## Search found 57 matches

Sun Mar 19, 2017 12:21 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 2013 Final #3
Replies: 1
Views: 257

### Re: 2013 Final #3

cell potentials are always given.
Sat Mar 18, 2017 2:07 pm
Forum: Phase Changes & Related Calculations
Topic: Boiling point and Delta H of fusion
Replies: 3
Views: 549

### Re: Boiling point and Delta H of fusion

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.
Fri Mar 17, 2017 2:09 pm
Forum: Phase Changes & Related Calculations
Topic: Boiling point and Delta H of fusion
Replies: 3
Views: 549

### Re: Boiling point and Delta H of fusion

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...
Tue Mar 14, 2017 10:52 pm
Forum: Balancing Redox Reactions
Topic: 2013 Final #4
Replies: 4
Views: 523

### Re: 2013 Final #4

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
Tue Mar 14, 2017 2:09 am
Forum: Administrative Questions and Class Announcements
Topic: Saying Thank You to Dr. Lavelle
Replies: 275
Views: 108663

### Re: Saying Thank You to Dr. Lavelle

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...
Sun Mar 12, 2017 9:25 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Transition States
Replies: 2
Views: 336

### Re: Transition States

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...
Tue Mar 07, 2017 11:10 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: lnA: does this y-intercept exist?
Replies: 1
Views: 404

### lnA: does this y-intercept exist?

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!
Thu Mar 02, 2017 11:02 pm
Forum: *Alkanes
Topic: Confusion on naming on page 93 on Course Reader
Replies: 2
Views: 346

### Re: Confusion on naming on page 93 on Course Reader

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.
Mon Feb 27, 2017 5:35 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Determining Slow Step
Replies: 1
Views: 283

### Re: Determining Slow Step

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.
Mon Feb 27, 2017 12:33 am
Forum: *Organic Reaction Mechanisms in General
Topic: Catalysts
Replies: 1
Views: 257

### Re: Catalysts

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.
Sun Feb 19, 2017 3:31 pm
Forum: General Rate Laws
Topic: 15.9 [ENDORSED]
Replies: 4
Views: 555

### Re: 15.9[ENDORSED]

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...
Thu Feb 16, 2017 12:40 am
Forum: Student Social/Study Group
Topic: Post All Chemistry Jokes Here
Replies: 7951
Views: 1124188

### Re: Post All Chemistry Jokes Here

When you accept that you won't know for sure if you solved an entropy problem correctly:
Sat Feb 11, 2017 11:17 pm
Forum: Administrative Questions and Class Announcements
Topic: Taking 14BL and 14C at the Same Time
Replies: 1
Views: 313

### Re: Taking 14BL and 14C at the Same Time

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...
Sat Feb 11, 2017 7:30 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: which half reaction to use
Replies: 2
Views: 399

### Re: which half reaction to use

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...
Fri Feb 10, 2017 2:10 pm
Forum: Administrative Questions and Class Announcements
Topic: Midterm Review Session- February 9, 6-7PM
Replies: 2
Views: 503

### Re: Midterm Review Session- February 9, 6-7PM

Thank you!!!
Thu Feb 09, 2017 4:57 pm
Forum: Balancing Redox Reactions
Topic: Redox Memorization Tips
Replies: 4
Views: 612

### Re: Redox Memorization Tips

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

Cathode
Reduction

Anode
Oxidation
Sun Feb 05, 2017 7:14 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: How do you deal with 3 given elements for a half reaction?
Replies: 1
Views: 229

### Re: How do you deal with 3 given elements for a half reaction?

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,...
Sat Jan 28, 2017 12:54 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Problem 9.13
Replies: 1
Views: 231

### Re: Problem 9.13

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 ...
Sat Jan 28, 2017 12:36 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Q rev?
Replies: 1
Views: 3221

### Re: Q rev?

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...
Sat Jan 28, 2017 12:21 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Derivation of Entropy Equation (changing temperature)
Replies: 1
Views: 252

### Derivation of Entropy Equation (changing temperature)

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)

Mon Jan 23, 2017 1:34 am
Forum: Calculating Work of Expansion
Topic: Reversible and Isothermal
Replies: 4
Views: 443

