Search found 45 matches
- Sun Feb 10, 2019 1:06 am
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: midterm
- Replies: 3
- Views: 34
Re: midterm
If you search "Hot Dog" you can find the practice midterm that Lyndon, one of the UAs, made.
- Sun Feb 10, 2019 1:06 am
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: Delta U
- Replies: 8
- Views: 20
Re: Delta U
DeltaU = q + W. Therefore, the internal energy of a system does not change only when q and W are zero.
- Sun Feb 10, 2019 1:05 am
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: standard reaction enthalpy vs reaction enthalpy
- Replies: 1
- Views: 9
Re: standard reaction enthalpy vs reaction enthalpy
If reactants and products are at different states or reactions take place at different pressures, the enthalpy of the reaction can be different. Therefore, the standard reaction enthalpy refers to the enthalpy when all reactants and products are in their standard states at 1atm. Reaction enthalpy re...
- Wed Jan 30, 2019 11:23 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: Piston and value of W
- Replies: 3
- Views: 14
Re: Piston and value of W
When you move the piston up, the system is doing work on the piston so the system loses energy. Because the system loses energy, the W is negative. When you move the piston down, the opposite occurs.
- Wed Jan 30, 2019 11:21 pm
- Forum: Ideal Gases
- Topic: derivatives and integrals
- Replies: 5
- Views: 20
Re: derivatives and integrals
Work is the integral of the product of force and displacement. In class, he used that definition of work to derive the formula for work done by a piston. However, you can just use the formula using delta V because that is more applicable to the scope of this class.
- Wed Jan 30, 2019 11:21 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: Surroundings
- Replies: 11
- Views: 23
Re: Surroundings
No, there is generally no limit. Essentially, the "surroundings" refer to the environment of the reaction.
- Wed Jan 23, 2019 9:53 pm
- Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
- Topic: Hmwrk 11.79 6th edition
- Replies: 2
- Views: 18
Re: Hmwrk 11.79 6th edition
Since we can only use scientific calculators for this class, this question is beyond the scope of our tests.
- Wed Jan 23, 2019 9:51 pm
- Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
- Topic: Adding a Liquid or Solid to an Equation
- Replies: 2
- Views: 24
Re: Adding a Liquid or Solid to an Equation
This question was previously answered here: https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=51&t=40454. Essentially, because water is a liquid it is not included in the equilibrium expression. However, the addition of water will decrease the concentration of C6H12O6, thereby causing the reac...
- Wed Jan 23, 2019 9:32 pm
- Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
- Topic: 6th edition 12.45
- Replies: 1
- Views: 13
Re: 6th edition 12.45
For this question, the pKa represents the relative strength of the acids. The lower the pKa, the higher the conjugate Ka, and therefore the weaker the base. Arylamines and Alkyamines refer to the class of organic bases that anilines, methylamine, and ethylamine fall under. I don't think we need to k...
- Fri Jan 18, 2019 1:49 am
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: Net Ionic Equations
- Replies: 3
- Views: 15
Re: Net Ionic Equations
In general, the net ionic equation will be water + ion <=> conjugate acid / base + oh-/h3o+
- Fri Jan 18, 2019 1:48 am
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: Polyprotic Acids
- Replies: 1
- Views: 21
Re: Polyprotic Acids
Not always. For example, you use Ka2 for H2SO4 because Ka1 is very strong. However, in most cases, you can disregard the second because it is much smaller and basically insignificant.
- Fri Jan 18, 2019 1:45 am
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: Equilibrium Arrow
- Replies: 7
- Views: 33
Re: Equilibrium Arrow
For reactions involving a strong acid and base, you can assume 100% protonation / deprotonation, and the single arrow represents the heavy skew towards products.
- Mon Jan 07, 2019 11:14 pm
- Forum: General Rate Laws
- Topic: Visualizing Equilibrium
- Replies: 1
- Views: 52
Re: Visualizing Equilibrium
Both flask 3 and 4 are at equilibrium. The diagram displays the dissociation over time, so flask 3 is merely the first instance when the flask reaches equilibrium. The fact that flask 3 and 4 have the same ratios is a clear sign that the reaction has reached equilibrium in flask 3.
