CHEMISTRY COMMUNITY

Dayna Pham 1I

so motivational :’)

Dayna Pham 1I

Dayna Pham 1I

In class today, Dr. Lavelle calculated the change in standard free energy of formation by using the summation method, and his deltaG standard of formation was in kJ/mol. He said that the deltaG standard of the reaction is a per mole equivalent, that is, per mole of reaction. Now I'm confused, becaus...

Dayna Pham 1I

How can we use the Nernst Equation to calculate pH? Does it have to do something with ion-selective electrodes?

Dayna Pham 1I

k is the rate constant for the forward reaction.
k' is the rate constant for the reverse reaction.

Dayna Pham 1I

If K is less than 10^-3, there are more reactants at equilibrium. Therefore, the reactants are strongly favored. The reactants are more stable than the products. If K is greater than 10^3, there are more products at equilibrium. Therefore, the products are strongly favored. The products are more sta...

Dayna Pham 1I

Also, if you are only given the overall reaction rate law, I don't think you would be able to determine what the intermediates are, as intermediates and catalysts aren't included in the rate law equation.

I'm also wondering about this though, someone please verify!

Dayna Pham 1I

Intermediate species are formed in one of the elementary steps of the reactions then used up in another of the following elementary steps of the reaction. It is not included in the rate law.

Dayna Pham 1I

Can someone please explain why Gibbs free energy for the combustion of a gas used to heat a BBQ at a pool party is negative? Wouldn't it require be a non-spontaneous process for the combustion to occur? This question confused me as well. After the test, I realized it was spontaneous because combust...

Dayna Pham 1I

Hello!

All these people gave amazing resources, just wanted to add a graphic that helped me understand it!

The derivative is shown as the slope of the graph, while the integral is shown as the area under the curve of the graph.

Dayna Pham 1I

Question 15.3 reads: 15.3 In 20. s, the concentration of nitrogen dioxide, NO2, decreases from 450 mmol/L to 320 mmol/L in the reaction 2 NO2(g) -> 2 NO(g) + O2(g). (a) Determine the rate of reaction of NO2. (b) Determine the rate of formation of O2. (c) What is the unique rate of the reaction? Why ...

Dayna Pham 1I

Hello! Question 15.5 reads: 15.5 Ethene is a minor but important component of natural gas, and its combustion has been thoroughly studied. At a certain temperature and pressure, the unique rate of the combustion reaction C2H4(g) + 3 O2(g) -> 2 CO2(g) + 2 H2O(g) is 0.44 mol/L/s (a) What is the rate a...

Dayna Pham 1I

I think to find the order in general, you have to do the calculations. The m and n values, which are the reaction orders with respect to each reactant, are determined experimentally. The difference between the 1st and 2nd order of the overall reaction seems to be just a difference in the sum of m an...

Dayna Pham 1I

This is when you're trying to find the balanced half reaction in a basic solution. This would be the last step, after balancing all atoms outside of O and H, then balancing the O's with adding H2O's to the side in which you need them. Now, we will focus on balancing the H's. You add an H2O to the si...

Dayna Pham 1I

Hi! Hope this helps!

Dayna Pham 1I

I don't think it matters as long as you keep your units consistent throughout and don't add kJ to J.

Dayna Pham 1I

Hello! I can't seem to wrap my head around the concept of galvanic cells. In class today, we learned that galvanic cells change chemical energy, but what exactly is a galvanic cell? When "cells" are mentioned, am I suppose to think of biological cells or just pockets of space? I'm having t...

Dayna Pham 1I

The purpose of a salt bridge is to maintain a change balance, since electrons are moving from one side to another. Without the salt bridge, electron transfer from one "reaction beaker" to another will stop due to charge buildup. Adding a salt bridge (or a porous disk) will help the solutio...

Dayna Pham 1I

Why can you only calculate delta S, when the q for the reaction is reversible? The reaction does not have to be reversible in order to calculate delta S in this way. They just use q rev as a convention, but since delta S is a state function, the pathway doesn’t matter (therefore, it doesn’t matter ...

Dayna Pham 1I

Parth Mungra wrote:If K=0, then the reaction is neither spontaneous nor nonspontaneous

This sounds interesting! I’ve never heard of a case where K=0, can someone elaborate on this?

