CHEMISTRY COMMUNITY

Molly Posta 1H

To my understanding, the 2 are related but not the same. I believe the equation relating them is ΔG = -nFE* = Wmax, but someone please correct me if I'm wrong.

Molly Posta 1H

Thank you for two great quarters Dr. Lavelle! I really enjoyed your class and I appreciate how much time and effort you put into providing extra resources. It was so nice to have a professor who cares so much about the wellbeing of their students, even beyond our understanding of the curriculum.

Molly Posta 1H

True that's probably what I'll try to do. I just wasn't sure because I know for 14A last quarter we were still able to submit posts after the final.

Molly Posta 1H

Thank you! That's definitely an important one to have!

Molly Posta 1H

Steady-state approximation is the assumption that the net rate of formation of the rxn intermediates is zero. Pre-equilibrium is when an intermediate is formed in a rapid equilibrium run prior to a slow step in a rxn mechanism. Hope this helps!

Molly Posta 1H

From my understanding, the concentration is not included for the shorthand cell diagram. Someone please correct me if that isn't the case, though.

Molly Posta 1H

I was also confused by this, I'm not sure how we arrived at the actual oxidation half-ran given this. I would also appreciate it if someone could clarify the concept and then explain how we use that to arrive at the half-run shown.

Molly Posta 1H

Does anyone know if the current formula sheet posted on the Chem 14B website has been changed/updated at all since the first midterm? I'm wondering if I need to print out a new one or if the one I used for midterm 1 is the same.

Molly Posta 1H

When will our lest set of Chemistry Community posts be due for this quarter? Will they be due tonight at midnight since that's when the Sapling is due, or do we still have until midnight Sunday?

Molly Posta 1H

Will we need to be familiar with the Michaelis-Menten mechanism for enzyme reaction for the final? It's mentioned in the textbook under topic 7E but it's not listed in the objectives for the Chemical Kinetics outline so I just wanted to make sure.

Molly Posta 1H

I've tried switching up the order of the elements on either side and get different, unhelpful error messages each time, so if someone did this question right, let me know. Hello, this is what I was able to get hope it helps. Thanks so much, this worked! But can anyone explain why this is correct an...

Molly Posta 1H

For the Cu half reaction you just have to balance the charge. For Cr--->CrO2-4, the equation is also unbalanced in terms of oxygen, so you need to add H2O to balance the oxygen and then add H+ to the other side to balance H before balancing the charge. Then you can go through the normal process of c...

Molly Posta 1H

So the process for this problem is going to be pretty of similar to other balancing problems with redox reactions or half-reactions. If you're stuck on that part, try looking at number 3 from this sapling assignment because that should help you understand that portion of the problem. The most import...

Molly Posta 1H

Update I changed what I had before and I got to this but it's still not right. Does anyone know which part I'm doing wrong?

Molly Posta 1H

So for the elements in the problem it gave me, for chlorine it would be ClO-2 ---> ClO-4 + 2OH- ? And then add water to balance the oxygen?

Molly Posta 1H

Can someone help me with this problem? The feedback says the cathode equation is not balanced but I've tried a bunch of times and I keep getting it wrong.

Molly Posta 1H

Can someone explain how they balanced the half-reactions for this problem? I figured it out for the half-reaction involving iron, but I think I'm doing something wrong for the second one. I'm not sure how to balance the oxygen, so if someone could explain that part that would help!

Molly Posta 1H

The textbook talks about the EMF of a cell as the cell potential, and it's defined as the difference between the electrodes of an electrochemical cell when it is producing no current. In other words, it is the potential difference associated with a galvanic cell (electrochemical cell) that is workin...

Molly Posta 1H

Also, the book mentions both transition temperature and the temperature of a phase transition. To my understanding, these two terms are interchangeable, but just to make sure, can anyone verify that they mean the same thing?

