Search found 53 matches

by Clara Rehmann 1K
Tue Mar 20, 2018 10:36 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: kinetic data
Replies: 2
Views: 271

Re: kinetic data

One strategy is to experimentally determine what reactants are colliding with each other, since that is the most basic step of a mechanism. For example, if you are determining if A+B->C or A->C, one way to determine which mechanism is correct is by seeing if A produces C in the absence of B.
by Clara Rehmann 1K
Tue Mar 20, 2018 10:34 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Difference in cell potential
Replies: 2
Views: 413

Re: Difference in cell potential

The difference in cell potential would be the difference between E and Eº, yes
by Clara Rehmann 1K
Tue Mar 20, 2018 10:33 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell Potential sign
Replies: 4
Views: 272

Re: Cell Potential sign

Yes, mathematically you are also switching the Eº for each side.
by Clara Rehmann 1K
Sun Mar 11, 2018 1:56 pm
Forum: *Enzyme Kinetics
Topic: Catalysts and activation energy?
Replies: 2
Views: 169

Catalysts and activation energy?

I had to miss the second half of lecture on Friday - can someone quickly explain to me the relationship between catalysts and activation energy and how that is calculated?
by Clara Rehmann 1K
Sun Mar 11, 2018 1:54 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Galvanic Cells
Replies: 2
Views: 134

Re: Galvanic Cells

This video https://www.youtube.com/watch?v=wGUKtJZJZUk helped me understand it a bit better - basically, electric current causes the "free" electrons to move from atom to atom of the conducting material.
by Clara Rehmann 1K
Sun Mar 11, 2018 1:51 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Activation Energies
Replies: 1
Views: 106

Re: Activation Energies

I don't think so; or if so, the relationship isn't quantifiable
by Clara Rehmann 1K
Sun Mar 04, 2018 4:53 pm
Forum: Second Order Reactions
Topic: Second Order/Zero Order Half-Life
Replies: 2
Views: 142

Re: Second Order/Zero Order Half-Life

It possibly may have to do with the way molecules of these reactants interact with each other? I think we'd have to learn more about the mechanics of 0, 1st, and 2nd order reactions in order to understand the exact ways in which they differ, but I would guess that more of these molecules being in a ...
by Clara Rehmann 1K
Sun Mar 04, 2018 4:50 pm
Forum: First Order Reactions
Topic: Clarification on Integrated Rate Law
Replies: 1
Views: 124

Re: Clarification on Integrated Rate Law

I'd assume you use it until the reaction reaches equilibrium, at which point A is no longer being converted into products.
by Clara Rehmann 1K
Sun Mar 04, 2018 4:49 pm
Forum: First Order Reactions
Topic: half life
Replies: 2
Views: 146

Re: half life

I believe it depends on the order of the reaction.
by Clara Rehmann 1K
Thu Feb 22, 2018 2:14 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: HW 14.117
Replies: 2
Views: 183

Re: HW 14.117

Start with ΔG = -nFE and It = nF. Combining these equations, you get I=

Knowing that Eº = ~1 V, you can plug in values (ΔG = -10 x 106, E = 1, t = 86400) and and solve for a solution in amps
by Clara Rehmann 1K
Thu Feb 22, 2018 2:14 pm
Forum: Interesting Applications: Rechargeable Batteries (Cell Phones, Notebooks, Cars), Fuel Cells (Space Shuttle), Photovoltaic Cells (Solar Panels), Electrolysis, Rust
Topic: 14.117
Replies: 2
Views: 320

Re: 14.117

Start with ΔG = -nFE and It = nF. Combining these equations, you get I=

Knowing that Eº = ~1 V, you can plug in values (ΔG = -10 x 106, E = 1, t = 86400) and and solve for a solution in amps
by Clara Rehmann 1K
Thu Feb 22, 2018 2:07 pm
Forum: Balancing Redox Reactions
Topic: 14.11
Replies: 2
Views: 176

