CHEMISTRY COMMUNITY

Wasila Sun 2I

Also, remember daylight savings changes the time!!!!

Does anyone know how many questions there will be in those 90 minutes?

Wasila Sun 2I

Are there any other review worksheets like Marshmallow for Chem 14B? If so, can you send the link?

Wasila Sun 2I

I think for a single reaction we refer to anything not in the system as the universe or the surroundings, so for a single reaction the entropy can be negative. But, the overall trend of the universe's entropy over time will increase because it takes energy to make things more ordered but descending ...

Wasila Sun 2I

The nerst equation, calculating cell potential, calculating gibbs free energy in relation to E are mostly the equations we used in electrochemistry. All of them should be near each other lower on the equation sheet on Dr. Lavelle's website.

Wasila Sun 2I

Does k' have anything to do with the derivative of k, the rate constant, or is it just an indication of the pseudo rate constant?

Wasila Sun 2I

Maybe try going asking your TA during discussion section directly considering the email option isn't working, or maybe send a followup email. Also, I think other TA's can over your exam with you as well!

Wasila Sun 2I

I usually try to go to each of the TA sessions (at least one for each topic). Also some of my favorite UA's to go to are Justin and Rosa. I would recommend going to as many as possible because I think that's what really helped me in 14A feel confident with the material.

Wasila Sun 2I

I think Dr. Lavelle said that the units represented on the final will be equally culumative. But, I remember on the Chem 14A final that there was slightly more of the last unit on the final than any of the other units. I wouldn't worry too much about the amount of each unit on the test, and I will b...

Wasila Sun 2I

Here are the equations that I generally use:
Ka*Kb=Kw
pKa+pKb=pKw
Kw=[pOH][pH]

These principles are conceptually related to the conjugate seasaw, the idea that a stronger base will result in a weaker conjugate acid and a weaker base will result in a stronger conjugate acid and vice versa.

Wasila Sun 2I

The concentration of the anode is always less than the concentration of the cathode. So, when you further decrease the concentration of the anode by diluting it, the difference between the concentration of the anode and the cathode is greater. Therefore, when the concentration difference is greater,...

Wasila Sun 2I

Work done on the system is positive, and work done BY the system is negative. This is because when the system is doing work it spends/loses energy to expand against exteral pressure so it would be negative. The opposite is true because energy is then inputted into the system when work is being done ...

Wasila Sun 2I

Yes, you can post on Chemistry Community as much as you want. I posted around 55+ times last quarter in Chem 14A, and still received 50 points for posting. Chemistry Community is originally for learning and creating a community to answer your chemistry questions, so the more you use the greater bene...

Wasila Sun 2I

I have been trying to start lifting weights. I recently got a weight bench and a barbell from amazon in my basement so I've been learning the correct form for trying to deadlift and bench press.

Wasila Sun 2I

Has anyone gone to the drop in sessions and would you say they are similar to TA office hours or the step-up sessions? Do you think it is worth it to start going to those?

Wasila Sun 2I

I took Sociology 1 and even though the exams are a little bit difficult, I loved the content so much and it opened my eyes to social constructs that we take as fact. In addition, if you are a stem bio major like me then you will probably like taking MCDB 50- Stem Cells Ethics, Biology, and Politics....

Wasila Sun 2I

Q is calculated the same way as the equilibrium constant K. So we can figure this out by taking the Q=[products]/[reactants]. If Q=K then the system is at equilibrium.

Wasila Sun 2I

Unfortunately, I have another LS7B midterm on Monday, so I will be spending my weekend studying for that. :/ But, I plan on destressing a little but by watching reruns of Dance Moms in between.

Wasila Sun 2I

What does the term thermodynamically favorable mean? Is this just in reference to the Gibbs Free Energy value or is it something else?

Wasila Sun 2I

Ryan_Page_1J wrote:Delta G being negative means the reaction is spontaneous, and if it is spontaneous the product will be more stable than the reactants.

So this reaction is the formation of the compounds and not the breaking down of them? Like the delta G is in reference to formation?

Wasila Sun 2I

4J.13 Determine which of the following compounds are stable with respect to decomposition into their elements under standard conditions at 25 C (see Appendix 2A): (a) PCl5(g); (b) HCN(g);(c) NO(g); (d) SO2(g). From my understanding, the compounds with negative Gibbs free energy would be less stable ...

Wasila Sun 2I

Under what circumstances can we use the delta U=delta H+ w equation? Like what allows delta H to be interchangeable with q?

