CHEMISTRY COMMUNITY

aisteles1G

To arrange them you dont need an equation, you just have to look at their standard reduction potentials, the lower the reduction potential the higher the reducing strength and the higher the reduction potential the higher the oxidizing strength

aisteles1G

I have the 7th ed book so im not sure if this problem was asking about electrolysis, but if it is asking about products of electrolysis then for the reduction half reaction you have to look at your given compound's cation (Ni2+) reduction potential and that of waters and whichever one has the greate...

aisteles1G

I think its because it shows that the bonds are being broken because X2 becomes separate Xs and to break a bond it requires energy (endothermic), at least thats how I thought about it

aisteles1G

the problem says "the presence of a catalyst provides a reaction pathway in which the activation energy of a certain reaction is reduced from 125 kj to 75 kj, by what factor does the rate increase at 298, all other factors equal?" I tried to do this using the Kr2/Kr1= e^-(Ea2-Ea1)/RT but ...

aisteles1G

Youre right, the reverse is 1/K and the answer was 1/3.38*10^-4 = 2.96*10^3, maybe your k value from part A was wrong?

aisteles1G

Im not sure if this is exactly what youre looking for but maybe it will help, The equilibrium constant Kc is equal to the forward reaction rate/ the reverse reaction rate (Kc=Kr/Kr'), so if you heat up an endothermic process (higher forward Ea than reverse Ea) Kc will increase since the forward reac...

aisteles1G

Half life is referring the time it takes for half of the current sample to die, so its not a set amount of substance that will die since 1/2 of 200 is different than 1/2 of 300 but the time for the half of the material to 'die' is the same regardless of the initial amount so t1/2 (half life) is a co...

aisteles1G

the problem says "the presence of a catalyst provides a reaction pathway in which the activation energy of a certain reaction is reduced from 125 kj to 75 kj, by what factor does the rate increase at 298, all other factors equal?" I tried to do this using the Kr2/Kr1= e^-(Ea2-Ea1)/RT but k...

aisteles1G

If you find the rate constant on a table but the rxn you have is multiplied by 2 of the table rxn, would you mulitply the Kr (rate constant) by 2, or raise it to the 2 power? Or is Kr independent of the reaction?

aisteles1G

If you have the correct rate law, rate=k[A][B]2[C]2, then any experiment should give you the rate constant, the differences should be minimal decimal values if anything. The solutions manual solves for the rate constant using two different experiments showing that either work, so just check over the...

aisteles1G

I was also really having trouble with this and found this old post: viewtopic.php?t=3354 the explanations in the answers are pretty helpful in understanding the Enaught value!

aisteles1G

I really don't understand how theyre calculating the water reduction potentials at pH=7 for when doing electrolytic cathode/anode, for example E=.82 at pH=7 but its different on the reduction table, are we just supposed to memorize this? Thanks for any help!

aisteles1G

i'm really confused on how to write out half rxns from a cell diagram that use H2 I HCL, in the book chapter there is an example and it writes it out as 1/2 H2--> H+ + e- but HCL isn't even incorporated into the half rxn? Do you always leave out the anion when dealing with hydrogen in an oxidation h...

aisteles1G

How do you determine n (number of moles of electrons) by simply looking at a redox rxn to us the equation G=-nFE? Or do you have to write out and balance the whole thing? Thanks!

aisteles1G

You have to look at the charge of the atom it's paired with so KMnO4 is +7 bc k is +1 and 0 is -2*4=-8 and the overall charge of the compound is zero so Mn has to be +7 to cancel the others out to zero. I'm not sure what the +2 compound is that you're referring to bc i dont have the book with me but...

aisteles1G

Sorry if this is a very simplistic question but Im not sure if 2Cl- means it has a -2 charge or just a -1 charge? Does the coefficient get multiplied to the charge or no? Thanks for any help!

aisteles1G

Sorry but so Monday's (the 25th) lecture won't be included with the material for test 2?

aisteles1G

first you should write the rxn, then balance P + Cl -> PCl3, then use the equation: n x (Cp) x (change in temperature)

aisteles1G

Clarissa Cabil 1I wrote:We just discussed electrochemistry because we finished thermochem!

