## Search found 65 matches

Fri Mar 15, 2019 9:04 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: residual entropy T=0
Replies: 2
Views: 268

### residual entropy T=0

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...
Fri Mar 15, 2019 2:14 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 4D.15
Replies: 1
Views: 253

### 4D.15

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?
Thu Mar 14, 2019 3:35 am
Forum: Phase Changes & Related Calculations
Topic: 4C3 7th Ed
Replies: 1
Views: 266

### 4C3 7th Ed

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?
Tue Mar 12, 2019 11:36 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: HW problem 4A. 13
Replies: 2
Views: 262

### Re: HW problem 4A. 13

if you use q=nCdeltaT, then do you simply convert the reaction substances into mols and add them together to get the total mols?
Tue Mar 12, 2019 5:36 pm
Forum: First Order Reactions
Topic: using mass or conc in rate laws
Replies: 2
Views: 105

### using mass or conc in rate laws

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?
Sun Mar 10, 2019 7:04 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Final Exam
Replies: 23
Views: 414

### Final Exam

Hi! Does anyone know if chem equilibria is going to be on the final? Is it cumulative?
Sun Mar 10, 2019 5:20 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: 7A.15
Replies: 4
Views: 120

### 7A.15

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 ?
Sun Feb 24, 2019 10:47 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: 6M7a 7th Ed
Replies: 1
Views: 48

### 6M7a 7th Ed

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 ?
Sun Feb 24, 2019 8:25 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 6M.1 7th Ed
Replies: 1
Views: 80

### 6M.1 7th Ed

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...
Sun Feb 24, 2019 8:21 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: SHE
Replies: 3
Views: 72

### Re: SHE

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 ...
Sun Feb 24, 2019 7:24 pm
Forum: Balancing Redox Reactions
Topic: 6L7 7th Ed
Replies: 1
Views: 55

### 6L7 7th Ed

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-?
Tue Feb 12, 2019 8:24 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: extensive & intensive properties and heat capacities
Replies: 1
Views: 74

### extensive & intensive properties and heat capacities

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...
Sun Feb 10, 2019 5:11 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Entropy change at temp transitions
Replies: 1
Views: 70

### Entropy change at temp transitions

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...
Sat Feb 09, 2019 5:50 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 4D.9 7th Ed.
Replies: 1
Views: 85

### 4D.9 7th Ed.

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?
Sat Feb 09, 2019 5:12 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 4D.7) 7th edition
Replies: 1
Views: 77

### Re: 4D.7) 7th edition

I think its because 298 K is the temperature for the standard state of gas reaction
Sat Feb 09, 2019 4:05 pm
Forum: Phase Changes & Related Calculations
Topic: 4C.13 7th Ed.
Replies: 2
Views: 92

### 4C.13 7th Ed.

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?
Fri Feb 08, 2019 7:29 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: monatomic ideal gas equation
Replies: 1
Views: 63

### monatomic ideal gas equation

Hi! Does anyone know if we'll need to know about the eqn Cv,m=(3/2)R or Um=(3/2)RT?
Thu Feb 07, 2019 3:51 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Replies: 2
Views: 89

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...
Thu Feb 07, 2019 3:48 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Degeneracy
Replies: 8
Views: 222

### Re: Degeneracy

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...
Thu Feb 07, 2019 3:45 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: What is q equal to in the second law of thermodynamics?
Replies: 5
Views: 322

### Re: What is q equal to in the second law of thermodynamics?

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?
Thu Feb 07, 2019 3:43 pm
Forum: Calculating Work of Expansion
Topic: irreversible and reversible
Replies: 5
Views: 356

### Re: irreversible and reversible

In general, are irreversible reactions more common than reversible ones?
Fri Feb 01, 2019 2:25 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Work Done By vs. On
Replies: 4
Views: 105

### Re: Work Done By vs. On

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...
Fri Feb 01, 2019 2:18 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Delta U vs Delta H
Replies: 6
Views: 195

### Re: Delta U vs Delta H

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.
Fri Feb 01, 2019 2:12 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Isolated Systems
Replies: 6
Views: 147

### Re: Isolated Systems

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...
Mon Jan 21, 2019 4:06 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: 5J13 7th Ed
Replies: 2
Views: 94

