CHEMISTRY COMMUNITY

Amanda Bueno-Kling

I figured this one out after a bit! It is rate = 0.600 = -/+ (1/coeficient)(delta[X]/delta t), with - signs for products since they decrease, and + signs for reactants since they increase.
So for B, it is 0.600= -1/4(delta[B]/delta t), and for C it is 0.600= 1/2(delta[C]/delta t).

Amanda Bueno-Kling

I am trying to work through this sapling problem and a bit stuck as I am not sure what equation to use. The problem I am given is: The rate of the given reaction is 0.600 M/s. A + 4B ---> 2C I figured out that delta [A]/delta t is -0.600 M/s, since every second all of the A breaks down. But I am not...

Amanda Bueno-Kling

I figured it out! It was an algebra issue and I got the correct answer of 411.2 after trying it again. I did set it up right, but just needed to make sure I did all the algebraic steps correctly.

Amanda Bueno-Kling

Hello, I have figured out the first two parts of the problem, but for some reason am having a very hard time with the last part. I have tried a few times and keep getting the wrong answer. This is the problem as I have it: "Dinitrogen pentoxide, N2O5, decomposes by first‑order kinetics with a r...

Amanda Bueno-Kling

The activated complex is "an arrangement of the two molecules that can either go on to form products or fall apart again into the unchanged reactants." (From the textbook, page 628) "In the transition state theory of reaction rates, it is supposed that the potential energy (the energy...

Amanda Bueno-Kling

The rate determining step is slowest step in a reaction mechanism. It limits the overall rate because it is the slowest step, so the rate law for the overall reaction is determined using the rate determining step. For example, the slow step in this mechanism would be the rate determining step.

Amanda Bueno-Kling

An endothermic reaction takes in heat. You have to provide energy for the reaction to occur. You can see this in the graph I attached called Endothermic Reaction, which shows that the products have more energy than the reactants in an endothermic reaction, so energy has to be taken in. High activati...

Amanda Bueno-Kling

The presence of a catalyst provides a reaction pathway in which the activation energy of a reaction is reduced by 74.00 kJ⋅mol−1. Uncatalyzed: A⟶B Ea=139.00 kJ⋅mol−1 Catalyzed: A⟶B Ea=65.00 kJ⋅mol−1 Determine the factor by which the catalyzed reaction is faster than the uncatalyzed reaction at 286.0...

Amanda Bueno-Kling

Thank you!
I did 9.8=k[16][175]^2 and solved for k to get 0.00002, which was correct.

Amanda Bueno-Kling

The presence of a catalyst provides a reaction pathway in which the activation energy of a reaction is reduced by 44.00 kJ⋅mol−1 . Uncatalyzed: A⟶B Ea=116.00 kJ⋅mol−1 Catalyzed: A⟶B Ea=72.00 kJ⋅mol−1 Determine the factor by which the catalyzed reaction is faster than the uncatalyzed reaction at 285....

Amanda Bueno-Kling

I have done most of this question, but I am stuck on the part that asks:
Calculate the rate constant, k, and identify its units.
I found that rate=k[A][B]^2
But I am not sure what do to find K. Do I plug in a set of values from the table?

Amanda Bueno-Kling

I am not sure what orders you have in your problem, but I have A= first order, B=second order, and C=third order. We can then insert the orders into the rate late as the exponents. This is the rate law: rate=k[A]^a[B]^b[C]^c so I would do: [A]^1[B]^2[C]^0 and then simplify, since we don't need to in...

Amanda Bueno-Kling

Thank you Katie! I appreciate your response. I'm glad to know that asking questions counts too, because I ask a lot of them sometimes!

Amanda Bueno-Kling

For this question, first we can group the graphs by zero order or first order. For a zero‑order reaction, the rate is constant and it does not change based on concentration of reactant. This will look like a straight line, so group the two straight lines together as zero order reactions. rate=k[X]^0...