Hope this helps!

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Thank you so much these notes are great! :D

aisteles1G

Hello! I wasn't able to attend today's lecture, can someone please let me know what was talked about or upload their notes? I'd greatly appreciate it!! Thank you in advance :)

aisteles1G

I was also really confused by this but the only explanation I could see is that the problem did not state the the pressure was constant so it would technically be incorrect to assume this and use the Cp in that equation, therefore they used the one that does not depend on pressure

aisteles1G

whenever its stated that a reaction is isothermal (reversible or nonreversible) you can assume that delta U = 0

aisteles1G

In some of the homework problems they say to assume 1 mole of gas if not specified but then in others they expect you to use the ideal gas law to solve for n, I'm not sure how to tell in a problem whether to assume 1 mol or solve for n?

aisteles1G

You're right the question did not state constant pressure so it wouldn't be 100% safe to assume delta H = q but theres an equation: delta H= delta U + delta nRT, so following this since you have delta H and delta nRT (work) you can rearrange and solve for delta U!

aisteles1G

I don't have a picture since I didn't really take good notes but all the problems he covered were homework problems so if you did all of the HW you didn't miss anything, he just went more in depth when solving/explaining them

aisteles1G

Hello, Unfortunately I won't be able to attend the review, will answers be posted? If not could someone please upload a pic of the answers after the review?? I would greatly appreciate it!! Thank you!

aisteles1G

I don't think we have to know it since it was not needed for any of the HW problems, but the way I understand it is that the theorem states that "molecules in thermal equilibrium have the same average energy associated with each independent degree of freedom of their motion" and for a mono...

aisteles1G

you multiply by 101.325

aisteles1G

Aqueous phase reactants/products are still included in all K calculations, only liquids and solids are excluded so removing it would cause a shift to the formation of products

aisteles1G

Sorry if these are hard to read! We basically just covered Entropy change equations and the entropy of reactions formula, started Gibbs but that won't be on the exam! I'd suggest reading the chapters in the book on entropy, hope this helps though!

aisteles1G

Entropy can never decrease, if it would then the 2nd law of thermodynamics would be violated, so when a system reaches equilibrium that means that entropy has maxed out, and it can't increase either because every state has been reached as you said, this is considering the whole "universe" ...

aisteles1G

Delta T will state the same so don't switch it! The change in temperature remains the same and the sign is accounted for by the negative already at the front of the equation, also temp changes are different from heat changes, temperature is a state function while heat is energy and depends on how yo...

aisteles1G

The enthalpy of formation of all elements in their most stable states will always be zero bc when you think about it Al-->Al will have a(delta)Hf of 0 per mole since it is a pure element. H2 is also in it's most stable state even though it is not the single H, O2 would also be in the most stable sta...

aisteles1G

w=-nRTln(v2/v1) is for a Reversible rxn, w=-p(deltaV) is for Irreversible reaction, be careful though because whenever you use w=-PV your answer will not be in joules and you have to multiply by 101.325 to get to joules whereas the w=-nRTln(v2/v1) will already be in joules. The question should state...

aisteles1G

I don't think you have to know the steps of the derivation since they involve calculus and calc was not a pre-req for the course but I think if you can't derive then memorize s=nRln(v2/v1) and s=nRln(p1/p2) since those are not given on the equation sheet since they are derivations of another equatio...

aisteles1G

The equation is a derivation, w=P*V and P*V=nRT (ideal gas law) but this time having the change in temp included, and H=U+w, so I think the book expects you to make the connection but honestly I think it would be better just to have it memorized. The book chapter 4C does explain this equation and it...

