CHEMISTRY COMMUNITY

Mary Gallo 1G

In kinetics, lowercase k represents the rate constant for a reaction. A larger rate constant means that the reaction will proceed at a faster rate. A smaller k means that the rate of the reaction is slower. In chemical equilibrium, we use uppercase K, which represents the equilibrium constant. K = [...

Mary Gallo 1G

Which problem is this? I believe this reaction mechanism is from Question 13 in the W9/W10 Sapling homework. In reaction mechanisms, a catalyst is a molecule that is consumed and then produced. In contrast, an intermediate is a molecule that is produced and then consumed. By this logic, would H2O(l...

Mary Gallo 1G

For the reversible, one‑step reaction: A+A⥫⥬ B+C the rate constant for the forward reaction, k1, is 285 L⋅mol−1⋅min−1 and the rate constant for the reverse reaction, k1', is 377 L⋅mol−1⋅min−1 at a given temperature. The activation energy for the forward reaction is 44.6 kJ⋅mol−1, whereas the activat...

Mary Gallo 1G

Thank you so much!

Mary Gallo 1G

The reaction A+B⟶C+D rate=k[A][B]^2 has an initial rate of 0.0860 M/s.
What will the initial rate be if [A] is halved and [B] is tripled?

What set-up should we use to solve this problem? Thank you!

Mary Gallo 1G

Dr. Lavelle sent some resources in an email that work through using half-reactions and balancing redox reactions: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Balancing_Redox_Reactions_Acidic_Conditions.pdf https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Balanc...

Mary Gallo 1G

In the Friday lecture, Dr. Lavelle used the example A+B+C --> P with rate = k [A]^N [B]^M [C]^L . Then we found that the pseudo rate law was k' [A]^N . I was wondering if we use k for regular rate laws and k' for pseudo rate laws? Or what the difference between k and k' is in this scenario. Thank you!

Mary Gallo 1G

The slowest step of a reaction is considered the determining state because the reaction will not finish until the slowest step is complete. Even if other steps of the reaction are faster, the reaction relies on the slowest rate to be finished.

Mary Gallo 1G

To study for the final I am going to try to attend a variety of UA and TA sessions this week to practice material and review old concepts. Additionally, I think the problems from the textbook are the best practice for exams so I plan to go through ones that I am unsure of.

Mary Gallo 1G

Rate constants are given for a reaction at a specific temperature. Thus, changing the temperature of the reaction would alter the rate law. Rate constants can also be changed by the removal or addition of a catalyst. This would alter the activation energy of the reaction and, subsequently, the rate ...

Mary Gallo 1G

It depends on how you are calculating E°cell. One option is to take: E°red (each option) - E°red (Zn to Zn2+) Subtracting the standard reduction potential of Zn to Zn2+ accounts for that reaction actually being oxidation. The other option is to take: E°red (each option) and + [ - E°red (Zn to Zn2+)]...

Mary Gallo 1G

Thank you, Shannon! I really appreciate it.

Mary Gallo 1G

Hi! I am having trouble finding the half-reactions for this problem:
The unbalanced equation for the reaction of gold with aqua regia is given. Add the stoichiometric coefficients to the equation to balance it.
Au(s) + HNO3(aq) + HCl(aq) --> HAuCl4(aq) + NO2(g) + H2O(l)

Thank you!

Mary Gallo 1G

A C-, or minimum passing grade, in this class would be receiving 200/400 points.

Mary Gallo 1G

I was having trouble with this problem and your walkthrough was super helpful! Thank you so much.

Mary Gallo 1G

Work done by the system is negative and work done on the system is positive. When a system does work, it is losing energy, thus the sign is negative. When work is being done on the system there is energy being transferred to it, making the work of the system positive.

Mary Gallo 1G

Open systems allow both energy and matter to be exchanged with the surroundings. Closed systems only allow energy transfer with the surroundings. Finally, isolated systems do not allow anything to be transferred with the surroundings.

Mary Gallo 1G

I am a self-proclaimed "stress baker", so I decided to bake some chocolate cookies this weekend. Baking takes my mind off of challenging academics and always comes with a tasty reward! :)

Mary Gallo 1G

Those modules are optional for the course. They teach you how to use certain tools in Sapling when answering homework questions.

Mary Gallo 1G

You can either view your grades on the Grades tab of CCLE or go to MyUCLA.