Amanda Bueno-Kling

Hello! I can help with this one.
One of the units is not used. This threw me off too, at first, so make sure to leave M^2/s in the answer bank, since it is not the correct unit for any of the reactions. Try again with the remaining four and if you are still stuck, I can help help you!

Amanda Bueno-Kling

305614361 wrote:Yeah like a lot of people have stated I believe this depends on how lenient your TA is

Do all the TAs assess chem.com posts the same way? Or do some of them not allow makeup posts? I think Lavelle said he was allowing makeup posts so I would hope all the TAs would consider them.

Amanda Bueno-Kling

Will all posts count or only specific ones? For example is asking a question counted, or only answering a question?

Amanda Bueno-Kling

Hi! I can help with this one. This is the advice the problem gives, so let's follow it step by step. "First, find the rate of change for A. rateA=delta[A]/delta t where delta[A] is the change in concentration of A, during the time period delta t." We can see from the table that delta[A] is...

Amanda Bueno-Kling

Thank you for explaining Valerie! I tried to follow the steps you had and did the following.
k=[OH-][HClO]/[ClO-]

[HClO]=k[ClO-]/[OH-]

k[I-][HClO]
=k[I-]*([ClO-]/[OH-])
=k[I-][ClO-]/[OH-]

Thank you for explaining to me how to arrive to this answer!

Amanda Bueno-Kling

I would assume it will take about a week, that's usually how long it takes for grades to come out for this class I think! It may be faster than that, but I would assume it will be graded around this Friday-Sunday.

Amanda Bueno-Kling

Usually I believe he does not curve or round, but instead he adds points to everyone's score if the final was challenging. I believe for 14A 10 points were added on the final for everyone.

Amanda Bueno-Kling

I am confused on how to do sapling 13, which is about rate laws. However, what I have tried is not working and I am a little stumped. This is the advice the problem gives me to help solve it. "The rate of the overall reaction is governed by the rate of the slowest step in the mechanism. In this...

Amanda Bueno-Kling

I've heard the practice problems in the book are really helpful. I also haven't had time to study, but that's what friends who took this class recommended. Do a few if you can!

Amanda Bueno-Kling

so, because it says "NH_3 is a weak base" we know to change the given Ka to Kb instead? We are given Kb and need to convert to Ka. We know to do this because it says: "the salt NH4Cl acts as a weak acid. What is the pH of a solution that is 0.019 M in NH4Cl at 25 °C?" Since it i...

Amanda Bueno-Kling

Thank you all. I have gotten it right now. I was wondering why the line diagram wants the chemicals in a different order than they are in in their respective reactions? Is there a convention for this?

Amanda Bueno-Kling

I am trying to figure out #7 with the diagram of the cell with an anode and cathode. I think I did the anode and cathode parts right, but the correction keeps telling me "The cathode should occupy the right‑hand position of the line diagram." I am not sure what this means or how to fix it?...

Amanda Bueno-Kling

Joyce Pang 2H wrote:By summing, it just means to make a net equation from both. So In + Cd2 =>ln2 + cd with whichever coefficients.

Ok, so I would add what I have where I multiple both equations by the coefficients? Like this?
2In+3Cd^2+ ---> 2In^3+ 3Cd

Amanda Bueno-Kling

I am having a hard time figuring out this problem. This is the problem I have: Write a balanced overall reaction from these unbalanced half-reactions. In--->In^3+ Cd^2+--->Cd The guidance it gives me is: First, balance the charge of the half‑reactions by adding electrons. Electrons should always be ...

Amanda Bueno-Kling

This is a really cool thread! I love reading the replies. I love to cook a lot and I find that learning chemistry helps me understand more about cooking. A while ago I learned about the Maillard reaction, which I'll admit I don't entirely understand the chemistry behind, but I wondered if anyone els...

Amanda Bueno-Kling

Is there any difference between Delta U and Delta E? I know the sapling mentioned they are used interchangeably, but is there any rule about when each is used or any difference when using one or the other?

