## Search found 55 matches

Sat Mar 17, 2018 9:35 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Pre equilibrium
Replies: 5
Views: 443

### Re: Pre equilibrium

Yes, both K and k/k' work, but k/k' would be the more accurate answer.
Sat Mar 17, 2018 9:31 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell Potential vs Standard Cell Potential
Replies: 2
Views: 202

### Re: Cell Potential vs Standard Cell Potential

Correct me if I'm wrong, but as far as I know, you are correct. The cell potential/voltage can never be larger than the standard cell potential because the standard cell potential describes the maximum potential difference.
Mon Mar 12, 2018 12:18 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Slow reaction
Replies: 2
Views: 241

### Re: Slow reaction

Also remember that Dr. Lavelle outlined 2 methods to write the rate law of a mechanism: steady-state approximations and pre-equilibrium conditions. He told us that we would be using the pre-equilibrium method in 14B, so I think approaching this problem with that method will make it a bit easier.
Mon Mar 12, 2018 12:12 pm
Forum: *Alcohols
Topic: functional groups [ENDORSED]
Replies: 4
Views: 769

### Re: functional groups[ENDORSED]

Are there any homework problems to go with this topic? I couldn't find a lecture outline for organic chemistry on the class webpage.
Mon Mar 12, 2018 12:11 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Catalyst Example from lecture 3/12
Replies: 3
Views: 237

### Re: Catalyst Example from lecture 3/12

Yes, NO(g) was the catalyst. It was used up in the first step of the mechanism, but then re-produced in the second step of the mechanism.
Mon Mar 05, 2018 4:06 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Termolecular Reactions
Replies: 2
Views: 239

### Re: Termolecular Reactions

Yes, Dr. Lavelle probably just wants us to know that the concepts between unimolecular, bimolecular, and termolecular reactions are similar. I also don't think we'll need to know this for test 3, because it falls under "Reaction Mechanisms."
Mon Mar 05, 2018 3:56 pm
Forum: General Rate Laws
Topic: Test #3 Problems
Replies: 3
Views: 291

### Re: Test #3 Problems

I also think you can do problems 83, 99, 103, and 105 for more practice!
Mon Mar 05, 2018 3:48 pm
Forum: Balancing Redox Reactions
Topic: Reducing power
Replies: 2
Views: 175

### Re: Reducing power

For these kinds of problems, use the chart for standard reduction potentials. The larger (more positive) the standard potential, the easier it is to be reduced. The species that is reduced is also the oxidizing agent. Hope this helps!
Mon Feb 26, 2018 3:47 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Standard Potential
Replies: 4
Views: 334

### Re: Standard Potential

I was wondering about this too, but the textbook doesn't mention anything about pH as an aspect of standard conditions for electrochemical cells.
Mon Feb 26, 2018 3:42 pm
Forum: General Rate Laws
Topic: Differential rate law
Replies: 3
Views: 222

### Re: Differential rate law

Deriving the integrated rate law from the differential rate law is more useful to us because we can plot ln[A] (and the natural logs of any other reactants present) against time and figure out the overall order of a reaction. As we discussed today, if the graph is linear, you know you are dealing wi...
Mon Feb 26, 2018 3:37 pm
Forum: General Rate Laws
Topic: Unique Rate of Reactions
Replies: 3
Views: 238

### Re: Unique Rate of Reactions

Keep in mind that we calculate unique rates because the average rates can be different depending on the time interval you are calculating for. When you calculate the unique rate, the coefficient is taken into account, so you have a more accurate answer.
Mon Feb 19, 2018 8:19 pm
Forum: Balancing Redox Reactions
Topic: Test 2
Replies: 3
Views: 248

### Re: Test 2

In terms of concepts, I'm sure we will have to know how the Gibbs free energy relates to the cell potential. Section 14.4 is especially helpful for this part.
Mon Feb 19, 2018 8:13 pm
Forum: Balancing Redox Reactions
Topic: Oxidation Numbers
Replies: 6
Views: 434

