## Search found 56 matches

Tue Mar 10, 2015 10:51 am
Forum: Phase Changes & Related Calculations
Topic: "Boil" imply phase change?
Replies: 1
Views: 525

### "Boil" imply phase change?

Doesn't the term "boiling" of water imply vaporization of liquid to gas? In the 2011 final exam, the questions says water at 25 degrees C is boiled, so I calculated the heat needed to raise water to 100 degrees C and then vaporize it. However, the solution only includes the heat needed to ...
Tue Mar 10, 2015 10:46 am
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Q, K, and Cell Potential
Replies: 2
Views: 517

### Re: Q, K, and Cell Potential

$E^{o}cell = \frac{RT}{nF}lnK$

and

$Ecell = E^{o}cell - \frac{RT}{nF}lnQ$

Therefore, if K is larger than Q, $E^{o}cell$ will be larger than $\frac{RT}{nF}lnQ$, and Ecell will be larger. The smaller Q is as compared to K, the more Ecell will increase.
Sat Mar 07, 2015 9:18 pm
Forum: *Cycloalkanes
Topic: Naming a cycloalkane
Replies: 4
Views: 793

### Re: Naming a cycloalkane

So I just explained this to Kayla in person but for the sake of openly accessible information.... Numbered naming is based on minimizing the number of the carbon to which the second substituent is attached. In the incorrect naming, the second substituent (propyl) is on the second carbon. In the corr...
Mon Mar 02, 2015 7:34 pm
Forum: *Organic Reaction Mechanisms in General
Topic: (picture)Electrons flowing to the oxygen seems very unlikely
Replies: 3
Views: 702

### Re: (picture)Electrons flowing to the oxygen seems very unli

The important part of this problem to recognize is that the central Carbon has a partial positive charge and the Oxygen has a partial negative charge because of the dipole that points from C to O because O is more electronegative than C. Therefore, the Nucleophile will be attracted to the partial po...
Thu Feb 26, 2015 5:23 pm
Forum: First Order Reactions
Topic: Self prep quiz 2013 question 7
Replies: 2
Views: 543

### Re: Self prep quiz 2013 question 7

For the second part of your question, the half life equation for a first order reaction is t_{1/2} = \frac{ln2}{k} Remember, the half life of a first order reaction is independent of reactant concentration. In the answer for #7, k = \frac{ln2}{32 d} is simply a rearrangement of the half life equatio...
Thu Feb 26, 2015 5:17 pm
Forum: First Order Reactions
Topic: Self prep quiz 2013 question 7
Replies: 2
Views: 543

### Re: Self prep quiz 2013 question 7

[A] t = [A] 0 e -kt simply comes from raising each side of the first order integrated rate law as a power of e. You know this question obeys the rules of a first order reaction because it says the compound has "first order decay." Integrated rate law for a first order reaction: ln[A] t = l...
Wed Feb 25, 2015 7:15 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Quiz 2 Prep, Q5
Replies: 2
Views: 903

### Re: Quiz 2 Prep, Q5

In the context of what we have learned in the course, activation energy does not reflect the order of a reaction. It is true that third order reactions, for example, occur more rarely than first order reactions because a termolecular slow step is less likely to have a successful collision of three m...
Tue Feb 24, 2015 7:28 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: KemiStry with Kayla and Sarah - Pre-Equilibrium/Steady State
Replies: 5
Views: 2200

### KemiStry with Kayla and Sarah - Pre-Equilibrium/Steady State

In this video, Kayla goes over the steady state approach and Sarah goes over the pre-equilibrium approach for reaction mechanisms.
Wed Feb 11, 2015 2:16 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Spontaneity in dissolving process
Replies: 5
Views: 742

### Re: Spontaneity in dissolving process

Hm, a spontaneous reaction = favorable reaction If a reaction, say, "favors products" it will be driven to production of products and will be spontaneous. You can relate K, which shows what the reaction favors, to G. Then based on that G value you can note precisely whether or not the reac...
Wed Feb 11, 2015 10:27 am
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Video: KemiStry with Kayla and Sarah KEG Party
Replies: 10
Views: 1688

### Re: Video: KemiStry with Kayla and Sarah KEG Party

Thanks for watching!
Wed Feb 11, 2015 9:25 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: State vs Path Functions
Replies: 4
Views: 800

