CHEMISTRY COMMUNITY

Marco Morales 2G

Thank you Lavelle for all your hardwork and time you put into the pre-recorded lectures. All your lectures were so thorough and helpful, it really helped me understand the concepts for this class. I wish nothing but the best for you! Thank you!

Marco Morales 2G

So my problem 12 states: A particular reactant decomposes with a half‑life of 101 s when its initial concentration is 0.338 M. The same reactant decomposes with a half‑life of 241 s when its initial concentration is 0.142 M. Determine the reaction order. a.)1 b.)2 c.)3 I was wondering how you would ...

Marco Morales 2G

I wanted to get other people's ideas of how they're studying for the final. I know Lavelle mostly uses textbook problems for the tests but I was wondering how you guys will be studying them? I was thinking of just going over the problems for one topic each day.

Thank you!

Marco Morales 2G

Lewis structures in 14A and in 14B Chemical Equilibrium and some thermo topics

Marco Morales 2G

Rachael Cohen 3G wrote:The equation we're working with is:
2H+(aq) + 2e− -> H2(g)
Q is products over reactants. H2 is a gas, so we use the pressure. H+ is aqueous, so we use the concentration. Hope this helps!

Thank you for this! I kept doing something else for Q and kept getting the wrong answer.

Marco Morales 2G

Same, I keep getting it wrong but I feel like I'm doing it right. Did you find out what you did wrong? To see what I'm doing wrong lol. Thank you! Nvm I know what we might've done wrong. The answer will be first given in joules, so you have to convert it to kj for deltaG and then you'll get the cor...

Marco Morales 2G

Same, I keep getting it wrong but I feel like I'm doing it right. Did you find out what you did wrong? To see what I'm doing wrong lol. Thank you!

Marco Morales 2G

Its being marked incorrect for me as well. Do you know what you did wrong? because I don't know what Im doing wrong with mine

Marco Morales 2G

rachelhchem1I wrote:Professor Lavelle sent an email yesterday with some text resources for balancing redox reaction in basic and acidic solutions! He also included some videos too so look at that email or his website if you haven't yet :)

thank you!

Marco Morales 2G

Does anybody know any youtube videos to balance redox reactions with H2O/OH and H ions, as shown in the textbook and sapling? I kinda understand it but I need little bit more help. Thank you!

Marco Morales 2G

Does anybody know up to what points in outline 3 were covered in midterm 1? To avoid them while I study for midterm 2. Thank you!!

Marco Morales 2G

Does anybody know up to what points in outline 3 were part of midterm 1? To not waste time on revising them while I study for midterm 2. Thank you!

Marco Morales 2G

I don't think 4F 11 was part of the assigned hw problems, so don't worry about it! x)

Marco Morales 2G

Jenny Chau 1I wrote:Will we ever use the integral equations like integral T1 to T2 dq/T and - integral V1 to V2 PdV?

I highly doubt it. Like everyone else said, he used it to just show us how we get the final equation and how everything relates.

Marco Morales 2G

Hi! Here is a super useful chart for this concept. Screen Shot 2021-02-10 at 2.24.49 PM.png The bottom two rows explore the temperature-dependent scenarios for Gibbs free energy. In these scenarios, a temperature can be above or below (or at, if in equilibrium and delta G is 0) a certain point of m...

Marco Morales 2G

This is such a clutch post. I got so many resources to use. Thank you!

But to answer your question, check out the step up sessions. They really do help.

Marco Morales 2G

Is the way you calculate the specific heat capacity of a calorimeter different from how you calculate specific heat capacity in general? Or does it always occurs with a calorimeter? I think it depends on the context you're given. If it asks for the specific heat capacity of a calorimeter than I thi...

Marco Morales 2G

Can someone explain who change in internal energy is different from q? The internal energy, U, of a system can be changed by q, which is energy transferred as heat. Thus, when internal energy of a system is changed by transferring energy in the form of heat (and no other processes of expansion or c...

Marco Morales 2G

An example can be like the one Lavelle gave in his lecture of a piston compressing a gas, thus resulting into an increase of energy.

Marco Morales 2G

Hi! For these calorimetry problems you should recognize which species is releasing heat and which is absorbing heat. Since ice is lower in temperature, this means that ice is absorbing the heat from the water. Since we assume no heat is lost to the surroundings, that means that the heat absorbed by...

