CHEMISTRY COMMUNITY

Pauline Tze 3B

cell potentials are always given.

Pauline Tze 3B

Based on this question, it looks like the grams of gas are being combusted so you would need to use delta h to get the heat generated from that. It can't be simply a temperature change.

Pauline Tze 3B

Hi Vera, You account for the enthalpy of fusion when the state of your system is going from a solid to a liquid. I assume you mean enthalpy of vaporization, which you would account for when the state is changing from a liquid to a gas. I don't know what question you are referring to, but I suggest l...

Pauline Tze 3B

2Fe(s) + 2H2O --> 2Fe(OH)2(s) + 4e-
pay attention to the charge for Fe as Fe is oxidized from 0 to +2
O2 + 4e- ----> 2Fe(OH)2(s)
pay attention to the charge for O as O is reduced from 0 to -2

Pauline Tze 3B

Dear Dr. Lavelle, I'm incredibly thankful to have had you for both 14A and 14B. It's honestly been such a pleasure attending your lectures, office hours, and review sessions. I am thankful for all the resources that you've provided, as they remind me that I don't need to struggle alone in my endeavo...

Pauline Tze 3B

Transition states are at the top of the "humps" of your energy profile diagram and intermediates are in the "valleys." Both are unstable, but transition states are more unstable than intermediates because they have more energy (from distorted/partial bonds). In the context of the...

Pauline Tze 3B

The linear version of the Arrhenius equation is lnk = (-Ea/R)(1/T) + lnA
When you consider the Arrhenius plot, does lnA actually exist as the y-intercept (where (-Ea/R)(1/T) would equal zero)? If so, under what conditions would it exist?

Thank you!

Pauline Tze 3B

Hi Jamie,

If you write out the numbers in numerical order you get
224
355
You look for the one with the lowest number, which is the group with 2.This is regardless of its position in the molecule.

Pauline Tze 3B

Hi Eliza,

If you're referring to K as in the equilibrium constant, the larger K indicates that the reaction favors products.
If you're referring to k (lower case) as in the reaction rate constant, the larger k does mean the reaction is faster.

Pauline Tze 3B

Hi James,

A catalyst speeds up a reaction by providing an alternative route for the reaction with a lower activation energy. It shows up in the reaction mechanism as something added (the reactant side) although it is not actually a reactant, so it doesn't appear in the overall reaction.

Pauline Tze 3B

Hi Erik, The overall rate of reaction has the units mol.L -1 .s -1 . The rate constant k depends on the reaction's activation energy and temperature. Problems that ask you to find rate constant k will require you to find its numerical value and units given the chemical reaction and initial rate. Say...

Pauline Tze 3B

When you accept that you won't know for sure if you solved an entropy problem correctly:

Pauline Tze 3B

Hi Omer! There's no particular issue; I know quite a few people who are taking 14B with 14BL this quarter. If you do take 14BL with 14C, however, I highly recommend only taking 3 classes (and ideally a 3rd class with a comparatively small workload). I'm taking 4 classes (including 14BL) and a semina...

Pauline Tze 3B

I understand that permanganate is the strongest oxidation agent, however, based on the Standard Reduction Potentials, potassium (potential = -2.92) is a stronger reduction agent than iron(II) (potential = - 0.41). Using this information, how do we know whether we need to write a reaction for potassi...

Pauline Tze 3B

Thank you!!!

Pauline Tze 3B

For cathodes and anodes, the UA Ashley recommended remembering consonants go with consonants and vowels go with vowels:

Cathode
Reduction

Anode
Oxidation

Pauline Tze 3B

Hi Andrew, Regardless of what equation you're being given, you need to identify which elements within those compounds are being oxidized/reduced and separate the oxidation/reduction reactions into two equations (at that point they will be unbalanced). You balance the elements (changing coefficients,...

Pauline Tze 3B

Hi Nicole, When a question asks you to assume ideal behavior, it can also mean that they want you to consider the ideal gas law PV = nRT. In the calculation for change in entropy using temperature, I believe they combine the relationship of V and T (in that they are directly related) in PV = nRT to ...

Pauline Tze 3B

Hi Grace! rev stands for reversible and you can get this value by solving for q (heat) in different ways depending on the problem you're being asked. For example, you might combine it with q = C delta T or if it's an isothermal reversible problem, then you can deduce change in internal energy (delta...

Pauline Tze 3B

Hi,
Does anyone know how to mathematically derive the formula delta S = nR ln (T2/T1)?

I can't seem to work it out from PV = nRT and delta S = q(reversible)/T though I presume that it works out because T is directly related to V which has the equation delta S = nR ln (V2/V1)

Thank you in advance.