Wasila Sun 2I

I think I'm going to take Chem BL with Chem 14C next quarter because I read on Bruinwalk that you might need some concepts from 14B for 14BL, but that might not be true. I think Chem BL isn't too much to handle with 14C, hopefully.

Wasila Sun 2I

In a reversible pathway, the changes in q occur little by little so in order to find the entropy value of the whole thing we would need to add up these small entropy changes to find the total change in entropy (delta S). This is different in an irreversible pathway because you wouldn't need an integ...

Wasila Sun 2I

This is because gas has the capacity to significantly expand and be compressed. Liquids and solids on the other hand aren't affected as much by external pressure that would result in any non-negligible change in volume. For example if you exert a 1 atm amount of pressure on a solid, the volume doesn...

Wasila Sun 2I

Standard molar entropy is just the entropy for one mole of the substance. We don't use this as much because in thermodynamics we are often more concerned about the overall work, temperature, etc. than the molar quantities of a singular mole of a substance.

Wasila Sun 2I

About text anxiety, my best tip would be to breathe. I know this sounds simple, but I panicked a lot during the Chem 14A exams last quarter that hindered my ability to read the question without immediately trying to frantically solve it. My anxiety always lessens when I prepare for the exam harder a...

Wasila Sun 2I

What are the outlines/textbook problems that are going to be tested for Midterm 2? How much have we gone up to so far?

Wasila Sun 2I

They should be back in the next week or two weeks because they have to check to see if the problems have no mistakes and assess the difficulty of the exam relative to our class performance. In the meantime, I wouldn't stress about.

Wasila Sun 2I

q= delta H is used when you are heating a gas at constant pressure.

Wasila Sun 2I

In addition is V1 being initial and V2 being final (similar to the variables used in the dilution equation M1V1=M2V2), this is useful when calculating change in volume = V2-V1 for the work equation w=-P(change in V).

Wasila Sun 2I

To calculate the work we would use the formula work=- $\int_{V1}^{V2}P_{ex}dV$

Therefore in a reversible change, when the system is in equilibrium and the external pressure is 2atm the work is different from when there is a irreversible change and the external pressure is 1atm.

Wasila Sun 2I

For part a, we can use method 3, the standard enthalpy of formation, to find the standard enthalpy of this reaction and determine if it is exothermic or endothermic. \bigtriangleup H_{RXN}^{\circ}=\sum \bigtriangleup H_{f}^{\circ} (Products) - \sum \bigtriangleup H_{f}^{\circ} (Reactant...

Wasila Sun 2I

Determine the reaction enthalpy for the hydrogenation of ethyne to ethane, C2H2(g) + 2 H2(g) to C2H6(g), from the following data: delta H(C2H2, g) = -1300 kJ/mol; delta H (C2H6,g)= -1560 kJ/mol; delta H (H2,g)=-286 kJ/mol. So, I did sum or products-sum or reactants and got 312 kJ/mol, but the answer...

Wasila Sun 2I

Hi! Could someone explain where to start with this question? Hi Aaina, Enthalpy of a reaction is determined as the (sum of enthalpies for products) - (sum of enthalpies for reactants). Keep in mind that you should multiply the enthalpy of a substance by the number of moles they have in the reaction...

Wasila Sun 2I

Rachel Kwan 1B wrote:Do you mean Table 5G.2? I don't see Table 5E.2 mentioned in 5.39.

Haha, false alarm! Late night chem textbook problems have my eyes hallucinating I guess :)

Wasila Sun 2I

I can't seem to find Table 5E.2 mentioned in this problem 5.39 from Outline 1, can someone point to a page number or even better attach a picture of the table? Thanks!

Wasila Sun 2I

How do I go about reading table 5G.2 from the textbook (i.e. what do I need to know from it)? Why are there 3 different rows of values for each reaction? Overall, I'm confused on which values to use when doing a problem.

Wasila Sun 2I

Dr. Lavelle mentioned in today's lecture that a perfect system was required to use qsys+qsurr=0. Will we ever have to calculate using a non-perfect system? Should we generally assume a perfect system always?

Wasila Sun 2I

I think your confusion is in the units. -2.9 kJ= -2900 J because 1000 J equals 1 kJ. I think Dr. Lavelle said the 2900 number just as an alternative way to write your answer.

Wasila Sun 2I

Lavelle sent out an email saying that the only problems that we had to do from this week's Sapling to prepare for the exam are questions 4-8. The exam does only cover week 1-3 content only.