Marco Morales 2G

Can someone explain why the H2O from the product side is disregarded when solving for delta-H using the (sum of all products' standard enthalpies)-(the sum of all reactants standard enthalpies)? Thank you!

Marco Morales 2G

For 4.E.5, Looking at the lewis structure for C2H2 gives us 1 C-C triple bond, which has an enthalpy of 837, and also 2 C-H bonds which have a combined value of 824. Add these 2 numbers and multiply by the moles, which is 3, and you get the total enthalpy for C2H2, 4983. For benzene, the table of b...

Marco Morales 2G

Determine the reaction enthalpy for the hydrogenation of ethyne to ethane, C2H2(g)+2H2(g)→C2H6(g), from the following data: ΔHc°(C2H2, g)=−1300.kJ⋅mol−1, ΔHc°(C2H6, g)=−1560.kJ⋅mol−1, ΔHc°(H2,g)=−286kJ⋅mol−1. Hello, can someone explain how to approach this problem and why do we need to use O2 when ...

Marco Morales 2G

Sorry this isn't an answer lol, but how come you get to see the explanation of the answer in the solutions manual? I only see the final answer we need to get.

Marco Morales 2G

Determine the reaction enthalpy for the hydrogenation of ethyne to ethane, C2H2(g)+2H2(g)→C2H6(g), from the following data: ΔHc°(C2H2, g)=−1300.kJ⋅mol−1, ΔHc°(C2H6, g)=−1560.kJ⋅mol−1, ΔHc°(H2,g)=−286kJ⋅mol−1. Hello, can someone explain how to approach this problem and why do we need to use O2 when ...

Marco Morales 2G

I'm not completely sure what balanced reactions are in the answer key, I only see -312 kJ.mol^-1 as the answer. But, to solve this problem I identified that I needed to make the delta H value for C2H6 negative because it accounted for the bonds formed. I kept the sign the same on H2 and C2H2 as the...

Marco Morales 2G

I'm not completely sure what balanced reactions are in the answer key, I only see -312 kJ.mol^-1 as the answer. But, to solve this problem I identified that I needed to make the delta H value for C2H6 negative because it accounted for the bonds formed. I kept the sign the same on H2 and C2H2 as the...

Marco Morales 2G

This may seem like a dumb question lol, but since we haven't really covered all of thermochemistry and thermodynamics yet, should we just focus on the 4D HW problems that Lavelle sent in his email for the Midterm? Or is it possible to do all the chapter 4D questions with the readings he told us to f...

Marco Morales 2G

Does anybody know if Lavelle has talked about a problem like this? Cuz I feel like he hasn't yet? Just asking to make sure I didn't miss it.

Marco Morales 2G

The temperature remains constant during phase changes because the heat energy which is supplied to change the state is used in breaking the intermolecular forces and other attractive forces; so, it remains constant as all the heat is used up and no external heat is released or absorbed. To add to J...

Marco Morales 2G

It's endothermic because heat is going into the system to break the intermolecular forces of the molecules present to change them into another state. For example, if water was being heated at 100 degrees, and the water needed to go to a vapor phase, the intermolecular phases of water have to be brok...

Marco Morales 2G

I don't think you have to worry so much on the proper definition. I think the main point to know is that for a state property, all you need to know is the initial state and the final state to calculate the difference. Also know that delta H is a state property, meaning that enthalpy only depends on ...

Marco Morales 2G

Hi! I actually answered this for someone else, so I'll just copy and paste it here. So for this, I think it's good to know that Cl won't be included in this whole thing, as it doesn't affect the pH. Al3+ is the (weak) acid, here, as it attracts water molecules enough that said water's hydrogen atom...

Marco Morales 2G

Can someone explain in the chemical formula for AlCl3 in water, if all three Cl molecules will dissociate from Al? or just one? Because if its all 3 wouldn't Cl cancel on both sides and we just have Al on the left side and Al(H20)6^3+ on the right, and when solving for x from the ICE table, wouldn't...

Marco Morales 2G

I had a quick question about when the chem community posts start getting counted again for the new week? For example, if I post a question this coming Sunday (of week 2), that post will be counted for Week 3 right? Because I do know that Saturday Midnight is the last day you can post something for i...