aisteles1G

I think it's because the problem does not state that it was at constant pressure and volume and pressure are inversely related so using C for constant pressure would not work in this case, where as S=nRln(v2/v1) is not pressure dependent.

aisteles1G

This question in the 7th ed is 4F.13: Use data in Table 4C.1 or Appendix 2A to calculate the entropy change for (a) the freezing of 1.00 mol H 2 O (l ) at 0.00 I did it correctly but my answer was positive not negative; for H(fus) the book uses -6.01 when solving for S(fus) and I thought we could ju...

aisteles1G

Yea so for this one you have to combine 2 separate equations from the given table to make your final equation (kind of how you do with rxn enthalpies as well). Two things to note: when you multiply an equation through by a number, you raise the K value to the power of that number and when you add eq...

aisteles1G

At constant volume you have to use the delta H= deltaU (internal Energy) + work equation. The thing you have to know though is that deltaU=q when there is constant volume so you can use the q from part a as your delta U in part b. To find enthalpy you still need work though and the equation to use i...

aisteles1G

the constants and equation sheet posted on the chem class website has Cv and Cp for ideal gasses so as long as the problem says its an ideal gas you can just use the given ones I'm pretty sure so all you'd have to know is that Cv means specific heat capacity at constant volume bc of the subscript an...

aisteles1G

In the solutions manual it uses J/celsius but would it be wrong to use J/kelvin? Basically would we get marked down on the test for switching up the celsius and kelvin? Thanks for any help!

aisteles1G

the q=-Ccal*deltaT when you are calibrating the calorimeter; q=-q(cal), another way is to note that the specific heat of a calorimeter should not be negative so if you get a negative value you should make it positive. hope this helps!

aisteles1G

Im kind of confused as to when heat is in J and when in KJ, for example a solution gives: q=mCΔT= (80.0g)(4.187J/C/g/)(20C-0C)=6.69kJ but the specific heat is given in J so I don't understand why the answer is in KJ? Is this just an error or am I missing something? Thanks for any help!

aisteles1G

One of the examples in the 7th edition text book in chapter 4G gives an example of calculating entropy and they get 11.52 which they say is "reasonably close" to the given S=10.1, my question is what qualifies as being reasonably close for the two entropy values to determine if the molecul...

aisteles1G

So the class website says everything up to the 'end of entropy' will be covered on the midterm. Does this mean all of "2nd and 3rd Law of Thermodynamics" section off the syllabus will be on the midterm?? I feel like that's leaving a lot of material for one week before the midterm and I'm g...

aisteles1G

We haven't covered it in class but the book talks about how specific heat at constant volume and pressure is given by 3/2R, 5/2 R, etc. so you would still use the formula q=mC(deltaT) but for C look in the book for the Cv or Cp formula to plug in. the fractions you just have to know I think, maybe t...

aisteles1G

I think we can still continue with Thermochemistry and the 1st Law, but also he mentioned to do the HW that you feel is the most beneficial for you to get more practice so I'm sure they would not mark down if you did problems from other areas you still needed work on since it's for your benefit!

aisteles1G

Entropy is basically a measure of the random activity within a system, enthalpy is a measure of the overall amount of energy in the system (random or not). The more overall energy there is in a system, the more chaos there will be (higher entropy). Hope this helps!

aisteles1G

The SI unit for Pressure is technically Pascals but the book uses atm for most calculations, and I don't think they expect you to know the conversion factor from atm to pascals which is 1 atm = 101325 Pa

aisteles1G

The -nRT is for reversible reactions, so if the problem states that the reaction is irreversible or reversible you should know which one to use based off that. Another good thing if youre stuck is to just look at what is given (moles or atm) to see which one you should use but be careful in going ab...

aisteles1G

In most of the homework problems it seems to tell you if the reaction is reversible or not so I don't think you have to know if it is or not, I think the important thing to know concerning reversible reactions is that they use a different equation to calculate work and also that reversible reactions...