Re: 14.11

Yes, but the catch with this problem is that the battery does not have a positive voltage, as the Eº of the anode is greater than that of the cathode. Pt(s) | O 2 (g) | H + (aq) || OH - (aq) | O 2 (g) | Pt(s) Overall reaction: 4H 2 O(l) → 4H + (aq) + 4OH - (aq) Anode: O 2 (g) + 4H + (aq) → 2H 2 O(l)...
by Clara Rehmann 1K
Thu Feb 15, 2018 7:33 pm
Forum: General Science Questions
Topic: Battery voltage
Replies: 4
Views: 367

Battery voltage

It seems as if most of the redox reactions that we are studying don't produce very high voltage - in my homework problems, voltage always is under about 2V. How do things that we use in our everyday lives (batteries, electronics) achieve such high voltage?
by Clara Rehmann 1K
Thu Feb 15, 2018 7:27 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Variation of Standard Potential in the Periodic Table
Replies: 1
Views: 86

Re: Variation of Standard Potential in the Periodic Table

Standard potential follows the same trend as electron affinity - an element that holds onto its electrons more tightly will generate more energy when those electrons are taken away.
by Clara Rehmann 1K
Wed Feb 07, 2018 1:41 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Needing a conductor or not?
Replies: 2
Views: 129

Needing a conductor or not?

Today in lecture Lavelle mentioned that if there is a conductive material present in the cell, then both products and reactants can be in the same solution rather than being kept separate with a conductor and salt bridge in between (at least this is what I understood him to be saying; that could be ...
by Clara Rehmann 1K
Wed Feb 07, 2018 1:38 pm
Forum: Phase Changes & Related Calculations
Topic: Endothermic/exothermic
Replies: 4
Views: 358

Re: Endothermic/exothermic

Regarding the question about the sun, you would know that there is heat exchange because the sun is causing the water to evaporate - it's transferring Joules of energy into the water to the point where the water reaches Hvap. As far as processes being exothermic or endothermic, for the sun the proce...
by Clara Rehmann 1K
Wed Feb 07, 2018 1:36 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Half Reactions
Replies: 6
Views: 216

Re: Half Reactions

I think either is okay, but to be thorough maybe label them as "reduction" and "oxidation" or "anode" and "cathode."
by Clara Rehmann 1K
Sat Feb 03, 2018 7:02 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Which value of R to use?
Replies: 3
Views: 129

Which value of R to use?

Hi, I'm a bit confused as to deciding which value of R to use when calculating entropy - Lavelle's cheat sheet has a couple different values (based on changes in units). I try to base my choice of R off of the other units I am provided with when solving a problem, but that doesn't seem to always wor...
by Clara Rehmann 1K
Sat Feb 03, 2018 6:57 pm
Forum: Calculating Work of Expansion
Topic: Reversible vs Irreversible [ENDORSED]
Replies: 3
Views: 191

Re: Reversible vs Irreversible [ENDORSED]

A reversible process does more work because it is working against a constant pressure, meaning the pressure in the system changes slowly. An irreversible process, on the other hand, involves fast pressure changes that limit the system's ability to do work. I found the graphs that Lavelle showed duri...
by Clara Rehmann 1K
Sat Feb 03, 2018 6:51 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Using Appropriate Equation
Replies: 2
Views: 86

Re: Using Appropriate Equation

I think you are correct because Gibbs' free energy is temperature dependent, and thus the standard Gibbs' free energy should be at a certain temperature.
by Clara Rehmann 1K
Thu Jan 25, 2018 2:12 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Bond enthalpies vs enthalpy of rxn - conceptual [ENDORSED]
Replies: 3
Views: 177

Re: Bond enthalpies vs enthalpy of rxn - conceptual [ENDORSED]

No matter how we're calculating enthalpy, we want the total "additive" energy in the reaction. When calculating enthalpy using bond enthalpies, I look it like this: breaking bonds requires energy, contributing positive values to my total (energy going in), while making bonds releases energ...
by Clara Rehmann 1K
Mon Jan 22, 2018 12:53 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Week 3 Homework [ENDORSED]
Replies: 1
Views: 136

Re: Week 3 Homework [ENDORSED]

Monday's class gives a good baseline for some of Chapter 9's problems; I would do the ones you can given the equations Lavelle gave us and then fill in the rest of the seven with problems from Chapter 8. My understanding is that as long as it's relevant to what we're doing in class, we can do whatev...
by Clara Rehmann 1K
Mon Jan 22, 2018 12:51 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.45- why do you multiply?
Replies: 5
Views: 176

Re: 8.45- why do you multiply?

