Search found 54 matches

by David S
Fri Mar 15, 2019 5:29 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 6.N.15 7th Ed
Replies: 1
Views: 53

Re: 6.N.15 7th Ed

Looks like a concentration cell to me. If we want our concentration cell to do work, Ecell has to be positive. Since Ecell = -(RT/nF)lnQ (E° = 0 for conc. cell), and the ln of numbers less than 1 is negative, we know that we want a Q that is less than one. For Q to be less than one, [P]<[R]. Thus, m...
by David S
Fri Mar 15, 2019 5:20 pm
Forum: Polyprotic Acids & Bases
Topic: Ka and Ka2
Replies: 1
Views: 254

Re: Ka and Ka2

You first set up ice table and Ka expression for the diprotic acid, and then you solve for [H+]and [HA-]. You then set up separate ice table and Ka2 expression for the dissociation of HA- into A2- and H+ using the [H+] and [HA-] found in part 1. Solve for new [H+] and then turn it into pH. Note that...
by David S
Fri Mar 15, 2019 5:11 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: using q=n(deltaH) vs q=mc(deltaT)
Replies: 4
Views: 277

Re: using q=n(deltaH) vs q=mc(deltaT)

The total heat involved in the process = heat needed to melt the ice into liquid (q=n∆H melting) + heat needed to raise temperature of liquid from 0°C to 20°C (q=nC∆T)
by David S
Sun Mar 10, 2019 6:31 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Self-Test 11.11A in textbook 6th Edition
Replies: 1
Views: 47

Re: Self-Test 11.11A in textbook 6th Edition

I got 0.3 and here is how: 1) K=\frac{(P_{Br_{2}})(P_{Cl_{2}})}{(P_{BrCl})^{2}} 2) After setting up ice table and finding expressions for equilibrium pressures in terms of x, you get: K = \frac{x^{2}}{(3.30-2x)^{2}} 3) multiply both sides by denominator and then simpl...
by David S
Sun Mar 10, 2019 5:52 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Approximation
Replies: 7
Views: 128

Re: Approximation

You can only use the approximation when equilibrium constant K is notably small. In class, lavelle told us that in general, when K < 10^-3, we can assume the amount of reactant lost to form product (x) is small enough to ignore. After using this approximation to solve for x, you have to check it usi...
by David S
Sun Mar 10, 2019 5:25 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Reaction Profiles
Replies: 2
Views: 44

Re: Reaction Profiles

The adjective "spontaneous" is specifically used in this case to describe a reaction with a negative delta G (Greactant > Gproduct). The activation energy has to do with kinetics, which only has to do with rates of reaction, so you dont have to pay attention to it when determining whether ...
by David S
Sun Mar 03, 2019 3:54 pm
Forum: Balancing Redox Reactions
Topic: Acidic vs basic solutions
Replies: 10
Views: 163

Re: Acidic vs basic solutions

I'm assuming you are referring to balancing redox reactions in acidic or basic conditions. When you are balancing equations, you want to make sure that the same amount of atoms are on both sides of the equation right? This applies to hydrogen as well, and when you are balancing redox reactions that ...
by David S
Sun Mar 03, 2019 3:26 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Corrosion and Ion Selective Electrodes
Replies: 4
Views: 93

Re: Corrosion and Ion Selective Electrodes

Looking at Professor Lavelle's learning outcomes for Electrochemistry (you can access this on the 14B class website), I do not see any explicit emphasis on those types of electrodes so you don't have to really know about it in detail. However, you should know about inert electrodes (e.g. Platinum, G...
by David S
Sun Mar 03, 2019 3:12 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: delta G
Replies: 5
Views: 93

Re: delta G

Delta G, or the change in Gibbs free energy, represents the change in the max amount of energy available to do non-expansion work at a given Temp and Pressure. In other words \Delta G = w_{max} @ const. T and P. Exergonic processes have negative Delta G values, which corresponds to the release of en...
by David S
Sun Feb 24, 2019 3:48 pm
Forum: Balancing Redox Reactions
Topic: Oxidizing and Reducing Agents
Replies: 2
Views: 60