Dayna Pham 1I

Last quarter it took around a week!

Dayna Pham 1I

In the constants and equations sheet, the values for various specific heats of water are listed as: Specific heat capacity ice = 2.03 J.°C-1.g-1 Specific heat capacity water liquid = 4.184 J.°C-1.g-1 Specific heat capacity water vapor = 2.01 J.°C-1.g-1 Molar heat capacity water liquid = 75.29 J·K-1·...

Dayna Pham 1I

What are the main differences between reversible and irreversible expansion of an ideal gas? Is it just that reversible is with changing pressure and irreversible is with constant pressure?

What about temperature?

Dayna Pham 1I

If we use W = -P * deltaV for when it's under constant pressure, what formula do we use when it's not?

Dayna Pham 1I

Kobe_Wright wrote:when a problem says there is 3 moles or 6 moles of a certain molecule do we just use that value to find the concentration or do we put the moles given in the exponent of our equilibrium constant equation?

I think we use those values of moles to find Q and then compare it to equilibrium.

Dayna Pham 1I

Karina Jiayu Xu 4E wrote:On the test I used bar and it was fine

My TA said she would’ve accepted both atm and bar on the test, but I’ve also heard that atm is more standard.

Dayna Pham 1I

Also, I thought standard reaction enthalpy was per mol, so why didn't they divide by two at the end, since there's two moles of HNO3?

Dayna Pham 1I

Just regarding 8.63 a), why is the standard enthalpy of formation for NO positive and not negative? The solutions manual said it was +90.25 kJ/mol, but I thought that since it was a product, it would have negative enthalpy?

Dayna Pham 1I

If a weak acid deprotonates and releases less than 10^-7 M of hydronium, then you would consider autoprotolysis. This is because if you ignore autolysis and calculate pH of an acid that creates 10^-9 M hydronium, then the pH would be basic. This would not make sense since there is an acid in the so...

Dayna Pham 1I

For 12.21 in the Sixth Edition, we are suppose to solve for either [H3O+] or [OH-] using the Kw relationship, [H3O+][OH-] = 10^-14. Can we use this relationship to solutions other than 100% water? I thought that Kw was only for water but it doesn't say in the question whether the solution is water-b...

Dayna Pham 1I

Does adding a liquid or solid to a system evoke a shift to restore equilibrium? I thought it wouldn't, especially if you add more of the solvent, since there is already so much of it. However, the Post-Test contradicts my thoughts, help?

See the attached picture for what I'm talking about.

Dayna Pham 1I

Hello! I find myself having trouble writing out my own chemical equation for those questions in which you are only given some initial information. How do you know what combines with what? For instance, the example we went over in class on Week 2, Lecture 3 (1/18/19), the given information was: Given...

Dayna Pham 1I

Adding onto what they all said, I’d like to relate this to Le Chatelier’s Principle. When pressure/volume/concentration change, it is Q that changes, not K. However, when temperature changes, then K changes.

Dayna Pham 1I

Hello!
Question 11.87 is about the dissociation of a diatomic molecule, X_s $\rightleftharpoons$ 2x.
How do we know this reaction is endothermic?

Dayna Pham 1I

Hello! In the example given on Wednesday, we did the ICE table on ATP hydrolysis. It was given that the initial molarity of ATP in healthy muscle tissue is 8.435 x 10^-3. We determined that the change in molarity was - 8.435 x 10^-3. I thought that if we subtracted the change from the initial, we wo...

Dayna Pham 1I

K is found using concentrations at equilibrium, BUT to calculate Q do we just use the initial concentrations or concentrations at any given time? Yes. Q can be calculated at any time in the reaction, and then you would compare it to the given equilibrium constant, K, to determine if the reaction is...

Dayna Pham 1I

Pure substances, which are solids and liquids, are omitted from equilibrium constant calculations. This is because the molar concentration of pure substances does not change in a reaction. This does make the calculation for K an approximation, as there may actually be a reaction going on with the so...

Dayna Pham 1I

Carbonato and sulfato both seem to be bidentate because they both have single bonded oxygens, but the textbook doesn't say they are polydentate... Any insight?

Dayna Pham 1I

Chlorate is ClO3 -1, and adding a Hydrogen would make it neutral, so I understand that HClO3 should be neutral.
Why is perchloric acid (HClO4) neutral and not negatively charged since there's another oxygen on it compared to neutral HClO3?