In the original equation, we have 1 mol of S8. We want to calculate the enthalpy when there's 1.25 mol of S8. To get 1.25 mol S8, you multiply 1 mol of S8 by 1.25. Multiply the reaction's enthalpy by the same amount to get the enthalpy for 1.25 moles.
by Clara Rehmann 1K
Wed Jan 17, 2018 12:36 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.51 energy "released"
Replies: 4
Views: 184

Re: 8.51 energy "released"

I assumed that the value was positive because we were looking at the energy from the perspective of the surroundings rather than the perspective of the system. Technically the TNT is losing energy by releasing it, but when observing that process from the outside, a positive amount of energy is being...
by Clara Rehmann 1K
Tue Jan 16, 2018 12:30 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Ch. 8 #67
Replies: 2
Views: 145

Re: Ch. 8 #67

You are calculating the enthalpy of formation, which means you need to calculate the enthalpies for each bond broken and formed. Approach the calculation for C6H6 the same way you did parts (a) and (b) - in part (c), take into account the bonds in a standard structure, and in part (d), take into acc...
by Clara Rehmann 1K
Tue Jan 16, 2018 12:26 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Pressure in enthalpy vs heat capacity
Replies: 2
Views: 120

Re: Pressure in enthalpy vs heat capacity

I think that unless stated, you assume standard pressure. Otherwise, there's really no way of knowing how to calculate it.
by Clara Rehmann 1K
Fri Jan 12, 2018 1:16 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Standard Enthalpy of Formation vs Standard Enthalpy of Reaction
Replies: 3
Views: 133

Re: Standard Enthalpy of Formation vs Standard Enthalpy of Reaction

To add on to what other posters have replied, standard enthalpy of formation requires that reactants must be at their most stable states.
by Clara Rehmann 1K
Fri Jan 12, 2018 1:13 pm
Forum: Administrative Questions and Class Announcements
Topic: Times to post per week
Replies: 5
Views: 278

Re: Times to post per week

Yes, three posts/replies per week.
by Clara Rehmann 1K
Thu Jan 11, 2018 12:50 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Relating energy and heat [ENDORSED]
Replies: 2
Views: 136

Relating energy and heat [ENDORSED]

Are the terms "energy" and "heat" interchangeable? And, if so, how does the measurement of temperature relate to the measurement of energy applied to/released by a substance?
by Clara Rehmann 1K
Thu Jan 11, 2018 12:38 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Predicting Molar Heat Capacity
Replies: 2
Views: 161

Re: Predicting Molar Heat Capacity

Because NO2 has more bonds and atoms, it has a higher heat capacity. The additional matter means that there is greater ability to contain energy, i.e. heat.
by Clara Rehmann 1K
Mon Dec 04, 2017 3:32 pm
Forum: Bronsted Acids & Bases
Topic: Identifying an acid vs. a base
Replies: 1
Views: 141

Identifying an acid vs. a base

Is there a trick for telling if a molecule will give off a proton or accept one? It's sometimes difficult to tell, especially when there are a lot of H's already in the molecule.
by Clara Rehmann 1K
Mon Dec 04, 2017 3:29 pm
Forum: Dipole Moments
Topic: Polar molecules [ENDORSED]
Replies: 2
Views: 198

Polar molecules [ENDORSED]

If all molecules have dipole moments, how do you tell when the dipole moment is great enough to create a polar molecule without having access to specific numbers?
by Clara Rehmann 1K
Tue Nov 28, 2017 1:19 pm
Forum: Hybridization
Topic: Hybridization and lone pairs
Replies: 3
Views: 194