Mary Gallo 1G

Midterm 2 will cover Outline 3 and Outline 4, but Outline 3 will only include topics that were not on Midterm 1 (so no ways of calculating reaction enthalpies).

Mary Gallo 1G

First you will need to find ∆G of the reaction, where ∆G = sum∆G(products) - sum∆G(reactants) (Make sure if you multiply the ∆G of each molecule by the amount of moles of it there are in the reaction when summing them.) Next you will use the equation: ∆G= - RT ln(K) Which can be arranged as: K=e^(-∆...

Mary Gallo 1G

I agree with others as I found the final significantly harder than the midterms. I prepared the same way for both types of exams (except a lot more for the final), but it was still very challenging. I think I will spend more time practicing questions from the textbook this quarter.

Mary Gallo 1G

You can go to your TA's Office Hours to review your midterm!

Mary Gallo 1G

I find that when I am struggling with certain topics in this class, the UA sessions always help me understand them better. If you need help conceptually I suggest the Step-Up sessions and if you're looking for practice problems I would go to the Workshops!

Mary Gallo 1G

Nevermind, I redid all my calculations and got the correct answer. Not sure where I slipped up but thank you so much for the help!

Mary Gallo 1G

Hi! So for the first step, the volume remains constant so there is 0 work being done. In the second step you could use the w=-(Pexternal)(deltaV). Your answer will come out with the units of L x atm, so you want to convert that into Joules. Then since the work from the first step was 0, your total ...

Mary Gallo 1G

Hi! So for the first step, the volume remains constant so there is 0 work being done. In the second step you could use the w=-(Pexternal)(deltaV). Your answer will come out with the units of L x atm, so you want to convert that into Joules. Then since the work from the first step was 0, your total ...

Mary Gallo 1G

Thank you so much for the help!

Mary Gallo 1G

Hi! I am unsure how to go about solving question #12 from the Sapling homework. What equations should we use to solve this? Part a: At constant volume, the heat of combustion of a particular compound, compound A, is −3761.0 kJ/mol. When 1.527 g of compound A (molar mass=120.79 g/mol) is burned in a ...

Mary Gallo 1G

In addition to everyone else, I think it is helpful to write down the units of each thing in your calculations. This ensures not only that you choose the correct R value, but that your answer is also solved in the right units.

Mary Gallo 1G

If you combine 230.0 mL of water at 25.00 C and 100.0 mL of water at 95.00 C, what is the final temperature of the mixture? Use 1.00 g/mL as the density of water.

What equation should we be using to solve this problem?
Thank you!

Mary Gallo 1G

I had the same issue with this problem, thank you! Could someone please explain what Kekule structures are? I wanted to make a post about this but I thought it would be helpful to add it onto this thread. Hi! Kekule structures were the original structures proposed for the structure of benzene. This...

Mary Gallo 1G

Thank you for the guidance! A follow up question- So to "calculate the lowering in molar energy" would we do the delta H of the structure with resonance subtracted by the delta H of the structure without resonance? I got 3108-2880 = 228 Is this correct? Does it mean that there is a 228 kJ...

Mary Gallo 1G

Dr. Lavelle just sent an email saying there will be 12-14 questions on the midterm.
For more information about the midterm you can look here:
https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Chem14B_Midterm1.pdf
Hope this helps!

Mary Gallo 1G

Hi! In a benzene structure without resonance, there would be 3 C-C double bonds and 3 C-C single bonds. For the benzene structure with resonance, there are 6 resonance-stabilized bonds. So for this problem you are calculating the difference in energy between the two types of benzene structures. You ...

Mary Gallo 1G

Thank you so much! I wasn't sure how to solve the last bit, your explanation really helped!

Mary Gallo 1G

Hi! I am having trouble understanding how to do problem 4D.23 from the textbook: Calculate the standard enthalpy of formation of dinitrogen pentoxide from the following data: 2NO(g)+O2(g) --> 2NO2(g) ∆H = +498.4 kJ 4NO2(g)+O2(g) --> 2N2O5(g) ∆H = -110.2 kJ and the from the standard enthalpy of forma...

Mary Gallo 1G

Hi! To further clarify, when steam (vapor) touches the skin, it goes through the phase change of condensation, which is an exothermic reaction (the opposite of vaporization). When a reaction is exothermic, ∆H is a negative value. This release of energy goes into the skin all at once during the phase...