### Re: Oxidation Numbers

I think O2 has an oxidation number of 0 because it is in its elemental form. This is the same case for other elemental forms such as H2 or Fe.
Mon Feb 19, 2018 8:07 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Word equations
Replies: 2
Views: 231

### Re: Word equations

Dr. Lavelle usually gives us the molecular formulas for complicated molecules, but don't depend on it for every reactant/product. We may have to find the compound by its name (covered in 14A).
Mon Feb 12, 2018 3:27 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: TEXTBOOK 11.83
Replies: 1
Views: 153

### Re: TEXTBOOK 11.83

You can calculate the deltaG with the Gibbs Free Energy of Formations in Appendix 2 only for questions involving standard conditions. In other words, for 11.83, you can only use Appendix 2 for 25 degrees.
Mon Feb 12, 2018 2:48 pm
Forum: Calculating Work of Expansion
Topic: 8.16 Constant Volume
Replies: 1
Views: 147

### Re: 8.16 Constant Volume

This is what I got when I tried the question: 1. True only when heat isn't transferred to/from the system (q=0) 2. Always true (because volume isn't changing) 3. Always false (work=0 because delta V=0) 4. Always true (because work=0) 5. True only when heat isn't transferred to/from the system (q=0);...
Mon Feb 12, 2018 2:40 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Test 1, #7 and General Question about Heat Transfer
Replies: 2
Views: 181

### Re: Test 1, #7 and General Question about Heat Transfer

Conceptually, that would make sense, but when you're solving the problem, I don't think it really makes a difference (because you can technically just multiply both sides of your equation by -1 and get the other version).
Mon Feb 05, 2018 8:44 pm
Forum: Administrative Questions and Class Announcements
Topic: Midterm
Replies: 5
Views: 420

### Re: Midterm

Dr. Lavelle confirmed in lecture today that the midterm would cover only chapters 8, 9, and 11. Electrochemistry will not be on the midterm.
Mon Feb 05, 2018 8:41 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Salt Bridges
Replies: 2
Views: 175

### Re: Salt Bridges

Salts that are poorly soluble are not effective in a salt bridge because the ions would react to form a precipitate, instead of balancing the charges.
Mon Feb 05, 2018 8:34 pm
Forum: Balancing Redox Reactions
Topic: Salt Bridges
Replies: 14
Views: 528

### Re: Salt Bridges

Without a salt bridge to manage the unequal charges, the electrons flowing to the cathode will eventually stop because the buildup will cause electron-electron repulsion. Likewise, the anode would have such a high positive charge that it will want to hold on to the negative electrons.
Wed Jan 31, 2018 10:32 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Definition of Entropy
Replies: 10
Views: 561

### Re: Definition of Entropy

To add some foundation, entropy is an extensive property (it depends on the number of particles we are calculating for), so when the number of particles increases, the degeneracy increases as well (W = 2^n). Thus, based on the entropy equation, S = kb (ln W), as W increases, so does S.
Wed Jan 31, 2018 10:16 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Relationship between ΔG⁰ AND K
Replies: 2
Views: 180

### Relationship between ΔG⁰ AND K

Hi,

Can someone explain the relationship between ΔG⁰ AND K? In other words, how do we know that we can relate the equilibrium constant directly to changes in enthalpy and entropy?

