CHEMISTRY COMMUNITY

Brian J Cheng 1I

Drinking a nice warm beverage and getting adequate sleep would help greatly. I've found that to be very helpful on occasion.

Brian J Cheng 1I

AKatukota wrote:Is molecularity then just the amount of reactants that interact?

Yep! Unimolecular = 1 molecule
Bimolecular = 2 molecules need to collide to react
etc etc etc.

Brian J Cheng 1I

We cannot thank you enough for your service Lyndon! Thanks for making Chemistry something we can enjoy, and for making your review sessions something we could all look forward every quarter! Love you 3000 king

Brian J Cheng 1I

As Naneeta says, they're uncommon. Dr. Lavelle specifically told us to focus on 0, 1, and 2 order reactions (zero order, unimolecular, and bimolecular). Zero order == nonmolecular?

Brian J Cheng 1I

what is the significance of the Boltzmann distribution? It plots the number of molecules against energy and shows how many molecules have a certain amount of kinetic energy at various stages of a reaction. At higher temperatures, there are more molecules with more kinetic energy than at lower tempe...

Brian J Cheng 1I

That tripped me up too. It's the unit conversions that cause it to have 10^-12.

Brian J Cheng 1I

Mai V 4L wrote:Will the test focus more on the first or second half of the class? (take home final sounds kinda lit)

Final exam for this class is cumulative meaning it will cover the entirety of this quarter's material. In other words, it will focus on everything :)

Brian J Cheng 1I

By replacing t with t1/2, you're basically freezing the equation at the moment when the half life of the reactant takes place. At this moment (t1/2), the original concentration is exactly half of its starting value which explains the replacing of [A0] with 0.5[A0].

Brian J Cheng 1I

Kallista McCarty 1C wrote:I was not able to make it to lecture on Friday, can someone explain what was covered?

If you would like some notes from that lecture, reply with your email and I can send them over :)

Brian J Cheng 1I

Pretty sure some of the exams should have the solutions already on them as they can be student submissions of old exams

Brian J Cheng 1I

There are definitely other errors in the solutions manual that may take some time to dig out. At this point we could either confirm with others that we got the same answer, consult Dr. Lavelle himself or the TAs, or just simply ignore the answer and say as long as we did the process right, we're fine.

Brian J Cheng 1I

Judging by the tests we have been given, there should be a large question for each of the major topics (maybe 1-2 depending on how broad the subject is). I would expect 1-2 solely on thermo, 1-2 on kinetics, and 1-2 on the things we learned at the beginning of the quarter with some other smaller mis...

Brian J Cheng 1I

Helpful way to remember it that I have used is to realize that the names of the agent and the rxn are opposites. For example: the species present in reduction rxns are the oxidizing agents, and vice versa.

Brian J Cheng 1I

Weird setup, but keep in mind there are exceptions to the rules we are taught. Here is one of them :) doubt we will see this on our exam though as it is almost eerily specific

Brian J Cheng 1I

Test 2 will cover all material from Focus 5 (material not on the midterm) to the end of electrochemistry. It will not have any overlapping material but most likely some concepts will carry over from thermodynamics as Focus 5 is still within the thermo outline.

Brian J Cheng 1I

Spontaneous reactions have positive cell potentials, so going off of this logic it technically contributes to a "higher" net cell potential. But as Sarah said the E cell value should determine spontaneity, not the other way.

Brian J Cheng 1I

Agreed. My TA recommended us know the process derivation of course but he also said not to depend on just the derivation on the exams as they would take up time. Just focus on the final formula and we should be good

Brian J Cheng 1I

Adding on to that, galvanic cells are spontaneous, meaning they have a positive net cell potential. Electrolytic cells are NOT spontaneous, which require electrical energy to run.

Brian J Cheng 1I

Make sure the charges that are reduced matches the amount of charges that are oxidized.

Brian J Cheng 1I

The glass bit is the pH meter which gauges the concentrations of products vs reactants and converts that in terms of [H+] to find the pH of the general reaction.