Mary Gallo 1G

To add on, pH tells us how acidic or basic a solution is. A solution that has a low pH is more acidic , and this is due to a higher concentration of H30+ ions. A solution that has a high pH is more basic due to a higher concentration of OH- ions. When we take the negative log of the H30+ concentrati...

Mary Gallo 1G

I would suggest using concentrations in ICE boxes because we use concentrations when working with K values, pH and pOH, or compositions of solutions.

Mary Gallo 1G

Thank you so much for the help!

Mary Gallo 1G

Calculate the pH of (a) 0.19M NH4Cl(aq); (b) 0.055M AlCl3(aq). I understand how to solve part a, but I am struggling with part b. I am not sure how we get the dissociation equation shown in the solutions manual as well as where the Ka value comes from. I would appreciate some help with this problem,...

Mary Gallo 1G

Hi! Although these images come from the solution pages of these two problems, I find them really helpful in understanding these problems conceptually: Screen Shot 2021-01-14 at 10.55.08 AM.png For acidic solutions, the diagram is saying that when the pH is less than the pKa the solution is neutral b...

Mary Gallo 1G

Hi! Could someone explain why we calculate [BH+]/([BH+][B]) instead [BH+]/[B] for the last step? I'm trying to understand it conceptually. Thank you!

Mary Gallo 1G

Hi! Dr. Lavelle just sent an email saying there will be no class this Monday.

Mary Gallo 1G

Hi! I had very little background in chemistry when I started 14A with Dr. Lavelle last quarter. I was nervous about falling behind, but Dr. Lavelle always made sure to go over the basics/review fundamentals in lectures. Initially I was not utilizing the UA Sessions for extra help. However, I would h...

Mary Gallo 1G

When Q<K, this means that there is a greater concentration of reactants than there would be at equilibrium. Therefore, the reaction would actually proceed right, towards the products, to achieve equilibrium.

Mary Gallo 1G

I find that completing practice problems is the best way for the material to stick. They require you to do critical thinking, but eventually the problems become almost mechanic. I would suggest doing all of the textbook problems from the outlines, as they best reflect test material. If you still wan...

Mary Gallo 1G

Hi! To add on, you would not want to pick a value for x that is greater than your initial concentrations because when you factor this in to find the concentrations at equilibrium, you would get a negative value. A negative concentration value would not make sense for the problem, so always choose th...

Mary Gallo 1G

Hello! I am also having difficulty with this question. I understand how to find x, but where do I factor in the new concentration of NO given in the problem (0.800 for my problem)? The feedback just says to determine the effect of adding NO and go from there, but I am unsure how to do this mathemat...

Mary Gallo 1G

I believe we are using concentrations because of the information we were given in this problem. Since we were given Kc in the problem, and not Kp, we do not have enough information to find the composition of the equilibrium mixture using partial pressures. Instead, we must use the given moles of PCl...

Mary Gallo 1G

I find that using visuals, similar to the ones Dr. Lavelle drew in lecture today, helps me understand this concept. For an endothermic reaction (where heat is absorbed) the ∆H is positive: Screen Shot 2021-01-08 at 12.38.07 PM.png Since a reaction favors the thing with the greatest enthalpy (H), hea...

Mary Gallo 1G

When K>1 then the chemical reaction will favor the products. However, K needs to be greater than 10^3 to strongly favor the products.

The reverse goes for K<1, where the chemical reaction favors the reactants. In this case, K needs to be less than 10^-3 to strongly favor the reactants.

Mary Gallo 1G

The most helpful things for me in 14A were doing the textbook problems. They were often similar to questions that would appear on exams so it made for good practice. I also would recommend going to UA sessions for extra help whenever you have the time. Step-Up sessions are useful if you need a littl...

Mary Gallo 1G

To solve this problem you are going to use formula: M1V1=M2V2. Your M1 is 0.20M and your V1 is 1.0L (as given in problem). Since 3.0L of water is added, your V2 will be 4.0L (1.0L+3.0L). Now you can solve for the final molarity value. To convert the molarity into moles you use the formula: M=mol/L T...

Mary Gallo 1G

A neutral H atom has 1 proton and 1 electron. When it is ionized to become H+ the electron is lost so there is just 1 proton remaining. Therefore, the H+ ion can be referred to as a proton.

Mary Gallo 1G

Is there some kind of sheet or something in the textbook that lists all of the strong acids/bases we should know for the final?

