit...

Jocelyn Thorp 1A

Connie Chen 1E wrote:Delta H refers to the total heat in a system whereas q is the heat that is being transferred. Delta H is equal to q when the pressure is constant, but if the pressure changes, then they are not the same.

this explanation makes a lot of sense, thanks!

Jocelyn Thorp 1A

the values are experimental and are an estimate based off of a ton of different possible bond pairings!

Jocelyn Thorp 1A

it's isolated if energy cannot be exchanged with the surroundings thus, if the system is insulated it is often isolated.

Jocelyn Thorp 1A

Hey guys, what should I look for to help me determine which way (Hess's Law, Bond Enthalpies, Standard Enthalpies of Formation) to solve a problem? I know this is old material, but I'm still pretty confused.

Jocelyn Thorp 1A

crash course is good for concepts, khanacademy is better for understanding the math imo

Jocelyn Thorp 1A

no need to memorize, only convert between and analyze

Jocelyn Thorp 1A

heat can be considered a product (in an exothermic reax) or a reactant (in an endothermic reax), and thus adding or removing heat can cause the reaction to shift

Jocelyn Thorp 1A

changing the pressure will cause the reaction to shift to minimize the change, as per le chatlier's principle

Jocelyn Thorp 1A

if the pressure is changed without a change in volume (i.e. by adding an inert gas), then the reaction does not shift. If it is changed due to a volume change, then the reaction will favor the side with the fewest overall moles of gas.

Jocelyn Thorp 1A

pH is a -log[H+], which simply means that a lower pH is actually associated with a higher number of H+ ions. So saying a weaker acid has a higher pH actually means it has a lower concentration of H+ ions.

Jocelyn Thorp 1A

Changing the concentration or pressure can cause the equilibrium to "favor" the products or reactants, but in both cases the equilibrium constant K will stay the same. Only when temperature is changed is K changed because of a permanent change in the system's dynamics (depending on if it'...

Jocelyn Thorp 1A

You're not really "omitting" units, K is unitless (since it is a ratio, the units "cancel")

Jocelyn Thorp 1A

It might be super helpful to watch some videos, on youtube or khanacademy, that walk you through the ICE table; It's a bit tough to understand from just an explanation, and easier to learn by walking through one.

Jocelyn Thorp 1A

K is y a ratio (between products and reactants) and so it does not have units- but so does this mean that all ratios we use in this class will be unitless? Is it safe to apply this rule widely? If it's simply a ratio then most likely, yes. Constants will come with their units but ratios should rema...

Jocelyn Thorp 1A

for exothermic, heat is released which makes it a product.
for endothermic, heat is absorbed which makes it a reactant.

Jocelyn Thorp 1A

Yes, you need to take the ΔH into account. Since ΔH < 0, the forward reaction is exothermic and the reverse reaction is endothermic. In other words, the forward reaction gives off heat and the reverse reaction uses up heat. Since heat is added, Le Chatelier's Principle states that the system will t...

Jocelyn Thorp 1A

An ICE table is a method to keep track of initial and final concentrations during an equilibrium reaction. You will write down all your reactants and products and mark their initial concentrations (the I in ICE). Then you will look for the change, which is denoted by x. Reactants will lose concentr...

Jocelyn Thorp 1A

it's just a constant representing the ratio of the concentrations. Looking at the units, they would cancel each other out :)

Jocelyn Thorp 1A

What exactly is the ideal gas constant in R? When I looked it up, I realized that there are two different ones to use. How do we determine which one to use based on what information is given to us in a problem? the way to determine which constant to use would be to look at the units and find which ...

Jocelyn Thorp 1A

Can someone explain why the trend makes sense with details? ok so as I understand it, a strong acid and a strong base dissolve COMPLETELY in a solution. A compound is will dissolve more easily if the bond is easier to break. The bond strength goes down as you go down in a group because the ionic ra...

Jocelyn Thorp 1A

Coordination number refers to the number of points to which ligands are attached to the central atom. For example, in [NiCl4]2-, the nickel is attached to 4 Cl- ligands, so the coordination number is 4. so if there's only one molecule attached, but it's attached at 2 sites (chelating and bidentate)...

Jocelyn Thorp 1A

I'm still having difficulties with coordination compounds and am looking for good material to help me learn. The slides are never posted, the textbook isn't always the best way to learn information, and I haven't found much on Khan Academy. Does anyone have any good online resources for learning abo...

Jocelyn Thorp 1A

Hello, I'm having some trouble with coordination compounds, and so I have a few questions. For problem 9C.3D from the text it's asking to write the formula for sodium bisoxalato(diaqua)ferrate(III). I've been mostly able to do the previous parts of this problem, but I have a few questions regarding ...

Jocelyn Thorp 1A

You should memorize a few of the common ligands for naming purposes (my TA recommended that we memorize I-, Cl-, Br-, OH-, CN-, C2O4-, H2O, NH3, CO, en). You should know which coordination compounds/ligands can be polydentate due to structure. For coordination number you just need to know how many ...

Jocelyn Thorp 1A

805306082 wrote:
VLi_1L wrote:Will there be a curve on the final?

