CHEMISTRY COMMUNITY

zfinn

i usually find it easiest to just split into half reactions, finding the number of e- by looking at change in oxidation numbers

zfinn

i look at the atom to determine it, but when asking for the species put the whole molecule

zfinn

i think the book says they're the same thing, just different names. the two different types would be galvanic/voltaic and electrolytic cells.

zfinn

it's useful in situations where you want to see how the rate, temperature, and activation energy all relate

zfinn

If there isn't a viable metal electrode listed in the equation then platinum can be used/put in the diagram

zfinn

thank you for such a fun past two quarters! we've all learned so much and are so lucky to have such an amazing professor.

zfinn

if i understand correctly, raising temp will affect the rates most of reactions that are endothermic (heat using) with large activation energies.

zfinn

if you have sabina i think they're in her mailbox!

zfinn

when w is positive work is done on the system

zfinn

with acidic you add h2o to balance oxygen and then simply h+ to balance the hydrogen, but with basic you add h2o to balance oxygen but then need to balance the hydrogens with oh-, while adding h2o to balance any additional oxygens.

zfinn

i doubt we’ll have to guess/make up any intermediates but you should know what they are and how to identify them

zfinn

what's the relationship between Ecell and the value of K? (i.e. if K > 1 or K <1)

zfinn

can someone explain problem 5J.15? even after looking at the solutions guide i'm still a bit confused on how to start, and also just the steps to solve the equation. thanks so much!

zfinn

what sections of the book/which problems correspond to the second page of the thermodynamics outline?

zfinn

the oxidizing agent is being reduced because it is recieving the e- from reduction process, and the reducing agent is being oxidized because it is giving e- for the reduction process.

zfinn

in the process C4+ --> C3+ one electron is gained.
the balanced equation has 2C4+ so therefore 2*1e- = 2e-, and n=2. hope this makes sense!

zfinn

this is how we did it in my discussion

zfinn

a comma is used if elements are in the same phase, for example of both are aqueous

zfinn

yep! unless one half reaction has 2e- and the other has 4e-, then n=4 because it is the lowest common multiple.

zfinn

if there is no salt bridge in an electrolytic cell it's because the anode and cathode can be in the same solution, so a salt bridge wouldn't be necessary (i think).

zfinn

when the standard reaction potential is positive, the rxn will spontaneously proceed in the forward direction

zfinn

1 atm and 273.5K, although in the midterm it was written on the board so i'm assuming if you don't memorize it it could be okay (?)

zfinn

the 2nd page of outline 4 will definitely be covered, but personally i might to brush up on some of the concepts beforehand too just in case it's implicitly tested!

zfinn

oxidation is loss of electrons, which means the charge of the oxidized atom will increase - the acronym OIL RIG is helpful for me (Oxidation Is Loss (of e-), Reduction Is Gain (of e-))

zfinn

it is balanced when there is an equal number of electrons on each side that can cancel out

zfinn

that's my best guess, yeah. but you should probably check with ur ta in discussion this week just to make sure!

zfinn

what homework should we turn in to discussion for week 7?

zfinn

my understanding is that step-up is in detail teaching of the topic, while workshop is more about going over questions that students may have (not as step by step/in depth)

zfinn

Drew Myers 4G wrote:Can someone explain how to use calculus to prove
w = -PΔV?

does anyone know if we will be tested on this?

zfinn

isothermal reactions mean there is no change in temperature, and therefore it can't have a change in internal energy

zfinn

I think the way it was last quarter was material from midterm to test 2

zfinn

For Pizza Rolls 5e why is the answer false? is it only crystals that have 0 entropy at absolute zero?

zfinn

can someone please explain #5 i am very confused. thank you

zfinn

the rxn will favor the side with less moles of gas, although Dr. Lavelle mentioned that this rule is a quick sub for what is actually happening due to changes in concentration.

zfinn

when x is <5% of the initial concentration you can omit it

zfinn

i'm kind of lost on the concept of how to calculate if work is positive or negative in respect to different things, and also the concept of work being "done on" a system. for example, homework problem 4A 3b. thanks!

zfinn

state functions only depend on the final value, not the way you got there. for example in a state function it wouldn't matter if you walked down bruinwalk or through the tennis courts to get to Lavelle's lecture, it only matters that you showed up at the lecture and that the lecture is your current ...