Amanda Bueno-Kling

A sample of an ideal gas in a cylinder of volume 3.44 L at 298 K and 2.07 atmexpands to 8.29 L by two different pathways. Path A is an isothermal, reversible expansion. Calculate the work for path A. For #14, why is q not 0 for reaction A? I am not sure if it asking for the work the gas does on the ...

Amanda Bueno-Kling

For week 3&4's sapling #8, I'm confused on why I have to divide the standard enthalpy of reaction by number of moles of Carbon in the second part of the question. Can someone explain what standard enthalpy of reaction is and the solution of this problem? Thank you! Hi! The reason we have to do ...

Amanda Bueno-Kling

Would we still disregard water if it was in the gas form, but still very abundant? I know we disregard it if it is a liquid.

Amanda Bueno-Kling

I think the best strategy is to memorize strong acids and bases, and consider all others as weak. I'm not sure what tips there are other than: I know most acids with COOH (carboxyl group) in them are generally weak acids, like acetic acid for example.

Amanda Bueno-Kling

Generally, we use 10^-3 as a metric, I believe.
If K is smaller than 10^-3, then K is small, and reactants are favored.
If K is greater than 10^-3, then K is large, and products are favored.

Amanda Bueno-Kling

I am stuck on #7 for week 3/4. It asks: Calculate the standard enthalpy change for the reaction at 25 degrees C. MgCl2(s)+H2O(l)⟶MgO(s)+2HCl(g) I looked up the standard enthalpy of formation values in the table and plugged them into sum(products) - sum(reactants). However, I keep getting the wrong a...

Amanda Bueno-Kling

How do we take the anti-log to go from pKa back to Ka? Will this be important for the rest of the class?

Amanda Bueno-Kling

The way I did it was to first find the pOH (14-pH), then find the concentration of the -OH by using 10^-pOH. This will act as the X in the ICE table Next, we can solve for our missing/needed value by setting up the equation Kb=[BH][OH]/[B] where be is our original base and BH is its conjugate acid,...

Amanda Bueno-Kling

Thank you everyone! I am assuming later we will see problems where the temperature changes and thus has an effect? I just wanted to make sure I wasn't missing something at this time. I appreciate all your replies.

Amanda Bueno-Kling

The way I did it was to first find the pOH (14-pH), then find the concentration of the -OH by using 10^-pOH. This will act as the X in the ICE table Next, we can solve for our missing/needed value by setting up the equation Kb=[BH][OH]/[B] where be is our original base and BH is its conjugate acid,...

Amanda Bueno-Kling

A lot of the problems give a temperature in them, but I don't know when we have to take it into account and when we don't. When reactions take place at the same temperature, like for example in #7 and #8 on the Week 2 Sapling the Ka and Kb take place under the same conditions, I've been told we can ...

Amanda Bueno-Kling

Do we ignore both aqueous and liquid and only consider it when it is a gas?

Amanda Bueno-Kling

Hi Lexy! This personally how I went about solving this problem: 1. Since Kb is given to you in this problem and the constant for Kw is known as 1.0 x 10^-14, I solved for Ka using the equation: Ka x Kb = Kw. 2. Once you calculate the value for Ka, you should notice that it is significantly small; t...

Amanda Bueno-Kling

I am not sure how to start solving #5 on the week 2 Sapling. I am wondering if anyone can walk me through the steps? I know I need to set up an Ice table, but I'm not sure exactly the order of how I should approach the problem. The Kb for an amine is 7.624×10^−5. What percentage of the amine is prot...

Amanda Bueno-Kling

For the other questions on the Week 2 homework, I was able to figure them out, but I am not sure how to approach #7 and #8. This is what I have for #7: HClO is a weak acid (Ka=4.0×10−8) and so the salt NaClO acts as a weak base. What is the pH of a solution that is 0.065 M in NaClO at 25 °C? And thi...

Amanda Bueno-Kling

If the question gives you the initial concentration of the products and reactants, then they add more moles to the reactants and let it go to equilibrium, which way does it shift and why? It helps me to imagine the reaction like one of those old fashioned scales. When you put more on one side, the ...