- Mon Jan 07, 2019 11:12 pm
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: Lecture 1
- Replies: 1
- Views: 27
Re: Lecture 1
Hello! today we went over the basics of equilibrium including the definitions of equilibrium and equilibrium constants. You could skim through the textbook as outlined in the syllabus for a review. Also, Dr. Lavelle has video modules on his website that cover the same material as lecture.
- Mon Jan 07, 2019 11:11 pm
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: Liquids not used in Kc
- Replies: 2
- Views: 21
Re: Liquids not used in Kc
No. In a Kc expression, you do not need to include the solute. The change in the concentration of any solute will be insignificant.
- Thu Dec 06, 2018 11:41 pm
- Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
- Topic: strong vs weak acids
- Replies: 2
- Views: 26
Re: strong vs weak acids
To calculate the pH of a weak acid solution, you need to use Ka and set up an equilibrium equation, which is beyond the scope of this quarter. For questions involving strong acids and baes, you can solve them stoichiometry as a neutralization reaction and then calculate the pH using the remaining H+...
- Thu Dec 06, 2018 11:39 pm
- Forum: Polarisability of Anions, The Polarizing Power of Cations
- Topic: Distorted electrons
- Replies: 2
- Views: 26
Re: Distorted electrons
Essentially, electrons that are more distorted are more likely to be attracted / repelled by neighboring dipoles, making them more polarizable.
- Thu Dec 06, 2018 11:31 pm
- Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
- Topic: Calculating pH of Weak Acid
- Replies: 1
- Views: 42
Re: Calculating pH of Weak Acid
To calculate the pH of a solution with a weak acid, you need to use the Ka equilibrium constant and set up an equilibrium equation, which is not covered in this course.
- Sat Dec 01, 2018 5:04 pm
- Forum: Hybridization
- Topic: pi bonds
- Replies: 5
- Views: 43
Re: pi bonds
Pi bonds are formed with the unhybridized orbitals. Usually, they are the p-orbitals, but they can also be formed with d-orbitals.
- Sat Dec 01, 2018 5:03 pm
- Forum: Hybridization
- Topic: Rating Polarity
- Replies: 3
- Views: 35
Re: Rating Polarity
Essentially, polarity is directly connected to electronegativity. You can also think of it in terms of the size of the atom. As you move down a group, the valence electrons are further away from the nucleus and experience more shielding. As a result, the atom will be less able to pull electrons in t...
- Sat Dec 01, 2018 5:02 pm
- Forum: Hybridization
- Topic: identifying hybridization of a central atom
- Replies: 4
- Views: 36
Re: identifying hybridization of a central atom
Essentially, to determine if a molecule is polar, you must first look to see that the bonds are polar and that the molecule is asymmetrical. To tell if the bonds are polar, you look at the difference in electronegativity. On notable nonpolar bond is C-H. Then, look at the shape of the molecule and t...
- Sun Nov 25, 2018 1:22 am
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Lewis Structure of IO3 (-)
- Replies: 1
- Views: 30
Re: Lewis Structure of IO3 (-)
Both structures are correct lewis structures, and the one you are suggesting would have a lower energy and would be the dominant lewis structure. Because this question was asking about shape, you don't necessarily need to draw the lewis structure with the lowest energy. Both lewis structures would r...
- Sun Nov 25, 2018 1:19 am
- Forum: Octet Exceptions
- Topic: Why Doesn't I Break Octet in IO3-
- Replies: 1
- Views: 25
Re: Why Doesn't I Break Octet in IO3-
Both structures are correct lewis structures, but the one you are suggesting would have a lower energy and would be the dominant lewis structure. Because this question was asking about shape, you don't necessarily need the lewis structure with the lowest energy. Both lewis structures would result in...
