CHEMISTRY COMMUNITY

Xinying Wang_3C

I think in this case, BC is the intermediate because it is formed and used in the later step, and C is the catalyst since it serves as a pathway. I am not sure about a systematic explanation for this.

Xinying Wang_3C

Intermediates are not always present in a reaction, usually it can be identified by formed as a product in the previous steps and gets used up in the later steps as a reactant.

Xinying Wang_3C

I think in most cases, some kind of experimental data will be provided to assist in finding slow/fast steps, and in other cases, the question will tell you which is the slowest step, so that one can determine the rate-limiting step.

Xinying Wang_3C

The intermediate product should not appear in the rate law, and I think you are supposed to use other equations provided to substitute its position in the rate law.

Xinying Wang_3C

A catalyst will not appear in the overall reaction equation. It is usually presented as intermediates in the steps and gets canceled during the process.

Xinying Wang_3C

I think it is just used to indicate the rate for the reverse reaction.

Xinying Wang_3C

you could plug numbers into the equation, t(1/2) = [A]0/2k

Xinying Wang_3C

the spontaneous reaction has a negative delta G, which indicates the product has a lower energy state than the reactant, however, the activation energy exists for both spontaneous and non-spontaneous reactions, and it can be lowered by adding catalysts or enzymes to facilitate the reaction.

Xinying Wang_3C

An intermediate is produced in the steps of reactions, but it will be the reactant for some follow-up chemical equations and is eventually canceled out after summing all the steps together to get the final formula.

Xinying Wang_3C

the average rate calculated by delta Y/delta Time, but the instantaneous rate is the one in which the derivative is needed.

Xinying Wang_3C

I would say redox reactions.

Xinying Wang_3C

I always start with O which is always -2, then H is often +1, any solids are 0. Then some really frequent ones, like Na,1+, Mg 2+ Ca 2+, Cl-, OH-, then you should be good for most of the cases to figure out the rest of elements' oxidation numbers.

Xinying Wang_3C

According to the question, you can put Au+HCl+HNO3 on the reactant side, and HAuCl4+NO2+H2O on the product side. Then determine, for example, Au-->HAuCl4, Au is oxidized since its oxidation number goes from 0 to +5, then do the same thing for N, then balance the transferred electron numbers on both ...

Xinying Wang_3C

Hi, I am stuck on question 18 in this week's sapling. I put Fe+O2+H2O for the reactants and Fe2O3-3H2O for the product, and I get 4Fe+3O2+6H2O⟶2(Fe2O3∙3H2O)+12e-, which is not correct. I am wondering which step did I do wrong.

Xinying Wang_3C

I was having the same question, and I saw Dr. Lavelle answered this same question in another post under this same topic page. Basically, because everything else is under the standard condition: 1 atm and 1M of the solution, we can use 1bar/1atm and 1molarity in the Nernst equation.

Xinying Wang_3C

Thank you! I was stuck on the same question and using copper(II) value.

Xinying Wang_3C

Because deltaS = q/T, when a reaction happens, Tsystem >Tsurrounding when the same amount of heat is transferred since the reaction is not at equilibrium, this difference between T results in deltaS system < delta S surrounding in absolute values, so when these two add up, the S total not equal to 0...

Xinying Wang_3C

A reaction is spontaneous when delta G is negative, given the equation delta G=delta H-T*deltaS.

Xinying Wang_3C

I guess the textbook might mean that the universal entropy always tends to increase, as the definition of entropy states. So disregarding other factors, when the total entropy change is positive, meaning it fits with the universal rule, so the reaction will be spontaneous.

Xinying Wang_3C

Yes, when K represents a variable, it means the equilibrium constant.

Xinying Wang_3C

Hi, you got the right approach, maybe just check if you multiply the coefficients before calculating values or if you forget the negative signs.

Xinying Wang_3C

Breanna Ouyang 1I wrote:Was there a minor calculation error? I tried it and got -507.3 from (2*109.3) - ((3*223.1)+(2*28.3)), but that might be wrong lol (your process is right though)

Thank you! Your answer is right, it might just be some calculation mistakes I made.

Xinying Wang_3C

Hi, I think left and right are just ways to distinguish these different microstates, that you can have them positioned up and down, as long as there are two different outcomes of the microstates.

Xinying Wang_3C

Since O2 is consisting of two identical molecules, if you place them on different sides, they will still be the same, so there is only 1 microstate possible for this molecule, so the degeneracy is 0.

Xinying Wang_3C

AJForte-2C wrote:are states/micro states/atomic states all basically the same thing when calculating degeneracy?

I think they are basically the same thing, just different names used in different circumstances.

Xinying Wang_3C

Hi, does anyone know how to find the entropy for the system in question 9? I used sum of products -sum of reactants, but the answer is wrong.

Thanks in advance.