Pauline Tze 3B

Hi Coco! A reversible system is typically isothermal because the heat absorbed by the system is canceled out by the heat released by the system (as is the nature of systems at equilibrium). We use those two specific equations you listed as follows: w= -P∆V is to find work in a system where there's c...

Pauline Tze 3B

All systems at equilibrium are reversible processes because any heat absorbed or work done on the system is canceled out by heat released or done by the system. This occurs such that infinitesimal changes in pressure do not increase entropy (because if entropy increases, it cannot be reversed accord...

Pauline Tze 3B

I'd be interested :)

Pauline Tze 3B

Hi Patrick,

Condensation is one phase change (vapor to liquid). This phase change occurs because energy (or heat) is being released from the container (the system) to its surroundings; it is exothermic not endothermic.

I hope that helps!

Pauline Tze 3B

So then HCl would have less covalent character than say CF4 right? Or do the 4 F's make any difference? Hello, The fact that there are 4 F's does not make a difference to the electronegativity difference in individual C-F bond. That would, however, influence the electric dipole moment and the polar...

Pauline Tze 3B

Hello, I'm also stuck on this concept. If it said n=2 and l=0, there's the possibility of boron, carbon, nitrogen, oxygen, fluorine, and neon all have valence electrons with those quantum numbers because, for example, the valence electron configuration of Boron is [He]2s 2 2p 1 . The 2s orbital woul...

Pauline Tze 3B

This was really helpful! Thank you Alison :)

Pauline Tze 3B

The pH for the stoichiometric point for a strong acid-strong base titration is always 7 because they neutralize completely. There are no calculations needed. However, we do need the volume of HCl added at that point for the next part.

Pauline Tze 3B

Hi Parsia, I believe K + is a weak acid in general and Br - is the conjugate base to a strong acid (HBr) (so the reaction doesn't really go to HBr because strong acid reactions go to completion and not the reverse) so therefore, neither react significantly in water to change the pH. I hope that helps.

Pauline Tze 3B

Hi Parsia, If you look carefully at (CH3)2NH2 and NH2OH on Table 12.2, they are bases (we are given the conjugate acids in the problem (Ch3)2NH2+ and +NH3OH respectively) so therefore, in the table we are given pKb values. (Intuitively, you can also predict that these will be the weakest acids becau...

Pauline Tze 3B

Hi Grace, Because Q is significantly larger than K, the reaction favors reactants, so the solutions manual assumes that you push the reaction as far left as possible. This means that all of H2 is consumed because it is the limiting reactant (based on the given partial pressures). So because two mole...

Pauline Tze 3B

@Crystal From looking at what Belicia wrote, I believe I misinterpreted the question. I was finding the conjugate acid and conjugate base and the Bronsted acid and base at the same time and somewhere down the line I associated them as the same thing, so I thought then that it was necessary for 2NH3 ...

Pauline Tze 3B

Can someone explain how they wrote out the complete ionic equation for this part of the question? d. NH 4 I + KNH 2 --> KI + 2NH 3 I understand the breakdowns of: NH 4 I --> NH 4 + and I - KNH[sub]2 --> K + NH 2 KI --> K + + I - But I don't understand why 2NH 3 would break down into 2NH 2 - + 2H + ....

Pauline Tze 3B

Confirming what Brandon said, you do not multiply the initial concentration by the number of moles the molecule has. The number of moles the molecule has is relevant when you're calculating the change in molarity (C part of the table) as well as in your equilibrium constant (K) calculations.

Pauline Tze 3B

For ICE box equations in general, if the equation uses solids and liquids (especially if one side doesn't have gases/aqueous compounds only solid/liquid), would you include those in your calculations or would this simply mean that you can't use ICE box? Or, a similar question would be, do ICE box eq...

Pauline Tze 3B

If there's an H2O outside the complex, do we refer to it as hydrate or aqua? For example: K3[Fe(CN6)]3H2O Actually, since H 2 O is outside the coordination sphere (represented by the brackets), it is therefore not directly bound to the metal atom/ion (iron). Since ligands are defined as ions or mol...

Pauline Tze 3B

Since it's a bidentate, it has two binding sites, and therefore only contributes 2 bonds to the coordination number. Remember that the coordination number is the number of bonds of the ligands directly attached to the central metal atom or ion.

Pauline Tze 3B

Oxalato is 2 because it is bidentate

Pauline Tze 3B

Screen Shot 2016-11-13 at 8.48.42 PM.png I understand how the solution manual answers this question (though is the first step where it converts 25.0g of the reactant to moles seems irrelevant), but I'm not understanding why my approach didn't work. I tried to find the concentration of CO 2 and NH 3...