zfinn

my impression of the example was that the displacement of the piston (moving out) can be used to calculate the change in energy of the system; if you calculate the work function of the piston movement then the energy would've decreased by an amount equal to the energy lost as work done by the system.

zfinn

^ also was it covered in lecture?

zfinn

expansion means the volume of the system increases (it expands)

zfinn

it will affect the problem if it's a bomb calorimeter, because then it's an isolated system

zfinn

work and heat are related to the path taken to the final state, not simply just the final state

zfinn

you can call me K because i don't change under pressure

zfinn

Micah3J wrote:Pressure doesn't affect the equilibrium constant though right? It will only shift it one way or the other?

yep! heat is the only measure that affects the equilibrium constant

zfinn

i don't think we need to know that; if in any way we would it's probably given

zfinn

the reaction will move away from the side of the concentration increase

zfinn

If the pressure is increased the reaction will go towards the side with fewer gas particles. If you decrease the pressure the reaction will go to the side with more gas particles. this is a good quick way to do it, but Dr.Lavelle also emphasized that the reason behind this shift is how pressure cha...

zfinn

[h3o+][oh-]=Kw and [Ka][Kb]=Kw

zfinn

how do we know which equilibrium constant of the three per equation column (the ones decreasing in value) to use? for example in the Kc column for H2+I2--> 2HI do we use 794, 160, or 54? thanks so much!

zfinn

i think we'll usually be able to calculate it, and if not i'm pretty sure it will be given to us, like in the practice problems Dr.Lavelle did in class!

zfinn

in lecture I remember him mentioning that equilibrium hw for both weeks is okay!

zfinn

how do we know when a problem wants us to use Kp versus when we should use Kc? thanks!

zfinn

Q is the calculation of a system not at equilibrium while K is the calculation of a system at equilibrium (the equilibrium constant).

zfinn

it is ideal because it's assuming that the gas molecules have no attractive forces between themselves, and no molecular volume.

zfinn

AGaeta_4E wrote:I think my number of posts is off by one, is this a problem for anyone else?

sometimes for me the number of posts by my name is wrong, so i have to go to "your posts" and that's usually correct!

zfinn

The pH is the negative log of the solution's H3O+ concentration. The pOH would be the negative log of its OH- concentration, and this related to pH because pH = 14-pOH.

zfinn

Q: What do you do with a sick chemist?
A: If you can't helium, and you can't curium, you might as well barium!

zfinn

Ashley Osorio wrote:How would one phrase number 36C?? Also, 36B is both Amphoteric and amphiprotic, right?

" the atoms that give glycine these properties are the N, with a lone pair that can accept a proton, and the -COOH, which can donate a proton." and for 36b it is both!

zfinn

do lone pairs count towards the number of groups attached to the central atom? for example would so2 have 3 groups attached to the central atom? thanks!

zfinn

How does the hybridization of orbitals change their shape?

zfinn

the lewis structure that has a lone pair ready to donate would be a base, while the lewis structure carrying a positive charge is an acid.

zfinn

Which concepts or values should we have memorized for the final (that won't be on the equation sheet)? thanks!

zfinn

strong acids inherently have a weak conjugate base, which means that its properties would not change the pH of a solution.

zfinn

bronsted acids and bases identify the exchange of protons while lewis deal with the exchange of electrons. they aren't necessarily two separate groups, just different ways of viewing acids and bases.

zfinn

bent will have 1 or 2 lone pairs (AX2E oe AX2E2) while linear will have none or 3 (AX2 or AX2E3)!

zfinn

The bond angles would be less because lone-bonding pars have more repulsion

zfinn

there's no need to write the "1" subscript for X and E!

zfinn

hydrogen bonding sites are wherever an H can bond with a F,O, or N

zfinn

in a VSEPR shape with more than one angle size, i.e. seesaw, would all the angles decrease because of the lone pair (so angles are <90, <120, <180)? or do just the angles next to the lone pair bonding region decrease?

zfinn

I'd recommend memorizing them just because it makes converting a lot less seamless from my experience! it's helpful if you can, for example, see 10^-9 and automatically know it's nm.

zfinn

If I remember correctly I think that Dr. Lavelle said it would not be on the test.

zfinn

Which specific VSEPR angle measurements do we need to know for test 2? Thanks!