Re: Hybridization and lone pairs

Yes - hybridization refers to regions of electron density so lone pairs contribute to it. The C atom in CH3 for example is sp3 hybridized because it has four regions of electron density (three in its bonds with H atoms and one as a lone pair).
by Clara Rehmann 1K
Tue Nov 28, 2017 1:17 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Electron Arrangement and Molecular Shape
Replies: 4
Views: 175

Re: Electron Arrangement and Molecular Shape

Electron arrangement refers to the position of electrons as well as the position of atoms in a molecule. Molecular shape essentially ignores extra electrons and just looks at the atoms in the molecule - for example, H2O is "bent" rather than tetrahedral because you look at just the atoms, ...
by Clara Rehmann 1K
Mon Nov 20, 2017 3:59 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 11.11 Part b
Replies: 2
Views: 224

Re: 11.11 Part b

There's more O2 in the same size space (.1 mol in one L vs. .5 mol in one L), thus the second container will always have a higher concentration of O2.
by Clara Rehmann 1K
Mon Nov 20, 2017 3:56 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Coordination Number
Replies: 2
Views: 150

Re: Coordination Number

A coordination number refers to the number of bonds formed rather than the number of ligands, so if a ligand is polydentate it will be contributing [number of bonds] to the coordination number rather than just the one ligand.
by Clara Rehmann 1K
Mon Nov 20, 2017 3:54 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Homework Question 11.7 [ENDORSED]
Replies: 4
Views: 246

Re: Homework Question 11.7 [ENDORSED]

Notice that you can see which molecules have decayed within the flask. Start with counting out how many molecules have split apart!
by Clara Rehmann 1K
Tue Nov 14, 2017 12:22 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Sigma vs. Pi Bonds
Replies: 5
Views: 255

Re: Sigma vs. Pi Bonds

Sigma bonds only have one field of electron sharing, so the atoms involved in a sigma bond can rotate independently of one another on that sigma-bond axis and still remain bonded. However, because pi bonds have two areas of electron sharing, no rotation is allowed: If an atom tried to rotate on one ...
by Clara Rehmann 1K
Tue Nov 14, 2017 12:19 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: "more or less than 109.5 degrees"?
Replies: 4
Views: 343

Re: "more or less than 109.5 degrees"?

Lone pairs are a little more electronegative than bonded pairs, so they "push away" a little more as well. This results in the angle of the bonds being smaller, since they are being pushed together by the electronegativity of the lone pairs.
by Clara Rehmann 1K
Tue Nov 07, 2017 7:15 am
Forum: Dipole Moments
Topic: How do Dipole moments determine polarity?
Replies: 4
Views: 272

Re: How do Dipole moments determine polarity?

Dipole moments represent the charge difference in a molecule. The arrow points toward the δ- atom. If the dipole moments do not cancel each other out (ie, if the arrows do not point at each other) then the molecule is polar because there is a charge difference across the molecule.
by Clara Rehmann 1K
Tue Nov 07, 2017 7:12 am
Forum: Molarity, Solutions, Dilutions
Topic: Test 1 Question Q2B
Replies: 4
Views: 615

Re: Test 1 Question Q2B

If you know the molarity of the stock solution, you can use the equation MiVi = MfVf, with Mf being .650M and Vf being .25 L.
by Clara Rehmann 1K
Mon Oct 30, 2017 6:11 pm
Forum: *Shrodinger Equation
Topic: "Describe the interpretation of atomic orbitals in terms of probability"
Replies: 1
Views: 243

Re: "Describe the interpretation of atomic orbitals in terms of probability"

That sounds about right. The most important thing to note is that atomic orbitals are three-dimensional areas where the probability of finding an electron is above a certain threshold. I'm not sure that just writing out the wave equation will be enough; you should demonstrate that you understand tha...
by Clara Rehmann 1K
Mon Oct 30, 2017 6:09 pm
Forum: Ionic & Covalent Bonds
Topic: determining bonds
Replies: 4
Views: 239

