CHEMISTRY COMMUNITY

Kelly Ha 1K

Chemical equilibrium made the most sense to me so I enjoyed that topic the most.

Kelly Ha 1K

Last quarter in Chem14A final test scores were released about a week after we took the final, so I would assume it's probably around the same time for this quarter.

Kelly Ha 1K

Q is the reaction quotient, or the amount (in terms of concentration or partial pressures) of products over the amount of reactants at any point in the reaction. K is the equilibrium constant, which is calculated the same way as Q and is essentially Q at equilibrium.

Kelly Ha 1K

How do I calculate the n in NFE? Do I have to balance the half-reactions? What if the electrons is 0? Would that ever happen? You can calculate n (number of electrons transferred) by balancing the redox reactions. I'm pretty sure the number of electrons wouldn't be 0 since it wouldn't be a redox re...

Kelly Ha 1K

Catalysts speed up the reaction (increasing k) by lowering the activation energy. Although catalysts provide a new pathway for the reaction, they don't affect the reaction thermodynamically such as altering ΔG or ΔH.

Kelly Ha 1K

ΔU = 0 in isothermal reactions and q= -w or -q= w since energy lost from work is replenished by heat.

Kelly Ha 1K

Use the R that corresponds with the units used in the equation you are using. The units should cancel out correctly if you used the right R value. It helps to always write out the units so it doesn't get too confusing!

Kelly Ha 1K

When doing pseudo-rate laws you're basically making one reactant concentration small and the other reactant concentrations in excess so that the reaction rate is only dependent on one reactant (the very small one)- since the larger concentrations are essentially constant. When you rewrite the pseudo...

Kelly Ha 1K

Intermediates are produced then consumed. Catalysts are present in the beginning and then regenerate at the end. They both do not appear in the overall reaction.

Kelly Ha 1K

Personally, I find both types of sessions incredibly helpful. But if you want to focus more on practicing problems with the benefit of input from people who have taken the class before (the UAs), I think you should try to attend more UA sessions. As stated above, TAs don't really have worksheets tha...

Kelly Ha 1K

Lowercase k is associated with kinetics, where k is the rate constant that relates the rate of the reaction to the concentration of reactants. A large rate constant k would mean that the reaction is relatively fast, while a smaller rate constant k means that the reaction is relatively slow. Uppercas...

Kelly Ha 1K

Definitely finish the textbook problems in the outlines and attend as many review sessions possible this week! Dr. Lavelle has the list of review sessions on his website, or you can see it through this link: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Chem14B_Week_10_Review_Ses...

Kelly Ha 1K

If K is less than 10^-4, then x is negligible. You can check if this approximation was valid with the 5% rule.

Kelly Ha 1K

Winter quarter was pretty rough for me to be honest, but trying to stay positive! Lately I've been trying to plan out my day on the Google Calendar app with the tasks I need to complete by the end of the day and it has helped me prioritize and visualize my assignments and studying periods. I also ma...

Kelly Ha 1K

Oxidation takes place on the anode side while reduction takes place on the cathode side. (A mnemonic to help memorize this would be AnOx RedCat). The anode is usually on the left while the cathode is on the right in typical cell diagrams.

Kelly Ha 1K

The instantaneous rate decreases as the reaction proceeds since the more reactants are used up, the less is available to form products.

Kelly Ha 1K

I believe you have to look up the standard reduction potentials on the table to determine that. It's easier to find the one you are looking for if you use command F and the search bar (on a Mac)!

Kelly Ha 1K

Since electrons are flowing left to right from anode to cathode, the anions must flow right to left from cathode to anode to prevent the buildup of charge.

Kelly Ha 1K

Q is [products]/[reactants] at any point during the reaction and K is [products]/[reactants] at equilibrium. Q = K at equilibrium.

Kelly Ha 1K

K is the equilibrium constant. However, if it is mentioned in the context of units, K most likely refers to temperature in Kelvin.

Kelly Ha 1K

Yes, larger and more complex molecules have more disorder since there is a greater number of possible microstates/positions available.

Kelly Ha 1K

Hi, 4C.3 is not an assigned textbook problem. I believe the ones listed in Outline 3 for 4C are: 1, 7, 9, 11, 13, 15 so we don't need to know how to do it. Hope this helps!

Kelly Ha 1K

I believe isolated systems are almost entirely hypothetical because perfect systems can't exist in nature or in experiments since it would have to be completely insulated and have a fixed volume.

