CHEMISTRY COMMUNITY

Summer de Vera 2C

Hi! for residual entropy, it's defined as being the entropy of a sample at T=0 from the remaining positional disorder @ that temp, but I'm a bit confused as to what T=0 refers to. Because it's not deltaT=0, which means it's isothermal, and the units aren't defined (T=0 K/˚C). So what does this refer...

Summer de Vera 2C

For this problem, I am using the standard enthalpy of formations given and doing the summation of the products minus the summation of the reactants, however I'm producing a positive rather than negative value. Can anyone tell me why this is happening? Am I using the correct method?

Summer de Vera 2C

Hi! For this problem, there is no answer given in the back of the textbook and in the solution manual for the answer of deltaH, so does that you can't calculate it? Or is this jut a typo?

Summer de Vera 2C

if you use q=nCdeltaT, then do you simply convert the reaction substances into mols and add them together to get the total mols?

Summer de Vera 2C

Hi! Just to confirm, can you either use mass (g) or concentration (M) in the rate laws if it's a sealed vessel? If so, does someone mind explaining this further?

Summer de Vera 2C

Hi! Does anyone know if chem equilibria is going to be on the final? Is it cumulative?

Summer de Vera 2C

Hi! For this problem, when you're trying to find the order of A & B, do you not consider C because you find that C is zero order and will have the same rate through the experiment ?

Summer de Vera 2C

Hi! Does anyone know why Zn is a stronger reducing agent than Fe and Cr even though it's farther to the right on the periodic table ?

Summer de Vera 2C

For this problem, do we only use the first piece of info given for the Cu/Cu2+ reaction to determine if it's being oxidized/reduced? I'm just confused because the later info tells us that for the overall reaction, Cu/Cu2+ is on the cathode side, but the first piece of info seems like it's in the ano...

Summer de Vera 2C

I believe the SHE is used to calculate the standard potential contribution for a single half cell. From what the book explains, you usually can't calculate the E˚ of a single half-cell because the standard potentials given to us in tables are used to calculate the overall cell potential. So because ...

Summer de Vera 2C

Hi! Why is it that for a), Ag is the one being reduced while Br is used for the oxidation reaction? Isn't it being oxidized because it's going from AgBr(s) --> <-- Ag+(aq) + Br-?

Summer de Vera 2C

Hi! So I understand that an extensive property depends on the amount of substance while an intensive property doesn't, but I'm just confused as to why heat capacity is an intensive property while molar & specific are extensive. Aren't molar and specific heat capacities dependent on grams/mols wh...

Summer de Vera 2C

Hi! In 4F.5 7th Ed, the book explains that for calculating entropy change at transition temperature, you have to break it into 3 parts. For example, to calculate deltaS(vap) of H2O at 25˚C & 1 bar, you have to 1) heat the liquid from 25˚C --> 100˚, 2) allow to vaporize, and 3) cool the vapor bac...

Summer de Vera 2C

Hi! For this problem, the enthalpy of formation of TNT= -67 kJ/mol, its density = 1.65 g/cm^3, and the chem eqn is 4 C7H5N3O6 (s) + 21 O2(g) --> 28 CO2(g) + 10 H2O(g)+ 6N2(g). The problem asks to find the enthalpy density of TNT, but I'm not really sure where to start. Can someone please explain?

Summer de Vera 2C

I think its because 298 K is the temperature for the standard state of gas reaction

Summer de Vera 2C

Hi! I understand how to do the problem, however the small part that I'm confused about is which specific heat of water I should use to calculate q(ice cube). Shouldn't Csp=2.03 J/gC instead of 4.184 since it's changing temperature in the solid state still?

Summer de Vera 2C

Hi! Does anyone know if we'll need to know about the eqn Cv,m=(3/2)R or Um=(3/2)RT?

Summer de Vera 2C

I think the degeneracy of one mole equals 2 raised to avogadro's number because N equals the number of atoms/molecules. Why is the degeneracy of one mole specifically equal to a base of 2 ( and then raised to Avogadro's number)? Is it just because for the specific example, there were 2 possibly ori...

Summer de Vera 2C

A good equation mentioned today in lecture for calculating the number of possible degenerate state was the formula W = 2^n , useful to determining all possible microstates of n different and separate particles. Does W = 2^n apply specifically to the problem we did in class? Or is it a general formu...

Summer de Vera 2C

Lauren Huang 1H wrote:The second law of thermodynamics states that delta S=q/T=nRln(V2/V1). So q=nRTln(V2/V1) if you multiply both sides by temperature. I believe the unit for q is Joules.