Dayna Pham 1I

thanks :P

Dayna Pham 1I

Nevermind. I looked it up in the textbook and it was just a subtle difference: Coordination compounds include what is inside and outside the brackets. They do not have a net charge, as whatever is inside the coordination sphere (surrounded by brackets) cancels out with whatever is outside the bracke...

Dayna Pham 1I

What's the difference between a coordination compound and a coordination complex? Does this have to do anything with the charges?

Dayna Pham 1I

Look at the formal charge of the atoms involved in the Lewis structure. Formal charges should add up to 0, unless there is a charge indicated on the molecular formula, and it is more favorable to have a formal charge on the terminal atoms.

Dayna Pham 1I

Dissociation energy is the energy required to break a covalent bond. I believe that dissociation energy is positive because a positive change in energy is required to break a bond (gains energy) while a negative change in energy is required to make a bond (loses energy).

Dayna Pham 1I

6.1 Identify the kinds of intermolecular forces that might arise between molecules of each of the following substances: (a) NH2OH; (b) CBr4; (c) H2SeO4; (d) SO2.

For SO2, why can it create dipole-dipole interactions between molecules of each? I thought it was polar. Is it because of the lone pairs?

Dayna Pham 1I

Rachel Yoo 3E wrote:Nitrogen is in row 2 which means it cant disobey the octet rule. By adding a double bond to chlorine, it would make nitrogen not follow this rule.

Thank you! Makes total sense now.

Dayna Pham 1I

Why does the lewis structure for ClNO₂ have only one double bond? Can't it have two and have zero formal charge on all of the bonds?

Dayna Pham 1I

So is any element in the first 3 groups without a d-orbital in its shell unable to fulfill the octet rule? Under my current understanding, I believe any element in the first three groups don't have to meet the octet rule of maximum 8 electrons, it can exceed the octet rule since it has access to th...

Dayna Pham 1I

Attached is the solution to question 4.95, which asks: Consider the bonding in CH2-CHCHO. (a) Draw the most important Lewis structure. Include all nonzero formal charges. (b) Identify the composition of the bonds and the hybridization of each lone pair—for example, by writing sigma(H1s,C2sp2). My qu...

Dayna Pham 1I

In this example, it shows the image of how a the bonds form.

Why is it C sp² instead of C 2sp²? As well as O sp² instead of O 2sp²? Aren't oxygen and Carbon in the second row?

Thanks!

Dayna Pham 1I

With regards to the dsp 3 hybridized orbitals as well as the d 2 sp 3 hybridized orbitals, octet expansion involves the d-orbitals, which starts at phosphorus. Why does octet expansion begin at phosphorus? I thought anything in the third row could have an expanded octet; why not Aluminum or Silicon?...

Dayna Pham 1I

For the bent shape, does this include instances where there's one lone pair, as well as instances including two lone pairs?

Thank you!

Dayna Pham 1I

Question 4.109 is: 4.109 Estimate the bond angles marked with arcs and lowercase letters in peroxyacetylnitrate, an eye irritant in smog. It adds an image of the molecule, which is attached. But why is angle b around 109.5 degrees? It only has two regions of electron density, so I expected it to be ...

Dayna Pham 1I

I agree with 705 and Clarissa that the reason that the bond lengths are the same is because of the resonance structures. This is due to the delocalization of electrons, creating a single-double bond hybrid, so all the bond lengths are actually the same.

Dayna Pham 1I

It will include hybridization, which is the focus of the UA's peer learning sessions this week.

Dayna Pham 1I

Hello!
I like to associate Pauli with the word "pair" since they both start with P! That's just an extra thing that helps me remember that Pauli Exclusion Principle is about how electrons that are spin paired must be of opposite spin.

Hope this helped!

Dayna Pham 1I

Hello! Square planar is with regards to molecular geometry/arrangement, that is, it considers the lone pairs and how their repulsion affect the bond angle. However, tetrahedral is with regards to the shape of the molecule, only considering the atoms and not the lone pairs.

Dayna Pham 1I

I also agree that the lone pairs affect the angular geometry of the molecule. This is because of electron repulsion. However, lone pairs only are accounted for in the geometry of the molecule. The atoms in the molecule are what determines its shape.