Brian J Cheng 1I

Thank you! Will add it to my chemistry jam list :))

Brian J Cheng 1I

As many have said above, Free Energy is the amount of energy available from a reaction or a process that can be used to do work. Delta G is change in this free energy, and Delta G naught is this value at equilibrium and standard conditions.

Brian J Cheng 1I

OR you could think of it in terms of positive charge as follows:

REDUCING the positive charge == gaining electrons
OXIDIZING the charge == losing electrons to the addition of an Oxygen (this may not be correct, but is still helpful in remembering it nonetheless)

Brian J Cheng 1I

Thanks Mac! That was very helpful.

Brian J Cheng 1I

To summarize, you can think of this in two ways: thinking about it in terms of concentration as Victoria Zheng--2F explained above, or the quicker way of gases occupying areas naturally as CameronDis2K and Natalie Benitez 1E have said. Whatever floats your boat!

Brian J Cheng 1I

Isolated systems cannot exchange heat or matter with the surroundings. They are closed in nature meaning they have constant pressure and volume (unless otherwise stated).

Brian J Cheng 1I

But how are calculations different for each? and how will we tell when to use delta G or delta G naught in calculations?

Brian J Cheng 1I

As many have said, look at the units you need for a certain calculation and choose the R value accordingly. For example: you're given values in Liters, pressure, temperature, and mols, then you would use R = 0.08206 L*atm/K*mol

Brian J Cheng 1I

Best course of action would be to take 14A-D and BL with CL. I would try to aim to finish all of those sometime during your time at UCLA.

Brian J Cheng 1I

Same here! It just felt like the material we had focused on studying was not as heavily represented as we thought. There was a more general distribution of concepts as compared to the often-meticulous detail that the homework/practice problems entailed. Guess this just means we must consider studyin...

Brian J Cheng 1I

Hope you find them! Have you tried emailing your TAs or Dr. Lavelle? They might be able to spread the news further.

Brian J Cheng 1I

Reversible processes can be "turned" in any direction at the very slight perturbation of the system. The common one we went over in class was the slow isothermic expansion of gas in a piston chamber due to the addition of heat in gas which causes it to expand. Isothermic systems have tempe...

Brian J Cheng 1I

Steam causes a larger energy change/transfer than water when it touches your skin. Larger energy transfer == more damage.

Brian J Cheng 1I

Constant volume as many have said. It's not really constant pressure because the reaction inside can keep changing the pressure.

Brian J Cheng 1I

I would know why the equation exists. It exists in irreversible conditions (temperature constant, P and V changing at infinitesimally small intervals, so dw = -PdV. P becomes nRT/V, and if you take the integral of both sides you're left with w = -nRTln(V2/V1). It's just helpful to know in terms of l...

Brian J Cheng 1I

They will be provided in the question or on a separate resource I believe. There are just so many values for standard enthalpy of formation it's unreasonable to memorize them all.

Brian J Cheng 1I

Any problems from outlines covered recently in class are applicable though I believe it would be best if we do problems correlating with lecture material.

Brian J Cheng 1I

As everyone has said, they're on his website. You'll also find many useful resources there besides the math ones!

Brian J Cheng 1I

The defining characteristic of closed system is that there is no matter exchange between the system or its surroundings. Almost anything else is applicable (i.e. heat transfer, volume changes, etc). Traditionally volume changes are incurred via the depression/compression of a piston as many have sai...

Brian J Cheng 1I

Average grades aren't posted to us. Ask your TA or Dr. Lavelle for the averages!

Brian J Cheng 1I

Depends on the information given and the setup of the question. For example: if multiple rxn equations are given with their respective enthalpy changes, then you might suspect Hess's Law. If bond energies are given, then you might use bond energies.

Brian J Cheng 1I

Yes, I would assume so eventually (maybe during today's lecture!)

Brian J Cheng 1I

Short answer: because steam has more phase changes than water, it releases more energy when it comes into contact with your skin. It transfers more energy to your skin resulting in greater damage.