Molly Posta 1H

This statement has been brought up both in lecture and the textbook, but I'm struggling to understand it conceptually: At the transition temperature, the temperature of the substance remains constant as heat is supplied. I know Professor Lavelle discussed this in an example during lecture, but I'm s...

Molly Posta 1H

Also, to find the number of moles each liquid that were vaporized for the first part, you have to divide the mass difference between them by the molar mass of each.

Molly Posta 1H

Start by dividing the energy supplied by the number of moles to find the molar enthalpy of vaporization for the first part. Then, you can divide that value by the temperature, T, to get the molar entropy of vaporization for the second part. Then just repeat this for the other compound. Hope this hel...

Molly Posta 1H

For this question, I found ΔG∘r and I know that I need to use the equation ΔG∘r = -RT ln (K) to get the final answer, but I keep getting stuck on how to solve for K. I plugged in the respective values and tried to solve for K a few times, but none of my answers were correct. Could someone explain ho...

Molly Posta 1H

Entropy increases when matter spreads into a greater volume or is mixed with another substance. Entropy also increases when a system is heated. So in summary, increasing volume or temperature both cause an increase in the entropy of the system.

Molly Posta 1H

Do all of the thermodynamic equations for Gibbs free energy calculations use kilojoules instead of joules? I noticed this conversion occurring frequently while working on the homework and I couldn't remember the units used in lecture, so I just wanted to clarify.

Molly Posta 1H

I believe it's just topics after midterm 1, and then the final is cumulative.

Molly Posta 1H

For 14A last quarter, if I'm remembering correctly I don't think there were ever more than 2 sapling assignments open at a time if that makes sense. So he'll probably post the next assignment pretty soon now the the week 3/4 one is closed, but then the one after that might not be posted until the we...

Molly Posta 1H

I solved it using this equation and solving for Tf:

(mass of ice)x(4.18)x(Tf) + standard enthalpy of fusion = - (mass of water)x(4.18)x(Tf-Ti)
Remember the negative sign on the right side of the equation! Hope this helps!

Molly Posta 1H

Cv = heat capacity of a gas at constant volume, Cp = heat capacity at a constant pressure. The corresponding molar heat capacities are these quantities divided by the amount of substance (in moles), giving you Cvm and Cp,m. The relationship between these values then is that Cp,m = Cvm + R. Hope this...

Molly Posta 1H

The energy heat lost gained by the ice cube is equal to to the the energy lost by the surrounding water. The ice cube requires 6.01x10^3 j/mol to change from ice to water, so for 42.6 g of water, using the molar mass of water you can find the energy required to change all the ice to water. Once the...

Molly Posta 1H

So to find work you're going to want to use the equation w = -nRTln(Vfinal/Vinitial), and to do that you have to first use the ideal gas law to find n, the moles of gas in the container. So your first step should be find n using the ideal gas law, and then plug that into the equation to solve for w....

Molly Posta 1H

It might help to start by drawing the Lewis structure, since you need to find the appropriate expression for molar heat capacity at constant volume first, and this is based on molecular shape. Then you just use that to calculate heat using q = n*Cvm*deltaT. Hope this helps!

Molly Posta 1H

Can someone explain the basic process of how to solve this problem? I tried to set up the equation to find Kc, excluding C because it is a solid, but I wasn't able to get the correct answer. 30. A mixture of 2.5 moles H2O and 100 g of C are placed in a 50 L container and allowed to come to equilibri...

Molly Posta 1H

I got this question wrong on one of the post-assessment modules and I'm not sure where I messed up. I attached a picture of my work if anyone can help my find the error. Q: A mixture initially consisting of 0.250 N2 (g) and 0.500 M H2 (g) reacts to form NH3 (g) which is 0.15 M NH3 (g) at equilibrium...

Molly Posta 1H

Can someone help me figure out where I went wrong solving this problem from the Chemical Equilibrium Part 2 module? I balanced the chemical equation and then set up the equation to solve for Qc, excluding As(s) because I thought solids and liquids aren't included when solving for Q or K. Is this cor...