Wed Jan 31, 2018 10:11 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Gibbs free energy of reaction vs. standard Gibbs free energy of reaction
Replies: 2
Views: 273

### Re: Gibbs free energy of reaction vs. standard Gibbs free energy of reaction

(ΔG) = (ΔH⁰) - (T)ΔS is the equation you use to calculate Gibbs free energy under standard conditions. You use this equation when the free energy change associated with the formation of the substance from the elements in their most stable forms exist under the "standard conditions" of 1 at...
Tue Jan 23, 2018 2:05 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: 3/2RT
Replies: 4
Views: 312

### Re: 3/2RT

TarynLane2J wrote:I don't think so because I don't remember going over it in class but I could be wrong

I know for a fact that we didn't go over the formula in Dr. Lavelle's 11am lecture, but I'm not sure if it was brought up in the 1pm lecture.
Mon Jan 22, 2018 10:15 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Example 8.8
Replies: 1
Views: 103

### Example 8.8

For example 8.8 in the textbook, can someone explain why the answer has 4 significant figures? Since the temperature we use (298 K) has the least number of significant figures (3), shouldn't the answer have 3 sig figs as well?
Mon Jan 22, 2018 9:31 pm
Forum: Phase Changes & Related Calculations
Topic: Heating Capacity from Heating Curve
Replies: 2
Views: 175

### Heating Capacity from Heating Curve

Hi, I was reading the textbook, and on page 287, it says "The steeper the slope of a heating curve, the lower is the heat capacity." Can someone explain why this relationship is true?

Sun Jan 21, 2018 5:14 pm
Forum: Calculating Work of Expansion
Topic: w=-P*deltaV
Replies: 3
Views: 183

### Re: w=-P*deltaV

Take a look at this response; it really helped me understand the conceptual reasoning behind it:

viewtopic.php?t=2341
Wed Jan 17, 2018 9:32 pm
Forum: Phase Changes & Related Calculations
Topic: Thermochemistry Sections in Ch 8
Replies: 2
Views: 217

### Thermochemistry Sections in Ch 8

Hi, I know Dr. Lavelle mentioned that students tend to understand the chapter 8 material if they flip the order of the chapter and read the thermochemistry sections of the text before the thermodynamics sections. Which specific section of chapter 8 starts the thermochemistry part?

Thanks so much!
Wed Jan 17, 2018 9:29 pm
Forum: Student Social/Study Group
Topic: Post All Chemistry Jokes Here
Replies: 8047
Views: 1412846

### Re: Post All Chemistry Jokes Here

Here's a haiku on Aluminum:

Spent kindergarten
One i or two i's?
Wed Jan 17, 2018 9:15 pm
Forum: Phase Changes & Related Calculations
Topic: Compression (Question 8.3)
Replies: 4
Views: 177

### Re: Compression (Question 8.3)

Can someone explain what Dr. Lavelle was trying to clarify with the tire example and the compression of the bike pump he was talking about in class today? I don't think I completely followed it.

Wed Jan 10, 2018 5:51 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: tables in chapter 8
Replies: 5
Views: 265

### Re: tables in chapter 8

I'm pretty sure the problem will give us whatever values we need, since Dr. Lavelle usually doesn't expect us to memorize specific values from tables.
Mon Jan 08, 2018 6:13 pm
Forum: Administrative Questions and Class Announcements
Topic: Having the textbook in discussion
Replies: 2
Views: 200

### Re: Having the textbook in discussion

The way my discussion worked in chem 14A was that the TA would put up specific homework problems on the board for us to work on in small groups. So I agree that the PDF is good enough, since you'll just be using it to refer to problems in the textbook!
Mon Jan 08, 2018 5:44 pm
Forum: *Making Buffers & Calculating Buffer pH (Henderson-Hasselbalch Equation)
Topic: aqueous equilibria
Replies: 2
Views: 651

### Re: aqueous equilibria

I don't see aqueous equilibria on the Chem 14B syllabus, so I don't think we'll go over it this quarter.
Mon Jan 08, 2018 5:18 pm
Forum: Administrative Questions and Class Announcements
Topic: Week 1 HW
Replies: 3
Views: 215

### Re: Week 1 HW

In lecture one, I believe Dr. Lavelle said that for Week 1, we turn in homework during Friday's lecture.
Sat Dec 09, 2017 10:54 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Test 4, #3
Replies: 1
Views: 327