- Sun Nov 25, 2018 1:17 am
- Forum: Hybridization
- Topic: Single, Double, and Triple Bonds
- Replies: 1
- Views: 18
Re: Single, Double, and Triple Bonds
When determining hybridization, it is important to know how many areas of electron density are around an atom. Like we discussed in class, CH4 and NH3 have the same hybridization, even though one is tetrahedral and one is trigonal pyramidal. As a result, you need to know what type of bonds are aroun...
- Sun Nov 18, 2018 9:41 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Lone pairs
- Replies: 5
- Views: 39
Re: Lone pairs
Yes. Although both the tetrahedral and see-saw shape have four atoms bonded to the central molecule, the lone pair in the see-saw shape causes the difference in name. Every VSEPR structure (AX4 vs. AX4E1) has a different shape name.
- Sun Nov 18, 2018 9:39 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: Question 6.19 (Sixth Edition)
- Replies: 2
- Views: 41
Re: Question 6.19 (Sixth Edition)
a. Xenon has more electrons, so it is more polarizable and has stronger LDF than Ar. Because of the stronger LDF, the melting point will be much higher because it requires more energy to break the stronger bonds. b. H2O has hydrogen bonding while C2H5OC2H5 is nonpolar and only has LDF. Because LDF a...
- Sun Nov 18, 2018 9:35 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Polar molecules
- Replies: 5
- Views: 42
Re: Polar molecules
If there is a lone pair, there will likely be a net dipole pointing in the opposite direction. In general, if a molecule has lone pairs, it is likely to be polar.
- Fri Nov 09, 2018 4:28 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: Induced dipoles
- Replies: 2
- Views: 41
Re: Induced dipoles
For every molecule, there are a lot of negatively-charged electrons held around a positively-charged nucleus. If a small shift in this electron cloud occurs, it will create a small dipole whereby one part is more positive and one more negative. This small dipole will then induce a change in nearby m...
- Fri Nov 09, 2018 4:26 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: Hydrogen Bonding versus other Intermolecular forces
- Replies: 2
- Views: 33
Re: Hydrogen Bonding versus other Intermolecular forces
Hydrogen bonds are incredibly polar because of the significant difference in electronegativity between hydrogen and O, N, F. They're essentially just very strong dipole-dipole bonds.
- Fri Nov 09, 2018 4:24 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: Boiling point
- Replies: 12
- Views: 164
Re: Boiling point
You can think of "rod" vs. "sphere" as straight-chain vs. branched molecules. Essentially, straight chain molecules are going to have fewer atoms "sticking out." so they can pack closer together, increasing the LDF.
- Sun Nov 04, 2018 3:44 pm
- Forum: Formal Charge and Oxidation Numbers
- Topic: Formal Charge and Radicals
- Replies: 2
- Views: 38
Re: Formal Charge and Radicals
Essentially, if the formal charge does not add up to the overall charge of the molecule, that lewis structure is not valid. Then, you can use process of elimination to figure out where to expand the octet.
- Sun Nov 04, 2018 3:43 pm
- Forum: Formal Charge and Oxidation Numbers
- Topic: formal charge on most electronegative atom
- Replies: 3
- Views: 59
Re: formal charge on most electronegative atom
Yes, because the most electronegative atom will draw the most electrons, so should have the most negative formal charge.
- Sun Nov 04, 2018 3:41 pm
- Forum: Octet Exceptions
- Topic: What are the octet exceptions?
- Replies: 9
- Views: 99
Re: What are the octet exceptions?
Essentially, the elements in rows 1&2 will never have more than 8 valence electrons. In rows 3+, n=3 so there are p shells, which allow atoms to expand their octet.
- Fri Oct 26, 2018 12:24 am
- Forum: Trends in The Periodic Table
- Topic: CH 2 6TH EDITION 2.81
- Replies: 2
- Views: 45
Re: CH 2 6TH EDITION 2.81
When it comes to these types of questions, always write out the electron configuration then look where the electrons are and if they're any different than the ones that follow the expected behavior.
- Fri Oct 26, 2018 12:23 am
- Forum: Trends in The Periodic Table
- Topic: CH 2 6TH EDITION 2.85
- Replies: 2
- Views: 62
Re: CH 2 6TH EDITION 2.85
In general, we don't really need to worry about heavier transition-metal elements because they don't usually follow normal patterns. But in general, the prime reason for their behavior will be their abnormally large size because they have so many more protons and electrons than elements in the first...