Kelly Ha 1K

Energy can be exchanged with the surroundings in a closed system. Nothing can be exchanged with the surroundings in an isolated system.

Kelly Ha 1K

Expansion: negative work (work done by system)
Compression: positive work (work done to system)

Kelly Ha 1K

An extensive property depends on the amount of substance involved whereas an intensive property does not.

Kelly Ha 1K

As someone said above, the idea behind the second law of thermodynamics is that the total entropy of the universe can never decrease. The entropy of one part of the universe (such as S sys or S surr ) can decrease, but the other part must increase by just as much so that the change in entropy of the...

Kelly Ha 1K

An adiabatic system means no heat enters or leaves the system, meaning that q = 0. Since ΔU = q + w, the change in internal energy (ΔU) would only be dependent on work (w), therefore ΔU = w in an adiabatic system.

Kelly Ha 1K

I kept getting this one wrong too because I forgot to convert kJ to J. Maybe that's the error since you are right in saying that entropy of surroundings is -139kJ/298K?

Kelly Ha 1K

When volume is constant (ΔV = 0), w = 0 because w= -PΔV.
Since ΔU = q + w, if w =0 at constant volume, then ΔU = q.
Therefore, ΔU = q at constant volume.

Kelly Ha 1K

Step 1 of Path B is at constant volume so ΔV = 0 and therefore w = 0.
Step 2 of Path B is at constant external pressure so you would use w = -P(ex)ΔV to solve for w. Since your units will be in L atm, use the 101.325 J = 1 L atm conversion given on the constants sheet so convert to J.

Kelly Ha 1K

I definitely had this problem at first too! Basically just always check your units; make sure the correct units cancel out to leave you with your desired value. It helps to write out all the units of the values when calculating so you can double check you picked the right R to use.

Kelly Ha 1K

This reaction shows the combustion of CH4 (combustion reactions involve O2 as a reactant and CO2 and H2O as products) so the reaction enthalpy will equal the enthalpy of combustion of CH4.

Kelly Ha 1K

Method 2 (bond enthalpies) is the least accurate method to calculate reaction enthalpies. The other two- Method 1 (Hess's Law) and Method 3 (standard enthalpies of formation) are pretty comparable in accuracy.

Kelly Ha 1K

ΔH (enthalpy) is defined as q (heat) absorbed or released at constant pressure.

Kelly Ha 1K

Your TA will most likely address in your next discussion session how you can review your midterm, but if not you can always email them to ask when you can meet with them to review it with you.

Kelly Ha 1K

I believe the general cutoff for this class is:
93-100%- A
90-92.9%- A-
83-90%- B
80-82.9%- B-
and above 50% is a C-
This is just an estimate though I'm not too sure if it is official since it was not listed in the syllabus.

Kelly Ha 1K

The change in X can be considered negligible whenever K is less than 10^-4. This approximation isn't just limited to solving cubic equations since we can do this with quadratics too. (Keep in mind this approximation is only valid if percent dissociation is less than 5%).

Kelly Ha 1K

Enthalpy is the amount of heat released or absorbed at a constant pressure. Heat describes energy transfer.

Kelly Ha 1K

Change in pressure only applies to gases because changing pressure in solids and liquids doesn't change their volume. Increasing pressure/compressing (by decreasing volume) will make the system favor the side with less moles. Decreasing pressure/expanding (by increasing volume) will make the system ...

Kelly Ha 1K

Endothermic reactions have a positive delta H because the system is absorbing heat from the surroundings. Exothermic reactions have negative delta H because the system is releasing heat into the surroundings.

Kelly Ha 1K

The change in X is negligible when K is < 10^-4. You can check if X was supposed to be considered negligible by using the 5% rule (if percent dissociation is less than 5% the approximation was valid).

Kelly Ha 1K

I agree, there are definitely tons of resources Dr. Lavelle provides such as step-up sessions and UA workshops that are extremely helpful. His website also has audio-visual modules that you can use to review. For Chem14A Dr. Lavelle had a review lecture right before the final and went over several s...

Kelly Ha 1K

This is how I understand it:
Forward reaction (reactants --> products): reactants have -x, products have +x
Reverse reaction (products --> reactants): reactants have +x, products have -x

Kelly Ha 1K

At equilibrium, the concentrations of the products and reactants do not change (which is why K is called the equilibrium constant ). If concentrations do change, the reaction will "shift to the left/right" to offset the stress on the system (Le Chatelier's Principle) and the reaction will ...

