CHEMISTRY COMMUNITY

Alison Perkins 2B

not doing homework in my bed and forcing myself to wake up early definitely help

Alison Perkins 2B

ok thanks !

Alison Perkins 2B

I always take hand written notes because I find the muscle memory helps me to remember more equations and important phrases.

Alison Perkins 2B

If the lectures are live and recorded does anyone know if we are required to attend?

Alison Perkins 2B

do you guys remember if there is a curve?

Alison Perkins 2B

I make a packet out of all of the recommended syllabus problems and work through it the week before =

Alison Perkins 2B

E˚cell is under standard conditions while Ecell can have varying conditons. Ecell = E˚cell - RT ln(Q)

Alison Perkins 2B

Sum the order of each reactant to get the overall reaction order

Alison Perkins 2B

It is theoretical because it is so difficult to actually create perfect conditions. I think the bomb calorimeter is a real isolated system but I am unsure how close it is to perfect.

Alison Perkins 2B

You can use the equation Ecell = E˚cell - RT ln(Q) where Q is the concentrations of your substances.

Alison Perkins 2B

K can either stand for Kelvin or the equilibrium constant, which should be fairly easy to distinguish. The Boltzman constant is the other k I can think of appearing, but it's lowercase and often has a B subscript. Be careful with kB for the Boltzman constant and KB for an equilibrium constant consid...

Alison Perkins 2B

I usually make all of the practice problems that Dr. Lavelle puts on the syllabus into a packet, and treat it like an open note midterm to gauge how much I know and what areas I'm lacking in. It also helps with muscle memory of repeated problems on the exam.

Alison Perkins 2B

Standard equilibrium constants for the half reactions are always written in a favorable direction. When you combine the half reactions, one usually is flipped to balance the electron transfer on each side, so you will have to flip its sign.

Alison Perkins 2B

This refers to the moles of electrons that are being transferred in the reaction. To find this, you can write out the half reactions for each molecule, and then make sure you take into account coefficients in the equation if necessary.

Alison Perkins 2B

from here if you know E° you can then solve for the equilibrium constant (K) with logK= (nE°)/(0.0592V)

Alison Perkins 2B

yes exactly!

Alison Perkins 2B

congrats on surviving midterm 2

Alison Perkins 2B

A spontaneous reaction always has a -ΔG, and since ΔG = -nFE, when there is a spontaneous reaction there will be a +E value for those negative symbols to cancel out.

Alison Perkins 2B

the moles will cancel between n (mol) and F (C/mol) so it will be in kJ

Alison Perkins 2B

n is the moles of electrons transferred in the reaction.

Alison Perkins 2B

For this one you should solve for two ΔS values and sum them.
1. ΔS = nRln(V2/V1)
2. ΔS = Cv ln(T2/T1)

Before adding make sure you multiply the second value by the number of moles in the equation.

Alison Perkins 2B

ΔG = ΔG° + -RTln(Q)

The degrees on the delta G means that this is the standardized value.

Alison Perkins 2B

ΔG = ΔG° + -RTln(Q)

The degrees on the delta G means that this is the standardized value.

Alison Perkins 2B

I had the same problem, just make sure the inequalities are on the bottom and spontaniety is on the top!

Alison Perkins 2B

Qrev is the Q of the reverse reaction, which is used for entropy. They're both heat transfer values.

Alison Perkins 2B

I am doing all of these steps and am still getting it wrong and don't know why. Can anyone help with C2H2(g) + 2H2(g) --> C2H6 ??

Alison Perkins 2B

Cv is the specific heat at a constant volume, and Cp is the specific heat at a constant pressure.
Since solids and liquids can't change volumes, we know for all that Cp = Cv.

Alison Perkins 2B

ΔU refers to the change in the internal energy of a system.
This is the effect that the heat/work done on the system has on its initial internal energy state.

Alison Perkins 2B

Only gasses can change volumes, so for solids and liquids Cp = Cv.

Alison Perkins 2B

I'm quite positive that the Boltzman Constant is always 1.3806 × 10-23 m^2 kg /s^2 K, it isn't dependent on the molecules you're dealing with.

Alison Perkins 2B

I am confused on when to use which value for the gas constant: R = 8.3145 J/mol·K. and R= 0.08204 liter·atm/mol·K.
Can anyone help?

Alison Perkins 2B

I felt the same way when I BOMBED the first 14A midterm, but it happens to everyone so don't get down on yourself...just study harder every time you think you can do better...because you can. All that matters is you do your best. I believe in you!

Alison Perkins 2B

Increasing temp in an exothermic reaction will shift to the left, meaning it favors reactants.