aisteles1G

A state property is basically a quantity, amount, numerical value that does not depend on how the substance being quantified/measured was prepared. So like temperature for example, the temperature does not differ based on how it got to that temperature where as something like work does depend on the...

aisteles1G

When the difference in Ka1 and Ka2 values is over 10^3 you can ignore the second Ka and just use the one for ka1 but if its like ka1= 1*10^-2 and ka2=1*10^-3 then you have to use the second K value and can't ignore it. Hope this helps! It talks about this in the 6E chapter in the textbook

aisteles1G

I got this too and I think it might just be an error? It wouldn't make sense for it to be a negative value but going by the formulas and exact calculations that's what it turns up as on my calculator as well :/ I think maybe just a mistake

aisteles1G

You use two reactions and multiply their K values to get the one for this new one, there is an example of such a problem in the 5H chapter of the book. You would use H2+cl2->2HCl and then 2BrCl-->Br2+Cl2 and multiply their K values

aisteles1G

The amount of products has to adjust to increased concentration on reactants in order to keep the equilibrium constant K, so if you have 1S/4H ratio and now you put in 8H there will have to be 2S to keep the 1/4 ratio of products to reactants (equilibrium constant K). Hope this helps!

aisteles1G

I dont't understand how to find the ph of such a solution since theres no hydrogen in it? At first I was thinking it would be 7 because maybe it's neutral, anion and a cation with balanced charges, but it's not. This is question 6D.15 in the 7th edition book. The solutions manual puts in H20 in plac...

aisteles1G

In one of the examples in the 7th edition for 6D.2, they equate the concentration of [h30+] = [A-] but it doesn't really explain why you're allowed to assume this is true so I'm confused. Will this always be true with all weak acids in a aqueous solution?
Thanks for any help!

aisteles1G

You would have to make an ICE table and assume that -x amount dissociates and therefore +x amount protonates to form the products. Then you use the relationship between pKa and pKb to determine the concentrations. I watched a pretty helpful video on this: https://www.khanacademy.org/science/chemistr...

aisteles1G

I think they might be referring to the initial concentration of that molecule, in science an 0 can sometimes represent the initial value. I have the 7th not 6th edition so I can't refer to the exact question you are asking about but hopefully this makes sense!

aisteles1G

I think we always use molarity within ICE tables since molarity gives the concentration in the solution (moles per liter), whereas moles would just give the amount of substance without taking into account the volume of where it is in and therefore not qualifying as a 'concentration' value. Im pretty...

aisteles1G

Looking online it came up as, "Activity is a measure of the effective concentration of a species under non-ideal (e.g., concentrated) conditions. This determines the real chemical potential for a real solution rather than an ideal one." I think activity is the concentration (either in mola...

aisteles1G

Hello, for part C I don't understand how they are finding the equilibrium concentrations for X2, in the solutions manual they take the initial pressure and multiply it by whats left over after decomposition, why? Thanks for any help!

aisteles1G

It is different when going across a period and going down a group. Across a period, you are correct, the more electronegative one makes a stronger acid. However going down a group, the larger one with the bigger atomic radii is the stronger acid. That is why HF is less than HCl. The strength of the ...

aisteles1G

I got this right and I put: "the electron gains energy when it is excited, because it goes up an energy level. To be excited it must absorb the energy of a photon that exactly matches the delta E between energy levels."

aisteles1G

I think you should know the differences between Arrenhenius, Bronsted and Lewis Acids and Bases and determine which compound in a reaction acts as which Arrenhenius (in water) Acid : A compound that supplies the H+ in water Arrenheius base: A compound that supplies the OH- in water. Bronsted Acid: A...

aisteles1G

You have the right idea. Polydentate means one ligand can bind to the same metal atom at different binding sites. The chelate is the complex that is produced when a polydentate ligand binds to a metal atom and forms a ring. Chelation is just this process, and the polydentate ligand can be referred ...