Dayna Pham 1I

Hello!
Can someone explain what the see-saw shape is please? Is it another name for something in our notes? I know it involves a lone pair, but I cannot conceptualize how it looks like a see-saw.

Thanks!

Dayna Pham 1I

Polarizability is a measure of how easy an electron cloud is distorted by an electric field. The electron distortion is proportional to the polarizability of the electron.

Dayna Pham 1I

I assumed that both the bond-lengths were the same, that they were each single-double bond hybrids.
Therefore, I thought it was 130.

Dayna Pham 1I

I think it may be strict on the midterm. On the constants and formula page, it says 2 pts. for sig figs.

Dayna Pham 1I

Hello!
I thought energy could only be positive, why does the equation for energy at a specific principle quantum number have a negative sign in front of it? Is this applied to find change in energy?

En = -hR/n^2

Dayna Pham 1I

Isoelectronicity is the phenomenon when two different atoms/ions have the same number of electrons. For example, Na and Mg+ are isoelectronic, since Na and Mg+ both have 11 electrons. (Mg has 12 electrons in its neutral state, then it loses an electron, having 11). Isoelectronicity is an important c...

Dayna Pham 1I

Copper and Chromium are exceptions to the common electron configuration methods because they are one electron away from reaching a more stable state (a half-filled 3d subshell for Chromium with 5 electrons under Hund's Rule, and a filled 3d subshell with 10 electrons under the Pauli Exclusion Princi...

Dayna Pham 1I

Hi, Stevin! I recall someone said that this is because of the proton to electron ratio. I don't understand it in full, but this is along the lines of what I remember... Ca2+ has 22 electrons and 20 protons, while Na+ has 10 electrons and 11 protons. I guess since calcium as a cation has the most cha...

Dayna Pham 1I

Hello! Here is Problem 3.5: 3.5 Give the ground-state electron confi guration expected for each of the following ions: (a) Cu+; (b) Bi3+; (c) Ga3+; (d) Tl3+. Regarding part a, for Cu+, why does the electron configuration go one more element to the right on the periodic table, when it loses one elect...

Dayna Pham 1I

Does anyone know where the midterm is located? Thanks!

Dayna Pham 1I

The step-up sessions and workshops are really helpful! Look up the practice workshops’ worksheets on chemistry community.

Dayna Pham 1I

E in this case is E(photon). The work function is the energy required to eject an electron in a 1:1 photon:electron interaction.

Dayna Pham 1I

Do we have to know the quantities of wavelengths for each category of light?

Such as gamma = 10^-12 m, and Radio waves = 10^3 m?

Thanks!

Dayna Pham 1I

I remember Dr. Lavelle saying something about not wanting us to use Rydberg's equation but to use another one in place of that. Can someone clarify or explain this?

Thank you!

Dayna Pham 1I

Hello!

I checked on CCLE and MyUCLA, and grades are not on there. I am also not sure where to access this information.

Dayna Pham 1I

I think it would be useful to remember the order of the wavelength classifications from largest to smallest energy, but not necessary to remember the exact length of each category, such as 10^-12

Dayna Pham 1I

Hello! I agree that there will be a formula sheet in the front. Common equations, such as E=hv and c= lambda•v will be given. However, derivations of common equations that are necessary for some problems, such as E = hc/lambda will have to be determined by the test taker. All constants should be on ...

Dayna Pham 1I

Hello! My TA said that if material is not covered in class, we will not be held accountable for it and it is not on the test.

Dayna Pham 1I

Hello!

Does Dr. Lavelle offer Bruincast/podcast lectures? If so, where can I find them?
I know he has the audio/video modules, but that only goes to the Heisenberg Uncertainty Principle.

Thanks!

Dayna Pham 1I

Under the Properties of Light slide, it said that "fields oscillate in direction & strength with frequency, v, (per second)..." but under the Properties of Electrons slide, it said that wavelength = h/(mass x velocity).

Does v stand for velocity or frequency?

Thank you in advance!

Dayna Pham 1I

It doesn't matter which notation you use, as long as you get the number of significant figures correctly. Hope this helped!

