CHEMISTRY COMMUNITY

Namita Shyam 3G

Thank you so much Minahil!! This was very helpful!! :D

Namita Shyam 3G

Are there any good acid/base problems from the textbook that people recommend reviewing?

Namita Shyam 3G

How can you easily identify amines? For example, if you're given NH2NH2, you know it's an amine because it has NH2 in the end, but how would you know that (CH3)3N is an amine?

Namita Shyam 3G

Thank you, that makes sense!! Also, how do you know if something like (CH3)3N is an amine? How can you tell (since it doesn't have NH2)?

Namita Shyam 3G

What are some easy ways to identify acids and bases? (For example, if we're given NH2NH2, how would we know that this is a base?). Any help would be greatly appreciated!!

Namita Shyam 3G

Thank you so much guys!!

Namita Shyam 3G

How do we know CH3COOH is an acid, and C6H5NH2 is a base? Can someone please walk me through this?

Namita Shyam 3G

Wow that makes so much more sense!! Thank you so much guys!!

Namita Shyam 3G

When finding DeltaS, why did we do DeltaH/T? Why can't we do q/T (in general, isn't DeltaS=q/T)? For example, for part a, why can't we do q/T=99360J/351.3K?

Namita Shyam 3G

I understand that we use w=-PDeltaV when pressure is constant, but then why don't we use this equation on Sapling (week 3 and 4 hw) number 14 for path A? I know they said that Path A is an isothermal, reversible expansion, but why would it be like that if the pressure is constant (2.62 atm)?

Namita Shyam 3G

Tysm guys!! Just to make sure I'm understanding this correctly---for a problem like Focus 7.23 in the textbook, if the elementary steps had coefficients (they don't, but just imagine), we would use them as the orders when writing the rates for each step as well as the overall rate, correct?

Namita Shyam 3G

When given a problem, how do you know whether or not to write the rate for an elementary reaction versus an experimental reaction?

Namita Shyam 3G

Thank you!! To follow up on that, how can we distinguish an elementary step? How can we identify elementary reactions and differentiate them from experimental reactions?

Namita Shyam 3G

When determining the order (exponent) for a reaction, when are we able to use the coefficients of equations? For example, in the ENDGAME review pdf, number 12, why are we able to write [A]^2 by looking at the coefficients in step 1?

Namita Shyam 3G

That makes sense! Thank you!!

Namita Shyam 3G

In textbook problem 6E.1, how do we know that reactants are decreasing and products are increasing when we start with .15 mol of one reactant and one product?

Namita Shyam 3G

In the "Change" row of the ICE table, how do we figure out whether products or reactants will increase? Do we always need to find Q and compare it with K to figure out if the reaction will form products or not? For example, if the initial concentration of one product and one reactant is gi...

Namita Shyam 3G

In textbook problem 7C.9, mechanism 3, they included NO3 in the overall rate expression even though it is an intermediate. In Sapling #13, they said that intermediates cannot be included in the overall rate expression. So why did they include the intermediate NO3 (in mechanism 3) in textbook problem...

Namita Shyam 3G

Sapling #14 uses a slightly different version of the Arrhenius equation than the one that's in our constants and formulas sheet. The formula is ln(k2/k1)=Ea/R(1/T1−1/T2). Will this formula be necessary on the final? If so, are we allowed to add it onto our constants and formulas sheet?

Namita Shyam 3G

While kinetics is focused on reaction rates and speeds, thermodynamics is focused on Gibbs free energy (delta G) and stability.

Namita Shyam 3G

As time goes on, the instantaneous rate decreases, until it approaches zero near equilibrium.

Namita Shyam 3G

Chemical equilibrium was my favorite!

Namita Shyam 3G

Is knowing the units important for the exam? Will we need to memorize this?

Namita Shyam 3G

On the final, will questions specifically state whether or not they want Ecell or E°cell? (I think in a few textbook problems they haven't explicitly stated which one to find...)