Re: Oxidizing and Reducing Agents

I believe so, because the oxidizing agent is the reactant in the reduction half reaction, and the reducing agent is the reactant in the oxidation half reaction. Since the total redox reaction is the simple sum of the balanced half reactions, both the oxidizing and reducing agents end up in the react...
by David S
Sun Feb 24, 2019 3:44 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Electrodes
Replies: 2
Views: 71

Re: Electrodes

I don't believe we need to use Platinum as an electrode here because the copper and zinc can react with their solid electrodes to be reduced and oxidized respectively, making both metals decent conductors of electricity. We would need an inert Platinum electrode to act as an electricity conductor if...
by David S
Sun Feb 24, 2019 3:20 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Not at Equilibrium
Replies: 3
Views: 66

Re: Not at Equilibrium

I think you use
by David S
Sun Feb 17, 2019 9:58 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Van't Hoff Equation
Replies: 1
Views: 165

Re: Van't Hoff Equation

The Van't Hoff equation is meant to show the Temperature dependance of K, and while it is derived from an expression that relates G to K, in its ultimate for the equation excludes G. Derivation: 1) Given \Delta G = -RTln(K) and \Delta G = \Delta H - T\Delta S : \Delta H - T\Delta S = -RTln&#...
by David S
Sun Feb 17, 2019 9:16 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Differences in Gibbs Free Energy
Replies: 3
Views: 53

Re: Differences in Gibbs Free Energy

K < 1 implies that at a given temp, the reactants are more favored than products, and thus the forward reaction to form more products is unfavorable. This is confirmed by a Positive Delta G, which tells us that the forward reaction is not spontaneous (endergonic), and needs energy/work input to be d...
by David S
Sun Feb 17, 2019 8:46 am
Forum: Third Law of Thermodynamics (For a Unique Ground State (W=1): S -> 0 as T -> 0) and Calculations Using Boltzmann Equation for Entropy
Topic: Derivations
Replies: 3
Views: 238

Re: Derivations

We use this equation to calculate the equilibrium constant K for a reaction at a different, new temperature (K is dependent on temperature). Specifically, K1 is your currently known equilibrium constant at initial Temperature (T1). Then, given that you know the reaction enthalpy, you use this equati...
by David S
Fri Feb 08, 2019 9:08 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: 6th edition Question 8.67
Replies: 2
Views: 82

Re: 6th edition Question 8.67

The opposite would be the negative value of \Delta H_{vap} . This is because if you put in X amount of heat energy to get a certain liquid to become a gas, that gas must lose the same amount of energy to return to being a liquid. Losing energy => exothermic process => negative enthalpy for the proce...
by David S
Fri Feb 08, 2019 8:58 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Degeneracy
Replies: 1
Views: 58

Re: Degeneracy

The degeneracy of a system is basically the number of ways you can arrange the components of the system (molecules, atoms, etc.) while keeping the energy level of the system constant. In the simple cases we covered in class (Atoms A and B being in the left or right flask), degeneracy (W) = (# of pos...
by David S
Fri Feb 08, 2019 8:35 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Systems
Replies: 2
Views: 65

Re: Systems

An open system allows both matter and energy to enter/leave the system. Think of a cup of tea with no lid on; the water is leaving the cup in the form of vapor, and the tea cools over time because there is heat transfer from the system to the surroundings. A closed system on the other hand is when e...
by David S
Sun Jan 27, 2019 1:09 pm
Forum: Phase Changes & Related Calculations
Topic: State Property
Replies: 3
Views: 61

