CHEMISTRY COMMUNITY

Alvin Lai 3J

Yes, joseph was referring to kforward and kreverse as the reaction rate constants! :)
Also, Brian, I thought there was supposed to be a negative sign on the right side of your equation?

Alvin Lai 3J

An intermediate is first formed in one step, then consumed as a reactant in the next step. It does not show up in the overall reaction.

Alvin Lai 3J

After doing all the textbook problems, there were not really any problems that gave the frequency factor or required us to solve for the frequency factor. Though, I would assume that if it were given to us, that we could use the equation to solve for k, Ea, or T depending on what else was given.

Alvin Lai 3J

I would think that the problem has a typo, the anode is always on the left side of the cell diagram.

Alvin Lai 3J

Yeah, it seems like you have it down pretty good. I guess I would add that for the basic conditions, make sure you know which side you are adding H2O and which side you are adding OH-. I believe that depending on how many more H's you need on one side in a basic solution, you add instead H20's. Then...

Alvin Lai 3J

The first equation is for zero order reactions and the second equation is for first order reactions. On Dr. Lavelle's outline for Kinetics, he has the equations with their respective order. Hope this helps! https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Chem14B_Outline6_Kinetics.pdf

Alvin Lai 3J

A salt bridge is important in a galvanic cell because it allows charges to transfer to prevent build up of charges. For example, at the anode, there would be a build up of + charges because electrons are leaving to go to the cathode, where a build up of - will also occur. The salt bridge allows for ...

Alvin Lai 3J

At equilibrium, the E naught will be zero!

Alvin Lai 3J

Determine the rate constant for each of the following first-order reactions, in each case expressed for the rate of loss of A: (a) A → B, given that the concentration of A decreases to one-half its initial value in 1000. s; (b) A → B, given that the concentration of A decreases from 0.67 mol⋅L−1 to ...

Alvin Lai 3J

You could use the equation Ecell = E^ocell-(RT)/(nF)ln Q, where more products makes a larger Q, which leads to a lower Ecell. More reactants would lead to a lower Q, which would make the Ecell larger. As for your question, I would think that depending on what -(RT)/(nF)ln Q is, it would lead cause E...

Alvin Lai 3J

For the zero and first-order reactions, the slope = -k. For the second-order reaction, slope = +k

Alvin Lai 3J

6O.1 A 1.0 M NiSO4(aq) solution was electrolyzed by using inert electrodes. Write (a) the cathode reaction; (b) the anode reaction. (c) With no overpotential at the electrodes, what is the minimum potential that must be supplied to the cell for the onset of electrolysis? How do determine whether Ni ...

Alvin Lai 3J

6L.3 Write the half-reactions and the balanced equation for the cell reaction for each of the following galvanic cells:
(d) Pt(s)|O2(g)|H+(aq)∥OH−(aq)|O2(g)|Pt(s)
Can someone show me how to do the half reaction for the anode of (d)? I do not understand how the answer key got 2H2O -> O2 + 4H+ + 4e-

Alvin Lai 3J

Thank you!

Alvin Lai 3J

For topic 6L #3, where do we find the Ecell information that the answer key has? Are we supposed to look them up?

Alvin Lai 3J

I think it is the moles of electrons transferred, which you can find my looking at the redox reaction?

Alvin Lai 3J

When give kPa, convert it to atm using 1 atm = 101.325 kPa. Then you can use the gas constant of R = 8.206 * 10^-2 L·atm·K-1·mol-1 to cancel out the atm and the other units. Hope this helps!

Alvin Lai 3J

The Van't Hoff Equation can be used to find the K2 value at a different temperature, assuming that you know the temperatures of K1 and K2, and the deltaH. Then you could solve for the right side of the equation, put that result as an exponent of e, then multiply it by K1 to get K2. Hope this helps!

Alvin Lai 3J

You would not calculate the gibbs free energy of formation, but you can calculate deltaG rxn by doing the gibbs free energy of formation of the products - the gibbs free energy of formation of the reactants. I believe that the deltaG has to be negative for stability, not positive, since a negative d...