Xinying Wang_3C

Because CO2 only has one way of organizing its components, but NO has two ways.

Xinying Wang_3C

Hi, Can anyone explain what is a reversible system and what is an irreversible system? Dr. Lavelle uses an example of if the difference of pressure between the outside and the inside is infinitesimal, then it is a reversible system, and we should use integral and derivatives, but I am still confused...

Xinying Wang_3C

Hi, So open beaker indicate it is directly contacting the air, and in normal circumstances, the standard pressure is 1 atm so it is constant pressure.

Xinying Wang_3C

Enthalpy is the heat transfer while entropy indicates the degree of disorder in the universe. So Enthalpy change depends on the reaction, but naturally, entropy is tend to increase in the universe.

Xinying Wang_3C

Yeah, I think when the question says it is a closes system that we have to imply there is the only energy exchange, etc. There is an implication aspect of indicating types of system.

Xinying Wang_3C

Thank you!

Xinying Wang_3C

Can someone explain how to find ΔU in the Q18 of the Sapling homework? The question is: A 0.253 mol sample of NO2(g), initially at 298 K and 1.00 atm, is held at constant pressure while enough heat is applied to raise the temperature of the gas by 17.5 K. Calculate the amount of heat q required to b...

Xinying Wang_3C

An easier way to think is that system is the reaction itself, and the rest is the surrounding, eg: solvent, air, etc.

Xinying Wang_3C

I don't think so, since a perfect system is a foundation of knowing q value given one of the conditions in the system or in surroundings in questions, I don't think we will need to handle non-perfect systems since that will be much more complicated.

Xinying Wang_3C

neutralization is the kind of reaction where acid+base=salt and water. In this case, HCl is the acid and NaOH is the base, so when they react with each other, the NaCl which is the salt formed, as well as H2O molecules.

Xinying Wang_3C

I also got stuck on this, and the answers in this post are helpful. I guess we all forget to pay attention to the phase of MCL3.

Xinying Wang_3C

For these types of problems, you can compare the pH and pKa to determine which direction the reaction will want to go. For an acid: If pH is lower than pKa, the reaction does not want to produce more H+ ions to lower pH even more, so it will go towards the reactants to produce more of the acid itse...

Xinying Wang_3C

Hi, can anyone explain the reasoning behind Q9 and Q10 in week 2 Sapling?
I don't know how to compare the pH and pKa values to know if the solution is charged or not.

Xinying Wang_3C

Basically, the percentage ionized is the [dissociated]/[original reactant]. So you would use the ICE Table to solve for the [ H+] as usual. and just use that value to calculate [H+]/0.121 M. Hope this helps.

Xinying Wang_3C

Hi, I think pKa is more about the process of the chemical equation, e.g: how strong the reactant is, etc; whereas pH is more about the concentration of the [H+] at the present stage.

Xinying Wang_3C

Basically yes, Q is the equilibrium for a reaction measured at any time, and k should be a constant number for the same equation conducted under the same temperature.

Xinying Wang_3C

Ka is the equilibrium constant for acid and Kb is the equilibrium constant for the base. They often will be provided in the question just like the Kc value, but if not, you can solve for it by [product]/[reactant].

Xinying Wang_3C

When Q<K, means [product]< expected at equilibrium, so more products tend to form, and the reaction goes forward;
when Q>K, means [product]>expected at equilibrium, so less product will be made and more reactants will form, so the reaction goes backward. Hope this helps.

Xinying Wang_3C

Hi, I think it is because the molar concentration of a solid or liquid can not change very easily. For example, water is usually in great excess, and a chemical compound that is solid doesn't change its concentration when it is still in its solid form. So the change in concentration for pure substan...

Xinying Wang_3C

I think water as a pure liquid never participates in calculating concentrations. Since for aqueous solutions, water will always be there and present in both reactant and product side, so it cancels out. Hope this helps.

Xinying Wang_3C

The Sapling Q9 is "At equilibrium, the concentrations in this system were found to be [N2]=[O2]=0.100 M and [NO]=0.500 M. N2(g)+O2(g)↽−−⇀2NO(g) If more NO is added, bringing its concentration to 0.800 M, what will the final concentration of NO be after equilibrium is re‑established?" I am ...

Xinying Wang_3C

I was also confused about this during the lecture, but I guess it is because in the Ba(OH)2 chemical function, only [OH-] is involved. However, changing [OH-] will further cause the change of [H3O+] in the solution, as the acidity of the solution changes. Hope this helps.

Xinying Wang_3C

Q is the reaction quotient, and it is the [product]/[reactant] at any point of the reaction. When Q=K, the reaction is at its equilibrium.

Xinying Wang_3C

Kc is about the molar equilibrium concentration, Kp is the equilibrium partial pressure, and they are two different values.