Re: State Property

Imagine if you are to climb up a mountain. Let's say I ask you for the distance you traveled to get to the top, which is 400 meters above the starting point altitude wise. If you follow a trail the distance to the top might be 10 miles. But if you take a bunch of shortcuts, you might get there after...
by David S
Sun Jan 27, 2019 12:54 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Accuracy of Enthalpy Calculation
Replies: 3
Views: 55

Re: Accuracy of Enthalpy Calculation

This is because in reality, the strength and energy of a bond between individual atoms is influenced by other electrostatic forces in more complex molecules, and so therefore there is no single enthalpy value to easily describe most bonds. Take for example a bond enthalpy for O-H. Not all O-H bonds ...
by David S
Sun Jan 27, 2019 12:27 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: The 3 methods
Replies: 5
Views: 81

Re: The 3 methods

In theory, with enough reference tables, you could use whatever method, but in certain problems and situations, given info will make one method more preferable than another. For example, If I want to calculate the enthalpy of a reaction, and that reaction is a sum of other reations whose enthalpies ...
by David S
Sun Jan 13, 2019 3:17 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Result of Only Adding One of the Reactants
Replies: 4
Views: 49

Re: Result of Only Adding One of the Reactants

Yes, because by increasing the concentration of any one reactant, the denominator (i.e. the product of reactant concentrations) of the quotient Q will become greater than the denominator of K at equilibrium. Thus, Q < K, and reaction will proceed to balance this larger denominator out by creating mo...
by David S
Sun Jan 13, 2019 2:40 pm
Forum: Administrative Questions and Class Announcements
Topic: HW 2 Due Date
Replies: 4
Views: 91

Re: HW 2 Due Date

Homeworks are due on whatever day you have discussion, and you turn it in during discussion
by David S
Sun Jan 13, 2019 2:28 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 11.49 6th Edition
Replies: 2
Views: 33

Re: 11.49 6th Edition

Pure solids and liquids are considered to have an activity level of 1, which roughly equates to them having no effect on equilibrium concentrations
by David S
Thu Dec 06, 2018 4:22 pm
Forum: Bronsted Acids & Bases
Topic: Distinguishing bronsted acids vs bases
Replies: 3
Views: 108

Re: Distinguishing bronsted acids vs bases

HPO4^2- is an amphiprotic derivative of H3PO4. We know that with polyprotic acids, after the first proton is lost, subsequent amphiprotic acids formed are substantially harder to deprotonate (removing an H+ from an increasingly negative anion molecule is harder); the increase in pKa with each additi...
by David S
Thu Dec 06, 2018 3:53 pm
Forum: Calculating the pH of Salt Solutions
Topic: Ka Constant
Replies: 4
Views: 237

Re: Ka Constant

No, but along with what others mention, just know that higher Ka --> lower pKa = stronger acid, higher Kb --> lower pKb = stronger base.
by David S
Thu Dec 06, 2018 3:20 pm
Forum: Lewis Acids & Bases
Topic: Resonance and acids
Replies: 2
Views: 79

Re: Resonance and acids

Resonance definitely plays a role as a form of charge delocalization. To explain, I'll use the fact that when an acid has proton transfer reaction with water, it produces a conjugate base (can be anion like ClO4- or neutral molecule like NH3). If the conjugate base is strong, then a majority of the ...
by David S
Sun Dec 02, 2018 12:05 pm
Forum: Naming
Topic: If a ligand has a name with...
Replies: 5
Views: 94

Re: If a ligand has a name with...

Another example is Na[Fe(OH2)_2(C2O4)_2]. The ligand C2O4^2- is oxalate, which is bidentate. Since we have two of these polydentate anions, we use bis- prefix to describe them in the full name (even though the name of the anion doesn't contain di-, tri-, etc.): Sodium diaqua bis oxalatoferrate(III)
by David S
Sun Dec 02, 2018 11:50 am
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Oxidation number
Replies: 1
Views: 53

