Chem test questions

[Fractals]

Well, I have my chem final tomorrow, so I decided now would be a good time to figure out what I don't get :rolleyes: I have a fairly good understanding of voltaic cells, but how can you tell which half-cell is the anode and which is the cathode without measuring them? We made cells in the lab, but it the way the lab was written it seemed like you have to build it then check which direction the current is going in. On the practice final we got, it looks like we're supposed to be able to tell. He's not following the textbook and I missed a few lectures, so you and google are my only hope.

Edit: Another question, "Which is the pKa for the strongest acid?" He forgot to give us the possible answers, but it's multiple choice and he gave us the answers, so at least I know it's 'D'! :rolleyes: pKa=-log(Ka), and Ka=[A-][H+]/[AH], and the stronger an acid is, the more it dissociates. Theoretically, 100% dissociation is the highest, so A-=1, H+=1, and AH=0? That would be 1*1/0...
http://www.omjapan.se/wp-content/uploads/2009/03/divide_by_zero1.jpg

Edit: Can I get this thread renamed to "Chemistry test questions"? Thanks

[Butcher]

The cathode half reaction will have a higher voltage than the anode.

[Fractals]

I know that, but is there any way to figure out the voltage without measuring it?

[Butcher]

Well if you aren't given the Voltage then just remember that cathodes are reduced and anodes are oxidized.

[Butcher]

Edit: Another question, "Which is the pKa for the strongest acid?" He forgot to give us the possible answers, but it's multiple choice and he gave us the answers, so at least I know it's 'D'! :rolleyes: pKa=-log(Ka), and Ka=[A-][H+]/[AH], and the stronger an acid is, the more it dissociates. Theoretically, 100% dissociation is the highest, so A-=1, H+=1, and AH=0? That would be 1*1/0...


Do you mean the pH or the Ka? I don't know what a pKa is, but pH for the strongest acid is 0 and Ka for the strongest acid is infinite.

[Fractals]

Here is the problem from the practice test, no other information was included:

12. In the voltaic cell: Zn| Zn+2 || Cu+2 | Cu Copper is the anode
A. True B. False

Do you mean the pH or the Ka? I don't know what a pKa is, but pH for the strongest acid is 0 and Ka for the strongest acid is infinite.

pKa = -log(Ka) just like pH = -log(H+)

[Butcher]

Here is the problem from the practice test, no other information was included:

12. In the voltaic cell: Zn| Zn+2 || Cu+2 | Cu Copper is the anode
A. True B. False




Oh that's just notation for electrochemical cells.

it reads:

anode|anode cation||cathode cation|cathode

so its false

pKa = -log(Ka) just like pH = -log(H+)

I don't remember that but I'll look through my notes.

edit: from wikipedia
Acids with a pKa value of less than about −2 are said to be strong acids;

[Fractals]

Oh that's just notation for electrochemical cells.

it reads:

anode|anode cation||cathode cation|cathode

so its false


Thanks, that makes a lot more sense.

About the pKa, here is the question he gave us (with no possible choices, he forgot them on a couple other ?'s too):
5.Which is the pKa for the strongest acid?It's only a 20 question multiple choice exam, and we're allowed to bring in 2 reference papers, so I'm not too worried if I don't have the answer to 1 question. There's no way I can get less than an A on this.

[Butcher]

About the pKa, here is the question he gave us (with no possible choices, he forgot them on a couple other ?'s too):
It's only a 20 question multiple choice exam, and we're allowed to bring in 2 reference papers, so I'm not too worried if I don't have the answer to 1 question. There's no way I can get less than an A on this.

Well if its multiple choice then just go for the lowest number.

[Fractals]

Well if its multiple choice then just go for the lowest number.

:facepalm: Wow, I'm a dumbass. He obviously meant "which of the following is the strongest acid"... I though he meant "strongest acid possible."

[Fractals]

Ugh, another with no possible answers. 15. Which is a first order rate law in [B]?
16. Which is a second order rate law in [A]?
I understand 1st and 2nd order rate laws (depends on the concentration of 1 or 2 reactants), but what does the second part, "in [B]" mean? Am I supposed to factor [A] into the rate constant?

Edit: WTF? the B's in the brackets are capitol in the editor but lower case in my post... when I try to edit my post it shows them as capitol letters. :confused:

[asilentbob]

Red cat. Reduction occurs at the cathode.

[Fractals]

Red cat. Reduction occurs at the cathode.

Thanks, but I already figured that question out. Maybe a mod could rename this thread to 'chem test questions' or something similar?

Ugh, another with no possible answers.

15. Which is a first order rate law in [b]?
16. Which is a second order rate law in [A]?

I understand 1st and 2nd order rate laws (depends on the concentration of 1 or 2 reactants), but what does the second part, "in [B]" mean? Am I supposed to factor [A] into the rate constant?

Edit: I wasn't paying attention, 15 has an answer, C.
A. k[A]/[B]
B. k[A][B]2
C. k[A][B]
D. k[A]2[B]2

Would 16 be something like k[A]2[B]? I'm reading the wikipedia article and it's just getting me more confused, could someone explain first- and second-order rate laws?

[nshanin]

^ Correct. The exponent tells you everything.

To be a bit anal:

Theoretically, 100% dissociation is the highest

For monoprotic acids this is the case, but for diprotic acids or acids that decompose in water (i.e. Cl-SO3H, chlorosulfonic acid, which yields two strong acids--HCl and H2SO4 when in contact with water), things get funky and you start getting superacids. That obviously won't be on your exam but I thought it was REALLY cool when I was in gen. chem.

[Fractals]

Well, I took the exam earlier to day and I think I did pretty good. We were allowed to bring in 2 pages of reference material, as long as we wrote it ourselves. I copied the practice test to my reference paper and wrote how to do each one. It turned out he took the 25 questions from the practice test, added 5 more, and that was the final! He didn't even change the numbers or even the order of the answers! Oh yea, we had to name a few organic molecules and groups (alcohol and carboxylic acid, easy) for extra credit.

It's been a good day, in my sociology class the teacher accidentally handed out 5 finals with a few answers written on them, and I got 1. Out of 80 questions, I know at least 8 are right. She told us not to worry about it, but it was a mistake... lol