Marco Morales 2G

I'm just going to walk through the steps I took to check your answer: First, convert 0.43 g Ba(OH)2 into moles by dividing by the molar mass of Ba(OH)2 (which is 171.344 g/mol). Then divide that value (0.0025mol) by 0.100 L to get 0.025 M as your Ba(OH)2 concentration. I'm not sure where you got 0....

Marco Morales 2G

So for this problem I was able to get the initial molar concentration for Ba(OH)2 and I got 5.577x10^-3. To get that I first divided 0.43 grams by the molarity of Ba(OH)2, which I got as 171.327 (0.43g/171.327g.mol), and got 2.509*10^-3. Then I took that result and divided by 0.450L to get the initi...

Marco Morales 2G

Actually, since NH3 and H2S are on the product side of the reaction, you would add 0.200 to x (0.200+x) for NH3 and H2S would correspond to +x. If NH3 and H2S were on the reactant side, then you would subtract x from 0.200 for NH3. Using the quadratic formula, you should get 0.0008 for x. Once you ...

Marco Morales 2G

For this problem, I was wondering how NH3=0.2 and how H2S= 8.04x10^-4? doesn't NH3 start with 0.2 mol.L so we subtract 0.2 with our answer that we got from the quadratic formula, and that would equal 8.04x10^-4 for NH3, and since H2S isn't mentioned on how much it's initial molar concentration was, ...

Marco Morales 2G

For part b, I already have an understanding of what I need to do, but I keep getting an error for the square root part in the quadratic formula. I was wondering if someone can explain or point out if the numbers I got are wrong (which Im pretty sure are wrong, but I've done the problem like 4 times ...

Marco Morales 2G

"(a) In an experiment, 2.0 mmol Cl2(g) was sealed into a reaction vessel of volume 2.0 L and heated to 1000. K to study its dissociation into Cl atoms. Use the information in Table 5G.2 to calculate the equilibrium composition of the mixture." Can someone explain how to do this problem? I ...

Marco Morales 2G

So for the answer for 5.i #11, the textbook states Q=6.9 I was wondering where that number came from because when I do the reaction quotient formula (P / R), the product is (SO3)^2 and the reactants are ([SO2]^2 * [O2]) and after plugging in their respected concentrations ([0.10]^2 / [1.20]^2 * [0.5...

Marco Morales 2G

HannahRobinson3L wrote:You can also go watch the Audio Visual Focus Topics on Dr. Lavelle's website if you want to hear an explanation directly from the professor! I think it is the Chemical Equilibrium Part 2 where he goes over ICE charts.

I recommend doing what Hannah said. In the module, he goes over it super well!

Marco Morales 2G

Dr. Lavelle mentioned in lecture that temperature affect equilibrium, but how does it factor into the equation for the equilibrium constant? In one of the module's, I remember him stating that later on in Chem 14B, we'll be using temperature that affects the Kc. I don't think it's something to worr...

Marco Morales 2G

Hi! I believe you have to start by looking at the K values given in the table. These values are for the individual reactions that make up this broad reaction. What you have to do is combine the two reactions to create the overall reaction. When you combine the reaction though you must multiply the ...

Marco Morales 2G

a) they switched the placements of the reactants/products so you would just take the inverse of K, so K in this problem would be 1/41 b) the given equation is the given equation in the problem but the coefficients are just divided by 2 so you would take the square root of 41 as your answer (~6.4). ...

Marco Morales 2G

HCl is stronger because it has a longer bond than HF, due to trends in the periodic table. Lavelle also mentioned that the longer the A-H bond, the stronger the acid because a property of a strong acid is it gives off protons easily. So if a bond is long, the proton isn't easily held to the nucleus ...

Marco Morales 2G

Also, how do you know V1=.200L ? and V2=.250L ?

Is it because we initially "think" we used .200 L at first?

Marco Morales 2G

For the pH of the desired solution, you just use the concentration of HCl, which is desired to be 0.025M. 0.025 M of HCl is 0.025 M of H+ since HCl is a strong acid and completely dissociates into H+ and Cl-. pH= -log[H+], therefore, the pH of the desired solution is -log(0.025). For the second par...

Marco Morales 2G

Hi! Here's how I solved this problem: 1. the molarity of H+ ions is equal to the molarity of HCl because HCl is a strong acid so its ions will completely dissociate 2. pH of initial HCl solution = -log([HCl]) and pH of diluted HCl solution = -log(0.12 x [HCl]) 3. I divided the pH of the diluted HCl...