Re: Oxidation number

The oxidation state of the TM is often given in the name of the compound. If you only have the formula and not the name, then here are the steps. 1) If you aren't given the net charge of the coordination compound (everything inside the brackets), figure out the net charge of the coordination compoun...
by David S
Sun Dec 02, 2018 11:18 am
Forum: Shape, Structure, Coordination Number, Ligands
Topic: why do we care about coordination
Replies: 2
Views: 49

Re: why do we care about coordination

Coordinate covalent bonding is a key underlying concept in acid and base chemistry, as acids and bases interact via coordinate covalent bonds (think Lewis definition). Understanding the mechanisms of acids and bases allows us to better understand the way acids and bases behave in biological systems,...
by David S
Sun Nov 25, 2018 12:38 pm
Forum: Dipole Moments
Topic: *Hydrogen Bonding
Replies: 2
Views: 99

Re: *Hydrogen Bonding

Yes, hydrogen bonds typically occur when a hydrogen is attached to a very electronegative atom (N, O, F) because this results in a strong partial positive charge on the hydrogen. When a molecule containing this sort of H to N, O, F bond encounters another such molecule, a hydrogen bond will form if ...
by David S
Sun Nov 25, 2018 12:05 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Bonds
Replies: 2
Views: 37

Re: Bonds

By ratios, I'll take it that you mean a particular orbital in question doesn't strictly exist between two directly bonded atoms. You can definitely have delocalized π orbitals that spread over entire portions of molecules, and not belonging wholly between two directly bonded atoms. You will have who...
by David S
Sun Nov 25, 2018 11:38 am
Forum: Interionic and Intermolecular Forces (Ion-Ion, Ion-Dipole, Dipole-Dipole, Dipole-Induced Dipole, Dispersion/Induced Dipole-Induced Dipole/London Forces, Hydrogen Bonding)
Topic: Determining Bond Strength with Resonance
Replies: 3
Views: 127

Re: Determining Bond Strength with Resonance

Resonance essentially means that the actual model of a molecule is the hybrid of equally favorable lewis structures. One attribute of hybrid structures is that they have characteristics lying in between the attributes of the constituent structures. Let's take sulfate ion for example. In sulfate ion,...
by David S
Fri Nov 16, 2018 1:23 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Lone pairs
Replies: 2
Views: 43

Re: Lone pairs

Lone pairs remain closer to the central atom because they are not being pulled by another atom into a central bonding region and occupy a slightly more diffuse space. This icreased penetration by the lone pair makes it a better repeller (like how internal, non valence electrons are better at shieldi...
by David S
Fri Nov 16, 2018 11:45 am
Forum: Determining Molecular Shape (VSEPR)
Topic: Molecular Geometry vs Electron-Pair Geometry
Replies: 4
Views: 185

Re: Molecular Geometry vs Electron-Pair Geometry

Electron pair geometry accounts for lone pairs and bonding pairs. Since it discounts the differences in strength of repulsion between the different types of electron density regions, the electron pair geometry gives us the idealized (equal repulsion) bond angles around a designated central atom; bon...
by David S
Fri Nov 16, 2018 11:33 am
Forum: Determining Molecular Shape (VSEPR)
Topic: Octahedral
Replies: 3
Views: 83

Re: Octahedral

It's simply a matter of convention. Sure, you can name it "square bipyramidal," and that can make sense, but somebody along the way decided to call it octahedral, and that name became the popular choice among scientists.
by David S
Sun Nov 11, 2018 12:56 pm
Forum: Interionic and Intermolecular Forces (Ion-Ion, Ion-Dipole, Dipole-Dipole, Dipole-Induced Dipole, Dispersion/Induced Dipole-Induced Dipole/London Forces, Hydrogen Bonding)
Topic: Dipole vs Induced Dipole
Replies: 3
Views: 134

Re: Dipole vs Induced Dipole

A Dipole refers to a molecule that has a true and constant Dipole moment. Polar molecules, due to electronegativity differences between constituent atoms, will have permanent partial positive and partial negative regions. When these partial charge regions are oriented such that the resulting dipoles...
by David S
Sun Nov 11, 2018 12:03 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Angles of Shapes [ENDORSED]
Replies: 4
Views: 181