Kelly Ha 1K

Solids and liquids are not included in the K expression so they should not be included in ICE tables! If you want to include them in your reaction when drawing your ICE table, make sure to cross out the corresponding rows under it for the rest of the ICE (initial, change, equilibrium) spots.

Kelly Ha 1K

If the K value is less than 1.0 x 10^-4 then the change in x (-x or +x) can be omitted. (I usually use 10^-4 rather than 10^-3 as the cutoff to be safe). Also, as stated in a previous post sometimes it's safer to just do the whole calculation without worrying about whether the approximation is valid...

Kelly Ha 1K

Doing textbook problems and attending UA sessions and doing the worksheets they provide are really helpful! As many people have said before, make sure you fully understand concepts rather than just memorize the steps to do a problem.

Kelly Ha 1K

Although calculated the same way, Q is the reaction quotient while K is the equilibrium constant. K is basically just Q at a specific point (at equilibrium).

Kelly Ha 1K

I believe any K value 10^-4 and below would allow you to simplify the quadratic equation. (So you wouldn't simplify a quadratic equation with K value 10^-3). Like someone said above, 10^-4 is a safer cutoff.

Kelly Ha 1K

I remember when I did this problem, I made a mistake in not recognizing that the initial concentration of NO2 is the given concentration plus the extra moles of NO2 added into the solution, not just the first given concentration value. Also recognize that the reaction is reverse since Q>K so the cha...

Kelly Ha 1K

You can automatically consider any negative x values as extraneous since concentrations can't be negative. If both your x values are positive, you can rule out the x value that will give you a negative concentration when you plug it back into the equation for the equilibrium concentration.

Kelly Ha 1K

In one of the workshops last week a UA (I believe it was Michael) mentioned a very useful tool you can use to understand Le Chatelier's Principle. He said you can imagine the reaction as a scale and whichever side the reactants or products are added, the scale tips in that direction. In order to off...

Kelly Ha 1K

You should use ICE tables when doing calculations with equilibrium constants such as solving for the equilibrium composition of products and reactants if only the initial composition of the reaction is given. As stated above, equilibrium "composition" can mean either solving for concentrat...

Kelly Ha 1K

Susanna Givan 2B wrote:We haven't finished outline 1 yet? I thought Lavelle said that we were done with chemical equilibrium and were moving on to acids and bases. Am I mistaken?

I believe you are correct. We finished outline 1 (chemical equilibrium) last week on Friday and we're starting acids and bases tomorrow.

Kelly Ha 1K

Maddie Turk Disc 2C wrote:So do we ever need to convert to pascals or will it always be in atm.

I don't think we will have to convert to pascals since Dr. Lavelle hasn't mentioned it yet. We will most likely be given atm or bar units for pressure.

Kelly Ha 1K

Kc is an equilibrium constant for gases and is found through the use of partial pressure; Kp is an equilibrium constant which is found through the use of concentrations Someone correct me if I am wrong, but I believe it is switched! Kp is the equilibrium constant used for when all products and reac...

Kelly Ha 1K

Kc is used for concentrations while Kp is used for pressures. If you are given the concentrations of reactants and products, use Kc. If you are given partial pressures of reactants and products, use Kp. It's also important to note that you should use Kp if all products and reactants in the reaction ...

Kelly Ha 1K

Yes, since the question is asking in terms of a base, pH > pKa would mean the predominant species is neutral (opposite of what it would be for an acid).

Kelly Ha 1K

They haven't updated the Chemistry Community points yet since the deadline was extended to 10am on Thursday due to the final. I believe most people still only have 45 points in the grade book so far since week 10 was not counted yet so don't worry!

Kelly Ha 1K

I will definitely be attending most if not all of the UA sessions next quarter since they were so helpful during Chem 14A. Similarly to the other responses, I will also be doing the textbook problems with more emphasis on understanding the entire topic, not just the processes of doing the problem it...

Kelly Ha 1K

For remembering strong acids I’ve heard the mnemonic: “SO I Brought NO Clean ClOthes” where
SO is H2SO4
I is HI
Br is HBr
NO is HNO3
Cl is HCl
ClO is HClO4

For remembering strong bases I believe it is usually Group 1 and 2 oxides and hydroxides. Hope this helps!