Wouldn't you have to divide rather than multiply both sides by T?

Summer de Vera 2C

In general, are irreversible reactions more common than reversible ones?

Summer de Vera 2C

Just to double check, when work is being done by the system the amount of energy is negative right? Similarly, is the energy positive when work is being done on the system? I believe you're correct. The change in energy of the system when work is being done by the system is negative (because it's l...

Summer de Vera 2C

I think it's also important to note that in biological systems, deltaU typically =deltaH because biological systems don't have changes in volume. So, from what I remember Dr. Lavelle mentioning, we might be working with more examples where deltaU=deltaH, but you should still know the reasoning why.

Summer de Vera 2C

I would disagree with the above statement. I'm pretty sure something as a simple as a liquid in a thermos is an isolated system because it's insulated to prevent any heat from escaping and, while it's sealed, there's no transfer in any matter that occurs. Also, there are things called bomb calorimet...

Summer de Vera 2C

Why is it that less ammonia will be formed? Isn't the reaction endothermic, meaning that the products (ammonia) will be favored?

Summer de Vera 2C

Hi! So the problem tells you that NH4HS(s) and .400 mol NH3(g) are in a 2.0L vessel @ 24˚C and that the equilibrium for NH4HS(s) --> <-- NH3(g) + H2S(g) is Kc=1.6x10^-4. You're asked to find the eq [products], however when I do the ICE table, I keep getting that x=0. Is this even possible? Am I doin...

Summer de Vera 2C

Hi! For the example given for 5H.2 (Composite Eqns), why is the second reaction multiplied by 2?

Summer de Vera 2C

Hi! I'm not sure if Lavelle mentioned this in lecture, but will we need to know how to calculate cubic equations when figuring out concentrations & ICE tables? Because I don't remember him talking about it in lecture even though it was in the module.

Summer de Vera 2C

Does anyone know if bar is the standard unit that we'll be using in class and in homework problems? Or will it also be atm, etc.?

Summer de Vera 2C

My T.A. said that brackets indicate concentrations while parentheses are for calculating the K of partial pressure. It would be incorrect to do [Pco2] for example because you'd be trying to calculate the concentration of a partial pressure, which isn't correct.

Summer de Vera 2C

Does anyone know if we'll be given the actual ENs of atoms when deciding if a molecule is more covalent/ionic? Or will we just have to use the table and the EN trends to guess?

Summer de Vera 2C

Why is water monodentate? Isn't the double-bonded oxygen stable, which means it wouldn't want to share its electrons?

Summer de Vera 2C

If you're given the bond length of two bonds in a molecule (e.g. NO2-: N-O=140 pm & N--O=120 pm), how do you predict the NO bond lengths in the molecule? In the example I gave (from the midterm), don't they already tell you what they are?

Summer de Vera 2C

Hi! So I'm doing a problem in the textbook and have to draw the structure of H2SeO4. I was a bit confused though because the hydrogens were bonded to the O and not the Se (central atom). Is this because H can ONLY bond to NOF? Or is it because the attraction between the H and O is stronger than H an...

Summer de Vera 2C

Do higher equilibrium constants indicate a stronger acid/base? And would you only have to find this for weak acids?

Summer de Vera 2C

Hi! Should we memorize the formulas of all the polyatomic ions? I'm just asking because if we need to first write out the reactants and products for a reaction given the name of the molecule, I imagine we're gonna need to know the polyatomic ions. Thanks!

Summer de Vera 2C

It's just the rules/standards for naming compounds (IUPAC=International Union of Pure and Applied Chemistry). On Lavelle's website, there's a doc called Naming Coordination Compounds that has a table of the names you should know

Summer de Vera 2C

Hi! Just to clarify, when Bronsted acids "donate" protons, does this just mean it's donating a hydrogen atom (H+)? Because I know protons are bound to the nucleus and can't actually move.

Summer de Vera 2C

Will we need to know how to draw hybrid orbitals? If so, do you essentially pick the energy shell (n) with orbitals that match the number of electron densities around the atom?

Summer de Vera 2C

Hey! I was also really confused on this question. I found an answer to this question from 2011, so hopefully this clarifies things a bit. "Answer: s-character is the contribution of sigma type bond in a hybridization: sp3 = 25% s-character, 75% p-character sp2 = 33% s-character, 66% p-character...

Summer de Vera 2C

Schem_student wrote:I got confused by this one too! Just think about hybridization labels of central atoms in terms of amount of atoms bonded not if they are double or single bonds. Therefore there are only 3 atoms bonded around C so it would be sp2.

I think this is what I was getting confused by. Thank you!