Marco Morales 2G

Jordi M 2F wrote:Is that all the information the question gives?

Yes, it only gives that.

Marco Morales 2G

This may seem like a simple question but how do you determine an ion is acidic or basic?
Thank you!

Marco Morales 2G

I had question about the radical part. So they are not radicals because they at least have paired electrons? So they're only radicals if the central atom has 1 unpaired electron?

Marco Morales 2G

The question states:

" Identify the hybrid orbitals used by the atom in boldface red type in each of the following species: b) AsF3"

the back of the book says the answer is sp^3. Can someone explain how they got the answer? Please! Thank you!

Marco Morales 2G

I think Lavelle mentioned something about this in his lecture stating that some textbooks write it either way and that it's the same thing but that he prefers writing it the "sp^3d" way because it's the order of the energy states.
If I'm wrong someone correct me please.

Marco Morales 2G

One of the questions on my sapling problem asked about the relative positions of the atoms in the lewis structure.
What do they mean by the positions of the atom in the lewis structure? and how do you figure it out?
the answer mentioned something about being in the same plane.

Marco Morales 2G

Lavelle in his lecture from Monday (week 8) he gave a little key on how to find the hybridization of a molecule. He said that the # of regions of electron density around the central atom (or the least ionized element), equals the # of hybrid orbitals. So for example, if there are 2 regions of electr...

Marco Morales 2G

A lewis structure has delocalized Pi bonds when the lewis structure has resonance. Meaning there are delocalized electrons (aka shared electrons) throughout the whole molecule.

Marco Morales 2G

the question states: 2E.17 Predict the bond angles at the central atom of the following molecules and ions: (a) ozone, O3 I drew the Lewis structure for "a" and I thought it would have an angular shape due to the loan pairs on the Oxygen atom in the middle. So with the 4 regions of electro...

Marco Morales 2G

the problem states: Compounds having bonds with a high covalent character tend to be less soluble in water than similar compounds that have low covalent character. Use electronegativities to predict which of the following compounds is the more soluble in water: (b) MgO or BaO. In the back of the boo...

Marco Morales 2G

I don't know exactly why they put A as an answer as well. But I was reading some posts on chem community how O is a special exception to the radical rule because it always has two lone pairs. To back it up, i found this in the reading of the book: "One of the most important examples is the oxy...

Marco Morales 2G

I don't know exactly why they put A as an answer as well. But I was reading some posts on chem community how O is a special exception to the radical rule because it always has two lone pairs. To back it up, i found this in the reading of the book: "One of the most important examples is the oxyg...

Marco Morales 2G

Only elements from the 3rd period and down can have an expanded octet. Because they all have "n" states bigger than 2, which means their "l" states can be s, p, d, or f orbitals, depending on the particular element you're focusing on and what their "n" state is.

Marco Morales 2G

The question states: Draw the Lewis structures that contribute to the resonance hybrid of nitryl chloride, ClNO2 (N is the central atom). I know how to draw the lewis structure. But i've been confused on why N is in the middle. I know ionization energy decreases down a group and increases across a p...

Marco Morales 2G

Sinc Br is in the 4th period of the periodic table, it has the ability to have an expanded octet. This means n=4, so l=3 (because l is solved by n-1). So since l=3, Br has the s,p, d, and f orbitals. Meaning it has extra orbitals to gain extra electrons, apart of only 8 from the octet rule. just as ...

Marco Morales 2G

The following Lewis structure was drawn for a Period 3 element. Identify the element.

: textbook q.png (8.66 KiB) Viewed 120 times

the back of the book says Phosphorus. How does it get to that answer? can someone explain how they solved it? please! Thank you!

Marco Morales 2G

2A.17 Predict the number of valence electrons present for each of the following ions: a.) Mn^4+ I need help understanding why the answer is 3 valence electrons in the answer section. Because I saw a youtube video saying we count all the electrons of the highest "n" value in the electron co...

Marco Morales 2G

He said in lecture today that the elements from the 3rd period and down have the expanded octet. But then again, he said we'll generally only be focusing on the first three periods of the periodic table for the class because they're more widely used. So I wouldn't really worry about further periods.

Marco Morales 2G

I remember him sending an email or reading it somewhere that the points don't carry over for the next week. Each week your points reset, if that makes sense. So each week we have to get those 5 points.