Wasila Sun 2I

Extensive properties (like mass and volume) depend on the amount of a substance, while intensive properties (like density) are more universal and applicable because they don't depend on the amount of matter present. Here, and extensive property is less useful because we have to know the amount of su...

Wasila Sun 2I

Halogens exist in many different states of matter. Fluorine and Chlorine are the only gases (Br, I, and At are not) and I assume that they would expand similarly to any other element. I do believe that halogens have much lower boiling and melting points because of their reactivity though.

Wasila Sun 2I

Adding an inert gas, that doesn't react to the reactants or products in the specified reaction, doesn't change the ratio of concentration of products to reactants (Kc value) or the ratio of partial pressure of products to reactants (Kp value). Therefore, increasing the pressure in this way doesn't i...

Wasila Sun 2I

What is the signficance of total pressure? Why would we use total pressure rather than just needing the pressures all of the products, for example?

Wasila Sun 2I

Are all reactants or products indicated with (l) or (s), pure liquids and solids? Do we assume they are?

Wasila Sun 2I

The equilibrium constant tells us the ratio of the concentration of products to the concentration of reactants when the reaction is in equilibrium. This is important because no matter whether the amount of the products or the amount of the reactants are changed this ratio remains the same and the re...

Wasila Sun 2I

I haven't been having this problem. But the first time that I logged into Dr. Lavelle's website for 14B, I still had the password for 14A autosaved so it didn't work. Check to make sure that the password you are typing in is for 14A and not 14B and that you are on the right class site.

Wasila Sun 2I

If it helps, shifts left or right don't actually mean the reaction is moving. Shifts only indicate whether the reactants or products will be favored going forward.

Wasila Sun 2I

There should be an option when going to using/paying for Sapling for 14B that says "use credit", and that will mean that you don't have to pay.

Wasila Sun 2I

Hello, I believe that for complex polynomials like that the K value (given) is usually very insignificant, meaning less than 10^-3. As a result, the ICE box which will have for example 0.15-X the value will become simply 0.15. This is because the approximation will result in an error that is less t...

Wasila Sun 2I

I currently think about calculation of % ionization and % protonation in the same way, with the same steps. Am I right or are they different, and should I think of them differently?

Wasila Sun 2I

For large polynomials (that can't be done with the quadratic formula) that can be difficult to solve, if there any optimum way to solve for X? I usually just graph them and find the X that way, but is there a better way or a way that Dr. Lavelle wants us to use?

Wasila Sun 2I

Other than H20, are there any other solutes of pure liquids that we should know? If we don't need to memorize it, how can you tell something will be a pure liquid or solute and insignificant to the calculations?

Wasila Sun 2I

From what I understood, a greater stability of the reactants means that a higher temperature is required to break them and form products. In Monday's lecture, Dr. Lavelle mentioned that he would go over temperature in relation to Kc in later lectures, so I don't think you need to know that now.

Wasila Sun 2I

I think what the reply meant was that if you were in both Chem 14A and Chem 14B then the total # of post you should have at the end of this class would be 100 (the posts last quarter do not count this quarter essentially), but since you didn't take Chem 14A with Lavelle your total posts at the end o...

Wasila Sun 2I

Are there groupme's made for Winter 2021 Chem 14B Lec 2 or Disc 2I? If so, would anyone be willing so send the links?

Wasila Sun 2I

Dr. Lavelle mentioned in yesterday's lecture that pure substances (specifically solids and liquids) have molar concentrations that don't change in a reaction. Why are pure liquids in a reaction insignificant? I know that they are large relative to the others, but can't their change still be measured...

Wasila Sun 2I

To do well on the exams, it is important to watch all of his recorded lectures, do the Sapling homework on time, complete all of the textbook problems given in the outline (many of them show back up on midterms), do the audio-visual modules he provides, and if you are having trouble with a certain s...

Wasila Sun 2I

Cl- Na+ as a group 1 ion isn't strong enough to pull a hydrogen from the oxygen in water. For our purposes, as states in the textbook, group 1 and group 2 ions generally aren't strong enough.

Wasila Sun 2I

It was quite spotty this morning, but it should be having no problem now. I think it was a system issue and not an individualized issue.

Wasila Sun 2I

Are we expected to know the oxidation states/numbers of common ligands like the ones listed https://sites.google.com/site/chempendix/ligands?