Alvin Lai 3J

The reaction will be spontaneous above a certain temperature, since you would need a greater TdeltaS to subtract from deltaH to make deltaG negative!

Alvin Lai 3J

Under what conditions, if any, does the sign of each of the following quantities provide a criterion for assessing the spontaneity of a reaction? (a) ΔG°; (b) ΔH°; (c) ΔS°; (d) ΔStot.

For (d), can someone explain to me why if ΔStot. > 0, that the reaction would be spontaneous? Thank you!

Alvin Lai 3J

In Dr. Lavelle's lecture, why does he draw a curve facing upwards for deltaU and a downward facing curve for deltaS?

Alvin Lai 3J

Which would you expect to have a higher molar entropy at T=0, single crystals of BF3 or of COF2? Why?
So the answer is COF2 and not BF3 because the oxygen and fluorine can change places while the fluorine in BF3 will just occupy the same place? So there's more disorder in COF2?

Alvin Lai 3J

Calculate the change in entropy when the volume of 2.00molAr(g) is decreased from 10.00 L to 5.00 L, while the temperature decreases from 300. K to 100. K. Assume ideal behavior.
The answer is −38.9J⋅K−1, but I do not know how to get it. Can someone explain how to do this?

Alvin Lai 3J

When is the next midterm? Just to know so I can prepare myself with workloads. Thank you!

Alvin Lai 3J

4D.11 The oxidation of nitrogen in the hot exhaust of jet engines and automobiles occurs by the reaction N2(g)+O2(g)→2NO(g)ΔH°=+180.6kJ (c) When the oxidation of N2 to NO was completed in a bomb calorimeter, the heat absorbed was measured as 492 J. What mass of nitrogen gas was oxidized? Can someone...

Alvin Lai 3J

We know that the volume is constant in the equation because of the subscript V in Cv. Cv means the heat capacity at a constant volume.

Alvin Lai 3J

Estimate the contribution of motion to the molar internal energy of water vapor at 25 °C. [Answer: 7.44 kJ⋅mol−1]
Can someone explain this to me? I tried to understand how to calculate Um, but I do not get it still. Please help, and thank you!

Alvin Lai 3J

How are you guys remembering the various equations for Thermochemistry? Since the equations do not seem to be on the constants and equations sheet, are there any tricks to remembering all the equations?

Alvin Lai 3J

So since you know that for every 358.8 kJ absorbed, you get 4 moles of CS2. With this, you take the energy they give you times that conversion factor to get the # of moles: (379 kJ ) x (4 moles CS2/358.8 kJ) = 4.23 mol Cs2 Now that you have moles of CS2, you multiply the moles by the molar mass of C...

Alvin Lai 3J

Yep, N means the molecules in the system!

Alvin Lai 3J

The equilibrium constant, K, will only be affected by a change in temperature, not change in volume or pressure.

Alvin Lai 3J

Nope, you do not need to memorize bond enthalpies. The values will be provided for you :)

Alvin Lai 3J

The K for a reverse equation is the reciprocal of K, so 1/K. Hope this helps!

Alvin Lai 3J

In step 2, you need to do the -5000 kJ divided by -2878 kJ/mol, not the other way around. Everything else looks good, so try what I said instead.

Alvin Lai 3J

Carbon disulfide can be prepared from coke (an impure form of carbon) and elemental sulfur. 4C(s)+S8(s)⟶4CS2(l) ΔH∘=+358.8 kJ Calculate the amount of heat absorbed when 1.31 mol S8 is reacted at constant pressure. Can someone run by me all the steps for this problem? I do not know where to start, pl...

Alvin Lai 3J

Use the data in Tables 4E.2 and 4E.3 to estimate the reaction enthalpy for (a) N2(g)+3F2(g)→2 NF3(g) (b) CH3CHCH2(g)+H2O(g)→CH3CH(OH)CH3(g)
(c) CH4(g)+Cl2(g)→CH3Cl(g)+HCl(g)

For (b), why did the textbook solution say that a C-C bond was formed? Can someone explain this to me? Thank you!