Kelly Ha 1K

When determining oxidation numbers you have to take into account the charges of the whole coordination compound, not just the coordination sphere. Although the 2 chlorine atoms are not within the coordination sphere, they still contribute to the overall charge of the compound, so it must be included...

Kelly Ha 1K

I don't think he has said anything about the evaluations counting as extra credit. I think he just wants us to fill it out to give him feedback on what resources were helpful to us in this course. Just to repeat what was stated before, the evals are also submitted anonymously so I don't think there ...

Kelly Ha 1K

Yes, you are correct as long as that atom/molecule is within the coordination sphere (within the brackets) since a ligand forms a coordination compound by interacting with a transition metal (typically a transition metal cation).

Kelly Ha 1K

I'm having trouble watching the lectures as well since CCLE crashed this morning. The screen usually blacks out and has a "this video can't be accessed" message or something similar to that. I just wait a couple minutes, log out, and log back in. Definitely a bit time consuming but it's wo...

Kelly Ha 1K

I'm mostly going through the textbook problems and reviewing Sapling homework from previous units. I also watch Youtube videos on topics I might be struggling with. I'll definitely be going to as many review sessions as I can throughout this week too- those will be very helpful!

Kelly Ha 1K

Coordination number is the number of bonds between the ligands and the central atom (usually a transition metal cation) inside the brackets. The brackets represent what's inside the coordination sphere.

Kelly Ha 1K

Doing practice problems from the textbook (especially the ones specified in the outlines) and attending UA sessions has been the most helpful for me. I would also recommend going to the review sessions throughout this week on the topics you may be struggling in!

Kelly Ha 1K

105405231 wrote:Did Dr. Lavelle announce how many questions will be on the final?

To restate what was mentioned above, I don’t think Dr. Lavelle has mentioned how many questions will be on the final yet.

Kelly Ha 1K

305311217 wrote:So do we not get credit for attending section?

No, the only “weekly discussion” credit we get is from posting on Chemistry Community.

Kelly Ha 1K

Yes, but I believe it also depends on the differences in electronegativity between the different atoms and the central atom. For example, if the different atoms around the central atom have the same electronegativity (despite being different elements), then there won’t be a dipole moment.

Kelly Ha 1K

Susanna Givan 1L wrote:what determines how strong london dispersion forces are again?

Generally, larger and heavier atoms have stronger dispersion forces while smaller and lighter atoms have weaker dispersion forces, which is due to the differing levels of polarizability.

Kelly Ha 1K

Personally, reading the textbook has helped me understand the details of specific concepts since it goes into depth on certain topics. I've found that it helps to watch the lectures first to understand the basics on a topic and then reading about the topic in the textbook. Definitely do the textbook...

Kelly Ha 1K

First ionization energy refers to removing an electron from the outermost orbital (the one furthest away from the nucleus).

Kelly Ha 1K

Kelly Yun 3K wrote:Oh, I see so size of cations and polarizing power and size of anions and polarizability are the opposite right? Thanks!

Yes, larger anions have greater polarizability and smaller cations have greater polarizing power.

Kelly Ha 1K

Polar molecules have dipole moments due to the unequal distribution of electrons, so a dipole-dipole interaction consists of attraction between two or more polar molecules. You can usually tell if there is a dipole-dipole interaction if you can identify if the molecules involved are polar (for examp...

Kelly Ha 1K

Yes, hydrogen bonds are the strongest type of dipole-dipole. They include a hydrogen bonding to a highly electronegative atom (N, O, F).

Kelly Ha 1K

Yes, London dispersion forces are present in all molecules- regardless if they are polar/nonpolar or covalent/ionic. They are also called induced dipole-induced dipole or Van Der Waals forces.

Kelly Ha 1K

So just to clarify, all mollecules have London Disperson forces right? regardless of what molecule it is , it will always have that? All molecules have dispersion forces (LDF/Van Der Waals/induced dipole-induced dipole), whether they are nonpolar or polar, ionic or covalent. Larger atoms/molecules ...

Kelly Ha 1K

I believe the names and chemical formulas would be provided on the test- I don't think we're expected to have memorized them since Dr. Lavelle hasn't taught/tested us on nomenclature yet.

Kelly Ha 1K

The cation within the ionic bond exerts an electrostatic attraction on the electrons surrounding the anion. This force causes the anion's electrons to be pulled into the bonding region between the two atoms, distorting the anion's electron cloud density. The electrostatic attraction pulls the anion'...