Wasila Sun 2I

What is the difference between formal charges of an atom in a compound/salt/molecule and its oxidation number?

Wasila Sun 2I

Dr. Lavelle mentioned in today's lecture that "salts that contain the conjugate base (anion) of a weak acid will raise pH". What is a conjugate base, and why is anion in parentheses next to it?

Wasila Sun 2I

I'm a freshman, so this is going to be my first finals week at UCLA. How does it work? Do we still have to go to classes? Is there anything I should know about it?

Wasila Sun 2I

More stable resonance structures occur when the each atom (other than He or H) has a full octet and there is a small number of formal charges. For example, a -1 molecule could have a resonance structure with formal charges of -2 and +1 on their atoms. This molecule has 2 atoms with formal charge, bu...

Wasila Sun 2I

Dipole arrows point towards the atom with partial negative charge and more electronegativity and points away from the atom with partial positive charge and less electronegativity. We can use these to solve problems when drawing things out by cancelling out arrows of dipole moments pointing in opposi...

Wasila Sun 2I

I believe that the bond angles that are fixed are ones involving no lone pairs, just bonding pairs (single, double, and/or triple). These ones we can predict by ourselves (example: tetrahedral shape with only bonding pairs is always 109.5 degrees). Lone pairs cause repulsion and the bonding pairs to...

Wasila Sun 2I

As Dr. Lavelle mentioned in the earlier lectures, the octet rule is just a guideline. Most elements outside of the 2nd row of the periodic table can accomodate an expanded octet and don't need to follow the octet rule. The octet rule is only a strict rule for elements including carbon, nitrogen, oxy...

Wasila Sun 2I

Thanks for everyone's responses! I still don't really understand why hydrogen bonds are unique in that they are so much stronger relative to other dipole-dipole bonds (-20 kJ/mol vs. -2 kJ/mol). It doesn't totally make sense to me that hydrogen bonds are 10X stronger than other dipole-dipole bonds ...

Wasila Sun 2I

How will we know when to use the normal electron configuration something over their hybridized electron configuration? Which contexts would we use each in?

Wasila Sun 2I

I believe we are taking it the date on MyUCLA because the final exam is 1 hour and 30 minutes long. This means that it would not fit within our 50 minute discussion section, unlike all of our past midterms.

Wasila Sun 2I

Sigma bonds are when two orbitals, each with one e-, are interacting END TO END. Sigma bonds allow for rotation because there is e- density symmetry around the internuclear axis (think Venn diagram shape). Pi bonds are when two orbitals, each with one e-, are interacting SIDE BY SIDE. Pi bonds can't...

Wasila Sun 2I

I wouldn't say I've done exceptionally well, but I do believe my study strategies are fairly decent. To study, I basically do all of the practice problems in the textbook and mark which ones I got wrong and/or were unsure about. A lot of these problems are similar to the ones on the actual exam, whi...

Wasila Sun 2I

What is the shape called when there are 2 lone pairs and 2 bonding pairs like H2O? Is "bent" the official shape name?

Wasila Sun 2I

I know that VSEPR is for qualitative predictions of molecular shape and real bond angles are experimentally determined, but I'm curious how does one experimentally determine bond angles?

Wasila Sun 2I

Electronegativity calculations aren't something we are expected to know because usually they are given. The only thing we would need to do with them is to interpret them, recognize the periodic trend, and compare them between molecules/atoms/salts/compounds.

Wasila Sun 2I

I think the bond angle is less than 180 degrees, more like around 109.5 because there is one non-bonding lone pair attached to the sulfur which would be more repulsive and make the other 3 bonding pairs be pushed down. Here, I think the shape is trigonal pyramidal similar to the example we did in le...

Wasila Sun 2I

In my lecture notes from October 28 (the lecture about electron configuration and the atomic properties), I have written down that s block elements have low ionization atoms and form cations. P block tends to gain electrons to form complete shells, greater electron affinity. For d- block I wrote tha...

Wasila Sun 2I

I am having some trouble with this question as well. In the explanation for why CO2 is a Lewis acid, Sapling says that molecules with polar double bonds accept electrons, but why are the double bonds in CO2 polar? Oxygen has a far greater electronegativity compared to Carbon, so the covalent bond i...

Wasila Sun 2I

I've heard that Chem 14BL would be better to take separate from Chem 14B because Chem 14BL is easier after learning the material in Chem 14B. Personally, I'm taking Chem 14C and Chem 14BL together in spring quarter.