Re: Angles of Shapes [ENDORSED]

While it is true that we probably should memorize the bond angles pertaining to the different electron geometries, you dont have to (and probably shouldn't memorize) molecular geometries. Overall it's best I think to memorize the electron geometries (linear, trigonal planar, tetrahedral, trigonal bi...
by David S
Thu Nov 08, 2018 2:19 pm
Forum: Sigma & Pi Bonds
Topic: four bonds
Replies: 3
Views: 247

Re: four bonds

If you are talking about 4 bonds between two atoms in a molecule, a quadruple bond, is pretty rare and usually only happens with transition metals that have valence d orbitals that can combine to form delta bonds. Quadruple bond consists of one sigma bond, two pi bonds, and 1 delta bond. If you are ...
by David S
Fri Nov 02, 2018 8:03 pm
Forum: Trends in The Periodic Table
Topic: The exception of ionization energies
Replies: 1
Views: 390

Re: The exception of ionization energies

In this case, the key here is that Oxygen and the other elements in it's group have just one p orbital with a paired electron. Since paired electrons experience more electron-electron repulsion, they are less tightly held by the atom and thus the ionization energy is lower (it is easier to remove th...
by David S
Fri Nov 02, 2018 7:42 pm
Forum: Ionic & Covalent Bonds
Topic: Writing Chemical Formulas
Replies: 1
Views: 57

Re: Writing Chemical Formulas

The nomenclature suggests that hydrogen is acting more like an anion in this case (hence the -ide ending in hydride) because Hydrogen is more electronegative than aluminum (2.1 vs 1.5). That being said, Aluminum is acting like a +3 cation in this case. Since each hydrogen is basically a -1, we need ...
by David S
Fri Nov 02, 2018 7:30 pm
Forum: *Shrodinger Equation
Topic: Probability Density
Replies: 1
Views: 214

Re: Probability Density

Think of mass density, where it's the amount of mass occupying a space divided by the volume of a space. When an object has higher mass density, that means there is more mass packed within the same sized space compared to something that is less dense. With probability density, it works the same way....
by David S
Tue Oct 23, 2018 11:28 am
Forum: Photoelectric Effect
Topic: HW Question 1.57
Replies: 4
Views: 194

Re: HW Question 1.57

First, we must recognize that emitted radiation wavelengths in the Balmer series correspond to electron transitions from n_i > 2 to n_f =2. Next, remember that a larger transition -> higher energy radiation ∝ smaller wavelength radiation. This means that you are going to be calculating a wavelength ...
by David S
Tue Oct 23, 2018 10:59 am
Forum: *Shrodinger Equation
Topic: Schrodinger's Equation
Replies: 6
Views: 484

Re: Schrodinger's Equation

What you need to know about the Schrodinger equation is: ⋅ It is a fundamental basis for Quantum Mechanics, integrating DeBroigle's concepts and Heisenberg's uncertainty principle. Some describe it as the "Newton's Laws" of Quantum Mechanics ⋅ The wavefunction, \psi , h...
by David S
Tue Oct 23, 2018 10:17 am
Forum: Properties of Electrons
Topic: Effective Nuclear Charge
Replies: 1
Views: 133

Re: Effective Nuclear Charge

Effective nuclear charge, Z_{eff} , describes the nuclear charge experienced by an electron (we usually focus on the valence electrons when talking about it), and is often less than the actual nuclear charge because it accounts for electron-electron repulsion. Take for example, Carbon, which has 6 p...
by David S
Sun Oct 21, 2018 6:28 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Energy levels of orbitals?
Replies: 10
Views: 143

Re: Energy levels of orbitals?