Namita Shyam 3G

I was also wondering why the temperature is 298k when its not written in the problem, and if we can assume it, in what other situations would we assume the temperature is 298k? I think that, if nothing is stated explicitly in the problem, you can just assume that the reaction occurs at SATP (25 deg...

Namita Shyam 3G

I think Lavelle covered it but I dont exactly remember. What exactly is the oxidation number and when is it altered? Thank you in advance for your help! The oxidation number is the number of electrons an atom gains or loses in order to form bonds. For this class, we need to memorize that "H&qu...

Namita Shyam 3G

Lorena_Morales_1K wrote:Would voltage difference be something we have to calculate on the final?

I think so, because there were a couple step up problems and sapling problems that involved finding the E naught value of the cell, which requires taking the difference in voltage of the Anode and Cathode.

Namita Shyam 3G

For state properties, the only thing that is important to know are the initial and final values, so that you can find the difference by doing final-initial.

Namita Shyam 3G

Yep, I think they basically mean the same thing for this class.

Namita Shyam 3G

For the Van't Hoff equation, usually if your given three of the four K1, K2, T1, T2 values, then you can use it to calculate the fourth (assuming H is given). If all four values are given, then you can use the equation to solve for H.

Namita Shyam 3G

STP: 1 atm and 273K (or 0 degrees C)
SATP: 1 atm and 298K (or 25 degrees C)

Namita Shyam 3G

Yep, I think so. If material isn't going in or out, and if heat isn't going in or out, then yes, this would be an isolated system.

Namita Shyam 3G

Does temperature change after volume changes? So, when temperature changes, is volume constant? I'm asking because in Justin's week 6 workshop, #2a, he used the equation delta S=nCvln(T2/T1), but why did he use Cv? Was it because after the volume changed, volume stayed constant afterwards (so while ...

Namita Shyam 3G

In problem 9 on Sapling, they said that: Delta S of System: ΔSsys=∑S∘(products)−∑S∘(reactants) Delta S of Surroundings: ΔSsurr=−ΔHsys/T I was about to memorize this "rule," until I came across 4I.7 on the textbook, which (on the answer key) states that entropy of surroundings is simply equ...

Namita Shyam 3G

A state function is something that does not depend on the "journey" taken to go from initial to final state. Therefore, a state function is simply the difference between initial and final state. Some examples of state functions are ∆S, ∆H, and ∆G. An example of something that is not a stat...

Namita Shyam 3G

So for part (a), you must have had to add the "q" values of both the copper kettle and water, meaning that you already have those separate "q" values already. So, for part (b), you simply take the "q" value of the water, and divide it by the total "q", and mul...

Namita Shyam 3G

Yup, bomb calorimeters are isolated systems. For closed system problems, coffee cup calorimeters are the usual example, and for isolated systems, bomb calorimeters are the usual example.

Namita Shyam 3G

Since K is just a ratio (of products to reactants), there are not units since they cancel.

Namita Shyam 3G

I'm planning on taking both together, just because I want to get things out of the way! I have also heard that it would be better to take BL while things are still online (bc labs and everything will be a lot easier and faster), so if I don't take BL in the spring, I definitely will during summer!

Namita Shyam 3G

I'm pretty sure A is 93% and above, while A- is 90%-92.9%

Namita Shyam 3G

Although there is no reset button, there is a drop down menu called "My Attempt" which lets you view the original Question, as well as each attempt. So if you click on the "Question" option, you can practice redoing the question without seeing the answer!

Namita Shyam 3G

Yep, Kc and Kp are solved the exact same way (products to the power of their coefficients over reactants to the power of their coefficients). For Kp, you just need to make sure that everything is in partial pressures, and for Kc, everything is in molar concentration. Also, don't include solids and l...

Namita Shyam 3G

Does anyone know if Lavelle is also strict on significant figures? I just want to make sure that I do not loose easy points on the midterm, so if anyone who has taken him previously could say, I would appreciate it! I didn't have him previously, but I asked a couple TAs and they told me not to worr...

Namita Shyam 3G

I agree with the previous post. To add, partial pressures only apply to gases (I think)---which is why you don't use them to find "K" for solids and liquids. For solids and liquids, we always use molar concentration...