aisteles1G

Im a little confused on if polyedentate means the molecule can form multiple bonds with different metal atoms or just with the same metal atom? As far as I understand chelating means forming the multiple bonds with the same central atom but does polydentate just mean the same thing then? Thanks!

aisteles1G

It's because this compound has resonance structures that allow for different bondings. In one of them, two O atoms can bond to the same central atom (bidentate) in the other resonance structure only one of the O atoms can bond so monodentate. I think the most helpful thing would be to draw out lewis...

aisteles1G

We went over "Salts as Acids and Bases" - Ions as acids: usually only Cations (+ charges) will donate H+ protons because they have more positive charges and therefore repel the H+ more so it's easier for them to donate protons than accept (be basic) -Salts (any ionic compound) that contain...

aisteles1G

An acid base reaction happens when a proton (H+) gets transferred from one to the other. In this case a hydroxide (OH-) was transferred and replaced with an NH2 bond. The CH3COOH did not donate a proton to NH3, it exchanged a bond that had a H atom in it but that is not the same as a proton transfer...

aisteles1G

The way I went about figuring this out was seeing that morphine was a stronger acid, more O atoms, so naturally if a compound is a stronger acid then it is a weaker base so hyperbromite is the stronger base then, this goes with pKb values too though, the pKa and pKb values are inversely related so i...

aisteles1G

A way I see it is react the compound with H20 to form an H30+ cation in the products and then react it with H20 to form a OH- in the products. If both of these equations are possible, then the compound is amphoteric since it can both donate a proton and accept a proton and the different equations wi...

aisteles1G

Hi, Sorry, this doesn't answer your question but I have a similar question so I figured I would add it to this thread. In the textbook, the naming of the compound says to drop the ending of ions and add "-ato" for -ate ions, "-ido" for -ide ions and "-ito" for -ite ion...

aisteles1G

What it means by no protons transfer is that the two reactants are not acting as acids and bases in this reaction. The two reactants form new molecules yes, but neither one is solely either an acceptor or donater of protons so neither one can be classified as acid or base in the reaction. Looking at...

aisteles1G

In the 7th edition book Fundamentals J.17 we have to write the proton transfer reactions of the ion with water. I wrote the equations almost correctly but the only thing that was wrong was I kept the ion in the equation and the solutions didn't, and I can't figure out why? The question states "...

aisteles1G

Do we have to memorize/know which elements have latin stems and what they are? Is there an easy way to tell if an element has a latin stem?? Thanks!

aisteles1G

Yes, the -ate ending is indicating the negative charge and the ferrum comes from the Latin stem. In the book it states that if the metal originates form a latin name, then the latin stem will be used for negative charges only though!! If the overall charge was positive you would not use the latin st...

aisteles1G

i wasn't able to copy everything down, and Professor Lavelle didn't have any important announcements. naming coordination compounds (greek prefix) ligand names alphabetical order then tm (transition metal) cation name (roman numeral) if there are anions, then anion name (greek prefix) hydrate [Co(N...

aisteles1G

Hello, So I missed lecture today, Wednesday the 28th, would anyone be willing to post their notes from lecture as well as any important announcements Lavelle might have made that I missed? I would greatly appreciate it, thanks in advance! :)

aisteles1G

If you draw the Hs across from each other and the Cls across from each other around the C atom the dipole moments will cancel out. Remember that you dont have to draw them in order of how they are written around the center atom so if drawn across from instead of next to the same atom the dipole mome...

aisteles1G

For the test, will we have to be able to draw the shape and structure of molecules or just name their geometry/shape? If we will be responsible for knowing how to draw them, when do you put the dashed lines instead of a straight line for a bond, and when do you put the bolder triangle type line to r...