Within the same energy level, the order goes s<p<d< f. If you want to predict order the orbitals for electron configurations in order of filling (without periodic table), a little trick is to compare the sum of n and l for two different orbitals. The one with a smaller sum goes first. If the sum is ...
by David S
Sun Oct 21, 2018 5:46 pm
Forum: *Shrodinger Equation
Topic: Use of Schrodinger equation
Replies: 3
Views: 118

Re: Use of Schrodinger equation

The shrodinger equation is conceptually important in chemistry, since we can use the equation to produce the mathematical model for orbitals (the wavefunction squared helps model a probability density for where electrons are likely to be found). In our class, we DO NOT need to know how to ccompute s...
by David S
Sun Oct 21, 2018 5:37 pm
Forum: Quantum Numbers and The H-Atom
Topic: Nodal planes and electrostatic interactions
Replies: 1
Views: 61

Re: Nodal planes and electrostatic interactions

The presence of a nodal plane (a zone where probability of finding an electron is zero) has imporant impacts on electrostatic attractions between electrons and the nucleus. In the case of p and d orbitals, for example, the nodal plane(s) pass through the nucleus. This means that electrons in these o...
by David S
Sat Oct 13, 2018 1:26 pm
Forum: Properties of Electrons
Topic: Atomic Spectra: Balmer & Lyman Series (energy levels)
Replies: 2
Views: 59

Re: Atomic Spectra: Balmer & Lyman Series (energy levels)

n_1 refers to the initial energy level of the electron, and n_2 refers to the final energy level of the electron. n_1 may be higher or lower than n_2. The negative sign vs positive sign doesn't matter in this case because we are solving for the frequency of the light emitted/absorbed. The signs only...
by David S
Sat Oct 13, 2018 12:49 pm
Forum: Limiting Reactant Calculations
Topic: Formula Units and Molecules
Replies: 2
Views: 271

Re: Formula Units and Molecules

The difference is that a molecule consists of a fixed number (at least 2) atoms bound together by covalent bonds (e.g glucose C6H12O6). So when I say I have 2 glucose molecules, I mean two separate, discrete pieces of the same C6H12O6; I'm not saying I have C12H24O12, which would be an entirely diff...
by David S
Sat Oct 13, 2018 12:04 pm
Forum: Properties of Electrons
Topic: Free vs. Bound Electrons
Replies: 6
Views: 141

Re: Free vs. Bound Electrons

Also, note that free electrons are at a higher energy state than bound electrons. While an electron is bound to an atom (via electrostatic forces) the system is most stable when the bound electrons are at their lowest energy levels allowable. That's why excited, bound electrons emit radiation to fal...
by David S
Sun Oct 07, 2018 5:44 pm
Forum: Quantum Numbers and The H-Atom
Topic: The Hydrogen Atom
Replies: 2
Views: 92

Re: The Hydrogen Atom

Since the answers of this question all have to do with energy gained/absorbed as a result of electron movement between energy level shells, we will use the following formula to determine the right answer: E = \frac{hc}{\lambda} = -R_{h}(\,\frac{1}{n_{i}^{2}}\, - \, \frac{1}{n_{f}^{2}} ) , wh...
by David S
Sun Oct 07, 2018 5:11 pm
Forum: Properties of Electrons
Topic: Energy Levels
Replies: 1
Views: 65

Re: Energy Levels

The mathematical way is the following: If 1 meter = 1,650,763.73 wavelengths, then divide both sides by 1,650,763.73. You should get 6.058 x 10^-7 meters = 1 wavelength. Looking at the answers, this means that A is out the window (which intuitively makes sense). To determine which region of the EM s...
by David S
Sun Oct 07, 2018 3:40 pm
Forum: Significant Figures
Topic: Rounding molar mass on the periodic table
Replies: 3
Views: 89

Re: Rounding molar mass on the periodic table

You can use the full number if you want as long as your final answer is with the correct number of sig figs. Or you can round to the number of sig figs in the problem and then solve, but I think the first way will get you a more accurate answer.

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