Namita Shyam 3G

Ya, this is just a rounding issue. I think you did the problem correctly, as long as you understood that for water, [H30+]=[OH-], you're good.

Namita Shyam 3G

So for this problem, you use PV=nRT and plug in (1 atm) for P, (.12 L) for V, (.082 L*atm*K^-1) for R, and (273 K) for T. Be careful what value you use for R, because the units have to cancel. So, you get n=.00536 mol H2. Now, since HBr has a coefficient of 2, we must multiply that with the number o...

Namita Shyam 3G

I agree with some of the previous comments---just remember that exothermic means forming bonds, and endothermic means breaking bonds. In your example, it looks like bonds are being formed, so it would be exothermic. I also feel like on the exam, the deltaH should be given to us, so I would suggest n...

Namita Shyam 3G

I'm also wondering why they would be equal...any help would be appreciated.

Namita Shyam 3G

When K>Q, the forward reaction is favored. When K<Q, the reverse reaction is favored.
And remember, Q is the value we get when the equation is not at equilibrium, while K is the value we get when we are at equilibrium.

Namita Shyam 3G

On the equation sheet on Dr. Lavelle's website, three R values are provided. Those are all we should need!

Namita Shyam 3G

When a reaction is reversed, we will get 1/K.

Namita Shyam 3G

During exams, we can use an equation sheet (which is on professor's website) that contains I believe three R-values... Those are the only R-values that we are given to use on exams, from what I've heard. However, those three given R-values are all we need to do most problems, so I would recommend pr...

Namita Shyam 3G

So the reason why we leave out solids is because solids don't have a concentration or pressure. However, gases and liquids do, which is why we can use them to find K. Also, to answer your second question, the reason why we don't include H2O is because H2O is a solvent (the stuff we have more of). Si...

Namita Shyam 3G

The ideal gas law is important because it allows us to convert from concentration to pressure, and vice versa. If in a problem, for example, we were given all concentrations and one partial pressure, we would probably need to change that partial pressure to a concentration so that we can solve our p...

Namita Shyam 3G

Endothermic reactions need heat, while exothermic reactions give off heat. So if we add heat to an endothermic reaction, it promotes product formation. The opposite is true for exothermic reactions, since heating favors reactant formation.

Namita Shyam 3G

Wow awesome trick/tip!! I have a graphing calculator which is starting to collect dust here at home haha, so now I guess I can start putting it to use! Appreciate the help :)

Namita Shyam 3G

Ya I've also run into this problem a couple times. What you can do is just take both of those values and plug it back into the expressions you made in the "equilibrium" row. If you get a negative value, you know that you can't use that #, but if you get all positive values (because concent...

Namita Shyam 3G

Temperature is the only thing that can make the value of K change. Changing volume or pressure won't affect K---only temperature does. As people have mentioned above, Q is the value we have currently---that is all that it means. K is the ratio of the concentrations at equilibrium, and increasing tem...

Namita Shyam 3G

The way I was thinking about it was that if we increase concentration on one side (either reactants or products), then that side will be -x and the opposite side will be +x in the "Change" row. Sometimes, when all values in the "Initial" row are not equal to zero, the question wi...

Namita Shyam 3G

Ya, I see what you mean, and I noticed that too when I was doing this problem. I don't know if this is correct, but the way I thought about it was that the water that they are adding is a part of the reactants, so we can basically do the problem by reasoning what would happen if reactants are added/...

Namita Shyam 3G

The reaction quotient, Q, is different from K, even though we are still dividing products by reactants. This is because the P and R values in Q are not concentrations at equilibrium. The only way K will ever change is if we change the temperature. We use Q mainly to find what the ratio is when somet...

Namita Shyam 3G

Calculate the concentration of CO(g) in mol.m-3 at 423 K and 0.8 atm using R = 8.206 × 10-5 m3.atm.K-1.mol-1 and assuming only CO(g) is present. Can someone show me the steps to do this problem? I am lost on where to start. Thank you! So I approached this problem by using the Ideal Gas Law equation...

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