Which, I'm trying to do right now x)

Marco Morales 2G

Anybody attending Lavelle's office hours right now, from 5-6 p.m., and still waiting to be accepted into the meeting? or is he taking students one by one?

Marco Morales 2G

Anh Trinh 1B wrote:Make sure to convert grams to kilograms before solving!

Yes thank you! I totally forgot about converting to kg x)

Marco Morales 2G

The question states: What would the speed of each particle be if it had the same wavelength as a photon of orange light ( "lambda" =625.0 nm)? proton (mass=1.673×10−24 g) speed: (blank) m/s neutron (mass=1.675×10−24 g) speed: (blank) m/s electron (mass=9.109×10−28 g) speed: (blank) m/s I k...

Marco Morales 2G

the question states: A violet line is observed at 434.0 nm in the spectrum of atomic hydrogen. Determine the values of "n" for the beginning and ending energy levels of the electron during the emission of energy that leads to this spectral line. n2= ? n1=? I know n1 is 2 but I'm having tro...

Marco Morales 2G

the question states: A violet line is observed at 434.0 nm in the spectrum of atomic hydrogen. Determine the values of "n" for the beginning and ending energy levels of the electron during the emission of energy that leads to this spectral line. n2= ? n1=? I know n1 is 2 but I'm having tro...

Marco Morales 2G

Is the Rydberg Equation given in the textbook (equation 2: v = R {1/n1 - 1/n2} ) the same as the equation given to us in lecture: ( En = -hR/n^2 )? I know they don't look the same lol, but it's because in one of the Sapling homework problems I had to use the the first equation, given in the book, wh...

Marco Morales 2G

the problem states: "As you may well know, placing metal objects inside a microwave oven can generate sparks. Two of your friends are arguing over the cause of the sparking, with one stating that the microwaves "herd" electrons into "pointy" areas of the metal object, from w...

Marco Morales 2G

I need help solving this problem:

A liquid is exposed to infrared radiation with a wavelength of 4.84×10−4 cm. Assume that all the radiation is absorbed and converted to heat. How many photons are required for the liquid to absorb 25.34 J of heat?

Marco Morales 2G

Assuming that you have already calculated the answer to part a), you would already know that the energy an excited sodium emits is around E= 3.37 x 10^-19 J Now, the next step is that we covert the 5mg of sodium atoms first to moles of sodium atoms and then from moles to total number of atoms by us...

Marco Morales 2G

I quickly skim my notes after every lecture. Then, every other day that we dont have lecture, I review my notes again and if I have time I just quickly skim the pre-recorded lecture to review the most important concepts. Then on weekends I just review all of my notes of that week. It helps retain th...

Marco Morales 2G

Why does professor Lavelle compare E (energy of photon) to (delta)E ? I'm having a hard time understanding why. Can someone give their explanation why? Please !!

Marco Morales 2G

Hi! So yes you are correct that the amplitude of the wave is the intensity of the wave, but increasing amplitude doesn't effect the frequency. As seen in the equation you were talking about C= wavelength*frequency, wavelength and frequency effect each other but not the amplitude. Also, if you graph...

Marco Morales 2G

Is it safe to say amplitude can be characterized as intensity of the wave?

if so, would it also be safe to say increasing amplitude (aka intensity) increases frequency?

if I'm wrong in any of this or mixed up any definitions please feel free to correct me! I'm trying to understand this x).

Marco Morales 2G

After reading some of the replies, I'm afraid I have a silly question. People are saying to "multiply 0.325 with 590.1g" but my question is what they did to get 0.325 and why. Thank you! Don't be afraid! Because I too was wondering the same thing. Thank you for asking before I did! We're ...

Marco Morales 2G

I know this wasn't part of the assigned problems on the syllabus but I just wanted to know how this problem is figured out [Part of Fundamentals G]: What volume of 1.25 x 10^-3 M C6H1206 contains 1.44 um (micromol) of glucose molecules? the answer was given: [Answer: 1.15mL] I know we have to use th...

Marco Morales 2G

I'm pretty sure it's always going to have the units you need to use in the question.

Marco Morales 2G

With stoichiometric coefficients you're always supposed to have the lowest whole number. So if you find that one of your coefficients is like 3.5 or 8.5, etc, you would multiply both sides of the reaction (reactants and products) by 2. That will give you the lowest whole number coefficients and kee...

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