Amanda Bueno-Kling

The three K terms are all different ways of saying the same thing. Kp is when you are using equilibrium partial pressures. Kc is when you're using the equilibrium concentration values. Keq is just a general equilibrium constant. All of these ways of denoting K are the same thing, and calculated the ...

Amanda Bueno-Kling

Hi! I can try to help. You need to take the given equations and combine them in a way that is similar to the original, which is 2NH3(g)+3I2(g)↽⇀N2(g)+6HI(g). I used the following to do so: H2(g)+I2(g)↽⇀2HI(g) Ka=160 N2(g)+3H2(g)↽⇀2NH3(g) Kc=3.6×10^−2 However, you have to make sure to reverse the sec...

Amanda Bueno-Kling

You multiply them, but make sure to inverse for a reaction going in reverse! Also make sure to put any exponents you need to scale the given values up. For my problem, I multiplied 27.7778 (the inverse of 3.6×10^−2) with 160^3 (the third power was to scale the 2HI in the given equations with the 6HI...

Amanda Bueno-Kling

Thank you Rachel! I tried both, but when used the smaller of the two solutions, I got the correct answer for the problem. I am wondering how one can tell which solution to the quadratic equation to use? Do you always use the smaller one? Or do you just use whichever seems to make most sense?

Amanda Bueno-Kling

For #10, I am getting two solutions when I do the quadratic formula. I get both 5.7247997665174 and 0.23232253598261 as solutions, but I don't know which one to use to find the equilibrium concentrations. Does anyone know what we should do if we get two solutions like this?

Amanda Bueno-Kling

I am confused on #3 for Week 1 Sapling. I tried to do the products/reactants = Kc, but I kept getting the wrong answer and sapling showed me the following message. Does anyone know why this is the case? I am confused as to why the numerator and denominator change like this after the square root is t...

Amanda Bueno-Kling

I took a screenshot of the feedback Sapling gives on how to set up the equation. I think it's a helpful example of how it should look. For me it is working with the balanced reaction: H2(g) + I2(g) <=> 2HI(g) and gives the Kc expression: Kc = [HI]^2/[H2][I2] It should be Kc = [products]/[reactants] ...

Amanda Bueno-Kling

How rare is it for neither products or reactants to be favored? I would think it would be very rare or almost impossible in reality, since Dr. Lavelle said that the more stable structure will have lower energy and be favored, and it seems unlikely that products and reactants will have the exact same...

Amanda Bueno-Kling

Typically, what counts as a large k value is >10^3 and what counts as a small k value is <10^-3. However, essentially I use 1 as a sort of threshold for k. Whenever k is lower than 1, the reactants are favored. Whenever k is higher than 1, the products are favored. I hope this helps! This is really...

Amanda Bueno-Kling

How can we tell if the products or reactants are favored using K? I know a large K value shows that the products are favored, but what qualifies as large or small K values?

Amanda Bueno-Kling

Why are liquids and solids excluded from calculations for K? My lecture notes say that it's because the molar concentration of a pure substance does not change in a reaction, but I am confused as to what this means. Could anyone explain this to help me understand?

Amanda Bueno-Kling

I'm going to get enough sleep and try to take it easy. I'll also be doing some gardening, watching movies with my mom and sister, and playing online games with my friends! I hope everyone else can relax and stay healthy too.

Amanda Bueno-Kling

I would assume so, but if you have concerns about your grade I would suggest emailing Dr. Lavelle. He would be able to explain or answer questions.

Amanda Bueno-Kling

I know course evals were extended, but when do they close? I also forgot to do them, but will do them if they're still open.

Amanda Bueno-Kling

Thank you so much Emma! I hope you're doing well too. I really appreciate you sharing that website, I'll definitely use it.

Amanda Bueno-Kling

Thank you! I really appreciate it.

Amanda Bueno-Kling

I like to use this one, as it has percentage breakdowns of the weight of each assignment. https://www.calculator.net/grade-calculator.html If this one doesn't work well for you or you prefer another one, make sure to search "final grade calculators with percentages" or something like that ...