Molly Posta 1H

One of the objectives for outline 2 states: Describe water autoprotolysis and derive pKW = pH + pOH. When it says be able to derive pKW = pH + pOH, what exactly does that mean, and to what extent? Do we just need to know that since Kw = [H3O+][OH-], and the autoprotolysis constant of water at 25*C i...

Molly Posta 1H

For other people in Westwood who don't have a printer in their apartment, where can I go to print out the periodic table and equation sheet for the midterm? If anyone knows somewhere that's close/cheap, please let me know!

Molly Posta 1H

The learning objectives don't explicitly say to memorize standard states, but it does say that you have to recognize when a substance in its standard state. If you can do this without memorizing a bunch of standard states, that's probably easier. Here's a link to a different post talking about how t...

Molly Posta 1H

Will we have to print out materials for the midterm as we did for 14A? I'm assuming we will be allowed to use the periodic table again, but is there a new equation sheet we should use or is the same one as 14A? I'm just trying to plan ahead what I will need to print because I don't have a printer in...

Molly Posta 1H

I'm confused by the following statement made in section 4D.3 of the textbook: The heat absorbed or released by a reaction can be treated as a reactant or product in a stoichiometric equation. What does this mean conceptually? I don't entirely understand how the known value would be used in this situ...

Molly Posta 1H

It might help to attach a picture of your work if you have one, so people can help look for what went wrong? I got confused by this problem as well because I kept messing up the conversions from mole to molarity in the ICE table. What worked for me was setting up my ICE table using the values given ...

Molly Posta 1H

Can someone explain how they created an ICE table for this problem? I'm struggling with what values to use for the initial concentrations and how to set it up to represent the change in concentrations if we're already starting at equilibrium.

Molly Posta 1H

I'm having trouble with question 3 on the first Sapling assignment. I attached a picture of my work. I think I set up the ICE table correctly, but I'm not sure because the equation it gives me seems different from what it usually looks like for these types of problems. If it is correct, how do I go ...

Molly Posta 1H

Courtney Situ 2B wrote:Hello!

For Kc, you used the mols O2 instead of the concentration of O2. I believe that's the only problem!

Hope this helped!

Thank you! That was the mistake, I was able to get the correct answer after fixing it!

Molly Posta 1H

I'm having trouble with question #2 from the first Sapling HW assignment. The questions states: At a certain temperature, 0.960 mol SO3 is placed in a 2.50 L container. 2SO3(g)↽−−⇀2SO2(g)+O2(g) At equilibrium, 0.160 mol O2 is present. Calculate Kc. I attached a picture of my work, could someone help...

Molly Posta 1H

Are students currently enrolled in 14B able to get participation points by replying to posts made by students currently taking 14A, or can we only interact with students also in 14B?

Molly Posta 1H

I don't know if there's a specific GroupMe for lecture 1 but I know there is one for all of Professor Lavelle's 14B classes this quarter. It's the same chat as the 14A one form the fall, the name is just different.

Molly Posta 1H

I got kind of lost during this example from Friday's lecture, can anyone explain where the number 5.40 is coming from and how we got to that?

Molly Posta 1H

In the example from today's lecture, the molar concentrations of the reactants and products were given when asked to calculate K for the reaction. For the sake of this class, will we always be given the molar concentration for these problems or will we at times need to find it ourselves? I know that...

Molly Posta 1H

I know that solids and liquids are not included in the K expression. In the example from lecture, a solid reactant was excluded but the aqueous products were used to calculate K. My understanding of this is that aqueous substances are liquid, but they don't count as pure substances because they are ...

Molly Posta 1H

Varsha Ravi 2C wrote:I heard that Organic Chem Tutor on youtube has good videos for the topics discussed!

