Official 2011 AP Chemistry Thread

ChickenOnRice · April 21, 2011, 11:02am

well I have officially started studying today.

chewydog · April 21, 2011, 1:40pm

It’s best to not be dependent upon one. Just study all of them and prepare for anything.

I agree with Abrayo on this one. Draw out the equation given to see if any of the substances fit the description of the Lewis acids/bases. However, I’m pretty sure that the octet rule doesn’t apply to every reaction, so just look to match situation with description of Lewis A/B.

chewydog · April 21, 2011, 1:45pm

@ChickenOnRice

Better late than never. I’m doing it over my spring break, and it sucks considering the nice weather outside and the presence of evil friends who try to seduce me into playing video games with them.

skateme · April 21, 2011, 5:36pm

What would you look for that “fits the description?”

Abrayo · April 21, 2011, 5:42pm

@skateme;

Take for example NH3. If you draw it, there is a lone pair on the central Nitrogen - this means that it is a Lewis Base, because another molecule, like O, can bond to them.
O would be the Lewis Acid, because it accepts NH3’s electron pair.

Actually, can a single element (i.e. O) be considered a base/acid or does it have to be a compound?

skateme · April 21, 2011, 7:07pm

Almost anything can be considered an acid or base. The Bronsted-Lowry definition looks at the ability to donate/accept, so if you were to have:

HCO3 + O2 –> CO2 + H2O (not balanced, I know)

HCO3 would be the Bronsted acid because it can give up the proton and O2 would be the Bronsted base because it CAN (and in fact, does) accept the hydrogen to become H2O. Oxygen is also reduced from a 0 charge to a -2 charge. Redox reactions and acid-base reactions often go hand-in-hand.

And thanks for elaborating above. I appreciate it.

kimberlyy · April 22, 2011, 12:38am

Someone explain how to do these please?

<ol>
<li>A sample of CaCO3 (molar mass 100. g) was reported as being 30. percent Ca. Assuming no calcium was present in any impurities, the percent of CaCO3 in the sample is
(a) 30% (b) 40% (c) 70% (d) 75% (e) 100%</li>
<li>Which of the following equations represents the net reaction that occurs when
gaseous hydrofluoric acid reacts with solid silicon dioxide?
(a) 2 H+(aq) + 2 F–(aq) + SiO2(s) → SiOF2(s) + H2O(l)
(b) 4 F–(aq) + SiO2(s) → SiF4(g) + 2 O2–(aq)
(c) 4 HF(g) + SiO2(s) → SiF4(g) + 2 H2O(l)
(d) 4 HF(g) + SiO2(s) → Si(s) + 2 F2(g) + 2 H2O(l)
(e) 2 H2F(g) + Si2O2(s) → 2 SiF(g) + 2 H2O(l)</li>
</ol>

16 is D, 19 is C

reach4thesky · April 22, 2011, 12:57am

Are absorbance and Beer’s Law tested?

Wahoo19 · April 22, 2011, 10:51am

yes, but i believe they were on last years free response so i wouldn’t worry about it

Abrayo · April 22, 2011, 10:54am

Are the approaches to solve them different?
Kb/Ka you would use pH equations, but what do you do differently between Ksp and Kc?

Abrayo · April 22, 2011, 11:01am

You start off with HF(g) and SiO2(s). Do a double-displacement sort of thing (not sure what you call it when water is produced? Condensation reaction?) and you get:
HF(g) and SiO2(s) → SiF4 + H20 (l)
Notice I did not put a state for SiF4 - I don’t actually know it.
Balancing the equation, you get :
4HF(g) and SiO2(s) → SiF4 + 2H20 (l)
Only c) and d) have 4HF. (notice that a) and b) can also be immediately eliminated because it says that HF is a gas, not an aqueous solution, and e) is just wrong.)

Now it’s between SiF4(g) and Si(s) + 2F2(g).
I have no real reason for choosing c) over d), just that I find it unlikely that the second would occur (i.e. a gas and a solid are produced, that isn’t an insoluble compound).
Could anyone provide insight on this? Does this actually ever happen?

skateme · April 22, 2011, 12:22pm

Ksp deals with solutions and is [products]^coefficients. Kc, Kp, and Keq deal with equilibrium of gases and solutions as [products]^coeff / [reactants]^coeff. Ksp is sometimes the same thing as Keq though:

AgCl (s) <–> Ag+ (aq) + Cl- (aq)

Ksp = [Ag+][Cl-]
Keq = [Ag+][Cl-]

Note that Keq is only similar to Ksp because the reactant was a solid. Solids and liquids are excluded from equilibrium expressions because their concentrations do not change (in terms of molarity).