Amanda Bueno-Kling

Does anyone want to make a groupme or study group and start reading the chapters for 14B topics over winter break together? Nothing too rigorous, because we all need a break, but just to get a head start on the concepts. We could do practice problems or share YouTube videos and study resources that ...

Amanda Bueno-Kling

I could be mistaken, but I thought there would be some for 14B as well? I just don't think we have access to them yet.

Amanda Bueno-Kling

Thank you all for providing lots of resources and support for students! The amount of time and work put into study sessions and worksheets by Dr. Lavelle and the TAs was amazing.

Amanda Bueno-Kling

If there isn't already a 14B groupme, we should make one! It was really helpful this quarter to have others share review videos and study tips that they found useful.

Amanda Bueno-Kling

There's plenty of online calculators that will allow you to put in grades and the weight of grades to estimate your final grade. Google something like "final grade calculator" or "percentage grade calculator". Use the syllabus and grade book to fill them out with the grades we ha...

Amanda Bueno-Kling

I think it depends on the TA, some TAs have said that you can make-up points I believe. I would email your TA if you are unsure.

Amanda Bueno-Kling

Once the outlines with recommended practice problems are up, my friend who took 14B previously recommended getting a head start on reading through the textbook and doing practice problems. You could probably try problems in the sections Dr. Lavelle just mentioned in the meantime.

Amanda Bueno-Kling

I had a difficult time learning to name coordination compounds. Does anyone know of resources or tips that make it easier? I saw flashcards suggested for memorizing certain parts of naming, but are there any good videos or websites to review the naming? I know it will come up again and I want to mak...

Amanda Bueno-Kling

Thank you for this! I am a visual learner so this is a great resource.

Amanda Bueno-Kling

This is such a cool idea! I think I would stick with purple but cautiously befriend green.

Amanda Bueno-Kling

I don't think this has been announced yet. I checked the syllabus and it does not specify the content distribution of the exam, so I think we will have to wait for Dr. Lavelle to notify us.

Amanda Bueno-Kling

I also find that drawing them out helps a lot. In my notes, I drew them as Dr. Lavelle showed them in lecture, since I tend to learn visually. It really helps me to draw them and practice seeing how they build up. Reviewing the lectures where Dr. Lavelle showed us the shapes with his 3D models might...

Amanda Bueno-Kling

A cyclic structure is a ring structure, or one that is not linear. A molecule like Benzene would have a cyclic structure, as it forms a hexagon shape with the carbons. A cyclic structure would form a triangle, hexagon, or other shape instead of a straight line (linear structures form more straight l...

Amanda Bueno-Kling

I counted one sigma bond for every bond in the molecule, since single, double, and triple bonds all have one sigma bond. Then, I counted one pi bond for every double bond, and two pi bonds for every triple bond. This gave me the right answer on that Sapling question.

Amanda Bueno-Kling

How can we tell which elements have expanded octets? Is there a specific region of the periodic table that tells us this, or is it based on the number of valence electrons each element has? Do we memorize them, or is there a rule or technique to figure it out?

Amanda Bueno-Kling

The lectures said that sigma bonds can rotate freely, whereas pi bonds cannot. How would this look in a molecule? If two halves of the molecule were bound by a sigma bond, could they rotate clockwise and counterclockwise from each other? I am a little unsure what this rotation would look like, and w...

Amanda Bueno-Kling

Both bonds and lone pairs are areas of electron density. Regions of electron density = the number of hybrid orbitals. So say you have two bonds and one lone pair. That would be three regions of electron density, and you would have three hybrid orbitals. This would be one s orbital combined with two ...

Amanda Bueno-Kling

Can anyone explain why NaCl is an ionic compound and HCl is a molecular compound? Can we figure this out just by looking at the periodic table?

Amanda Bueno-Kling

I believe any equations that we need will be provided to us. The exam is probably more focused on if we know how to use an equation than if we can memorize it.

Amanda Bueno-Kling

Thank you both! That is very helpful and I have figured it out now.