Xinying Wang_3C

I understand it as normally everything will eventually settle down to the lowest energy form, so eventually it will reach equilibrium.

Xinying Wang_3C

I think the pure substance he mentioned is solid (like metals) and liquid (like water, for example). Aqueous is often counting into the concentration since it is not "pure" I guess, if this helps.

Xinying Wang_3C

I had the same question while watching the lecture, and this answer clears it up a bit. Thank you!

Xinying Wang_3C

Hi, for question 13 in the sampling homework, I am wondering what exactly is pKa, and does it works the same way as pH values while determining the acidity of a solution?

Thanks in advance!

Xinying Wang_3C

Yes, basically pH+pOH = 14, so they have the same scale.

Xinying Wang_3C

Hi, you can find pOH through 14-pH, and then you can calculate the 10^(-OH) to find the [OH-].

Xinying Wang_3C

Hi, there are a couple of ways to find pH given [OH-]. First is to use the formula pOH = -log[OH-] and then use 14 -pOH = pH. You can also use 1.0*10^(-14)/[OH-] = [H+] and use pH = -log[H+] .

Xinying Wang_3C

I think [H30+] and [H+] are basically the same thing, they are just different ways to represent proton concentrations.

Xinying Wang_3C

Hi, I guess the convention is to put the central atom first, and then other ligands.

Xinying Wang_3C

It is triamminetrichlorocobalt(III). This question also took me several tries, the tri ammine is because (NH3)3, and the tri chloro is Cl3, when you calculate the net charge, the cobalt has to be +3 charge, so there is (III) at the end.

Xinying Wang_3C

In his previous email, Dr. Lavelle mentioned the periodic table and the constant sheet, but not the ligand one, so I guess not.

Xinying Wang_3C

Hi, because the overall compound is neutral, and Cl2 has a charge of -2, so the whole compound inside the bracket should have a net charge of +2. In the bracket, the (NH3)5 is neutral, and Cl has a -1 charge, which implies that cobalt has to be +3 charge to make the whole compound neutral as indicat...

Xinying Wang_3C

Hi, just as everyone else said, the chlorine inside the bracket and outside of the bracket do not combine. So there is only one chlorine in the bracket and therefore the name should be pentaamminechlorocobalt(III) chloride.

Xinying Wang_3C

Can someone explain why SF2 is not linear?
I thought there will be one lone pair above the S atom, and another lone pair below it, making it a linear shape.

Thank you in advance!

Xinying Wang_3C

CH2CL2 is polar because the four bonding regions give the molecule a tetrahedral shape, and because of the shape, there must be a dipole moment among the two ends of the molecule, so it is polar.

Xinying Wang_3C

Hi, CH2CL2 makes carbon to form four bonds, so there are 4 electron density regions, which makes the shape tetrahedral. And given the shape is tetrahedral, it is polar and the dipole couldn't be canceled.

Xinying Wang_3C

The 60 degree comes from the triangle that three carbons can form to make one of the possible structures. And resonance structures are not possible in this case. Hope this helps!

Xinying Wang_3C

Because as the whole molecule, the two ends are both oxygen atoms, which makes the molecule nonpolar.

Xinying Wang_3C

In some of these cases, the formal charge definite plays a role in deciding the double/triple bond, but when you have a lewis structure that every atom meets the octet rule and the drawing seems symmetrical, you are probably fine cause usually more bonds are added when there are incorrect numbers of...

Xinying Wang_3C

Hi, yes, lone pair electrons also count in electron density and will affect the shape of the molecules, but they don't count when naming the geometry of the molecule.

Xinying Wang_3C

Hi, London dipole, also called London dispersion force, is an induced dipole, meaning the atom by itself doesn't carry any charges if it has no contact with other atoms/molecules. Dipole-Dipole is permanent, meaning the molecule has a significant difference in electronegativity and it will show a di...

Xinying Wang_3C

Yes, hypothetically they can for two hydrogen bonds with two different hydrogen atoms.

Xinying Wang_3C

Thank you, Dr. Lavelle, I was so stressed out when I saw all of your classes are closed.

Xinying Wang_3C

Hi, yes, 3d will always come before 4s when writing the electron configuration. But 4s has actually slightly lower energy than 3d, so when it comes to losing electrons/adding electrons, there will be a situation like 3d^1 4s^2, since the lower energy level(4s) fills up first. I know it is really con...

Xinying Wang_3C

This question also confused me when I was doing it, and I took multiple attempts. I guess the confusing part is that the question is asking as a whole, not inside of the molecule, so you are trying to find which molecule exhibits overall non-polar characteristics, for example, CH4 will be one of the...

Xinying Wang_3C

Dipole-Dipole intermolecular force is stronger than induced-dipole-induced-dipole intermolecular force. The difference in negativities of the two atoms in dipole-dipole is permanent, meaning that the atoms have this difference by itself, whereas in induced-dipole-induced-dipole force the atoms don't...