### Re: Reversible and Isothermal

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...
Mon Jan 23, 2017 1:18 am
Forum: Calculating Work of Expansion
Topic: Reversible processes
Replies: 1
Views: 257

### Re: Reversible processes

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...
Sun Jan 22, 2017 11:59 pm
Forum: Student Social/Study Group
Topic: Quiz 1 Prep
Replies: 8
Views: 843

### Re: Quiz 1 Prep

I'd be interested :)
Sat Jan 14, 2017 6:24 pm
Forum: Phase Changes & Related Calculations
Topic: Water Phase Changes
Replies: 1
Views: 305

### Re: Water Phase Changes

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!
Sun Dec 04, 2016 10:20 am
Forum: Ionic & Covalent Bonds
Topic: least covalent character
Replies: 4
Views: 698

### Re: least covalent character

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...
Sat Dec 03, 2016 11:33 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Confusion with n, l, ml, ms.
Replies: 2
Views: 684

### Re: Confusion with n, l, ml, ms.

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...
Fri Dec 02, 2016 1:21 am
Forum: *Titrations & Titration Calculations
Topic: Calculating titrations beyond stoichiometric point
Replies: 3
Views: 811

### Re: Calculating titrations beyond stoichiometric point

This was really helpful! Thank you Alison :)
Fri Dec 02, 2016 1:10 am
Forum: *Titrations & Titration Calculations
Topic: Homework Help 13.27
Replies: 1
Views: 249

### Re: Homework Help 13.27

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.
Sun Nov 27, 2016 8:58 am
Forum: Calculating the pH of Salt Solutions
Topic: Homework 12.65
Replies: 5
Views: 948

### Re: Homework 12.65

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.
Fri Nov 25, 2016 5:29 pm
Forum: Acidity & Basicity Constants and The Conjugate Seesaw
Topic: hw 12.39
Replies: 1
Views: 399

### Re: hw 12.39

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...
Thu Nov 24, 2016 1:47 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: chapter 11 #67 help happy thanksgiving! :)
Replies: 2
Views: 372

### Re: chapter 11 #67 help happy thanksgiving! :)

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...
Tue Nov 22, 2016 9:34 am
Forum: Lewis Acids & Bases
Topic: Homework 12.9 [ENDORSED]
Replies: 5
Views: 773

### Re: Homework 12.9[ENDORSED]

@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 ...
Sun Nov 20, 2016 2:19 am
Forum: Lewis Acids & Bases
Topic: Homework 12.9 [ENDORSED]
Replies: 5
Views: 773

### Re: Homework 12.9[ENDORSED]

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 + ....
Sat Nov 19, 2016 8:37 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: ICE table question
Replies: 2
Views: 353

### Re: ICE table question

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.
Thu Nov 17, 2016 11:58 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: ICE box
Replies: 2
Views: 414

### ICE box

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...
Wed Nov 16, 2016 10:17 am
Forum: Administrative Questions and Class Announcements
Topic: Naming Coordination Compounds [ENDORSED]
Replies: 2
Views: 725

### Re: Naming Coordination Compounds[ENDORSED]

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...
Mon Nov 14, 2016 11:40 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Coordination Number for (en) [ENDORSED]
Replies: 6
Views: 1192

### Re: Coordination Number for (en)[ENDORSED]

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.
Mon Nov 14, 2016 8:11 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Coordination Numbers [ENDORSED]
Replies: 4
Views: 559

### Re: Coordination Numbers[ENDORSED]

Oxalato is 2 because it is bidentate
Sun Nov 13, 2016 8:58 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: HW 11.41
Replies: 1
Views: 239

### HW 11.41

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...
Wed Nov 09, 2016 10:33 pm
Forum: *Molecular Orbital Theory (Bond Order, Diamagnetism, Paramagnetism)
Topic: MO diagrams and Bond order [ENDORSED]
Replies: 2
Views: 495

### Re: MO diagrams and Bond order[ENDORSED]

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 ...
Sun Nov 06, 2016 9:30 pm
Forum: Hybridization
Topic: 4.95 Composition of Bonds/Hybridization of Lone Pairs
Replies: 1
Views: 471