Amanda Bueno-Kling

For the sapling question: "Add formal charges to each resonance form of HCNO." I am stuck and am having a hard time figuring out the charges. The hint it gives when I am wrong is cut off, so I cannot see the equation it gives for figuring out formal charges. Can anyone help me figure out t...

Amanda Bueno-Kling

I agree with familiarizing yourself with the SI units and their conversions. These were not given on the last midterm and I had to try to remember them, so memorizing them will be very helpful.

Amanda Bueno-Kling

Will Midterm 2 have the same format as Midterm, with a small number of questions each worth a large number of points? Or will the points be more broken up among more questions? I know on the last midterm, missing even one question really detracted from one's grade.

Amanda Bueno-Kling

Unfortunately, no. Dr. Lavelle said he'd go over some that want of us missed, but so far I have been unable to review my answers and have heard it will stay that way due to the online format of the exam.

Amanda Bueno-Kling

Hello,
I am wondering if it will be possible for us to view our Midterm 1 and see which questions we got correct and incorrect? Currently, all I can see is a score and not which questions were correct/incorrect, which I think would be helpful for us to learn from our mistakes.
Thank you!

Amanda Bueno-Kling

Can anyone explain the relationship between the amplitude of the wave and the intensity of the light? I am curious to know why they are related the way that they are. I know increasing the amplitude should increase the intensity, but I don't know why.

Amanda Bueno-Kling

I believe by the end of Week 5, since some people had their midterm on Friday of Week 4.

Amanda Bueno-Kling

Are we expected to memorize SI units? I have some of them memorized, but could not remember what power of 10 a picometer was on the exam. I didn't see SI units given on the exam or on the formulas sheet, so I am guessing we should memorize them? Does anyone have any tricks they use to memorize them,...

Amanda Bueno-Kling

I usually don't round until the end if possible, but if I do round, I try to keep at least 3 or more sig figs. It's best to not round if possible, but rounding a little bit should not throw your answer off entirely, since the moles of each element are usually just one step towards the final answer, ...

Amanda Bueno-Kling

The question asks: How many electrons in an atom could have these sets of quantum numbers? n=2 n=5, l=3 n=7, l=2, ml=-1 How do we figure this out? I had a hard time and was wondering if anyone is able to succinctly explain the process. I tried to look up a few YouTube tutorials and websites explaini...

Amanda Bueno-Kling

Thank you so much! That really helps and now I figured it out.

Amanda Bueno-Kling

I am trying to convert 1.029 Electron Volts per Atom to Kilojoules per Mole. I am having a hard time with this and wondered how I should be setting up the conversion? The problem says: "Calculate electron affinity in kilojoules per mole from the electron affinity in electron volts per atom by u...

Amanda Bueno-Kling

Sometimes when I am balancing chemical equations, I end up with coefficients that are not whole numbers. For example, there are 15 mols of O on one side, and on the other side I have to add a coefficient before O2. I can't put 7.5, even though 7.5 * 2 = 15, because I know coefficients have to be who...

Amanda Bueno-Kling

Hi, I had a question about the molarity of a solution. Am I correct in assuming that the molarity of a solution is the same in the whole sample as it would be in a small fraction of the sample? For example, if I had 100 mL of 0.04 Molarity solution, and I took 20 mL out, would the 20 mL still have t...

Amanda Bueno-Kling

The way vision works, when you see a certain color, you are seeing what is not being absorbed by the object you are looking at. If I look at a leaf and it is green, it's because the molecules of the leaf absorb all wavelengths of light except green. The green light that bounces off the leaf due to n...

Amanda Bueno-Kling

Do we need to know the details about the experiment needing to take place in a vacuum, how the detector works, or why a metal must be used? I am much clearer on the other concepts you mentioned, but am not as knowledgeable about the specifics of the experimental design.

Amanda Bueno-Kling

As electrons move down from a higher energy level to a lower one, they are "losing energy", as they emit photons while moving down through orbitals towards the ground state. Since the electron has an energy of zero at the reference point, how is it possible for them to move down and lose e...

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