Alvin Lai 3J

In example 4D.3, they did n(C4H10) = (350.kJ) × (2mol C4H10/5756kJ) = (350. × 2)/5756 mol C4H10 = 0.122 … molC4H10

Do we need to know how to convert heat output into moles of fuel molecules for the midterm? If we do, could someone explain this process to me since it is completely new?

Alvin Lai 3J

At equilibrium, concentrations should not change unless concentration is added or subtracted for the product or reactants. Hope this helps!

Alvin Lai 3J

You are also correct, you just included how the equilibrium reaction is specifically affected from products, in which the book just states that the equilibrium reaction is affected. You're good!

Alvin Lai 3J

Calculate the pH, pOH, and percentage protonation of solute in each of the following aqueous solutions: (a) 0.057 M NH3(aq); (b) 0.162 M NH2OH(aq); (c) 0.35 M (CH3)3N(aq); (d) 0.0073 M codeine, given that the pKa of its conjugate acid is 8.21. Can someone run down the steps on how to approach (a)? T...

Alvin Lai 3J

When you are using the ICE box to solve for x, when do you know to approximate for x? Does the Ka or Kb need to be *10^-4 or smaller? Thank you!

Alvin Lai 3J

I have not encountered any polynomials larger than x^3, because some equations when putting Products/Reactants can be square rooted to remove the squares on the top and bottom. For x^3, you would assume that x is a very small number, like 10^-10, and can make equations with a number + or - x to just...

Alvin Lai 3J

I think Evelyn means the partial pressure of A from around 27 to 17, not 35 to 25. But her explanation of how the change in partial pressure is around 10 for A and C, while B has only a change of 5.

Alvin Lai 3J

I think it would be best to read the textbook and do the problems, as they enhance what Dr. Lavelle discusses in lecture. Then, you could try to form a study group or attend UA sessions to review what you've learned. Also, make sure to read the section outlines to check what you know and don't know....

Alvin Lai 3J

Yep, you got it! What multiplies to the entire reaction equation is put as an exponent to the K value.

Alvin Lai 3J

While the first Sapling homework is due on the 17th, you should try to wrap up the section soon to start working on the next section, since we will be starting the next section soon.

Alvin Lai 3J

C is false, because the even at equilibrium, forward and reverse reactions are still occurring.

Alvin Lai 3J

I do not think there will be Audio-Focused modules on the upcoming topics, at least not for them all. In Chem 14A, there were not any for the last topics of the quarter, so I think the same will happen for this quarter. I do recommend doing the textbook problems, since they offer practice and have a...

Alvin Lai 3J

Calculate the concentration of CO(g) in mol.m-3 at 423 K and 0.8 atm using R = 8.206 × 10-5 m3.atm.K-1.mol-1 and assuming only CO(g) is present.
Can someone show me the steps to do this problem? I am lost on where to start. Thank you!

Alvin Lai 3J

I just have all my favorite songs in a playlist on YouTube playing at the lowest possible volume. It's perfect for keeping me relaxed without totally distracting me. Hope everyone has an amazing break!

Alvin Lai 3J

Can someone run down how to do this step by step? This does not make much sense to me.

Alvin Lai 3J

That makes sense, thank you!

Alvin Lai 3J

In each of the following salts, either the cation or the anion is a weak acid or a weak base in water. Write the chemical equation for the proton transfer reaction of this cation or anion with water: (a) NaC6H5O; (b) KClO; (c) C5H5NHCl; (d) NH4Br. How do you figure out whether to use the cation or t...

Alvin Lai 3J

In Dr. Lavelle's Friday lecture, he talked about KA and pKA. I do not really understand what these are, can someone explain to me? Also, are we going to be calculating these numbers for the final?

Alvin Lai 3J

Which of the following ligands can be polydentate? If the ligand can be polydentate, give the maximum number of places on the ligand that can bind simultaneously to a single metal center: a) HN(CH2CH2NH2)2; (b) CO32−; (c) H2O; (d) oxalate. Can someone explain to me how to approach this? I do not und...

Alvin Lai 3J

Thank you! I actually understand that now. So we can find the charge on the TM metal by using the compounds total charge? And we can find the total charge of the compound by adding all the charges of the elements?