Xinying Wang_3C

Hi, I thought it was SO2 when I was solving it. Since S and O has a difference in electronegativity, the force between two molecules would be dipole-dipole, as one end would be more negative than the other.

Xinying Wang_3C

For question 5 in the sampling 5/6, can anyone explain what is the correct formal charges for resonance structure A? It has a triple bond between N and O, and O has 1 lone pair of the electron, C has 2 lone pairs. Thank you very much!

Xinying Wang_3C

Hi, another resonance structure would be a double bond between C and O(C=O), basically switch the structure given by the question to another O molecule. And the O with 6 lone pairs will have a negative formal charge. Hope this helps.

Xinying Wang_3C

Hi, as everyone else said, it has to do with the distance on the periodic table, the further on the periodic table, which means it is metal to non-mental, the more ionic the bond is. Hope it helps.

Xinying Wang_3C

Hi, I would say first understand all the major concepts/formulas and then practice with the textbook questions and sapling. For me, doing practice problems is really helpful, and that also further deepens your understanding of the materials. Hope it helps.

Xinying Wang_3C

Can someone explain how to start on this? what is the molecular formula of these molecules and would formal charges be different? Here is the question: The formulas of the bromate ion, BrO−3, and chlorate ion, ClO−3, are very similar. However, the formulas of the nitrate ion and phosphate ion contai...

Xinying Wang_3C

My interpretation of the wording is "which lewis structure is most likely to happen/appear to this molecule", in order words, it is asking which one of the lewis structures is the most stable one. Hope this helps.

Xinying Wang_3C

I am also confused on the same concept, I guess it's just hard to visualize what happened to those delocalized electrons while switching between double and single bonds.

Xinying Wang_3C

I also had the same question, and this is really helpful! Thank you.

Xinying Wang_3C

Given the wavelength and ask to find the initial energy level and ending energy level, after using v=R(1/(n2)^2-1/(n1)^2), then I have (1/(n2)^2-1/(n1)^2)=some number, how do you find n1 and n2 from fractions or decimals?

Xinying Wang_3C

In the equation: v=R(1/(n2)^-1/(n1)^2), is n1 the higher one, or is n2 the higher one? I always got confused about these two.

Thanks in advance!

Xinying Wang_3C

For question 9 in the sampling homework for week 2,3,4, I got the wavelength, but I don't know how to test if the photoelectric effect is the cause of sparking or not.

Can someone explain the second part of question 9 please?

Thanks in advance!

Xinying Wang_3C

For Q4 in the homework, I understand I need to find E through E=hv, but how I proceed from there? I am quite confused about how to find the work function.

Thanks in advance!

Xinying Wang_3C

I had exactly the same question, and you worded it really clear. Thanks.

Xinying Wang_3C

I am also very confused about the equation Hψ=Eψ, especially what each of the variables stands for and what scenario should we use it. However, it seems Dr. Lavelle mentioned this majorly for introducing the idea of atomic orbitals. So I am not sure to what extent should we understand this equation ...

Xinying Wang_3C

I do believe the studying method varies from person to person, especially for chem. For me, I listen to lectures and practice on homework, there are definitely times that I have no clue how to start solving a question, then I will go to textbooks or chemistry community, sometimes you can find a simi...

Xinying Wang_3C

Yes, Planck's constant is 6.626 x 10^-34 J/s, and there is no need to memorize it as there are a lot of constants in this chapter, during the quiz/test, the value should be given.

Xinying Wang_3C

Hi,
I was working on the Sapling homework, and I got confused between the formula E=hv and E=hc/v. I am wondering what is the difference between these two formulas and when to use which?

Thanks in advance!

Xinying Wang_3C

I understand, the question was worded quite confusingly, but once you know what it's asking it's not that hard. Number of spectral lines would be equal to the number of times the electron emits energy, and the electron would do that for each complete energy level it moves down since energy is quant...

Xinying Wang_3C

Hi, for me it really depends on the requirement of the class. I am using the IPad for my math class, since it requires me to upload a pdf version of my homework on website, and I am not a big fan of using a scanner every time for my homework. Instead, apps like GoodNotes and notability are really us...

Xinying Wang_3C

Yeah, I think a scientific calculator is definitely necessary for problems like this, and just remember to pay extra attention to negatives, decimal places, etc, when plugging in the numbers.

Xinying Wang_3C

When electron drops to n=1, it emits UV light, so it is the Lyman series. When electron drops to n=2, it emits visible light, so it is the Balmer series.

Xinying Wang_3C

Usually the threshold energy, which is the same as work function, will be given to you in the question. The value of the threshold energy is not a fixed constant, and it varies for different metals. Hope this is somewhat helpful.

Xinying Wang_3C

Got it! Thank you!

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