- Fri Oct 26, 2018 12:21 am
- Forum: Trends in The Periodic Table
- Topic: CH 2 6TH EDITION 2.47
- Replies: 2
- Views: 46
Re: CH 2 6TH EDITION 2.47
In general, for these questions: 1. list out the electron configuration 2. start removing e- from the end of your list, because these are the electrons furthest from the nucleus and the least stable
- Sun Oct 21, 2018 8:33 pm
- Forum: Photoelectric Effect
- Topic: Conditions that allow electron to be ejected from metal surface
- Replies: 12
- Views: 181
Re: Conditions that allow electron to be ejected from metal surface
Remember that Ephoton = WorkFunction + KEejectede-. If Ephoton is less than the work function, the kinetic energy of the electron would be negative, and the electron would not be ejected.
- Sun Oct 21, 2018 8:32 pm
- Forum: Photoelectric Effect
- Topic: Kinetic energy
- Replies: 4
- Views: 43
Re: Kinetic energy
In general, the work function will always be the difference between the energy of the photon & the kinetic energy of the electron. Thus, if they give you ways to find these quantities, you can be responsible for finding the work function.
- Sun Oct 21, 2018 8:31 pm
- Forum: Photoelectric Effect
- Topic: Test #2
- Replies: 9
- Views: 147
Re: Test #2
My TA specified that the following would be on the test: photoelectric effect, deBroglie and wavelength, Heisenberg uncertainty, atomic spectra, and quantum numbers. We won't need to know electron configurations or multi-electron systems.
- Fri Oct 12, 2018 12:13 am
- Forum: Properties of Light
- Topic: Threshold Energies
- Replies: 5
- Views: 52
Re: Threshold Energies
Different metals have different threshold energies depending on the nucleic pull on electrons. There are many factors that affect the nucleic pull. ie: electrons far away from the protons will be "shielded" from the nucleic force and as a result, easier to remove.
- Fri Oct 12, 2018 12:11 am
- Forum: Properties of Light
- Topic: problem 1. 9 sixth edition
- Replies: 4
- Views: 35
Re: problem 1. 9 sixth edition
Here is my solution for that question: Mhz = 10^8 Hz
- Fri Oct 12, 2018 12:09 am
- Forum: Properties of Light
- Topic: Wave and particles [ENDORSED]
- Replies: 1
- Views: 44
Re: Wave and particles [ENDORSED]
Light as a wave has two key characteristics: amplitude and frequency. According to this model, the energy of a stream of light should be directly proportional to the amplitude. Essentially, "high-intensity" light should have high energy. However, with the discovery of the photoelectric eff...
- Wed Oct 03, 2018 9:43 pm
- Forum: Limiting Reactant Calculations
- Topic: hi stuck again
- Replies: 3
- Views: 60
Re: hi stuck again
Another way to think about reactants is to think about which one will produce the least product. Ie: calculate the product stoichiometrically assuming each reactant is limiting. The correct answer will always be the reactant that produces the least amount of product.
- Wed Oct 03, 2018 9:29 pm
- Forum: Administrative Questions and Class Announcements
- Topic: Advice from a Medical Student [ENDORSED]
- Replies: 86
- Views: 25108
Re: Advice from a Medical Student [ENDORSED]
Thank you so much for your advice! It's really reassuring to hear from a successful past student that what might seem to be a challenging class will be rewarding in the long run!
- Sun Sep 30, 2018 9:37 pm
- Forum: Balancing Chemical Reactions
- Topic: Short cuts/methods for balancing equations
- Replies: 15
- Views: 150
Re: Short cuts/methods for balancing equations
In general, I try to start with elements that appear in the least number of compounds. Then, I make sure there are no odd numbers of elements if there is an even number on the other side (ie: double H2O if there is an O2 on the other side). Lastly, try to balance single-element compounds (ie: O2) la...