There is no curve on the final. At the end of the quarter, the points we have acquired throughout the quarter will be given a grade. Lavelle takes the average points earned and assigns them grades.

ok so there is a curve class wide?

Jocelyn Thorp 1A

this class has been one of the hardest for me this semester, but really it all depends on how much work you put in and how efficiently you are using your time.

Jocelyn Thorp 1A

In terms of the grading, I don't know if we are getting graded on a week to week basis, or at the end of the quarter. Some people say that if we have 50 by the end of the quarter we get all the points, but others say that we need to do 5 a week or else we do not get the points. If anyone knows the ...

Jocelyn Thorp 1A

the test grade is up now for me, so I would assume that it would just depend on when your TA enters the points

Jocelyn Thorp 1A

one of the UAs said that it was cumulative

Jocelyn Thorp 1A

the shape affects the polarity (due to the possibility of dipole moments cancelling), but as far as i know it doesn't work the other way around

Jocelyn Thorp 1A

just 2E-2F!

Jocelyn Thorp 1A

Does anyone have any advice for remembering molecular shape? Hi Cassandra, I think it's good to start by remembering molecular geometry, where lone pairs are counted as regions of electron density. If there are 2 areas of electron density, the shape is linear and the angle 180 degrees. If there are...

Jocelyn Thorp 1A

the UA from the step-up i went to today said it would be IMF, VSEPR, and sigma/pi bonds :) good luck

Jocelyn Thorp 1A

induced dipole= van der waal's forces= london forces, right?

Jocelyn Thorp 1A

How are we supposed to approach this problem? the only thing I can think of is that if the bond lengths are shorter then the attraction is greater.

Jocelyn Thorp 1A

You can make a visual model of the intermolecular attractions by drawing the Lewis structures. Based on how close the electrons or nucleus of one molecule/atom can get to another nucleus or electron pair, you can distinguish what will be stronger. For instance, H forms Hydrogen bonds with lone pair...

Jocelyn Thorp 1A

Another way to remember the order of intermolecular force strength is to think about the strength of the charges on ions, dipoles, and induced dipoles respectively. Ions are charged atoms of elements, and thus, already have a full charge associated with them (+1, +3, -2 etc.) Thus, the interactions...

Jocelyn Thorp 1A

Ayush Ray 1I wrote:My midterm grade

this is like the SI derived unit for frequency.

it hertz

Jocelyn Thorp 1A

If I remember correctly, the types of intermolecular forces directly impact the strength of the bonds and thus influence properties such as melting points. Is there any way to remember which are stronger and which are weaker, other than just memorizing? If any part of my question is incorrect, pleas...

Jocelyn Thorp 1A

how can we know if they're likely to use the expanded octect though?

Jocelyn Thorp 1A

Jessica Tran_3K wrote:I also agree ^^ I think as long as the concept is still relevant, it should be fine.

So we're still good to do stuff from chemical bonds this week? I'm never sure about what we can do.

Jocelyn Thorp 1A

As anions get larger and larger, their polarizability increases. This is due to the decreasing effective nuclear charge on their valence electrons. As charge increases on cations and as they get smaller, their polarizing power increases since they can get close to the nucleus of anions and exert a ...

Jocelyn Thorp 1A

Which ones are initial and which ones are final don't really matter, what matters is that the correct molarity is paired with the correct volume :)

Jocelyn Thorp 1A

Chem_Mod wrote:I'm assuming c is concentration (ie Molarity), M is also molarity, n= number of moles, and v= volume. Can you elaborate on what "m" refers to?

I would assume that m is mass from the equation, since mass * molarity would result in n, the number of moles

Jocelyn Thorp 1A

a radical is if a compound has an unpaired lone pair electron. radicals tend to be very reactive (due to them being unstable, often one electron away from a full valence shell)

Jocelyn Thorp 1A

Kelvin Chung 1C wrote:In an ion, generally the formal charges should add up to the charge of the ion. In a molecule, I believe the formal charges should add up to 0.

exactly. for example Na has a formal charge of +1 and Cl has a formal charge of -1. Their combined compound NaCl has a formal charge of zero.

Jocelyn Thorp 1A

Jorge Ramirez_4H wrote:Are radicals that important for the midterm?

I'd like to know this as well. Will we be expected to select what compounds are radicals or not? And does this knowledge have some other application?

Jocelyn Thorp 1A

Megan Ngai- 3B wrote:If you look up periodic table charges on Google images, it shows the charges per group (it's easy to remember). So Mg has a charge of +2 and As has a charge of -3. Therefore it would be Mg3As2.

Oh, this is super useful! Do you know where the formal charge comes from?

Jocelyn Thorp 1A

for 2A.23, it asks to give the chemical formula based on the charges of the elements involved in the compounds. How do you know what the charge is in order to calculate this? In the problem, it asks to give the formula for magnesium arsenide, so if that can be used as an example, it'd be helpful.

Jocelyn Thorp 1A

[Ar]3d¹⁰4s²4p³, remembering to write 3d10 before 4s even though it comes after, because 3d10 is a lower energy level.