### Re: State vs Path Functions

Thanks for clarifying! Yeah, that is confusing. "q" denotes charge in a physics context when relating potential, work, and charge.
Wed Feb 11, 2015 9:21 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Spontaneity in dissolving process
Replies: 5
Views: 742

### Re: Spontaneity in dissolving process

In question 8.87, the sub-questions are very specific, using words like "favor" and "likely." These questions can be answered from enthalpy and entropy, but spontaneity can only be determined by \Delta G or \Delta S_{tot} . The solutions manual indicates you already know the proc...
Wed Feb 11, 2015 9:15 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Spontaneity in dissolving process
Replies: 5
Views: 742

### Re: Spontaneity in dissolving process

The only measurement on that list that spontaneity can truly be determined from is $\Delta S_{tot}$. The reaction is spontaneous if $\Delta S_{tot}$ is positive.
Tue Feb 10, 2015 9:51 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Why is Ka the square root of K
Replies: 3
Views: 1019

### Re: Why is Ka the square root of K

Tue Feb 10, 2015 9:23 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Winter 2012 Midterm Question 5
Replies: 3
Views: 625

### Re: Winter 2012 Midterm Question 5

You can certainly use those factors to predict some degree of spontaneity. However, the only complete indicators are Gibbs free energy and \Delta S tot For example, an exothermic reaction could actually be nonspontaneous (positive \Delta G) if its entropy was a large enough negative value based on \...
Tue Feb 10, 2015 8:23 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Writing a cell diagram, electrodes
Replies: 7
Views: 4766

### Re: Writing a cell diagram, electrodes

You don't need Pt (s) if the electrode is a conducting, solid metal such as Zn (s) and Cu (s). So look for the clues--first you need something in the solid phase. Then, check if it is a metal. In the case where the electrode is a conducting solid metal, the electrode participates in the redox reacti...
Tue Feb 10, 2015 7:28 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Winter 2012 Midterm Question 5
Replies: 3
Views: 625

### Re: Winter 2012 Midterm Question 5

Say you have an internal energy to start X J. So X J is your "original" internal energy.

In step 1, you increase it 50 J so you have X + 50 J. In step 2, to get back to X J, it is (X J +50 J) + -50J = X J. Take away the 50 J you added in step 1 and you get back to your original, X J!
Tue Feb 10, 2015 6:08 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Writing a cell diagram, electrodes
Replies: 7
Views: 4766

### Re: Writing a cell diagram, electrodes

Only place a line if the compounds/anions/cations/elements are in different phases! A comma is used in-between if the pieces are in the same phase. Between ions, there should be commas because ions are aqueous.
Mon Feb 09, 2015 10:39 pm
Forum: Phase Changes & Related Calculations
Topic: State Functions
Replies: 4
Views: 733

### Re: State Functions

Hm, forgive me if I am being obtuse here put on p 11 of the course reader it says state properties are U, P, V, T, Density, and Heat Capacity and that path functions are w and q. Where do you see E?
Mon Feb 09, 2015 8:44 pm
Forum: Phase Changes & Related Calculations
Topic: State Functions
Replies: 4
Views: 733

### Re: State Functions

Mon Feb 09, 2015 8:36 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: State vs Path Functions
Replies: 4
Views: 800

### Re: State vs Path Functions

Potential is equal to work over charge. E = w/q (you can check this because volts = joules per coulomb.) **IN THIS CASE "q" IS NOT HEAT, but CHARGE. Because E can be related to q in this way, which is a path function, E is also a path function. Also, remember that to find the E o of a half...
Mon Feb 09, 2015 2:38 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Thermodynamically Stable
Replies: 2
Views: 475

### Re: Thermodynamically Stable

To answer this question you need a formation reaction that combines the two compounds: 4Fe 3 O 4 (s) + O 2 (g) ---> 6Fe 2 O 3 (s) The standard Gibbs free energy for this reaction is negative. Negative G r o indicates a spontaneous reaction so the product formation of Fe 2 O 3 is favorable. If you re...
Mon Feb 09, 2015 2:27 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Writing a cell diagram, electrodes
Replies: 7
Views: 4766

### Re: Writing a cell diagram, electrodes

My response to this previous question https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=140&t=5207&sid=a8c15a447679e63652a57835cb025e98 answers the second part of your question.
Mon Feb 09, 2015 2:25 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Writing a cell diagram, electrodes
Replies: 7
Views: 4766