Brian J Cheng 1I

Hess's Law encompasses all enthalpy changes that accompany a general reaction no matter how many intermediate steps said reaction has. Each individual change in enthalpy is taken into account.

Brian J Cheng 1I

Expect them to be returned this week or next week during discussion sections.

Brian J Cheng 1I

The energy change granted by the steam is higher (more energy is released) resulting in greater energy.

Brian J Cheng 1I

Technically you're allowed to use problems from any material that is going to be on the exam, but like others have said it's better to remain on top of the lecture material as homework problems.

Brian J Cheng 1I

Test is during your discussion period this week, so it will occur at different times for every discussion section.

Brian J Cheng 1I

If the equilibrium concentration divided by the initial reactant concentration is less than 0.05, then the equilibrium constant is valid. Basically this means: the change in concentration of product can be ignored while solving the equilibrium constant equation as long as it is less than 5% of the i...

Brian J Cheng 1I

X value was ignored because the K value was so small, meaning the amount of product formed compared to the reactants was very small. This means that the amount of products (x) can technically be ignored as a change factor which is why you can eliminate it.

Brian J Cheng 1I

I study by doing homework problems and reading the textbook. Just make sure you're well adapted to the situations that might arise and you will be fine.

Brian J Cheng 1I

As stated already, inert gases are exempt as they are stable and seldom interact with species involved in reactions. You can think of them as "pure" substances like liquids and solids which are also exempt from the equilibrium constant.

Brian J Cheng 1I

When K is in the intermediate region, neither forward nor reverse reactions are favored. You can say the rxn is in an "intermediate" stage

Brian J Cheng 1I

Recap: Q can be thought as the products-to-reactants constant at any time of the reaction. If Q < K, this means that the [products]/[reactants] ration at this time is smaller than the ratio at equilibrium; i.e. the [reactants] at this time are greater. Thus, the forward reaction will be favored as t...

Brian J Cheng 1I

I just had a random thought: since it's Week 1 and we've barely started, how about we set up an anonymous Chemistry confessions forum like the UCLA/UC Berkeley Facebook Confessions pages? Personally I think it would be a fun way to get to know each other more. Thoughts on this?

Brian J Cheng 1I

Just got an email about discussion sections and thought I would confirm with you guys.

Are there going to be discussion sections this week for Chem 14B?

Brian J Cheng 1I

Amphoteric means the compound can act as an acid or base, whereas amphiprotic means that the compound can donate and accept protons.

Building on to this, amphiprotic compounds are always amphoteric by definition.

Brian J Cheng 1I

What is the difference between Ka, Kb, and Kw? Also, what is the difference between Ka1 and Ka2? Do we need to calculate Ka and Kb in the final? Ka * Kb = Kw = 1.0 x 10^-14 pKa + pKb = pKw = 14 Ka1 and Ka2 only appear in polyprotic acids (i.e. H2so4). Ka1 is the dissociation constant when 0 H+ are l...

Brian J Cheng 1I

pKa is the negative log of the Ka value. Right now it's used in comparing acid strength as many have stated above.

Brian J Cheng 1I

Acids are by definition both Bronsted and Lewis. The two different "types" are just different ways of looking at acid function. Acids traditionally combine with water molecules and result in the loss of an H+ proton, which binds to water to form the acidic H3O+ hydronium ion. However, this...

Brian J Cheng 1I

Vitamin B12 has a center of Cobalt with 5 Nitrogen ligands and one CH2. It has an overall octahedral shape centered at Cobalt. According to the book, the cobalt prevents pernicious anemia and mental illnesses, and classmates have said it also participates in synthesis of DNA and myelin, metabolism o...

Brian J Cheng 1I

Yes, we're allowed to use abbreviations (Lyndon and many TAs have confirmed this). en, edta, and ox (for oxalato) are all allowed.

Brian J Cheng 1I

Think of polydentate molecules having two different bonding sites on two different atoms. As mentioned above, "spacer" atoms are need to give the atoms adequate space to bind more than once. The two electron LP's on the oxygen of H2O do not provide enough space to form two bonds to a centr...