Wasila Sun 2I

I think this is related to the charge. An cation is a positively charged ion, which means that it gives an electron to an anion. This means that the cation is polarizing because it has the ability to polarize and give it's electrons away. The anion, a negatively charged ion, is polarizable because i...

Wasila Sun 2I

Adding to what has already been said, if the d-orbital already has 10 electrons then they aren't considered valence electrons because that shell would be full. If we remember the example in the lecture, the d orbital of the Scandium element can be described as 3d1 4s2 and not 4s2 3d1. So here, the d...

Wasila Sun 2I

I think usually the preferred unit would be coulombs*meter, as that is the SI unit, rather than using debye. But, since our exams our multiple choice, you would never need to decide which unit to use (either would be correct).

Wasila Sun 2I

I remember Dr. Lavelle referring to covalently bonded elements as molecules and ionically bonded elements as compounds/salts. Does this mean compounds can't be covalently bound and molecules can't be ionically bound (are they exclusive?)?

Wasila Sun 2I

The points aren't posted to CCLE or MyUCLA, but you can just check by yourself to see if you've posted the right amount of times by taking the week # and multiplying it by 5 and comparing it to the number of matches in the "your posts" section.

Wasila Sun 2I

Midterm 2 will again be 50 minutes long, and I'm pretty sure it's multiple choice but I could be wrong.

Wasila Sun 2I

I believe they would be considered different resonance structures because in the example in the lecture from today Lavelle was talking about nitrate with the double bond on the left and on the right as separate structures/variations. While these are mirror images, they are considered separately.

Wasila Sun 2I

In the lecture, Dr. Lavelle mentioned that the brackets are just for clarity for the student's learning rather than being official notation, I believe.

Wasila Sun 2I

So, this morning I clicked the link on CCLE for the Sapling HW for this week, and the website came up with the message, "Sorry, this activity is currently hidden". I was just wondering when it will open or if this is just a me problem.

Wasila Sun 2I

Just making sure, frequency and wavelength are ALWAYS positive, right?

Wasila Sun 2I

I think all of the questions except the ones on quantum numbers and periodic trends on the Sapling assignment should be on the midterm.

Wasila Sun 2I

For part c, I would begin by changing the mg of Na2CO3 into moles of Na2CO3. This can be done by changing mg into grams by dividing by 1000 and then changing grams to moles by dividing grams by the molar mass of Na2CO3 which is 105.99 g/mol. The resulting asnwer should be around 0.000472 mol Na2CO3....

Wasila Sun 2I

I believe variable valency occurs when an element displays different valency (the number of valence electrons it has) when combining with various elements depending on the nature of a reaction.

You can read more about in this link: https://sciencing.com/variable-valency-11372429.html

Wasila Sun 2I

Heisenberg's Uncertainty/Indeterminacy equation is (delta p)(delta x) \geq h/4 \Pi . Usually in a question, like the one done in class, they would give you either the uncertainty in position (delta x) or the uncertainty in momentum (delta p). From there you can use the equation to solve for the vari...

Wasila Sun 2I

When you go to Dr. Lavelle's website, https://lavelle.chem.ucla.edu/class-websites/chem14a/, there are tabs that say "Constants and Equations" and "Periodic Table". Dr. Lavelle sent out an email about this a couple of days ago I believe.

Wasila Sun 2I

Does the E=hv and E=hc/wavelength equation only work for the energy of an incoming photon and not the energy of the emitted electron?

Wasila Sun 2I

I know deBroglie suggested that the wavelength=h/p equation worked for any particle with momentum and that electrons have wavelike properties. But, why is it so important? What are the applications of knowing that electrons behave like both waves and particles?

Wasila Sun 2I

To begin, I realized that they gave us the frequency of the photons and the kinetic energy and wanted us to find the work (threshold energy) of the metal. I then decided to convert the frequency of the photons to energy of the photons by using the equation E=hv. With the energy of a photon, I used t...

Wasila Sun 2I

Intensity is proportional to the number of photons and can be described as the number of photons that pass a given area per second. So as the intensity increases, the # of photons increases too. In the lecture, Dr. Lavelle mentions that increasing the intensity of a light source DOES NOT increase th...

Wasila Sun 2I

Given that the metals in both instances are the same, I know that the work of both instances will be equal. The equation for work is work=E(photon)-E(kinetic). The equation for energy of a photon can be expressed as E=ch/wavelength by combining E=hv and c=v*wavelength, and the kinetic energy is give...

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