Mary Gallo 1G

Octahedral involves 6 bonded atoms attached to the central atom (AX6). Tetrahedral involves 4 bonded atoms attached to the central atom (AX4). Square Planar involves 4 bonded atoms and 2 lone pairs attached to the central atom (AX4E2). While both tetrahedral and square planar have 4 bonded atoms, sq...

Mary Gallo 1G

An acid will accept an electron (Lewis acid) / donate a proton (Bronstead acid). A base will donate an electron (Lewis base) / accept a proton (Bronstead base). So Lewis' discusses electrons and Bronsteads refer to protons. They are just two different ways of thinking about what will happen to a mol...

Mary Gallo 1G

Bronstead acids will donate protons and Bronstead bases will accept protons. I think that drawing the Lewis structure can be helpful to visualize what will happen (lone pairs, H+, OH-). Another thing to look out for that hasn't been mentioned is that OH- often indicates a Bronstead base because it i...

Mary Gallo 1G

Thank you both so much for the help! I really appreciate it.

Mary Gallo 1G

Are dentates just used to determine the coordination number of a compound? Also, what is a good way to determine if a dentate is mono, bi, tri, etc?

Mary Gallo 1G

Lewis acids accept electrons while lewis bases donate electrons. The acids will usually have a positive charge and want electrons to neutralize the charge. Lewis bases will usually be negatively charged and willing to donate the extra electrons.

Mary Gallo 1G

CHI3 is larger than CHF3 and therefore has stronger London Dispersion forces. There is more energy required to dissociate the atoms in CHI3 so its melting point is higher.

Mary Gallo 1G

One common square planar molecule is XeF4 (we saw this molecule in lecture). Due to the geometry of this molecule, the polar covalent bonds have dipoles that cancel (equal and opposite directions) and the electron lone pairs are on opposite sides of the central atom. This means that all charges canc...

Mary Gallo 1G

Yes! Hybridization correlates to the energy level of the atom's orbital that has hybridized electron(s).

Mary Gallo 1G

Delocalized pi bonds refer to molecules that have resonance structures. This means that the pi bond can be between different atoms in the same molecule as long as it has resonance.

Mary Gallo 1G

Yes, when there are 4 regions of electron density and 2 are bonding then the VSEPR shape can be referred to as bent or angular.

Mary Gallo 1G

I think I am going to review textbook and Sapling homework problems to review for the final. I also want to attend more UA sessions.

Mary Gallo 1G

I think that reviewing the textbook and Sapling hw problems from each section will be helpful in preparing for the final. This way you can focus on what questions you get wrong and rewatch lectures or read through any notes. I also want to take more advantage of the workshops we have available to us...

Mary Gallo 1G

In question 3F.13 in the textbook we are asked to choose the formation of CH2Cl2 atoms that would create the strongest bonds. I understand alignment II is correct because there are more dipole-dipole bonds where the negative dipole of Cl connects to the positive dipole of H. However, I was wondering...

Mary Gallo 1G

I really like doing the textbook problems to study for midterms. It is also nice to have the solutions in the back of the book so I can focus on certain topics if I am not getting those questions correct.

Mary Gallo 1G

Wow, I really like the balloon and dart analogy! Thank you both for the explanations, they really help!

Mary Gallo 1G

Chlorine has a lower ionization energy than oxygen because it is easier to remove an electron from a higher orbital. To ionize chlorine an electron is removed from 3p subshell, which is higher in energy than the electron, from the 2p subshell, that would be removed from oxygen. Chlorine also has ano...

Mary Gallo 1G

Dr. Lavelle prefers that we write the 3d subshell before the 4s subshell because once the 3d subshells starts filling it is lower in energy than 4s. This order of writing is also helpful if you are starting with the ground state electron configuration and finding the configuration for a cation. You ...

Mary Gallo 1G

Hi! Does anyone have a good way to remember what makes an atom/ion more polarizable and which atoms/ions have more polarizing power? I'm having a little trouble with those concepts.

Mary Gallo 1G

Question 17 of the W5/W6 Sapling homework asks: Which molecules exhibit only London (dispersion) forces? I understand how to determine that if a molecule is fully nonpolar it exhibits only London dispersion forces. However, with the other molecules, a knowledge of molecular structure is required to ...

