Official 2011 AP Chemistry Thread

Test Preparation AP Sciences
501

<p>lmao another question:
the standard enthalpy of formation, Hf, of HI(g) is +26KJmol^-1. Which of the following is the approximate mass of HI(g) that must decompopse into H2(g) and I2(s) to release 500 KJ of energy?
i haven’t done these questions.</p>

<p>answer is 2,500 g.</p>

<p>2nd question: WHat mass of Cu(s) would be produced if .4 mol of Cu2O(s) was reduced completely with excess H2(g)?
answer: 51 g
I got this right but i don’t know why:</p>

<p>Which of the following substances exhibit significat hydrogen bonding in the liquid state?
answer:
n2h4 (is it cause of HFON? 2 other choices also had HFON so i guess i got lucky…)</p>

<p>next question:
In an aq soluition with a ph of 11.5 at 25 Celsius the molar concentration of OH- is approximately?
how to approach this problem?</p>

<p>answer is 3.2x10^-3 M</p>

<hr>

<h2>next question is by me, why doesn’t Keq change when more reactants are given to the system?</h2>

<p>next question is by me, to figure out highest bioling point is it reliable to see which solution has the greatest mass?</p>

<hr>

<p>next question:
A certain reaction is spontaneous at temperatures below 400K but it is not spontaneous at temperatures above 400k. If dH for the reaction is -20 kj/mol and if dH and dS don’t change with temperature, then the value of dS for the reaction is?</p>

<h2>i was talking to some people and they said that dG becomes zero and then some other stuff happens but i want a decent explanation that goes over the negatives and positives of entropy/dH/gibbs please~</h2>

<p>next question When .6 mol of CS2 (l) reacts as completely as possible with 1.5 mol of 02(g) according to the equation: CS2(l) + 3O2(g) –> CO2(g) + 2 SO2(g), the total number of moles of the reaction products is:</p>

<hr>

<p>Here is the bond order question:
Which of the following molecules contains bonds that have a bond order of 1.5?</p>

<h2>answer was O3. how do u figure thiss out?</h2>

<h2>can someone give me a run down of hybridization?</h2>

<h2>how do u find the average molecular speed?</h2>

<p>last question:P :
XY2(aq) <–> X^2+ (aq) + 2 Y^-
a soluble compound XY2 dissociates in water according to the equation above. In a .050 m solution of the compound, the XY2(aq) species is 40% dissociated. In the solution the number of moles of particles of solutes per 1.0 Kg of water is closest to?</p>

<hr>

<p>these were all the questions i still have for now after i took a released exam…</p>

502

<p>@ChickenOnRice
2 K(s) + Br2 (l) -> 2KBr (s)</p>

<p>That’s oxidation reduction because K becomes K+ and Br2 becomes Br-. K lost an e- and Br2 gained one :)</p>

<hr>

<p>Idk how to do all of them -.- but…

  1. In an aq soluition with a ph of 11.5 at 25 Celsius the molar concentration of OH- is approximately?
    how to approach this problem?</p>

<p>you find pOH 14 -11.5 = 2.5…so the conc of OH- = 10^-pOH =3.16 x 10^-3</p>

<p>2) why doesn’t Keq change when more reactants are given to the system?
Keq is a ratio. When you add more reactants to the system, the reaction shifts to the right to produce more products. So, the ratio would still be the same.</p>

<p>3) to figure out highest bioling point is it reliable to see which solution has the greatest mass?
this has to do with van hoff factor. More molecules mean higher boiling points/lower freezing points</p>

<p>4)f dH for the reaction is -20 kj/mol and if dH and dS don’t change with temperature, then the value of dS for the reaction is
dG = dH - TdS. There is a table for it, but I think it’s easier to think logically? if dH is negative & T is positive, then dG would be negative if dS is positive, and dG + if dS is - . But if T is negative, then dG would be negative if dS is negative, and + if dS is +. What I usually do for these problems is plugging in 1, 2,3 to each of the variable XD with the different signs just to make sure it’s correct.</p>

<p>5) the total number of moles of the reaction products is:
This is a limiting reactant problem. O2 is the limiting reactant. So, from there you just use stochiometry. I think the answer is 1.5 mol? O.o</p>