Jocelyn Thorp 1A

Emma Popescu 3D wrote:The 4s is filled first because it is at a lower energy than 3d. However, Dr. Lavelle wants us to put them from lowest to highest energy level (3d and then 4s) because 4s is ionized first.

ok so, 4s is filled first, and 3d is written first? thanks.

Jocelyn Thorp 1A

First of all, s-subshell is l=0, p-subshell is l=1, d-subshell is l=2, f-subshell is l=3 For (a), it is 4p-subshell, so l= 1. If l=1, ml can be -1, 0, 1. Therefore, 4p-subshell can have 3 orbitals. For (b), it is 3d-subshell, so l=2. If l=2, ml can be -2, -1, 0, 1, 2. Therefore, 3d-subshell can hav...

Jocelyn Thorp 1A

it's easier to think about whether an elememt wants to gain or lose an electron to become stable. elements in group 1 and 2 want to lose their electrons to become stable so the ionization energy is low. elements in groups further along the period are closer to becoming stable (filling their electron...

Jocelyn Thorp 1A

Mariah wrote:Im kind of confused about this, what would cause an electron to transition into a higher energy level in the first place? Do they do this on their own or because of some outside force?

they can get excited by light, and the energy from the photon allows them to make the transition.

Jocelyn Thorp 1A

When doing your calculation, the Δv you should use is 10m/s, which is your maximum uncertainty. Why do we have to use the maximum uncertainty velocity in the Heisenberg Indeterminacy equation? because the maximum uncertainty is the entire range that you are uncertain. when you say plus or minus fiv...

Jocelyn Thorp 1A

To add on, atomic radius increases down a group with each new period because the outermost electrons occupy shells with increasing principal quantum number and therefore lie farther from the nucleus. Atomic radius decreases across a period because new electrons are in the same shell of the atom and...

Jocelyn Thorp 1A

I would recommend checking out the crash course video "Light Is Waves: Crash Course Physics #39" . The entire video is very helpful.
https://www.youtube.com/watch?v=IRBfpBPELmE&t=

Jocelyn Thorp 1A

So primarily talking about the E= hv, c= λv, r = h/n^2, Ek = 1/2mv^2, λ = h/mv So in essence, ryberg equation, energy of a photon equation, speed of light equation, de broglies, etc. When do we use such equations? Which equations should I primarily memorize or understand the most? What are the spec...

Jocelyn Thorp 1A

I am a bit confused about diffraction patters and wavelike properties. Do both light and electrons have wave like properties? If so, does that mean they both have Difraction patterns. Also, what are diffraction patters? ok so basically, light/electrons acting as waves means that there will be some ...

Jocelyn Thorp 1A

505106414 wrote:What is a real-world application of this equation?

the v for frequency is actually nu (fun fact), and this is used to calculate wavelength, where c=lambda*v.

Jocelyn Thorp 1A

Doris Cho 1D wrote:Not sure what the question is even asking... I understand what the answer is saying but I don't understand the question (?) if anyone knows what I mean...

it's asking how the lines are related, in this case it's asking you to make a connection the the energy levels

Jocelyn Thorp 1A

I matched up the right radiation to each event but I only figured out the 3.3 x 10^-19 J to microwaves because of process of elimination... does it need to be converted to Hz or nm for me to see that it goes with microwaves? I've seen charts with Hz or m but never any with Joules, so that might be ...

Jocelyn Thorp 1A

Chem_Mod answered your question already but if you want the equation it's used in, here you go

The work function is the Φ in the equation $\frac{1}{2}m_{e}v^{2}=h\nu -\phi$ that relates the kinetic energy of the ejected electron to the energy supplied by a photon and the work function.

Jocelyn Thorp 1A

Rydberg Equation: 1/λ=R(1/n(1)^2−1/n(2)^2) You can change n(1) and n(2), which refer to different energy levels in the Hydrogen atom. For example, you can use n(1)= 1 and n(2)=2 to find the wavelength of light emitted when an electron drops from the n=2 to the n=1 energy levIel (or the wavelength o...

Jocelyn Thorp 1A

Hi, to solve this problem, first the equation needs to be fully balanced. There's already a stoichiometric coefficient in front of AgNO3, however there also needs to be a 3 in front of AgCl. So, the resulting equation should be: C6H9Cl3 + 3AgNO3 ---> 3AgCl + C6H9(NO3)3 Then, find the molar masses o...

Jocelyn Thorp 1A

Hey all, I was hoping for help on one of the problems from the module that I simply wasn't able to get even a remotely close answer for. Any explanation would be very helpful. The problem is as follows: 22. According to the following equation, 0.750 g of C 6 H 9 Cl 3 is mixed with 1.000 kg of AgNO 3...

Jocelyn Thorp 1A

Jesse H 3 wrote:precision is consistent results while accuracy is how close to the true value we can get.

this response is succinct and how I'm going to think of it from now on. thanks

Jocelyn Thorp 1A

if it helps, think about how it would be written in scientific notation. for example .085 would be 8.5*10 -2 (meaning the 0 after the decimal is not significant) while .850 would be written as 8.50*10 -1 (so the zero is significant :). for a number like 100 or 200 the zeros aren't considered signifc...

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