Mary Gallo 1G

Thank you so much Crystal, this helped a ton! I really appreciate the picture too. :)

Mary Gallo 1G

From Sapling HW Question 13: Urea is an organic compound widely used as a fertilizer. Its solubility in water allows it to be made into aqueous fertilizer solutions and applied to crops in a spray. What is the maximum theoretical number of water molecules that one urea molecule can hydrogen bond wit...

Mary Gallo 1G

There are more electrons shared between atoms in a double bond. Therefore, the attractive forces between atoms with double bonds are stronger, thus pulling them in closer. Atoms that share a single bond have smaller attractive forces in comparison, and are usually longer.

Mary Gallo 1G

I do not have prior knowledge on determining molecular formulas from their names. I was also wondering about this, and if it is something we need to know how to do.

Mary Gallo 1G

I'm lucky to have my midterms decently spread out among my classes, but the pace is definitely picking up. The content we are starting to go over in class is surpassing my previous knowledge so I've been getting a little frantic about keeping up. The concepts after Midterm 1 are not sticking with me...

Mary Gallo 1G

The symbol is the Greek lowercase letter delta, δ, and is used to indicate partial charge. The atom with a greater electronegativity has a partial negative charge, and the atom with a lesser electronegativity has a partial positive charge.

Mary Gallo 1G

The Walking Dead, How to Get Away With Murder, and Psych (not on Netflix, but one of the most genius shows ever) are some of my favorites! I just started The Queen's Gambit and wasn't sure if I'd like it, but it's really captivating!

Mary Gallo 1G

Could someone explain a bit more on why atoms with high electron affinities would be more stable with a negative formal charge and vice versa? Thanks!

Mary Gallo 1G

Boron is in group 13 of the periodic table so it only has 3 valence electrons. In order to maintain a formal charge of zero, it can only form 3 covalent bonds. It is actually less stable to have boron follow the octet rule because its formal charge would no longer be zero.

Mary Gallo 1G

Adding on to above, when using the formal charge equation, make sure you are only applying it to one atom in the structure. So in terms of L (or the lone pairs) just count the dots around one specific atom because you are only calculating the formal charge for one atom at a time.

Mary Gallo 1G

Thank you both so much for the help! :)

Mary Gallo 1G

The sulfur atom in the sulfate ion can hold more than 8 electrons because atoms in period 3 or higher of the periodic table have d-orbitals in their valence shell. The octet guideline follows that the 8 electrons are filled in s and p orbitals (s2 p6 = 8 electrons). However, with the d-orbital in th...

Mary Gallo 1G

When drawing Lewis Structures, what is a good rule of thumb for determining the amount of electrons to use for each element?

Mary Gallo 1G

I think completing practice problems is one of the best ways to study because it mimics the calculations we have to complete on exams. This ensures that the steps for different kinds of problems are drilled into your mind. It's also a good idea to look over the lectures, or any notes you took on the...

Mary Gallo 1G

Do you think we will be able to see any questions we get wrong to review for the final?

Mary Gallo 1G

I believe that the Rydberg Equation can only be used for hydrogen atoms (or hydrogen-like species containing only one atom) because it stems from the Bohr model of a hydrogen atom. The Bohr model doesn't account for more than one electron or electron-electron interactions. Because He+ only has one e...

Mary Gallo 1G

I also keep clicking on my name to view my points but it is not showing. Where do we click again to view our points? If you click on "Quick links" at the top right of the page you can select "Your posts" from the drop down menu. This will take you to a page that shows all of the...

Mary Gallo 1G

I love making chemistry jokes, but I only do so periodically.

Mary Gallo 1G

I think doing art (or something that you're passionate about) is a really nice way of relaxing. I find that it is nice to be focused on something other than school for a while.

Mary Gallo 1G

Velocity measures the speed of an object (in a certain direction) and momentum is used to measure how much mass in in motion (as represented by mass x velocity). They both involve movement in a specified direction, however, momentum depends on mass whereas velocity is independent of mass.

Mary Gallo 1G

I was confused on the same thing but I got a lot of clarification on this forum:
https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=17&t=64748&sid=5cfe4a54eb50695e3b57ca5605232d1f
The responses were really helpful to me so I hope they can benefit you too!

Mary Gallo 1G

Dr. Lavelle has recommended that we print out the periodic table and sheet of equations and constants that he emailed to us because we are not required to memorize that information for the midterm. Both of these sheets can be found on his website as well. I also put a link to them below: https://lav...

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