Pauline Tze 3B

I highly recommend calculating bond order primarily through MO diagram. If you do lewis structure, you'll miss it if the bond order is not a whole number (like 0.5 or 2.5). This is because the bond order equation specifically asks for 1/2( # bonding orbitals + # anti-bonding orbitals) and you can't ...

Pauline Tze 3B

Consider the bonding in CH2=CHCHO. (a) Draw the most important Lewis structure. Include all nonzero formal charges. (b) Identify the composition of the bonds and the hybridization of each lone pair—for example, by writing (H1s,C2sp2). Hi, I'm confused by part b. Can someone explain how you determine...

Pauline Tze 3B

Why do bonding molecular orbitals have lower energy than anti bonding molecular orbitals? seems like it should be the opposite to me Bonding and anti-bonding molecular orbitals are necessary to ensure the conservation of energy of orbitals. Bonding orbitals act to make the atomic orbitals overlap i...

Pauline Tze 3B

When writing out the molecular orbital configuration for quizzes/exams, do we list (pi_px)² and (pi_py)² as separate entities or is it okay to simplify it to (pi_p)⁴?

Also, why do (pi_px) and (pi_py) have the same energy level? What exactly do they represent?

Thank you in advance!

Pauline Tze 3B

Hi Jasleen, If you observe the electron configurations on the periodic table, you would notice that valence electrons come from the s- and p-orbitals of the highest energy shell for a given element (for the s and p blocks). The s orbital can hold 2 and the p orbital can hold 6 which add up to 8 (for...

Pauline Tze 3B

Both abbreviations are correct; though when you do the problem you'll realize that you can't use Argon for all of them.

Pauline Tze 3B

If you think you missed some of the material, you can watch Professor Lavelle's lectures on bruincast:
http://bruincast.ucla.edu

Pauline Tze 3B

What is the difference between electron affinity and electronegativity? They seem similar to me. If you would review the notes under "electron configuration and periodicity of atomic properties" and "ionic vs. covalent bonds" and read sections 2.11 Electron Affinity and 3.12 Cor...

Pauline Tze 3B

Hi, can someone please explain why the photoelectric effect does not apply to this question and what principle you would actually apply? Question: "5. Iron is an important element in the environment and water systems. Given its environmental significance, we know it emits 510. nm light and abso...

Pauline Tze 3B

The answer in the solution manuals reads [Kr] 4d^10 5s^1

Make sure you list the electron configurations in order of increasing energy levels (ex n=4 before n=5) then within each set, your orbitals should be in order of s, p, d, f.

Hopefully that helps.

Pauline Tze 3B

I think your error stems from reading the periodic table incorrectly. Rf and Th do have different electron configurations, which are Rf: [Rn] 5f^14 6d^2 7s^2 and Th: [Rn]7s^2 6d^2. The periodic table's trends follow increasing atomic number. You need to read the f-block before the rest of the d-bloc...

Pauline Tze 3B

I think Chem_Mod explained it pretty thoroughly, but this diagram helps for visualizing it:

Pauline Tze 3B

NinaSheridan wrote:For later quizzes, should we have specific polyatomic ions memorized?

My TA mentioned that we should at least memorize the ones in the course reader (Check under "Naming Simple Compounds" in the "Review of the Chemical & Physical Principles.")

Pauline Tze 3B

A problem asks for the energy of a photon emitted with a wavelength of 5.50x10^2. I multiplied it by plank's constant and got the answer. However, I don't understand why that is if the original equations is E= h x frequency, not wavelength. Hi! I'm not understand how you got the answer just by mult...

Pauline Tze 3B

Wang_Pong_Chan_1L wrote:In the Rydberg Equation, does n1 refer to the higher energy level or the lower energy level?

n1 refers to the initial energy level. Whether it is the higher or lower one depends on the problem.

Pauline Tze 3B

You probably wouldn't be able to find a "perfect" black body except in a laboratory setting because it would need to absorb and emit ALL radiation. Examples of a general black body would be the sun, a furnace, lightbulb, etc. If you check online you can find quite a few examples; this site...

Pauline Tze 3B

I'm not sure where you got the .6 from, or even what molarity equation you used, but I think it'd be a good idea for you to review Mixtures and Solutions Fundamentals Section G. You're supposed to use (Molarity) initial (Volume) initial = (Molarity) final (Volume) final In this problem, 2.00 M is th...

Pauline Tze 3B

The sig fig rule is that non-zero digits are always significant, so your answer is correct in having 4 sig figs.
The 3 sig fig answer is probably a typo in the solutions manual (which may be because all the other parts in the question have 3 sig figs).

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