### 4.95 Composition of Bonds/Hybridization of Lone Pairs

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...
Fri Nov 04, 2016 12:31 pm
Forum: *Molecular Orbital Theory Applied To Transition Metals
Topic: MO Theory
Replies: 1
Views: 574

### Re: MO Theory

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...
Thu Nov 03, 2016 12:34 pm
Forum: *Molecular Orbital Theory (Bond Order, Diamagnetism, Paramagnetism)
Topic: Molecular Orbital Configuration: px/py?
Replies: 1
Views: 330

### Molecular Orbital Configuration: px/py?

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

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

Wed Oct 26, 2016 3:43 pm
Forum: Trends in The Periodic Table
Topic: Grouping of Valence Electrons [ENDORSED]
Replies: 3
Views: 681

### Re: Grouping of Valence Electrons[ENDORSED]

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...
Tue Oct 25, 2016 11:58 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Electron configuration with noble gas abbrevation
Replies: 2
Views: 527

### Re: Electron configuration with noble gas abbrevation

Both abbreviations are correct; though when you do the problem you'll realize that you can't use Argon for all of them.
Tue Oct 25, 2016 11:53 pm
Forum: Administrative Questions and Class Announcements
Topic: Quiz Info
Replies: 2
Views: 399

### Re: Quiz Info

If you think you missed some of the material, you can watch Professor Lavelle's lectures on bruincast:
http://bruincast.ucla.edu
Fri Oct 21, 2016 3:56 pm
Forum: Trends in The Periodic Table
Topic: What is the difference between electron affinity and electronegativity?
Replies: 3
Views: 700

### Re: What is the difference between electron affinity and electronegativity?

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...
Thu Oct 20, 2016 1:33 pm
Forum: Properties of Light
Topic: Quiz 1 #5 [ENDORSED]
Replies: 1
Views: 329

### Quiz 1 #5[ENDORSED]

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...
Tue Oct 18, 2016 10:04 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: d and s orbitals
Replies: 2
Views: 320

### Re: d and s orbitals

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.
Tue Oct 18, 2016 6:04 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Ch. 2 Textbook Question 45d. [ENDORSED]
Replies: 1
Views: 367

### Re: Ch. 2 Textbook Question 45d.[ENDORSED]

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...
Fri Oct 14, 2016 4:57 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Problem 2.17
Replies: 2
Views: 311

### Re: Problem 2.17

I think Chem_Mod explained it pretty thoroughly, but this diagram helps for visualizing it:
Fri Oct 14, 2016 3:46 pm
Forum: Properties of Light
Topic: QUIZ 1 [ENDORSED]
Replies: 7
Views: 981

### Re: QUIZ 1[ENDORSED]

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.")
Sun Oct 09, 2016 10:54 pm
Forum: Bohr Frequency Condition, H-Atom , Atomic Spectroscopy
Topic: Fall 2014 Quiz Prep
Replies: 11
Views: 965

### Re: Fall 2014 Quiz Prep

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...
Sun Oct 09, 2016 10:44 pm
Forum: Bohr Frequency Condition, H-Atom , Atomic Spectroscopy
Topic: Fall 2014 Quiz Prep
Replies: 11
Views: 965

### Re: Fall 2014 Quiz Prep

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.
Sun Oct 09, 2016 10:30 pm
Topic: Examples of Black Body Radiation
Replies: 4
Views: 2826

### Re: Examples of Black Body Radiation

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...
Sun Oct 09, 2016 1:15 am
Forum: Molarity, Solutions, Dilutions
Topic: Quiz 1 Prep: Fall 2015, Question #1 [ENDORSED]
Replies: 1
Views: 314

### Re: Quiz 1 Prep: Fall 2015, Question #1[ENDORSED]

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...
Thu Sep 29, 2016 12:18 pm
Forum: Accuracy, Precision, Mole, Other Definitions
Topic: Fundamentals E.21: Amount in moles vs Number of molecules vs Formula units
Replies: 4
Views: 1748

### Re: Fundamentals E.21: Amount in moles vs Number of molecules vs Formula units

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).