Increasing temp in an endothermic reaction will shift to the right, meaning it favors products.

Alison Perkins 2B

In an exothermic reaction:
Increasing temperature will favor the reactants
Decreasing temperature will favor the products

In an endothermic reaction:
Increasing temperature will favor the products
Decreasing temperature will favor the reactants

Alison Perkins 2B

When the equilibrium constant is less than 10^-4 in your ICE table, you can assume that x is negligent in K calculations.

Alison Perkins 2B

I fill in every single assignment and test on the syllabus in a planner at the start of the quarter so that my schedule is already written out for me when each week begins. Having a schedule and mapping everything out definitely helped.

Alison Perkins 2B

a) to find out whether the reaction is endothermic or exothermic, take the ΔH°f(products) - ΔH°f(reactants). If you get a positive value, it is endothermic, and if negative it is exothermic.
b) now use these conditions to solve for ΔH (unsure about how exactly to do this step).

Alison Perkins 2B

rather than approximating, make an ICE table, and use the Ka value and equation to solve for your x value.
Then, take [H+]/[HA] at equilibrium x 100% for percent ionization.

Alison Perkins 2B

ΔH° = ΔH°f(products) - ΔH°f(reactants).

So for this, combine the ΔH°f for your products and reactants, and subtract for overall reaction enthalpy.

Alison Perkins 2B

to solve for OH-, make sure you are using an ICE table to solve for x from the Kb equation.

Alison Perkins 2B

I am unsure, but in my TA Reid Wilson discussion slides from week 3, there is a table from the textbook (4D.1).

Alison Perkins 2B

From the pH, you can solve for pOH because pH + pOH = 14.
From there, you can make an ICE table, and solve for your x with the Kb equation.

Alison Perkins 2B

The predominant species is just the strongest species present I believe.

Alison Perkins 2B

Once you know the pH you can subtract this from 14 to get the pOH. Then, by doing 10^(-pOH) you will get [OH-]

Alison Perkins 2B

A salt will be basic if it contains the conjugate base of a weak acid
A salt will be acidic if it contains the conjugate acid of a weak base.

Alison Perkins 2B

I wouldn't approximate, but rather set up an ICE table using another variable as the initial concentration. If you do 10^(-pOH) you will get an x value for your table. Then you can use the x and Kb to solve for an initial concentration, which you can use to set up the percent ionization equation. I ...

Alison Perkins 2B

Ka = [H+] [A-] / [HA] -> formula for the acidity constant of equilibrium, where HA is an acid and A- is it's conjugate base Kb = [BH+] [OH-] / [B] -> formula for the basicity constant of equilibrium, where B is a base and BH is it's conjugate acid Kw = [H3O+] [OH-] -> this is the formula for equilib...

Alison Perkins 2B

An inert gas would be one that does not affect the outcome of the reaction. For instance, if you added Cl2 to a reaction that does not involve Cl2 and will not react with it, the same exact concentration of CL2 would be in the reactants as well as the products, only changing the volume and thus the ...

Alison Perkins 2B

If you are given the initial concentration for products, then the reactants could be zero for initial concentration if you are asked to find the concentration of the reactants.

Alison Perkins 2B

When the problem contains all gases, you can use Kp if the pressure measurements are given. If you are given measurements for concentration rather than pressure, however, it is okay to use Kc.

Alison Perkins 2B

They can all be calculated in the same way as you would calculate K with products over reactants. The difference between them is that Q is general, Qc is concentration specific, and Qp is partial pressure specific. Just as is with K, Kp, Kc. To calculate Qp or Qc, you would need to use the appropria...

Alison Perkins 2B

"1.50 mol PCl5 is placed in a 500ml reaction vessel and decomposes at 250°C to form PCL3 and CL2. Kc= 1.80, all 3 compounds are gases at 250°C." Lavelle ended the second lecture today with this example and I have been trying to solve it, but am stuck on if I should be calculating Qc or Qp,...

Alison Perkins 2B

Grades are saturday for the Final I believe! I would assume quarter grades would follow soon after.

Alison Perkins 2B

Does anyone know if there is a Groupme for 14B next quarter yet? I was thinking that would be a good way to stay on top of chem and study/review so that I don't forget everything before 14B starts.

Alison Perkins 2B

A Bronstead Acid, or just an acid, is a species that can donate a proton. A Lewis Acid is a species that accepts an electron pair.
A Bronstead Base, or just a base, is a species that accepts a proton. A Lewis Base is one that donates an electron pair.