### 5J13 7th Ed

Why is it that less ammonia will be formed? Isn't the reaction endothermic, meaning that the products (ammonia) will be favored?
Sun Jan 20, 2019 2:20 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 5I.15 7th Ed
Replies: 1
Views: 46

### 5I.15 7th Ed

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...
Sun Jan 20, 2019 11:08 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 5H.2 E.g. 7th Ed
Replies: 1
Views: 32

### 5H.2 E.g. 7th Ed

Hi! For the example given for 5H.2 (Composite Eqns), why is the second reaction multiplied by 2?
Sun Jan 13, 2019 3:29 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Vid Module Part 3
Replies: 1
Views: 33

### Vid Module Part 3

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.
Sun Jan 13, 2019 1:35 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: "bar" meaning
Replies: 4
Views: 146

### Re: "bar" meaning

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.?
Sun Jan 13, 2019 1:32 pm
Forum: Ideal Gases
Topic: Denoting brackets/parentheses
Replies: 9
Views: 289

### Re: Denoting brackets/parentheses

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.
Fri Dec 07, 2018 12:40 pm
Forum: Ionic & Covalent Bonds
Topic: Ionic and Covalent Character
Replies: 3
Views: 193

### Re: Ionic and Covalent Character

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?
Thu Dec 06, 2018 1:57 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: H2O Denticity
Replies: 1
Views: 81

### H2O Denticity

Why is water monodentate? Isn't the double-bonded oxygen stable, which means it wouldn't want to share its electrons?
Wed Dec 05, 2018 7:55 pm
Forum: Bond Lengths & Energies
Topic: How to Predict Bond Lengths
Replies: 1
Views: 581

### How to Predict Bond Lengths

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?
Wed Dec 05, 2018 1:13 pm
Forum: Lewis Structures
Topic: H2SeO4 Structure
Replies: 1
Views: 97

### H2SeO4 Structure

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...
Tue Dec 04, 2018 4:11 pm
Forum: Acidity & Basicity Constants and The Conjugate Seesaw
Topic: Significance of Equilibrium Constants
Replies: 2
Views: 98

### Significance of Equilibrium Constants

Do higher equilibrium constants indicate a stronger acid/base? And would you only have to find this for weak acids?
Sun Dec 02, 2018 3:19 pm
Forum: Balancing Chemical Reactions
Topic: Final Exam Q: Memorizing Polyatomic Ions
Replies: 2
Views: 276

### Final Exam Q: Memorizing Polyatomic Ions

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!
Sun Dec 02, 2018 1:36 pm
Forum: Naming
Topic: IUPAC
Replies: 2
Views: 116

### Re: IUPAC

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
Sun Dec 02, 2018 12:13 pm
Forum: Bronsted Acids & Bases
Topic: Proton "Donation"
Replies: 4
Views: 160

### Proton "Donation"

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.
Tue Nov 27, 2018 10:13 pm
Forum: Hybridization
Topic: Drawing Hybrid Orbitals
Replies: 3
Views: 163

### Drawing Hybrid Orbitals

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?
Tue Nov 27, 2018 9:59 pm
Forum: Hybridization
Topic: HW Problem 2F.15
Replies: 1
Views: 60

### Re: HW Problem 2F.15

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...
Tue Nov 27, 2018 7:34 pm
Forum: Hybridization
Topic: 2F.13 7th Ed
Replies: 4
Views: 169

### Re: 2F.13 7th Ed

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!
Tue Nov 27, 2018 2:47 pm
Forum: Ionic & Covalent Bonds
Topic: Can an organic compound have an ionic bond?
Replies: 1
Views: 96

### Re: Can an organic compound have an ionic bond?

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.
Tue Nov 27, 2018 2:42 pm
Forum: Bond Lengths & Energies
Topic: Intermolecular Potential Energy
Replies: 1
Views: 125

### Re: Intermolecular Potential Energy

Hey! "r" is the distance between the two molecules
Tue Nov 27, 2018 2:32 pm
Forum: Hybridization
Topic: 2F.13 7th Ed
Replies: 4
Views: 169