Brian J Cheng 1I

I would review all the biological examples Dr. Lavelle has reviewed in class. As seen by Test 2, he might pull up something from his lectures that we considered to be unimportant or just another example.

Brian J Cheng 1I

Coordination numbers are basically the amount of bonding sites the central atom has. Take into account some ligands are polydentate and bind at more than one place. Can't give you the exact methods to solve this problem as I don't have my materials with me at the moment, but hope my reply helped.

Brian J Cheng 1I

I would just focus on the topics we've been taught in class. I don't think we've learned about the specifics behind molecular and atomic spectroscopy. We've learned that mass spectroscopy, however, gives you the exact masses of molecular compounds to help in making molecular formulas.

Brian J Cheng 1I

I believe Ka only focuses on the formation of acids (i.e. HCl -> Cl- + H+) so thus water won't really apply here.

Brian J Cheng 1I

As many have said above, it represents the (usually positive) charge of the atom it's coupled with. Usually only transition metals have this notation.

Brian J Cheng 1I

Thanks for the info! I would brush up on everything you can't remember off the top of your head and reinforce those topics via hw questions.

Brian J Cheng 1I

Quick question: does the final cover all topics throughout the entire school year? Or does it only cover material taught after the midterm?

Brian J Cheng 1I

I second the use of a visual aid to determine molecular shape. Sometimes I find it's helpful to visualize a molecule as if each electron density corresponds to a X value in the "AXE" VSEPR notation, then removing X's based on the number of lone pairs there are until the true molecular shap...

Brian J Cheng 1I

Dr. Lavelle will most likely give us an idea on which specific acids we should know but the most common acids (shown above) are a good place to start!

Brian J Cheng 1I

I second the statement above about the effect of pH on protein functionality. I would assume everything in the lectures that Dr. Lavelle goes into detail about is fair game.

Brian J Cheng 1I

Get a good night's sleep. Always helps to give your mind some proper rest

Brian J Cheng 1I

Ligands are basically "anything" that attaches to a metal atom to form a coordination compound. Ligands can be neutral in charge or negatively charged.

Brian J Cheng 1I

Think of this as a more convenient shorthand configuration. The bonding of the Hydrogens on either side of the Carbon is there at the molecular level but it's written as CH2 instead of H-C-H etc to save time and space.

Brian J Cheng 1I

Molecular shape is based on the arrangement of atoms and areas of electron density. Higher or lower Ionization Energies or Electronegativities of atoms will not affect structure.

Brian J Cheng 1I

It's a symmetric molecule and has even distributions of electron density; thus, it is nonpolar.

Brian J Cheng 1I

Yes, we need to memorize the names of the molecular shapes and the rough bond angles. I.e. trigonal planar = shape. 120 degrees = bond angle. OR i.e. trigonal pyramidal = shape, < 109.5 degrees = angle.

Brian J Cheng 1I

I would try to visualize this in 3 dimensions. If there were no lone pairs, the molecule would have a trigonal bipyramidal shape with 180 degree bond angle through the axis and 120 degrees equatorial. But because there are 3 lone pairs, you remove the three equatorial atoms, causing enough repulsion...

Brian J Cheng 1I

Brian J Cheng 1I

It's great to memorize the general conversion factors. pm (picometers, 10^-12) and nm (nanometers, 10^-9) are given but it's great to have a handle on the other prefixes in case they pop up in questions.

Brian J Cheng 1I

An excited electron in this sense may be deadly

Brian J Cheng 1I

Check the atoms' octets as well as the formal charge. More electronegative atoms would prefer negative formal charges and thus form more stable structures if this is accomplished via bond rearrangement.

Brian J Cheng 1I

Venus is correct. But in any case, the 4s orbitals fill first before the 3d orbitals. In copper and chromium, the pre-existing electron from 4s2 fills up a spot in the lower energy 3d5 and 3d10 orbitals, resulting in 4s1 configuration.