### Re: Writing a cell diagram, electrodes

Platinum is an inert conductor that must be in the cell with the half reaction that has no conducting solids. For example, Zn(s) is a conducting metal so Pt is not needed in the Zn cell. In the other cell, N and O aren't conducting metals so Pt added to conduct electrons from cell to cell. Pt doesn'...
Sun Feb 08, 2015 4:16 pm
Forum: Balancing Redox Reactions
Topic: Homework 13.11
Replies: 4
Views: 787

### Re: Homework 13.11

You should be able to. Some are straightforward electron gain. However, the ones that include compounds with oxygen and balanced with H2O and H+ or OH- it is helpful to have the reference sheet.
Sun Feb 08, 2015 3:54 pm
Forum: Balancing Redox Reactions
Topic: Homework 13.11
Replies: 4
Views: 787

### Re: Homework 13.11

Unless specified otherwise, the anode is on the left side of the salt bridge || and the cathode is on the right side of the salt bridge ||. You determine the half reactions by looking them up on the reference sheet that has half reactions and their standard reduction potentials. Remember, in a galva...
Sun Feb 08, 2015 3:48 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 2012 midterm #8
Replies: 2
Views: 490

### Re: 2012 midterm #8

Be careful! One uses log10 and the other is for loge (ln). 0.0257 V goes with ln and the number you found on the equation sheet goes with log10.
Sun Feb 08, 2015 1:29 pm
Forum: General Science Questions
Topic: Formation of metal ions when metals dissolve
Replies: 1
Views: 1019

### Re: Formation of metal ions when metals dissolve

Your question implies its answer :) When a solid metal is dissolving, it must be turning from a solid (standard state with oxidation number of 0) to its aqueous form (a cation with a positive oxidation number). In order for a pure metal to go from a solid to an aqueous ion, it must lose electrons. I...
Sun Feb 08, 2015 11:22 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: cell potential shift
Replies: 1
Views: 273

### Re: cell potential shift

In this question, you decrease [Fe 2+ ] which is a product of the total reaction (not a reactant): 2Cr 3+ (aq) + Fe (s) --> 2Cr 2+ (aq) + Fe 2+ (aq) When you decrease the product, there is a deficit that the reaction fills by shifting to products (LeChatlier's Princip...
Sun Feb 08, 2015 11:15 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Writing cell-diagrams with other products such as H+...
Replies: 4
Views: 1423

### Re: Writing cell-diagrams with other products such as H+...

I don't think it matters (as long as it is placed in the proper anode or cathode slot in the proper reactant or product slot). Just remember the anode is on the left and the cathode is on the right and each side is written in order of reactants and then products. Just make sure that the half cell re...
Sun Feb 08, 2015 10:49 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Determining cathode and anode from two half-reactions!
Replies: 3
Views: 706

### Re: Determining cathode and anode from two half-reactions!

So in 14A, we learned the general equation for a weak acid is HA <--> H+ + A- and so you know the general form for K a is ([H+][A-])/[HA] It can also be written as: http://www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/EquilibriumArt/Weakacid1.gif if you want to explicitly include water and the...
Sat Feb 07, 2015 10:52 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Homework Question 13.37A- Significant Figures for e
Replies: 2
Views: 570

### Re: Homework Question 13.37A

So when you do out the math, you get Q = 3.27x10 6 . However, some fancy significant figure rules are happening here. The solutions manual lists the last calculation as: lnQ = 15 Take e of both sides e lnQ = e 15 Q = ? When you raise each side as a power of e, you have special significant figure rul...
Sat Feb 07, 2015 10:30 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Writing cell-diagrams with other products such as H+...
Replies: 4
Views: 1423

### Re: Writing cell-diagrams with other products such as H+...

Okay, I did some serious Google searches and couldn't find anything explicit about H 2 O inclusion. After thinking about it (and chem mod/students, feel free to correct me on this if I am wrong), it seems that the elements/molecules are included that contribute to an oxidation number change or the o...
Sat Feb 07, 2015 10:17 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Determining cathode and anode from two half-reactions!
Replies: 3
Views: 706