Alvin Lai 3J

(b) Write the formula of sodium dichloridobis(oxalato)platinate(IV).
(Book help->) Two Cl− ligands and two C2O42− ions are attached to Pt4+. The charge on the complex is −2.

How did they figure out that Pt has a 4+ charge? Please help.

Alvin Lai 3J

If you're looking for the oxidation number of Pt, you do -1 times the number of Chlorines there are, which gives you -6. Then the equation is Pt - 6 = -2. The equation equals -2 because that is the charge of the compound. This leaves Pt to equal +4 to make -2. Hope this helps.

Alvin Lai 3J

Hey! So you are right in that there are three lone pairs on the middle Iodine. This makes five electron densities, and that would be trigonal bipyramidal. The three lone pairs take all the equatorial electron densities, leaving only the axial atoms as Iodine. This makes the linear shape. Hope this h...

Alvin Lai 3J

I believe it would be 12 sigma bonds and 3 pi bonds because each carbon is also bonded to a hydrogen. Hope this helps!

Alvin Lai 3J

Thank you, that really helped! I do not think the textbook talked much about it, so thanks for telling me.

Alvin Lai 3J

Noting that the bond angle of an sp3 hybridized atom is 109.5° and that of an sp2 hybridized atom is 120°, do you expect the bond angle between two hybrid orbitals to increase or decrease as the s-character of the hybrids is increased? Can someone explain to me what the question means by s-character...

Alvin Lai 3J

h1=s+21/2py h2=s+(32)1/2px−(12)1/2py h3=s−(32)1/2px−(12)1/2py

Do we need to know about the hybrid orbitals h1, h2, etc.? Like the equations at the beginning of 2F.3?

Alvin Lai 3J

For me, I just try to memorize it. I try to remember the geometry with no lone pairs on the center atom, and then think of the geometry and it's name when there are lone pairs.

Alvin Lai 3J

Yeah, you would put the charge on the outside, top right of the brackets. You are right!

Alvin Lai 3J

They are also recorded, so you should be able to take your fiat lux as well!

Alvin Lai 3J

Predict the shapes and estimate the bond angles of (a) the thiosulfate ion, S2O32−; (b) (CH3)2Be; (c) BH2−; (d) SnCl2.

For (b), do you predict the shapes with the carbons as the central atoms?

Alvin Lai 3J

2E.5 (a) What is the shape of a ClO2+ ion? (b) What is the expected OClO bond angle?

Is the lewis structure for ClO2+ drawn with one double bond or two? Can someone explain to me why? Thank you!

Alvin Lai 3J

Thank you! Also, would it be okay to take the Chem 14BL Lab section from 11am-1:30pm on Friday even though it goes into Chem 14B's friday lecture at 1pm, since the lecture is recorded?

Alvin Lai 3J

Hello, I am thinking of taking Chem 14B and Chem 14BL next quarter along with LS30B. Is the minimum units per quarter 12 or 13? If it is 13, then I would have to take another class, probably a GE since Chem 14B is 4 units, Chem 14BL is 3 units, and LS30B is 5 units which equals 12 units. Please help!

Alvin Lai 3J

My book gives me the wrong answers then. Is there a place that I could get the correct answers for the odd numbers?

Alvin Lai 3J

Also, the book says just (a) and (b) are radicals. Is (c) also a radical since the O has one electron by itself?

Alvin Lai 3J

Which of the following species are radicals? (a) NO2−; (b) CH3; (c) OH; (d) CH2O.

Can someone explain to me how (a) is a radical? Thank you!

Alvin Lai 3J

Hypochlorous acid, HClO, is found in white blood cells, where it helps to destroy bacteria. Draw two Lewis structures with different atom arrangements for HClO and select the most likely structure by identifying the structure with formal charges closest to zero. Consider only structures with single ...

Alvin Lai 3J

In the textbook solutions, it shows one of the F with only two pairs of electrons and one bond with Br. This is a book mistake right?

Alvin Lai 3J

When is Midterm 2 going to be? Thanks in advance.