So to answer your question, Ksp, Kc, and the functions you do with them are pretty similar, except Keq is always used in conjunction with an ICE table. Ksp is primarily compared to the Qsp to determine whether the solution is saturated or not.

My question: what is there in nuclear chem to learn? My teacher said it was all memorization and handed us a sheet to memorize the various types of decay, but the questions on the tests don’t look remotely similar. So what is your understanding of nuclear chemistry?

scholarshipFTW · April 22, 2011, 1:21pm

Is there a website with multiple choice explanations for the released exams? specifically 2004?

reach4thesky · April 22, 2011, 1:49pm

@ Wahoo19, are you sure? I haven’t studied that and I don’t even know what it is, so if it shows up on the exam, I’m screwed.

Abrayo · April 22, 2011, 4:09pm

Is Keq the only one that is used with the ICE table?

When you study the various types, make note of what happens to the original element.
ex. When something undergoes beta decay, what happens to its molar mass or atomic number? (Answer: Atomic number goes up by one.)
I believe that’s how they can be qualitatively tested.
Also, gamma decay is most penetrating and alpha is the least, because of its size/mass.
What kind of questions have you seen on the tests?

skateme · April 22, 2011, 9:51pm

All K’s (except Ksp) use an ICE table.

Keq/Kc uses ICE to organize your information regarding equilibrium.
Ka/Kb use ICE in the event that you have a weak acid or weak base, which results in equilibrium.

So if equilibrium exists, use an ICE table.

This question was the most recent I saw regarding nuclear chem:

<a href=“http://img856.imageshack.us/img856/8148/picture3j.jpg[/img]”>http://img856.imageshack.us/img856/8148/picture3j.jpg

</a>

And these I found while googling:

<a href=“http://img691.imageshack.us/img691/1571/picture5ca.jpg[/img]”>http://img691.imageshack.us/img691/1571/picture5ca.jpg

</a>

<a href=“http://img90.imageshack.us/img90/3729/picture4f.jpg[/img]”>http://img90.imageshack.us/img90/3729/picture4f.jpg

</a>

Oops last problem above wasn’t nuclear chem, but the answer was C right?

Abrayo · April 22, 2011, 10:14pm

^ First one is pretty simple. Nuclear equations are balanced, so look for the one with a balanced “top number” sum (i.e. molar mass sum) and with the “bottom numbers” (i.e. atomic numbers).

Second one:
You can simplify this into like, two steps (because order doesn’t matter in this case):
The total is 3 alpha emissions and one beta.
This means that on the right side, you’ll have 3 (4/2A) [I just put a slash for clarity, it is not a division sign) which is 12/6, and then a beta, which makes it 12/5 (because beta = 0/-1). So take 226/88 and put it on the left, and then put 12/5 on the right. You’ll see that the resultant is 226-12/88-5 = 214/83. That’s D.

72 is C. Just memorize that.
I believe 73 is A… or D. I know they represent electrons, not sure if they are electrons.
And yes, 74 is C.

kimberlyy · April 22, 2011, 10:28pm

The nuclear chemistry they test seems pretty basic, like integrated science stuff.

Mikeypz · April 22, 2011, 11:07pm

So I never really got a firm grasp on what exactly vapor pressure is.
For some reason I just couldn’t get the definition to make sense to me.

Someone please define it as layman-ly as possible? :)

Also, why does adding a nonvolatile solute lower vapor pressure, and increase boiling point?
What would adding a volatile solute do?

Thanks!

314159265 · April 22, 2011, 11:15pm

Vapor pressure: If you put a tight-fitting glass chamber around a beaker containing a liquid, the vapor pressure of the liquid is the pressure that the evaporated molecules (now in gas form) exert on the walls of the glass chamber.

Adding a non-volatile solute increases the intermolecular bonds between the solvent particles. That means more energy has to be added in order to allow solvent particles to escape the liquid (i.e. to boil), so BP increases. The fact that there’s more intermolecular force between the liquid particles means that fewer solvent particles can escape the liquid. That means that there’ll be less gaseous particles to create vapor pressure (so adding a solute lowers vapor pressure since fewer particles can escape).

I’m not 100% sure about this, but I think both non-volatile and volatile solutes will lower VP, lower FP, and increase BP. The formulas are just different.

Official 2011 AP Chemistry Thread

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