zfinn

It's octahedral because the shape of the whole molecule looks like an octahedron (8 faces). It's basically another way to describe the bipyramidal shape, which is two 4-faced pyramids put together, creating an 8 faced shape.

zfinn

Since it wasn't explicitly covered in class/discussion I doubt it'll be on the test!

zfinn

any atom that interacts with another atom will have these forces because it has electrons that are available for induced dipole movement. that being said, larger atoms have stronger van der waals forces because they have more electrons.

zfinn

I think the bond lengths do vary slightly depending on the atoms, based on their electron affinity.

zfinn

It should cover everything since the midterm up to friday's lecture (+the concepts from one slide on monday)

zfinn

Hi! a few more questions on the dino nuggets review - can anyone explain the reason behind the answers to 9.e, 9.f, or 9.h? also 9.j? Thanks so much in advance!!

zfinn

How do we tell when we should convert our answers into scientific notation? I'm never sure when or when not to - thanks so much!

zfinn

for dino nuggets #4 why is the answer 2 sig figs and not 3? is it because 250 mL has only 2 sig figs? i'm a bit confused if that's the reason or if it's something else - if someone could clarify that would be great! thanks so much

zfinn

1 cycle/second or in calculation basically s^-1

zfinn

these ones were on a worksheet my ta gave me!

zfinn

I usually just see if multiplying by a number will clearly make it even. i.e if it is 1.66, even though that is close-is to 2, it is easy to see that multiplying by 3 will get us much closer to a whole number. if the number is like 1.97 though it's definitely safe to round up!

zfinn

The octet rule means that most atoms are most comfortable/stable when they gain enough electrons to form an octet, and imitate the e- configuration of a noble gas.

zfinn

principle quantum # = n = energy & size (shell)
angular momentum = l = can be anything up to (n-1)
magnetic number = ml = orientation, -l....l
spin number = ms = -1/2 or +1/2

zfinn

The chemical bonds homework, because that's what we spent most of this week covering! and I think it'll translate better into what we discuss in discussion next week too.

zfinn

It would have 7 valence e- (from d and s), and would more readily loose e- from the 4s shell.

zfinn

Selena Yu 1I wrote:Dr. Lavelle was saying that you shouldn't put more charges on a lower electron affinity molecule because that would make the whole lewis structure less stable, which is what you don't want.

Does more charges include both negative and positive charges (for the FC)?

zfinn

In my homework I usually use c=3.00*10^8 as the constant for speed of light, but I've noticed that in the answer booklet they use c=2.99792*10^8. Due to sig figs rounding my answer is almost always the same as if I were to use c=2.99792*10^8 like the book, but I'm wondering if there will ever be a t...

zfinn

In homework problem 1B.15 (from the quantum section) were we expected to know the mass of the electron? Or is it okay to look that info up/will it be provided on a constants sheet for the test? Thanks so much!

zfinn

In today's lecture Dr. Lavelle mentioned that when creating Lewis Structures you didn't want to, "put more charges on an atom with lower electron affinity." I was wondering what this meant in the context of Lewis Structures, and the reasoning behind it? thanks so much!

zfinn

For our week four homework do we just do 5 more problems from "The Quantum World" section? Thanks in advance!

zfinn

If you purchased the loose leaf textbook you should be able to access the resources for free on the site! If you didn't purchase the book though you might have to pay (but I'm not too sure).

zfinn

I think my TA mentioned that just to be safe we should memorize them, in case they happen to be missing from the formula sheet.

zfinn

The inner electrons "shield" the outer ones from the nuclear charge, which causes the outer ones to be lost easier, since they aren't as strongly attracted anymore (due to shielding).

zfinn

The way I understood it is that the intensity is what effects the amount of photons ejected. The frequency determines if a photon will have enough energy to eject an electron. But if you increase intensity it will double the photons which will double the amount of electrons ejected (at least I remem...

zfinn

For this course do we have to have all the values in the electromagnetic spectrum memorized?

zfinn

You should keep as many sig figs as possible while you work on the problem in order to be the most accurate. Then at the end, you can round to the correct number!

zfinn

If two waves are "in phase" this means that they both peak at the same time (their highest points are at the same time). "Out of phase" would be when their peaks don't line up.

zfinn

c=(lambda)(v) is the wave equation so I think you can use it to solve for the wavelength or frequency of any wave (as long as you're given/find one or the other before using the equation)

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