Alvin Lai 3J

HI! It's 1 Hz = 1/second
I hope this helps!

Alvin Lai 3J

That really helps, thank you!

Alvin Lai 3J

Give the ground-state electron configuration and number of unpaired electrons expected for each of the following ions: (a) Sb3+; (b) Sn4+; (d) Br−; (e) Ni2+. For (e) Ni2+, the textbook says the answer is [Ar]3d8 and two unpaired electrons. I thought the answer would have been [Ar]3d6 4s2 and four un...

Alvin Lai 3J

Which M3+ ions (where M is a metal) are predicted to have the following ground-state electron configurations:
(a) [Ar]3d6; (b) [Ar]3d5

I'm confused on how to solve this correctly, can someone clarify the exceptions when it comes to the d orbitals? Thank you!

Alvin Lai 3J

Oh I see, thank you for figuring that out. Maybe I'm just bad at figuring things out, but that was a bit obscure. Thanks again though!

Alvin Lai 3J

Did Dr. Lavelle say that we do not have to think about nodal planes too much in his lecture? Maybe I'm wrong?

Alvin Lai 3J

Same, I can't find it anywhere. I tried searching google, but still didn't figure it out. Please help!

Alvin Lai 3J

I also really enjoyed the modules, since they taught me quick and easy, and that I could view them before a quiz or midterm. Too bad he's not making any more.

Alvin Lai 3J

Hi, apparently there was an error with one of the questions and each individual midterm is being looked at. This means it will probably be next week when we can see our scores.

Alvin Lai 3J

In case you need the formula sheet or periodic table:
https://lavelle.chem.ucla.edu/wp-conten ... _IUPAC.pdf
https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf

Alvin Lai 3J

Like the midterm is still proctored by our TA with our discussion group, but every student takes it on Friday at like 4pm PST? Realistically, we should just prep for our discussion section for the midterm.

Alvin Lai 3J

I should read section 1D, but I don't need to do any of the problems until after the midterm?

Alvin Lai 3J

So the coefficients in front of each compound is a mole. All you have to do is add up the total moles on the left and compare them to the total moles on the right. This should be 30->36, so the net moles is 6.

Alvin Lai 3J

I just watch the lectures, take notes on the modules/do the assessments, and do the problems in the textbook. If I get stuck on a problem, I look at Chemistry Community to get help. Hope this helps!

Alvin Lai 3J

I believe it is ultraviolet light. Hope this helps. And if you need it, you can paste your question into google and find a quizlet for the post-assessment.

Alvin Lai 3J

Yes work function is the same as the threshold energy.

Alvin Lai 3J

Hi! Basically when we calculate the empirical formula, we only want to use individual elements. In other words, instead of O2 you would simply use only O. Your calculations should look like this: 69.9 g Fe (1 mol Fe/55.85 g Fe)= 1.25 mol Fe 30.1 g O (1 mol O/16.00 g O) = 1.88 mol O Dividing by the ...

Alvin Lai 3J

I try not to round during the equations, and round the final answer with sig figs. That way, I get the most accurate answer. Hope this helps!

Alvin Lai 3J

Hey! On a different post, somebody said this acronym to help remember the prefixes:

Great Mr. Knight Doesn't Cook Me Macaroni Until Nine Pm
(giga, mega, kilo, deci, cent, milli, micro, µ, nano, pico)
(credit: Shreya Patel- 2D)

Hope this helps

Alvin Lai 3J

I read on a different post that the midterms and quizzes will be multiple choice, so I don't think we have to worry about sig figs and rounding.

Alvin Lai 3J

How did you find the molarity of Na2CO3?

Alvin Lai 3J

Prefix Name Meaning G giga 10^9 M mega 10^6 k kilo 10^3 d deci 10^-1 c centi 10^-2 m milli 10^-3 u micro 10^-6 n nano 10^-9 p pico 10^-12 I'll just leave this here :)

Alvin Lai 3J

You would convert the grams for each element into moles using the element's molar mass. So take the mass of Phosphorus and divide it by the molar mass to convert it to moles, and do the same with Te. Then you would compare the moles of P to Te and see which moles is bigger.

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