### Re: Test 4, #3

The value of K for the forward reaction is 0.23, so the K value for the reverse reaction would be the reciprocal of that (1/0.23), which equals 4.3.
Wed Dec 06, 2017 5:16 pm
Forum: Octet Exceptions
Topic: Octet Exceptions
Replies: 8
Views: 607

### Re: Octet Exceptions

Could someone please summarize all the exceptions we need to be aware of for chemical bonds? I know of: 1. S, P, Xe, and the halogens can have an expanded octet 2. Be and B can have less than 8 3. Radicals are unpaired electrons Is there anything I am missing? Another thing to remember is that Hydr...
Wed Dec 06, 2017 5:12 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Will we be given Table 11.2?
Replies: 2
Views: 202

### Re: Will we be given Table 11.2?

I believe Dr. Lavelle said that all values we need will be given to us, and do not have to be memorized.
Wed Nov 29, 2017 12:44 pm
Forum: *Molecular Orbital Theory (Bond Order, Diamagnetism, Paramagnetism)
Topic: Test 4 [ENDORSED]
Replies: 4
Views: 413

### Re: Test 4[ENDORSED]

The syllabus states that we only have to read 11.1 to 11.7. I don't think we will be tested on this topic because diamagnetism and paramagnetism falls under 11.8.
Mon Nov 27, 2017 11:24 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Product/Reactant Formation
Replies: 1
Views: 111

### Re: Product/Reactant Formation

The answer is D, because at equilibrium, the reactants are still forming products, and the products are still forming reactants. In other words, at equilibrium, the ratio [P]/[R] doesn't change, but the individual concentrations are still changing at equal rates. Hope that makes sense.
Thu Nov 23, 2017 8:57 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: K and Kc
Replies: 7
Views: 471

### Re: K and Kc

You can use Q for any reversible reaction that has not reached equilibrium yet. This value of Q can be compared to the value of K to decide whether the forward or the reverse reaction is favored. If Q>K, the reverse reaction is favored, and if Q>K, the forward reaction is favored. Finally if Q=K, th...
Wed Nov 22, 2017 5:34 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Aqueous in Kp?
Replies: 2
Views: 5997

### Re: Aqueous in Kp?

I think you only solve for Kp if the compounds in the given reaction are all gases, or if you are given the partial pressures (instead of concentrations) of every compound. Also, solids and liquids are the only states that are omitted from K calculations (Kc and Kp); thus the aqueous reactant/produc...
Wed Nov 15, 2017 9:37 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: trigonal planar vs. t-shaped
Replies: 5
Views: 1305

### Re: trigonal planar vs. t-shaped

Nope, a molecule with 4 regions of electron density, 2 bonded and 2 lone pairs, would be bent.
Wed Nov 15, 2017 9:32 pm
Forum: Photoelectric Effect
Topic: Midterm question [ENDORSED]
Replies: 6
Views: 481

### Re: Midterm question[ENDORSED]

That statement would only be true given that the frequency is high enough to eject the electron from the metal surface in the first place. For example, it could be possible that the increase in frequency (and thus energy) of the incident light is still less than the work function of the metal.
Wed Nov 08, 2017 5:26 pm
Forum: Bond Lengths & Energies
Topic: Homework Problem 3.87
Replies: 5
Views: 502

### Re: Homework Problem 3.87

CF4 has the shortest bond because F is the most electronegative among F, Cl, and Br. This basically means that F pulls on its surrounding electrons with more strength than Cl or Br can, making it so that in CF4, the atoms are all very close together (aka short bond lengths).
Wed Nov 08, 2017 5:18 pm
Forum: Lewis Structures
Topic: where do you place the unpaired electron of a free radical?
Replies: 2
Views: 299