Alison Perkins 2B

Make some tea and tidy up your room! Or change study locations every so often. Keeping a fresh scenery is important especially when you're stuck at home.

Alison Perkins 2B

I believe it is that it would be "dichloride" if the two chlorides were binding to the same central atom, but since they are not inside the brackets and thus not both binding to the same central atom these are just referred to as "chloride".

Alison Perkins 2B

Bronsted is concerned with proton presence/movement, and Lewis is concerned with electron presence/movement. I have found it helpful to remember that every Bronsted Base is a Lewis Base and every Bronsted Acid is a Lewis Acid, but these trends DONT work in reverse.

Alison Perkins 2B

Lone pairs have significant stronger repulsion strength, so they occupy a larger region of electron density as they push down on other surrounding bonds to push away from themselves and thus the other bonds become closer together, reducing bond angles and forming a bent shape.

Alison Perkins 2B

Bronsted defines based on proton characteristics and movement, while Lewis focuses on electron movement and characteristic.

Alison Perkins 2B

I know that for resonance structures you must determine which has the most stable formal charge, to ensure that this is the most stable atom. I am not sure of what else.

Alison Perkins 2B

Yes, more bonds would make London Dispersion Forces stronger, because a heavier atom with more bonds will have electrons extending further from the nucleus with a weaker connection, so they can temporarily form dipole connections.

Alison Perkins 2B

A ring or cyclic structure is a combination of the resonance structure arrangements of double or triple bonds in a circular formation (C6H6 for example). I hope that helps!

Alison Perkins 2B

I don't believe there is a specific order in which these bonds need to be numbered, however one must be sigma and any remaining must be pi bonds.

Alison Perkins 2B

Do you know if we will need to know only specific ones or the trick to determine the elements with expanded octets for the final exam? Because I am still unsure how to determine this.

Alison Perkins 2B

Melting points increase as the intermolecular strength of the atom increases. The more forces acting upon the nucleus to hold the atom together, the more energy required to break these.

Alison Perkins 2B

To find the shape with more than one central atom, separate the large molecule into pieces by each of the central atoms. Then, after determining the shape of each portion, you combine these to describe the overall shape of the molecule.

I hope that helps !

Alison Perkins 2B

Yes, each lone pair counts as its own region of electron density.

Alison Perkins 2B

It has a higher melting point because it has more electrons, making it a larger molecule with more electronegative forces acting upon it, making it more difficult to break the bonds.

Alison Perkins 2B

I'm pretty sure that it's because the number of regions of electron densities doesn't change when you draw different resonance structures. I am also confused by this because in the SO3 ^2- example in lecture, one structure had all single bonds and 4 regions of e- density, while the other structure ...

Alison Perkins 2B

Electron density refers to the probability that an atom would be found in any given region of an orbital. And to answer your second question, I believe that any area where an electron could be found would be considered a region of electron density. Does electron density also contribute to the polar...

Alison Perkins 2B

I totally feel you. I thought I had the first midterm in the bag....and i definitely did not.
Also felt like I had concepts for this midterm down and thought it was easier than the last... but I'm praying that the same thing doesn't happen. Scared for my grade.

Alison Perkins 2B

sp orbitals have a linear shape
sp2 orbitals have a trigonal planer shape
sp3 orbitals have a tetrahedral shape
sp3d orbitals have a trigonal bi- pyramidal shape
sp3d2 orbitals have octahedral or square bi-pyramid shape

Hope this helps!

Alison Perkins 2B

Are there any extra credit opportunities offered for this course?

I am concerned since we cannot make up Chemistry Community posts if we miss the hard midnight deadline, and because the midterms have been more difficult than I had expected.

Alison Perkins 2B

I thought this midterm was much easier than the last...however I thought the first midterm was fairly doable ... and really did not do well.
Does anyone know when the grades will be released?

Alison Perkins 2B

Linear structures should have bonds of 180 degrees

Alison Perkins 2B

jasmineculilap_2J wrote:You need 5 posts per week, so 35 posts are needed.

Do general questions and jokes count? Or strictly chemistry material?

Alison Perkins 2B

Summer_Corona 3F wrote:By this Sunday you should have 35 posts.

If we missed a week can we make extra posts in the next week to make up for it?

Alison Perkins 2B

I am not really sure...but what may help is to know the structures that go with different VSEPR notations...because if molecules have the same VSEPR structure they have the same shape.

Ex: CH4 and CCl4 are both notated as AX4...which always has a tetrahedral shape.

I hope that helps somewhat !

Alison Perkins 2B

Why do chemists like nitrates so much?
They're cheaper than day rates!