Thank you, I'll check that one out! Do you know if it cover topics through the entire 14 series?

Molly Posta 1H

Hi, just wondering what happens to our Chemistry Community posts for 14A once this quarter ends? Will they stay up or are they removed at the end of the quarter? I'm curious as to whether we will be able to look back on our old posts in the future.

Molly Posta 1H

Hi, just wondering if anyone found any outside resources particularly helpful throughout the quarter? I'm looking for additional ways to study for 14B in the winter.

If anyone has YouTube channels, videos, or websites that helped them this quarter, please link them!!

Molly Posta 1H

For people taking 14B in the winter, are you planning to do any review of particular topics to prepare? I'm just wondering if anyone knows what we will be starting with/what would be most important to review, or if anyone has good resources for reviewing.

Molly Posta 1H

Throwback to the election

Molly Posta 1H

Why is HBrO2 not an amphoteric substance? It could act as a Bronsted acid and donate a proton to a base because it has a hydrogen ion, so what is preventing it from accepting a proton to act as a Bronsted base as well?

Molly Posta 1H

I'm struggling with the following concept: The greater the electron affinity (and therefore the electronegativity) of A, the stronger is the acid. (Where A is referring to the second element in a binary acid) Previously, we learned that a shorter, weaker bond corresponds to a stronger acid. By this ...

Molly Posta 1H

Are pi-bonds always weaker than sigma-bonds, or is this dependent on molecular shape?

If this is true, is it also true that the sum of 2 single bonds of any homonuclear diatomic molecule will be stronger than one double bond of the same molecule? Or is this only true for certain elements like carbon?

Molly Posta 1H

I'm doing a practice problem from the textbook, why would the shape of AsF5 be trigonal bipyramidal instead of square pyramidal? In general as well, how do you differentiate between the 2 after drawing the Lewis structure and establishing the number of regions of electron density?

Molly Posta 1H

Why does the Lewis structure for Oxygen have 2 unpaired electrons, making it a biradical? Does it have to do with lower energy levels? And does this apply to any other elements in the same period?

Molly Posta 1H

For #3 on this week's Sapling assignment, the Lewis Structure I was shown was a trigonal bipyramid, meaning the smallest angles would be the axial bond angles of 90 degrees. However, the central atom had a lone pair of electrons. In lecture it was said that lone pairs force bonding electrons closer ...

Molly Posta 1H

I think this is because even though there is a negative charge, there is no net dipole, so the molecule isn't polar. Dipole moment is a measure of polarity of a molecule, so that's what I look at when determining whether a molecule is polar, rather than looking at the overall charge for that molecule.

Molly Posta 1H

The sp3d hybrid orbital corresponds to 5 bonded pairs and lone atoms. Since there are 5 Br atoms bound to P in this molecule, it corresponds to the sp3d orbital.

Molly Posta 1H

I think it would depend on what other information you are given. Sigma bonds interact end to end while pi bonds interact side by side, so if you know what kind of bond it is you know how it will interact. If you don't know the bond type, you can also figure it out if you are given information on whe...

Molly Posta 1H

The oxygen anion has a charge of 2-, not 1-, so you would add two electrons and get 2p^6. Maybe the 1- charge was a typo, because when oxygen forms an anion it carries a 2- charge. Thanks for your response, but I'm still confused now. The problem just said O-. Are we just supposed to know/assume th...

Molly Posta 1H

No, discussion attendance isn't graded. The only online discussion that counts towards final grade is posts on Chemistry Community.

Molly Posta 1H

I'm reviewing my TA's worksheet from week 6 and it says that the electron configuration for the anion O- is 1s^2 2s^2 2p^6. When I was trying the problem, I thought the configuration would be 1s^2 2s^2 2p^5, because the normal configuration for O is 1s^2 2s^2 2p^4, so I added one electron to account...