### Re: where do you place the unpaired electron of a free radical?

I think Chlorine gets the unpaired electron because Oxygen is more electronegative (meaning Oxygen wants its octet more than Chlorine does). Thus you would first complete the octet for Oxygen before distributing the electrons around Chlorine.
Thu Nov 02, 2017 2:03 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Electron configurations of noble gases
Replies: 1
Views: 132

### Re: Electron configurations of noble gases

Both are technically correct, but the reason we usually write it the second way (neon = [He]2s2 2p6) is so that we can see how many valence electrons are in the outermost shell.
Thu Nov 02, 2017 1:54 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Drawing the configuration
Replies: 4
Views: 268

### Re: Drawing the configuration

Nope it does not matter, although you have to be consistent. For example, if you start filling spin-ups in the 2px orbital, you have to continue filling spin-ups in the 2py and 2pz orbitals until you need to pair all the spins in the p-orbitals to complete the configuration.
Wed Oct 25, 2017 10:18 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Sequen of Orbitals
Replies: 4
Views: 329

### Re: Sequen of Orbitals

[Xe] 4f^14 5s^2 5p^6 5d^4 6s^2 wouldn't make sense in this case because the "[Xe]" already accounts for the electrons in the 5s and 5p orbitals. Thus the electron configuration would simply be [Xe] 4f^14 5d^4 6s^2.
Wed Oct 25, 2017 10:03 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Electron Configuration
Replies: 2
Views: 231

### Re: Electron Configuration

To clarify, the textbook says that when the 3d subshell is empty, its energy is greater than than that of the 4s subshell; thus for elements like Potassium (K), the s orbital is filled first. However, when the 3d orbital is occupied by one or more electrons, its energy becomes lower than the energy ...
Wed Oct 18, 2017 4:03 pm
Topic: Black body for quiz 2 [ENDORSED]
Replies: 11
Views: 1038

### Re: Black body for quiz 2[ENDORSED]

I know we don't have to know Wien's law for Test 2, but I had a question regarding significant digits for the textbook's example problem 1.3 displaying Wien's law: "The maximum intensity of solar radiation occurs at 490. nm. What is the temperature of the surface of the Sun?" The only valu...
Wed Oct 18, 2017 3:51 pm
Forum: Properties of Light
Topic: Homework Week 3
Replies: 3
Views: 241

### Re: Homework Week 3

When I asked my TA, she said that the problems from Chapter One can still be turned in for credit this week. :)
Thu Oct 12, 2017 2:03 pm
Forum: Bohr Frequency Condition, H-Atom , Atomic Spectroscopy
Topic: Bohr Frequency Condition
Replies: 2
Views: 7005

### Re: Bohr Frequency Condition

I'm a bit confused at the basics. Let's say an electron has been excited to the n=3 state from its original n=1 state. When it's returning to its original energy level, does it stop at n = 2 to give off radiation and then go to n = 1 to give off radiation? Or does the electron go directly from n = 3...
Thu Oct 12, 2017 9:23 am
Forum: Photoelectric Effect
Topic: Photoelectric Effect
Replies: 8
Views: 487

### Re: Photoelectric Effect

I think I understand the concept of the photoelectric effect, but I still don't completely understand why it can occur only in a vacuum. Can someone clarify that for me? Thanks!
Thu Oct 05, 2017 12:20 am
Forum: Molarity, Solutions, Dilutions
Topic: Question L.35
Replies: 2
Views: 318

### Re: Question L.35

From my understanding, you have to work backwards from the third reaction. It is given that you have 2.50 t of NaBr. Using stoichiometric coefficients, you can find how much Fe3Br8 was needed to produce the 2.50 tons of NaBr. Once you find out how much Fe3Br8 you have, you can turn your attention to...
Wed Oct 04, 2017 11:52 pm
Forum: General Science Questions
Topic: Sig Figs
Replies: 9
Views: 1086

### Re: Sig Figs

If no values are specified in the problem, I would use the molar masses and round to 4 sig figs, since the periodic table allows for that precision. AKA whenever possible, it's best to be more precise.