### Re: Determining cathode and anode from two half-reactions!

There are a few rules when it comes to determining the anode and the cathode. When you are given a reaction or the question indicates reactions and products, you have to fit your redox equation to what is given in the question. For example, in question 8, it is asking for the K a of HF which means H...
Sat Feb 07, 2015 10:03 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Midterm 2011 Q4 - Partial Pressures and K
Replies: 1
Views: 350

### Re: Midterm 2011 Q4

It all depends on what your equation comes down to. In the case of Q4, it is K = 10 = 2x/x 2 when the reactant pressure is set to x and the product pressure is set to 2x. 10x 2 = 2x An x can be crossed out from each side. 10x = 2 x = 0.2 bar. So, the reactant pressures are 0.2 bar and product pressu...
Sat Feb 07, 2015 9:57 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Finding Cv or Cp for entropy change due to volume change!
Replies: 1
Views: 499

### Re: Finding Cv or Cp for entropy change due to volume change

For our purposes, you can assume He has ideal gas properties because we have only worked with ideal gases. Dr. Lavelle mentioned this in his midterm review :)
Sat Feb 07, 2015 9:38 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Video: KemiStry with Kayla and Sarah KEG Party
Replies: 10
Views: 1688

### Video: KemiStry with Kayla and Sarah KEG Party

In this overview, we briefly go over the relationship between K, E, and G. It is a great review for the midterm!
Sun Feb 01, 2015 10:36 pm
Forum: Balancing Redox Reactions
Topic: Video: KemiStry with Kayla and Sarah - Redox Reactions
Replies: 1
Views: 582

### Re: Video: KemiStry with Kayla and Sarah - Redox Reactions

Here is part 2!
Mon Jan 19, 2015 1:46 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Video: KemiStry with Kayla and Sarah - Entropy
Replies: 1
Views: 480

### Video: KemiStry with Kayla and Sarah - Entropy

This week's episode of KemiStry with Kayla and Sarah synthesizes concepts from chapter 7 and chapter 8 by going over a phase transition from ice to vapor by calculating the overall change in enthalpy as well as entropy. Part 1 reviews enthalpy, and part 2 goes into important equations from chapter 8...
Fri Jan 16, 2015 3:42 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Ch 7: Reference Sheet for Units and Signs
Replies: 1
Views: 546

### Ch 7: Reference Sheet for Units and Signs

I find the hardest part of this chapter to be keeping track of units, conversions, and signs. I made a reference sheet to help us all keep them straight! Click on the attachment to open the PDF. To watch a video on completed step-by-step homework problems, check out KemiStry with Kayla and Sarah, ou...
Fri Jan 16, 2015 3:36 pm
Forum: Phase Changes & Related Calculations
Topic: Homework Problem #19
Replies: 5
Views: 909

### Re: Homework Problem #19

You are right, if something is giving off heat, the sign is negative. If something is absorbing heat, the sign is positive. However, in this question, the most important concept is that q system + q surroundings = 0. This means that one equals the negative version of the other. You can choose which ...
Mon Jan 12, 2015 9:15 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Video: KemiStry with Kayla and Sarah - Enthalpy Changes
Replies: 1
Views: 516

### Video: KemiStry with Kayla and Sarah - Enthalpy Changes

Watch Kayla and Sarah explain gas laws and work and the relationship between internal energy and changes in enthalpy! We answer textbook questions #50, 48, and 51. Part two is posted as the first comment to this video. Enjoy!
Tue Dec 09, 2014 10:56 pm
Forum: Amphoteric Compounds
Topic: Taking Autoprotolysis into Account
Replies: 1
Views: 696

### Taking Autoprotolysis into Account

Under what conditions do you take the autoprotolysis of water into account?
Mon Dec 08, 2014 11:12 pm
Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
Topic: -CH3 Group in Acids
Replies: 1
Views: 359

### Re: -CH3 Group in Acids

Adding Carbon groups causes more electron delocalization because Carbons have electron-pulling power (based on their electronegativity). This stabilizes the anion part of the compound which makes the compound easier to deprotonate. The more "willing" a compound is to separate from its prot...
Sat Dec 06, 2014 1:09 pm
Forum: *Making Buffers & Calculating Buffer pH (Henderson-Hasselbalch Equation)
Topic: Using the Henderson-Hasselbalch Equation
Replies: 1
Views: 442