Molly Posta 1H

I think it relates to both quantum number n and the effect of shielding. In my notes on this objective, I said: Larger principal quantum number = farther from the nucleus = higher energy Shielding effect: The decrease in attraction between an electron and the nucleus of an atom with more than one el...

Molly Posta 1H

Are the terms dipole and polar molecule interchangeable? Or is there some distinction between the two?

Molly Posta 1H

This isn't about ionic or covalent bonds but I wasn't sure where else to post it. I know that larger, heavier atoms/molecules have stronger London forces, but I was wondering if there are any other additional factors that affect the strength of the force? This is the only one I know of but I think t...

Molly Posta 1H

I wasn't entirely sure where to post this question, but it has to do with hydrogen bonding and question #16 from sapling. This question asks you to identify the hydrogen bond crosslink between the 2 polymer chains, but I wasn't sure how to differentiate between 2 of the answer choices. I thought tha...

Molly Posta 1H

I'm not 100% certain, but it isn't listed in the learning objectives we need to know for this midterm so I would assume not. I think we just need to know the different factors that effect bond length and how they change it, but not how to calculate an exact value.

Molly Posta 1H

Generally, I think the idea is that the ideal structure would minimize the amount of formal charges. So having less atoms with a formal charge would make the structure more plausible.

Molly Posta 1H

In general, electronegativity increases from left to right and decreases from top to bottom. I don't really have a specific way to remember this, but it might help to just think of a diagonal arrow going from the bottom left corner of the table to the top right, and knowing electronegativity increas...

Molly Posta 1H

I think it's the same amount of time, at least that's what the syllabus says.

Molly Posta 1H

Double bonds are actually shorter than single bonds, and triple binds are even shorter than double bonds. It's because triple bonds have more electrons, so they have a stronger force which pulls the atoms closer together, so the bond length itself is shorter.

Molly Posta 1H

I'm struggling with this concept from the textbook: Compounds of boron and aluminum may have unusual Lewis structures in which boron and aluminum have incomplete octets or halogen atoms that act as bridges. I was confused by the book's explanation of this statement. Is this just something we have to...

Molly Posta 1H

The Pauli exclusion principle is just saying that no more than 2 electrons can occupy each orbital.

Hund's rule states that due to the natural electron repulsion, electrons in the same sub shell occupy different orbitals with parallel spin because this is the lowest energy state.

Molly Posta 1H

I'm not entirely certain, but I would assume it will be the same number of questions as midterm I because it's the same amount of time given and both are out of 110 points on the syllabus.

Molly Posta 1H

Out of all the equations we've used so far in units 1 and 2, should mass always be in kg? I've been using g for mass for most of my work, but I noticed while reviewing the De Broglie equation that mass needs to be converted into kg for this example. Do some equations require mass in kg while some us...

Molly Posta 1H

One of the questions from the balancing equations post-assessment says: During a summer camping weekend 4 moles of butane (C4H10) gas were used for cooking. Chose the right balanced equation for the combustion of 4 moles of butane gas. What is the net number of moles of gas produced? I'm confused ab...

Molly Posta 1H

This question asked: If an electron (mass 9.11 x 10-31 kg) has an associated wavelength of 7.28 x 10-9 m, what is its speed? Is your answer reasonable, why? I calculated the speed to be 1.00x10^5 m/s, and I said this was not reasonable because the calculated speed is much slower than the speed of li...

Molly Posta 1H

What is the cut off of wavelength for a particle/object to have measurable wavelike properties? I know things like electrons and neutrons do while things like cars or baseballs don't, but I don't know if there's a numeric cutoff in terms of wavelength that determines this? I just have been assuming ...

Molly Posta 1H

The question states: When incident light (electromagnetic wave) hits a barrier with two or more holes does it give rise to an observed diffraction pattern? For this question, what is the significance if the barrier having two or more holes? Why would it not give rise to an observed diffraction patte...