Summer de Vera 2C

I believe so. For example, I think salts can have organic anions or cations. But for the most part, a majority of organic molecules are covalently bonded.

Summer de Vera 2C

Hey! "r" is the distance between the two molecules

Summer de Vera 2C

Hi! For 2F.13, why is the hybridization of CH2 and CH sp2? Because there are each 4 areas of e- density surrounding the molecules, shouldn't the hybridization be sp3?

Summer de Vera 2C

Hi! How can you compare the bond lengths of two molecules/ions to each other if in each molecule the central atom has different bond lengths connected to the atoms (e.g. one atom connected by single bond and other connected by double) ? For example, in 2D.13 b) 7th Ed, it asks you to put SO2, SO3, a...

Summer de Vera 2C

Hi! For test 3, does anyone know if we need to memorize the whole VSEPR chart or have basic knowledge about some of the shapes? Thanks!

Summer de Vera 2C

Hi! Are "bent" and "angular" synonymous for the same shape? Because in Topic 2E 7th Ed, I was doing some problems where I had to determine the shape of the molecule and was using a VSEPR chart online to help me, but the names from the answers vs chart were different even though t...

Summer de Vera 2C

Hi! I found someone who had the same question from several years ago, and I thought this person's answtper explained it pretty clearly: "Ba has a lower electronegativity than Mg does, so the difference in electronegativity would be greater for BaO, meaning it is more ionic, and thus, more polar...

Summer de Vera 2C

So do electronegativity trends essentially rely on electron shielding ? As do the other periodic table trends ?

Summer de Vera 2C

What does a formula unit refer to? In Fundamentals E..25, it asks to determine the number of formula units in KNO3. Is it basically like asking how many atoms or molecules a compound has?

Summer de Vera 2C

Would you have to draw all the resonance structures of an ion/compound/etc. in order to determine the formal charge?

Summer de Vera 2C

When we draw all the resonant structures, is that just a model for how the bond is actually somewhere between a single and double, or are there actually that many different structures? All the different resonance structures you draw are basically "one structure" that shows where and how t...

Summer de Vera 2C

When determining the type of ligand, should you write out Lewis structure ? Or can you just look at the chemical formula?

Summer de Vera 2C

Xingzheng Sun wrote:Will the concept of Black Body Radiation be tested?

I don't think it will be. Lavelle mentioned that it wouldn't because we didn't learn it, or rather discuss it in lecture.

Summer de Vera 2C

when determining the shape of a molecule, do you focus on the amount of lone pairs on the central atom as well as the number of electron dense areas? I'm just confused because it seems like there's so many factors.

Summer de Vera 2C

In order to determine which bonds are in a molecule, do you just have to write out the electron configuration of each atom and compare them?

Summer de Vera 2C

I know that on the periodic table you can determine the # of valence electrons for the s– and p-blocks by essentially looking at their group number, however how can you determine the amount of valence electrons for the d-block? Just a little confused by that.

Summer de Vera 2C

Yes, I believe you can use wave function and orbital interchangeably. I remember Lavelle mentioning this (unless I misunderstood what he said).

Summer de Vera 2C

It translates into kgm^2/s because a joule is kgm^2/s^2, and if you divide it by 1/s, it gets to this unit.

Summer de Vera 2C

I know that the s-, p-, d-, and f- represent how many electrons can fit in a certain shell, but would anyone also know why they're specifically labelled those letters? Does it stand for something ?

Summer de Vera 2C

Yes, you can use both equations. You may also want to combine them for some, though I don't know if it's needed.

Summer de Vera 2C

Hey! I'm having a bit of trouble deciding which equation(s) to use in order to figure out this problem from the post-module assessment. It seems like I have to do this in several steps, however I'm not sure where to start. 28. Light hits a sodium metal surface and the velocity of the ejected electro...

Summer de Vera 2C

I think we should still be using the SI units that we learned in high school (kg, m, L, etc.) just to be safe. Unless otherwise specified in a problem, those units are the standard for calculating chemistry (and other subjects') problems. Then again, I would also suggest asking your TA if you're rea...

Summer de Vera 2C

Hi! I just wanted to clarify the reasoning for depending on the amount of mols rather than grams used for dilutions/solutions. Do we focus on the specific mols of the reactant used rather than grams because diluting/creating solutions depends on the concentration of the reactants? This is what I've ...

Summer de Vera 2C

In one of the post-module assessment problems, I was asked to find the theoretical yield of a product using the limiting reactant and molar ratios. I found the limiting reactant to be 1.56 mols and the reactant–product molar ratio to be 4:5, so what would the next step be? Do I just multiply the 1.5...

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