### Re: Using the Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation will definitely be used when the salt and acid are not equimolar. In fact, it is a more useful estimate when there are concentrations to enter into the expression. If the salt and acid are equimolar, you can just recognize that pH = pK a and you don't even need to ...
Mon Dec 01, 2014 3:44 pm
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Tetrahedral vs. square planar
Replies: 2
Views: 1040

### Re: Tetrahedral vs. square planar

Square planar is most common for transition metals that have a d8 electron configuration such as Pd 2+ . However, there are some exceptions based on the size of the ligands (larger ligands may cause tetrahedral shape). If the TM does not have a d8 electron configuration, it is usually tetrahedral. T...
Thu Nov 20, 2014 9:05 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: "pushing the reaction to the left"
Replies: 2
Views: 299

### "pushing the reaction to the left"

When is it appropriate to determine that reactants are strongly favored so the original initial conditions should be changed by "pushing the reaction to the left as far as possible"? How do you decide on "new initial conditions"? This occurs in #10.69.
Mon Nov 10, 2014 4:44 pm
Forum: Accuracy, Precision, Mole, Other Definitions
Topic: Percent Yield --> Grams of a reactant
Replies: 1
Views: 8087

### Re: Percent Yield --> Grams of a reactant

If the percent yield of the reaction is only 95%, you know that you have to theoretically produce a larger amount of silicon nitride to actually get the 125 g. So, you know that 125 g = (.95)(X g). The "X" is the larger number of grams that the theoretical yield should be, so when the 95% ...
Sun Nov 02, 2014 1:19 am
Forum: Resonance Structures
Topic: Lone pairs on C #2.103
Replies: 3
Views: 470

### Re: Lone pairs on C #2.103

Thank you so much!
Sat Nov 01, 2014 1:45 pm
Forum: Resonance Structures
Topic: Lone pairs on C #2.103
Replies: 3
Views: 470

### Lone pairs on C #2.103

I know Carbon "likes" to form 4 bonds instead of having a lone pair left. However, do Carbon atoms with lone pairs count in Lewis resonance structures? For example, in #2.103 I could draw more resonance structures in part b) and c) than listed in the solutions manual if I left a lone pair ...
Thu Oct 30, 2014 7:10 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Trigonal Planar e- Configuration #3.1
Replies: 1
Views: 406

### Re: Trigonal Planar e- Configuration #3.1

There cannot be lone pairs on the central atom in trigonal planar geometry (AX 3 ). If a molecule has one central atom, two surrounding atoms, and one lone pair, its molecular geometry is bent (AX 2 E). If a molecule has one central atom, three surrounding atoms, and one lone pair, its molecular geo...
Thu Oct 30, 2014 7:00 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Molecular Geometry vs. Molecular Shape
Replies: 1
Views: 1842

### Re: Molecular Geometry vs. Molecular Shape

Hm, I believe that "molecular geometry" and "molecular shape" are interchangeable. What you might be referring to is finding "electron domain geometry" first to then find the molecular geometry. Essentially, only the position of atoms in a molecule are used to name the ...
Thu Oct 30, 2014 6:41 pm
Forum: Octet Exceptions
Replies: 1
Views: 386

For a biradical where both of the electrons are on the same atom, think of Hund's Rule. Hund's rule states that each orbital in a sublevel has to be filled once before the electrons can be paired up. Therefore, the oxygen atom, for example, would have two unpaired electrons as demonstrated by its co...
Tue Oct 28, 2014 11:34 am
Forum: Octet Exceptions
Topic: Hydrogen - Postive/Negative Properties
Replies: 1
Views: 352

### Hydrogen - Postive/Negative Properties

The cation of H is H+, however, in the tetrahydridoborate ion, BH4-, Hydrogen assumes a negative charge to compensate for B3+. Is Hydrogen the only element that can do this? What makes Hydrogen able to assume a negative charge in this case?
Mon Oct 27, 2014 4:51 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Polarity and Nonpolarity
Replies: 2
Views: 463

### Re: Polarity and Nonpolarity

In lecture we discussed BeCl 2 . This molecule is linear because the chlorines each have 3 lone pairs and are each bonded to Be with a single bond (Be has 0 lone pairs). The furthest apart the two regions of Cl electron density can be is directly opposite with Be in the center. However, there are tw...