Molly Posta 1H

I'm having trouble understanding this question conceptually from the Wave Properties of Electrons post-assessment: When the trough of one wave interacts with the trough of another wave is their interaction constructive or destructive? I know if the trough of one wave were to interact with the peak o...

Molly Posta 1H

Double check with your TA, but here's what mine said. Dr. Lavelle had originally said that we would need a non-graphing scientific calculator, but the last I heard we will now be allowed to use a graphing calculator. So any calculator you have should be allowed, since there's no restrictions on grap...

Molly Posta 1H

One meter is equal to 1x10^-9 nanometers, so to convert from meters to nanometers you would multiply the value by 10^9.

In the example you gave, 4.2X10^-7 m becomes 420 nm by multiplying the value in meters by 10^9.

Molly Posta 1H

I could be wrong because I took this assessment last week, but I'm pretty sure all you need to do is multiply your answer from the first part by Avogadro's constant to get the answer for the second part. Because the first part gives you the electron's uncertainty in kinetic energy, so multiplying th...

Molly Posta 1H

I asked my friend that took this class just to confirm, but yes basically what the other person said, you won't need to know how to derive it for any midterm/exam. Knowing it to some extent might help to better your understanding, though.

Molly Posta 1H

Will there be additional audio-visual focus-topics/assessments posted on the 14A website for Schrodinger Equation and the rest of the topics remaining in the quantum world unit? Or were those only available for the Review section and the first part of this section?

Molly Posta 1H

Is anyone else suddenly unable to access the textbook through Macmillan? I've been using it through the same link this whole time and signed up with my access code in the beginning, but now it's saying I don't have access to the book. Did this happen to anyone else/were you able to fix it?

Molly Posta 1H

This question from the Photoelectric Effect Module post-assessment asks: "In photoelectric experiments, typically what part of the electromagnetic spectrum is the incoming light?" Would the answer to this be visible light? I'm really not sure at all, I just feel like we've discussed visibl...

Molly Posta 1H

Regarding the questions about phones, has anyone actually been able to open respondus through their phone and connect to zoom through it? I tired during discussion section last week but it didn't work, so I'm wondering if other people were able to.

Molly Posta 1H

I'm very confused by this question in the textbook, even after looking at the answer. The problem states: "In the ultraviolet spectrum of atomic hydrogen, a line is observed at 102.6 nm. Determine the values of n for the initial and final energy levels of the electron during the emission of ene...

Molly Posta 1H

In section 1B of the textbook, there is a question that asks: From the following list of observations, select the one that best supports the idea that electromagnetic radiation has the properties of particles. Explain your reasoning. (a) Black-body radiation. (b) Electron diffraction. (c) Atomic spe...

Molly Posta 1H

Hi I also have a question about the 1:1 ratio, how would we solve to get that, and were we suppose to find the chemical equation just from the question? You can tell that there is a 1:1 ratio of 2-butanone to 3-methyl-3-hexanol by looking at the reaction given. Since the equation shown is already b...

Molly Posta 1H

Adding on to this question, are the 5 posts due by Friday or Sunday of each week to get points? I may have missed the answer to this somewhere, but I couldn't find it in the syllabus.

Molly Posta 1H

The instructions for the audio-visual focus topics say that we should finish the second set of modules, photoelectric effect through Heisenberg uncertainty equation, by the end of week 2. I'm just wondering if there are any recommended readings and exercises from the textbook associated with theses ...

Molly Posta 1H

I believe the reason it asks for formula units is because magnesium sulfate heptahydrate is an ionic compound, so that's the why formula units is used instead of molecules, as would be used for covalent bonds, or atoms, as would be used for single elements.

Molly Posta 1H

Question E.9 states: "Epsom salts consist of magnesium sulfate heptahydrate. Write its formula". I wrote MgSO4 as the formula for magnesium sulfate heptahydrate, but the answer key has MgSO4*7H20. Where did they get this? Why is water included in the formula and how do I know it should be ...

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