Re: determining bonds

You can predict the nature of the bond based on the elements in it and how electrons need to be shared. If both can fulfill the octet rule (or whatever rule they need to fulfill) by just giving/receiving an electron, then it's probably an ionic bond, but if it looks like electrons need to be shared ...
by Clara Rehmann 1K
Mon Oct 23, 2017 3:51 pm
Forum: Properties of Light
Topic: Coulomb” potential
Replies: 2
Views: 218

Re: Coulomb” potential

We haven't learned the equation for potential energy, but we know that it is proportional to (meaning it changes at the same rate as) (Q1)(Q2)/r where the Q's are the charges and r is the distance between the two. Because r is in the denominator, as it increases (the charges get farther apart), the ...
by Clara Rehmann 1K
Mon Oct 23, 2017 3:44 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: 2.43 (a) Silver
Replies: 4
Views: 237

Re: 2.43 (a) Silver

Jessica is right - there are more exceptions than Cr and Cu, but those are the only two we learned about. Having 5 or 10 electrons in the d orbital is more stable than having 4 or 9, so any d-block element that ends up with 4 or 9 electrons in the d orbital will actually have 5 or 10 (and then just ...
by Clara Rehmann 1K
Fri Oct 20, 2017 7:29 am
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Nodal Planes
Replies: 2
Views: 183

Re: Nodal Planes

Nodal planes are similar to nodes in that no electrons are found in that space. The difference is that a node is just a single point, while a nodal plane is a "sheet" (remember, we are dealing with 3D space) through which electrons don't travel. Think of it as just a really long/tall node,...
by Clara Rehmann 1K
Fri Oct 20, 2017 7:26 am
Forum: *Shrodinger Equation
Topic: Ψ^2
Replies: 2
Views: 178

Re: Ψ^2

Ψ^2 is the wave function squared, meaning that all values of the wave function are positive. It equals the probability of finding an electron in that space, since electrons behave like waves.
by Clara Rehmann 1K
Fri Oct 20, 2017 7:16 am
Forum: Photoelectric Effect
Topic: Problem in post assessment
Replies: 2
Views: 150

Re: Problem in post assessment

Does the question supply any other information? Also, does it specify that it is referring to the photoelectric effect?
by Clara Rehmann 1K
Fri Oct 13, 2017 7:19 am
Forum: Einstein Equation
Topic: Photoelectric Post-Module 29 [ENDORSED]
Replies: 2
Views: 204

Re: Photoelectric Post-Module 29 [ENDORSED]

The 150.6 kj is required to remove electrons from a mole of sodium. The question is asking the amount of energy needed for just one sodium atom, so some conversions need to happen - find the amount of energy required for just the one atom. Hope this helps you piece it together!
by Clara Rehmann 1K
Fri Oct 13, 2017 7:16 am
Forum: Properties of Electrons
Topic: Energy Levels
Replies: 6
Views: 360

Re: Energy Levels

Electrons hop up between discrete energy levels when they are excited by a photon. What connects them to spectral lines is that the "hopping" of an electron from one energy level down to a lower energy level releases a photon of a certain energy level and wavelength. The energy level, and ...
by Clara Rehmann 1K
Wed Oct 04, 2017 4:44 pm
Forum: Molarity, Solutions, Dilutions
Topic: G5
Replies: 1
Views: 257

Re: G5

1. Find the number of moles of sodium carbonate added (2.111g / molar mass) 2. Find molarity of the solution (M = n / V) - don't forget to convert 250.0 mL to L! a) 1. Convert mmol of Na+ to mol 2. Knowing that there are 2 Na+ molecules in each sodium carbonate molecule, the molarity of Na+ in the s...
by Clara Rehmann 1K
Wed Oct 04, 2017 4:29 pm
Forum: Significant Figures
Topic: If last digit is 5: round to nearest even number?
Replies: 2
Views: 430

Re: If last digit is 5: round to nearest even number?

Rounding to the nearest even number (eg. 2.5 -> 2, 5.5 -> 6) prevents there from being a significant shift up in the quantities you have. If you always always always round 5 up, especially with a big data set, you'll end up with the average of all your numbers being shifted up. Rounding to the neare...

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