Alison Perkins 2B

What's the dullest element? Bohrium!

Alison Perkins 2B

I don't exactly know the theory but... sigma bond: when 2 orbitals with 1 electron each interact end to end ... that is on their internuclear axes. these bonds are also cylindrically symmetric. pi bond: when 2 orbitals with 1 electron each interact side to side. These also occur in any case where th...

Alison Perkins 2B

Yes, however since the Chlorines are on opposite sides the dipole moments cancel in terms of direction. It is different for cis-dichlorethene because the Chlorines are on the same side, so their dipole moments do not cancel. Hope that helps !!

Alison Perkins 2B

I believe we will still have them seeing as they are pre recorded.
Also will there be discussions on thursday or friday?

Alison Perkins 2B

Will Dr. Lavell be telling us which ones we should memorize for the final then or should we assume all?

Alison Perkins 2B

I am confused as to how we are supposed to find the structure of SO3 ^-2 One resonance structure with all single bonds has a lone pair in a 4th region of electron density, however with a double bond there are only 3 regions. Why is it that it is still trigonal pyramidral instead of trigonal planar ....

Alison Perkins 2B

This is probably a simple answer ... but I forget why Xe is allowed to have 12 valence e- in molecules.
What makes Xe and As and other elements like this able to have more than 8 valence e- ?

Alison Perkins 2B

This problem asks for the ground state of listed ions.
I am confused as to why Sb^(3+) = [Kr]4d^10 5s^2, while Sn^(4+) = [Kr]4d^10...is this because 5s would be filled after 4d?

Alison Perkins 2B

Hi! Correct me if I'm wrong, but I think you have confused unpaired electrons with valence electrons. Since Ga, Ge, As, Se, and Br are all p-block elements, the number of unpaired electrons for each element respectively is 1,2,3,2,1. If you are looking for the number of valence electrons for each e...

Alison Perkins 2B

"On the basis of the expected charges on the monatomic ions, give the chemical formula of each of the following compounds: (a) magnesium arsenide; (b) indium(III) sulfide; (c) aluminum hydride; (d) hydrogen telluride; (e) bismuth(III) fluoride."

Can someone explain a, b, and e please?

Alison Perkins 2B

Can someone please explain how to find the ground-state e- configurations for Bi^(3+) and P^(3-)?

Alison Perkins 2B

The question reads: "The elements Ga, Ge, As, Se, and Br lie in the same period in the periodic table. Write the electron configuration expected for the ground-state atoms of these elements and predict how many unpaired electrons, if any, each atom has. Seeing as these all go across the period ...

Alison Perkins 2B

The textbook question reads: "Give the notation for the valence-shell configuration (including the outermost d-electrons) of (a) the alkali metals; (b) Group 15 elements; (c) Group 5 transition metals; (d) the “coinage” metals (Cu, Ag, Au)." I know how to give a notation for a single atom,...

Alison Perkins 2B

The atomic radius decreases because of the increasing amount of protons in the nucleus, which creates a more polar charge and pulls electrons closer to the nucleus in their orbit due to increased stability.

Alison Perkins 2B

I know that a Lewis Acid is an atom that can accept a pair of electrons, and a Lewis Base is an atom that can donate a pair of electrons. For instance, F + BF3 --> BF4 is a Lewis Acid-Lewis Base reaction, creating an acid-base adduct. I am unsure if we need to label these concepts in our Lewis Struc...

Alison Perkins 2B

Forgive me if I am wrong, but isn't spin a constant property of the electron or am I completely missing something. I believe youre right in that it is a constant property of an electron...meaning it always has some sort of spin. I think this spin can change depending on bonding, but I also think th...

Alison Perkins 2B

I also have a question. If they cannot change spin, then does that inhibit what kinds of atoms are able to bond together? I am actually not sure ... I am also wondering this. I thought this just depended on the kind of bond, so a borrowed or shared electron would fall into place in terms of spin wi...

Alison Perkins 2B

In lecture when Prof. Lavell was calculating the individual formal charges of S and O in SO4 I was very confused by the FC of the Oxygen in the molecule.Can someone explain how he did that ?

Alison Perkins 2B

Yes! A resonance structure can be any of the possible structures (ex: In C6H6 there are 2 resonance structures bc of double bond placement. A resonance hybrid is the combination of all of these structures (ex: in C6H6 the diagram with the circle in the middle represents the resonance of those placem...

Alison Perkins 2B

Why is it that when we are given an energy of E, frequency of v, and wavelength of λ,
E=100 while λ and v are not altered when applied to 100 photons??

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