### 2F.13 7th Ed

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?
Tue Nov 27, 2018 2:28 pm
Forum: Bond Lengths & Energies
Topic: Comparing Bond Lengths of Diff Molecules/Ions
Replies: 1
Views: 140

### Comparing Bond Lengths of Diff Molecules/Ions

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...
Fri Nov 16, 2018 10:30 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Test 3
Replies: 19
Views: 515

### Test 3

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!
Wed Nov 14, 2018 1:02 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Bent vs Angular
Replies: 3
Views: 117

### Bent vs Angular

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...
Wed Nov 14, 2018 9:24 am
Forum: Determining Molecular Shape (VSEPR)
Topic: MgO vs BaO
Replies: 5
Views: 944

### Re: MgO vs BaO

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...
Fri Nov 09, 2018 12:51 am
Forum: Electronegativity
Topic: electronegativity trends
Replies: 6
Views: 179

### Re: electronegativity trends

So do electronegativity trends essentially rely on electron shielding ? As do the other periodic table trends ?
Sat Nov 03, 2018 2:29 pm
Forum: SI Units, Unit Conversions
Topic: What is a formula unit?
Replies: 1
Views: 415

### What is a formula unit?

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?
Thu Nov 01, 2018 9:19 pm
Forum: Formal Charge and Oxidation Numbers
Topic: Formal Charge of Ions
Replies: 9
Views: 336

### Re: Formal Charge of Ions

Would you have to draw all the resonance structures of an ion/compound/etc. in order to determine the formal charge?
Thu Nov 01, 2018 9:12 pm
Forum: Resonance Structures
Topic: Definition of resonance
Replies: 7
Views: 214

### Re: Definition of resonance

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...
Sun Oct 28, 2018 6:14 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: 9C.5 7th Ed.
Replies: 2
Views: 218

### 9C.5 7th Ed.

When determining the type of ligand, should you write out Lewis structure ? Or can you just look at the chemical formula?
Thu Oct 25, 2018 9:11 pm
Replies: 4
Views: 458

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.
Mon Oct 22, 2018 8:09 pm
Forum: Lewis Structures
Topic: determining shapes of molecules
Replies: 2
Views: 127

### determining shapes of molecules

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.
Mon Oct 22, 2018 8:02 pm
Forum: Sigma & Pi Bonds
Topic: determining which bonds are in a molecule
Replies: 4
Views: 224

### determining which bonds are in a molecule

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?
Thu Oct 18, 2018 11:48 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Valence Electrons for d-block
Replies: 2
Views: 152

### Valence Electrons for d-block

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.
Thu Oct 18, 2018 11:43 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Wave Function v. Orbital
Replies: 3
Views: 81

### Re: Wave Function v. Orbital

Yes, I believe you can use wave function and orbital interchangeably. I remember Lavelle mentioning this (unless I misunderstood what he said).
Thu Oct 18, 2018 11:40 pm
Forum: Einstein Equation
Topic: Units
Replies: 17
Views: 772

### Re: Units

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.
Fri Oct 12, 2018 12:11 am
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Shells?
Replies: 7
Views: 361

### Re: Shells?

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 ?
Fri Oct 12, 2018 12:08 am
Forum: Properties of Light
Topic: Problem 1A.9 (7th edition)
Replies: 5
Views: 74

### Re: Problem 1A.9 (7th edition)

Yes, you can use both equations. You may also want to combine them for some, though I don't know if it's needed.
Tue Oct 09, 2018 4:27 pm
Forum: Photoelectric Effect
Topic: Post-Mod Problem #28
Replies: 4
Views: 154

### Post-Mod Problem #28

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...
Fri Oct 05, 2018 1:15 pm
Forum: SI Units, Unit Conversions
Topic: SI Units on tests
Replies: 5
Views: 206

### Re: SI Units on tests

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...
Wed Oct 03, 2018 8:56 pm
Forum: Molarity, Solutions, Dilutions
Topic: measuring mols vs grams in dilutions
Replies: 2
Views: 50

### measuring mols vs grams in dilutions

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 ...
Mon Oct 01, 2018 1:17 pm
Forum: Limiting Reactant Calculations
Topic: Limiting Reactant Calculation Question [ENDORSED]
Replies: 2
Views: 131

### Limiting Reactant Calculation Question[ENDORSED]

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...