Dayna Pham 1I

Hello! I asked Dr. Lavelle this question today in office hours, and he said for now, not to worry about it. Conventionally, for organic molecules, it usually goes in the order of C H O, but if it has lesser known elements, like Cl or Br, it can be in any order. Some order the elements in descending ...

Dayna Pham 1I

Hi! It depends what computations you are doing in the problem. Most problems require multiplication and/or division, in which case your answer should include the least number of significant figures from the problem. If the problem only requires addition and/or subtraction, then your answer should in...

Dayna Pham 1I

Never mind regarding this question. There was only one reactant.

But in other cases, would they give you the masses of both/all the reactants to determine which one is limiting? Thank you!

Dayna Pham 1I

Questions M3 reads: When limestone, which is principally CaCO3, is heated, carbon dioxide and quicklime, CaO, are produced by the reaction CaCO3 (s) -> CaO (s) + CO2 (g). If 17.5 g of CO2 is produced from the thermal decomposition of 42.73 g of CaCO3, what is the percentage yield of the reaction? Si...

Dayna Pham 1I

Hello! Question L39 reads: A 1.50 g sample of metallic tin was placed in a 26.45 g crucible and heated until all the tin had reacted with the oxygen in the air to form an oxide. The crucible and product together were found to weigh 28.35 g. A) what is the empirical formula of the oxide? B) write the...

Dayna Pham 1I

Hello!
I also start balancing with whatever element occurs the least. Sometimes, I leave the ones that are on their own (like O2) for the end, so I can just balance it accordingly with the other side, and it won’t affect any other element.

Hope this helped!

Dayna Pham 1I

If you are at 3.5, you can multiply by 2 to get 7! Hope this helps.

Dayna Pham 1I

What’s the threshold of rounding up and down when calculating the empirical formula versus multiplying to get a whole number?

Is it 0.75 and up, round up; 0.25 and down, round down? When do we know to multiply to get a whole number for the empirical formula?

Dayna Pham 1I

Abby-Hile-1I wrote:Is the homework due in discussion this week? For some reason I thought he said something about turning it in on Friday.

Hi! For this week, we will be turning in the homework in class. Moving forward, it’ll be in discussion.

Dayna Pham 1I

Hello! Does anyone know when the homework we turn in is given back to us, if at all? I did it on a separate piece of paper, but I want to start doing it in my notebook. However, if it turns out that the homework is collected and not given back, I'll just keep doing it on loose leaf. Thank you in adv...

Dayna Pham 1I

Hello! Would I be able to take it?

Dayna Pham 1I

I'm not sure exactly which periodic table we will be given, but Dr. Lavelle did say that we will surely be given a periodic table. Have a wonderful day!

Dayna Pham 1I

Hello! Here's a link to an old post on Chemistry Community that I found helpful when I was wondering this too. https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=1925#p3890 Basically, @Chem_Mod said that we used carbon because it set a common standard for everyone to work with because if different ...

Dayna Pham 1I

Can we leave our answer in pm, or do we have to convert it to km? Hi, Nicolette! Even though the textbook answer is in kilometers, I would write my answer in meters, since meters is the standard SI unit for length. In this case though, I assume they converted all the way to kilometers since it was ...

Dayna Pham 1I

Hello!
With regards to question E21, it asks for the answer in molecules and formula units. What is the difference between molecules and formula units?

Thank you!

Dayna Pham 1I

Hello!

I just completed the pre-assessment for the first module: Empirical and Molecular Formulas. Is there any way I can access my score to gauge how well I did on it?

Thank you in advance!

Dayna Pham 1I

Hello! I’m working on question E16, which asks to find the chloride of a mystery metal. The mystery metal is determined by using its molar mass, which we calculate. Anyways, the mystery metal turned out to be Ag. Since silver is a transition metal, how do we determine its charge? Do we just assume t...

Dayna Pham 1I

Hello! In my chemistry discussion yesterday, my TA did question E1 with us, which reads: The field of nanotechnology offers some intriguing possibilities, such as the creation of fibers one atom wide. Suppose you were able to string together 1.00 mol Ag atoms, each of radius 144 pm, into one of thes...

Dayna Pham 1I

I think solutions manual is not mandatory; however, it is highly recommended. It can be used to check your homework as well as trace back answers if you don't understand them. I believe this is because the textbook (sixth edition) only provides answers to the odd-numbered exercises, which starts on ...

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