aisteles1G

To tell which one has the greatest dipole moment, basically add the dipole moments in the way you add vectors (if you've gone over that in math) if you're not familiar with vector addition, simply just add together the arrows and see which ones cancel each other the least or add up to the biggest lo...

aisteles1G

From what I understand, its because pi bonds happen outside of the hybridized orbital, they happen in the p orbitals, hence just the 2p instead of 2sp. Pi bonds happen from the electrons left over in the p orbitals after hybridization so youre not going to have them as a part of the hybridized sp or...

aisteles1G

The s character of an orbital is simply what percentage of s makes up the hybridization, so in sp3 there are 4 orbitals, 1 s and 3 p, so s makes us 25% (1/4) of the hybridization and therefore it has 25% s character and 75% p character. An sp hybridization has 1 s and 1 p, so 50% (1/2) s character. ...

aisteles1G

Did midterm scores come out? I still am not able to see mine on MyUCLA, how do you know which ones you got correct?

aisteles1G

Hey so I just wanted to share a tip for studying for the final, Over the quarter there are 210 total hw questions assigned (7th edition book) and 3 weeks till the final so if you do 10 hw questions a day starting now you'll get through all of them by the time to the final comes around and the syllab...

aisteles1G

When the molecule has no lone pairs, the molecular shape and electronic geometry will be the same, but when there are lone pairs as in this example, the lone pairs will bend the molecule slightly but the electronic geometry will fit the predicted shape.

aisteles1G

I understand that the two are closely related but is there any defining differences in the terms?

aisteles1G

I think homework from 2D would still be acceptable, especially because some discussions happen earlier in the week then others, but they will not penalize you for going ahead of lecture with the homework either. Really just depends what you feel you need more practice on.

aisteles1G

O is more electronegative than N, and you put the least electronegative in the center so that's why, but yes also it helps maintain symmetry.

aisteles1G

Is delta v just 5 or is it 10? In a hw problem the book just used the number given for +-5 so 5 but in the practice midterm we had to double the +- number to use as our delta v so I'm confused.
Thanks for any help!

aisteles1G

A good formula I found for the number of lone pairs of electrons on the central atom is LP= total Valence- 8*(number of other atoms that are not central) all over 2. So for example: ICl4 (-), total valence is 7+7(4)+1= 36. the central atom is I and there are 4 additional atoms that will be connected...

aisteles1G

39.1 g is approximately the mass of a mole of K ions. You need to divide by avogadro's number to find the actual mass of a single ion. Hope that helps. I finally got the right answer using this, thank you so much I had spent like an hour stressing and trying to figure out where I went wrong, apprec...

aisteles1G

Okay so I use the de broglie equation to get v=1.509*10^-24, then plug that into (1/2)m*v^2 but keep getting 4.44*10^-50. Im using 39.1 as the mass of the potassium ion, what am I doing wrong???

aisteles1G

Hello, I was just wondering if you will still be posting solutions online after the review session? On the practice midterm I know it says there won't be detailed solutions but the answers will still be posted? Thanks!

aisteles1G

Youre right in that aluminum has a charge of +3 but Hydrogen actually has a charge of -1, remember that hydrogen is willing to gain not lose an electron because it is forming a duplet not octet therefore the -1 charge is meaning it needs to gain one while aluminum wants to give away 3 to fill its sh...

aisteles1G

First look at the electron configuration, for Mn+4 you see Mn has in its outer most shell 3d^5 4s^2 meaning at this moment it has 2 in its outer most shell (4s). then because of the +4 charge, subtract 4 electrons starting from the outer most shell, in this case the 4s one. You will end up with 3d^3...

aisteles1G

since 32 watts = 32 J/s and its for 2 seconds you do 32*2 to get 64 J/s for that time. From the given wavelength you use c=Freq*Wavelength and get V(frequency) = 7.14*10^14 Hz and using delta E you get the energy per photon to be (delta)E= 4.73*10^-19. now you divide the total E (64J) by your energy...

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