Kelly Ha 1K

Since bond length is proportional to atomic radius, the bond length trend is the same as the atomic radius trend: increase down a group, decrease across a period. This is because larger atoms/ions form longer bonds (since they're further away from each other), and smaller atoms/ions form shorter bon...

Kelly Ha 1K

Induced dipole means that temporary dipole moments are created when one molecule induces (causes) an electron density distortion in the other molecule. This can be in the form of two types of interactions: 1) an dipole- induced dipole (a permanent dipole moment molecule induces a non permanent dipol...

Kelly Ha 1K

Each post on Chemistry Community counts as one point and we should be posting 5 times a week before Sunday midnight each week. This adds up to a total of 5 points per week, and 50 points for the whole quarter.

Kelly Ha 1K

The most favored (most stable) Lewis structure among resonance forms is the one with the most formal charges on all atoms of the structure closest to zero. Structures with lower formal charge are also more favorable to structures with higher formal charge (for example -1/+1 is better than -2/+2).

Kelly Ha 1K

Do you think there's any way to check our overall grade? Like how many total points there are so far in the class. I've been hearing that the only way to find out your overall grade is to calculate it yourself. You can add up all the points you have in the grade book and divide it by the total poin...

Kelly Ha 1K

Elements such as P, S, Cl are exceptions to the octet rule and have expanded valence shells because the valence shell has enough orbitals to accommodate additional electrons. This applies to other elements in period 3 or beyond because the 3d subshell becomes available starting from n=3 and beyond (...

Kelly Ha 1K

To add on to the previous answer, the number of orbitals simply corresponds to the subshell (letter) you identified:
s subshell has one orbital
p subshell has three orbitals
d subshell has five orbitals
f subshell has seven orbitals

Kelly Ha 1K

You are correct, boron and aluminum are exceptions to the octet rule because they both only have 3 valence electrons, fewer than the usual 8 electrons that satisfy the octet rule.

Kelly Ha 1K

Everytime you post something course-related on Chemistry Community it counts as 1 point. According to Dr. Lavelle's website (on the Weekly Online Discussion page for reference), he says that questions, answers, comments to questions or answers, or just general discussion counts as a post.

Kelly Ha 1K

All of the given ions are isoelectronic with one another, which means that they all have the same number of electrons but different number of protons. For cations, when electrons are lost, electron-electron repulsion (shielding) decreases and the protons pull the electrons closer to the nucleus so t...

Kelly Ha 1K

I believe your TA will be checking what time you are posting each post so you should post 5 times every week before Sunday each week at 11:59PM.

Kelly Ha 1K

Yes, the Sapling homework is now due on Week 4, which includes Week 2, 3, 4 material (because Week 2 and 3 were supposed to be due Sunday of Week 3 but now Dr. Lavelle added 10 additional problems from Week 4 content). I would suggest to at least attempt to complete all the Sapling homework before ...

Kelly Ha 1K

In Dr. Lavelle's email he said that we can write on the front and back of the two sheets he gave us (Constants and Equations & Periodic Table sheets) for rough work and show our TAs that both sides are blank. I'm assuming, as everyone else said, that we can use more sheets as long as we show our...

Kelly Ha 1K

I believe the format of the test is all multiple choice but I don't think the exact number of questions was disclosed to us.

Kelly Ha 1K

I believe the waves themselves represent electron waves orbiting around the nucleus. The standing wave model shows that the electron waves constructively interfere with themselves and create a standing wave, proving that electrons have discrete quantized energies (can only exist in certain energy st...

Kelly Ha 1K

Knowing SI unit conversions would be very helpful on the test and just to know in general as a fundamental topic. I believe there was a SI unit conversion step in the practice test on Respondus last week (I know it's probably not the same type of problem but since it was on the practice test I think...

Kelly Ha 1K

So just to clarify: an increase in frequency increases the energy of the wave and the number of photons, but not the energy per photon? And this is what the photoelectric effect is? That the waves have particle like properties where only one photon interacts with an electron, and therefore changing...

Kelly Ha 1K

Black body radiation is a type of material that will absorb all wavelengths of light and emit at all wavelengths of light. Like the previous post stated, it is a hypothetical material that currently does not exist. Black colored materials are hotter because they absorb more wavelengths of light than...

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