<p>6) how do u find the average molecular speed?
There is a formula for it on the reference table :smiley: r1/r2 = sqrt (MM2/MM1)</p>

503

<p>thanks omchi23. i sorta lost it there xD the pressure is so much. 2mrw will be my first 2 AP exams ever in my highschool career and i am freaking out.</p>

504

<p>another quick question(i am freaking out too much! i can’t even focus on my review book),
difference between lewis acid-base and bronsted-lowry acid-base reaction?</p>

505

<p>lewis acids are electron pair acceptors and bronsted acids are proton donors.</p>

<p>and visa versa for bases</p>

506

<p>also at this point what do u rec me to review? anyone have some review sheets orsomething?</p>

507

<p>Stupid time is speeding like California wildfires.</p>

508

<p>honestly, just go on collegeboards website and do free responses and then read the answers. you can pretty much learn any concept you need from the score guidelines</p>

509

<p>I have a question about the length. I understand that it starts at 8 and the first part is 90 and second part is 95 minutes, about how long for you guys does it take to actually finish the whole AP testing (that is when you can leave the room now etc.) I ask this because I may need a ride depending on when we finsh I just want in estimate of how long it takes to set up etc. I now it is fairly subjective and contingent upon the proctor but just give me an estimate</p>

510

<p>yo devan1044 then for this question:
a lewis acid-base reaction that is not a bronsted-lowry acid base reaction:
why is this the right answer:
Zn(OH)2(s) + 2 OH^-(aq) –> [Zn(OH)4]^-2(aq)
Zinc (ii) hydroxide has a base and it gains more OH in the product…</p>

511

<p>11:30 a.m. - 12 pm is when we are dismissed. Correct me if I’m wrong.</p>

512

<p>Why is hydrogen bonding an intermolecular force?
Doesn’t it bond two different H2O (for example) molecules together?
Or does it bond the H to the O?</p>

513

<p>Hydrogen bonding occurs between polar molecules( i think so…) and anyways it occurs when a hydrogen molecule is bonded with Flourine, oxygen or nitrogen(HFON)</p>

<p>it is the strongest IM force( i think…)</p>

514

<p>Rather than going in deep, you can pick out lewis acid/base reactions by two things, a transition metal with an OH and the formation of a complex ion. The transition metal is Zn in this case and a complex ion is formed</p>

515

<p>Hydrogen Bonding is just a strong dipole dipole force.
Because the Hydrogen is basically just a proton (there’s little electron shielding, since the O is so electronegative) there’s a very strong attraction there.</p>

516

<p>Also, can anyone confirm if there is always a lab question on the Free-Response?
I just recently starting seeing some lab questions (I went oldest from newest) so I’m wondering if they recently changed it so there are?</p>

<p>If it’s a dipole-dipole force, how is it intermolecular? Wouldn’t that make it intramolecular? (inter means bonds within one molecular and intra means bonds with other ones, right?)</p>

517

<p>@devan1044:what does a metal with OH signify? i understand what a complex ion is though.</p>

<p>guys can u please check page 34 for my long list of questions~! i need ayudar pronto!</p>

518

<p>the bonding of hydrogen to oxygen in water is an intramolecular force (within molecules) and it is very polar. This intramolecular bond is super polar and produces strong dipole diple bonds within water, or any other molecule engaged in hydrogen bonding (only with flourine oxygen and nitrogen as chickenonrice said) and besides ionic and network covalent structures it is the strongest intermolecular</p>

519

<p>I don’t recall learning about Kp…</p>

<p>Can anyone answer this?</p>

<ol>
<li>2 NOBR(g) <-> 2 NO (g) + Br2(g)</li>
</ol>

<p>The reaction above came to equilibrium at a temperature of 100<em>C. At equilibrium the partial pressure due to NOBr was 4 atm, and the partial pressure due to Br2 was 2 atm. What is the equilibrium constant, Kp, for this reaction at 100</em>C?</p>

<p>a) 1/4
b) 1/2
c) 1
d) 2
e) 4</p>

<p>I think the equation for Kp is Kp= Kc(RT)^(deltaN)
No idea where to begin. Should I set this up like it’s a Keq equation?</p>

520

<p>well, the transition metal with the OH is just a red flag of lewis, so it can help you save time in multiple choice section. Is that what your asking?</p>