Brian J Cheng 1I

Not really sure why ms has -1/2 and +1/2 specifically but the positive and negative values are there to represent opposite electron spins.

Brian J Cheng 1I

The empty 3d block is higher in energy than the 4s block. Because electrons fill up lower energy shells first, the 4s fills before the 3d orbitals. But once one electron is present in the 3d orbitals, it drops to a lower energy than the 4s. This is why an atom with filled 4s and 3d orbitals has 4s e...

Brian J Cheng 1I

Theoretically speaking, all atoms in the 3rd energy level can break the octet rule because of their unused 3d orbitals that can accommodate extra electrons; however, in nature, only a few atoms consistently do this.

Brian J Cheng 1I

We will not need to know how to calculate the exact values in ionization energy or electronegativity; however, we will need to know what they mean and their respective periodic trends. We'll also need to know how they apply to bonding (i.e. the atom with the highest electronegativity prefers a negat...

Brian J Cheng 1I

n = principal number
l = orbital angular momentum number
ml = magnetic number

I would just try and hard-code it into my mind. I don't think the full names of the quantum numbers matter very much though--I'm sure the graders will know what you mean if you just use n, l, ml, and ms.

Brian J Cheng 1I

Adding on to the posts above, I find it helpful to remember the two separately by thinking of Electron Affinity (energy released when an element gains an electron) as the "affinity" or "attraction" that an elements have for an electron. This increases across a row and decreases d...

Brian J Cheng 1I

Expanded octets (otherwise known as exceptions to the octet rule) exist starting with elements with 4s and 3d orbitals. Due to the nature and arrangement of orbitals, the 4s orbitals fill up before the 3d ones do. This allows additional electrons to be added into the empty 3d orbitals, allowing thes...

Brian J Cheng 1I

I personally have not tried Sapling for myself yet, but I've heard some students are disappointed with what it offers. It's apparently just the same information you can find from the textbook and Dr. Lavelle's webpage. Still open to anything though! I'll give it a look and update my post if I find a...

Brian J Cheng 1I

Formal Charge for an element in a compound/molecule is calculated with this formula: FC = # of Valence electrons - (# of lone pair electrons + (#of shared electrons / 2) ) = V - (L + S/2). The S value refers to the # of shared electrons; i.e. the amount of electrons present in bonds. Remember that s...

Brian J Cheng 1I

I would personally focus on exposing myself to as many problems as possible to get used to applying the information we've learned in lectures in multitudes of ways. External resources (Professor Lavelle's webpage, Youtube, Khanacademy) can help clarify information, but the true essence of understand...

Brian J Cheng 1I

We haven't been taught how exactly to obtain measurements for bond lengths as of now. Professor Lavelle specified the different lengths to prove that double and triple bonds have higher attraction and thus shorter lengths than single bonds, and the bonds for resonance structures have lengths between...

Brian J Cheng 1I

Valence electrons of a specific element are based on the position the element is inside its period (row). For example, Oxygen is the 6th element in its period (not counting elements in d and f blocks), which means it has 6 valence electrons. Chlorine is the 7th element in its period so it has 7 vale...

Brian J Cheng 1I

Question: 1D.15: What are the principal and orbital angular momentum quantum numbers for each of the following orbitals:(a)6p,(b)3d,(c)2p,(d)5f?

principal # n = energy level
Angular momentum # l = 0,1,2,3,.... n-1

a) 6p: n = 6, l = 1
b) 3d: n = 3, l = 2
c) 2p: n = 2, l = 1
d) 5f: n = 5, l = 3

Brian J Cheng 1I

Below always holds true:
s: 1 orbital
p: 3 orbitals
d: 5 orbitals
f: 7 orbitals
g: 9 orbitals

Brian J Cheng 1I

Avogadro's number is used to convert mols to virtually any unit. Think of it as the number of "something" in one mole of this "something". It can be the amount of atoms in 1 mole of atoms, or even the amount of jelly beans in one mole of jelly beans.

Search found 115 matches