<p>Yup, London Dispersion Force depends on the number of electrons.</p>
<p>I was also talking about instantaneous dipoles. Our teacher just said that the larger mass meant more places for instantaneous dipoles. Looking back, this statement does seem too general since F2 has a smaller radius than O2 but has more london dispersion.</p>
<p>I just looked over my lecture notes.</p>
<p>They specifically state that LDF's are completely independent of mass... that is underlined, put in all caps, and boxed.</p>
<p>However, LDF does correlate to molar mass. That is not causation though.</p>
<p>My Chem teacher has given 2 practice tests so far. I didn't study for either of them and got a 3 on both. I have been studying today and I have a partice test tommorow morning and I hope it will be a 4. I've studied using Princeton Review and my teacher has us doing past MC questions since the beginning of the year.</p>
<p>time to check in with my first practice test. Got 75/75 on the multiple choice and all but one point on the FR. raaaaaaaaaaape!</p>
<p>question: can u LOSE points on the FRQ?</p>
<p>like on the reaction predictions, can u lose points if u dont predict it propoerly? and what bout the follow up question, can u lose points if u get it wrong? </p>
<p>and on the free response problems, can u lose points for an incorrect answer?</p>
<p>You don't lose points from the FRQ.</p>
<p>Even if your calculate one thing wrong, as long as you do the right process you're still supposed to get some credit.</p>
<p>"However, LDF does correlate to molar mass. That is not causation though."</p>
<p>That's why I selected that example. Frequently, correlations are taught to help students remember relationships (ex: atomic radius decreases across a period, or LDF's increase with MM), but correlations are not causal relationships, and often lead to misconceptions. (Many students take the MM/LDF relationship and form some misguided notion that it's because heavier things attract each other more strongly - presumably because of gravity). Because the mistakes are common, the rubrics often call them out specifically as ways to lose points.</p>
<p>"and on the free response problems, can u lose points for an incorrect answer?"</p>
<p>Within a specific part of an essay, if you answer correctly, then go on to say something blatantly wrong, you may negate the points you would have otherwised earned. You can't lose points, though, in the sense of getting a negative score for the part.</p>
<p>On the other hand, especially on the caculator questions, if you don't know how to answer part X and part Y depends on your answer for X, MAKE SOMETHING UP for part X! They will grade part Y based whether you did the correct process, albeit with the wrong input.</p>
<p>chat--apchem07</p>
<p>check in whenever, itll be active until tues... eek</p>
<p>AP Chem was quite easy last year. I drew polar bears over my exam last year for the last hour and got a 4. At our school, most people failed though, only 3 out of 24 passed...</p>
<p>I have a question.
How come HF has a high solubility in water?
This is the explanation my teacher wrote:
high solubility because both are polar covalent molecules which are caable of forming hydrogen bonds.</p>
<p>I thought HF was strongly bonded (hydrogen bonding) and that one of the reason it's such a weak acid is that it doesn't ionize in water.
The lattice energy will be huge (which is endothermic). So it won't be very soluble.</p>
<p>hmm i took the 2002 multiple choice and got raw score of 35, out of 75. Do I have any shot at all of getting a 4 or 5? LOL</p>
<p>Well HF is a weak acid, meaning it won't disassociate in water, however the polar covalent nature of the HF bond means that it can create several H bonds with the water molecules, which are also polar covalent in nature. Note: HF does not have to disassociate in order to be soluble.</p>
<p>Example: pentane and hexane are soluble with each other, but neither disassociates</p>
<p>make sure you know chiral molecules, enantiomers, and optical isomers in general...</p>
<p>(In case you didn't get it, I was joking)</p>
<p>to austin: just remember that like dissolves like, and since both molecules are highly polar, HF will be soluble in H2O.</p>
<p>how do i enter that chat room through aim?</p>
<p>"I thought HF was strongly bonded (hydrogen bonding) and that one of the reason it's such a weak acid is that it doesn't ionize in water."
Hydrogen bonding provides the strong attraction <em>between</em> HF molecules, leading to HF's relatatively high boiling point (and MP, dHvap, dHfus, etc.). The ability to ionize (or not ionize) is due to the strength of the bonds <em>within</em> molecules - a completely separate phenomenon.
"The lattice energy will be huge (which is endothermic). So it won't be very soluble."<br>
Solubility involves 2 steps.
(1) The solute particles separate from each other and the solvent particles separate from each other. Overcoming these attractive forces is endothermic as you pointed out. <a href="2">As an aside - you referred to this as the lattice energy - right idea, but wrong term.</a> The solute and solvent particles associate with each other - an exothermic process.
A substance is soluble when the energy input to separate the particles is roughly the same as the energy output when the new solute/solvent associations form. (Entropy then drives the process.) Thus "like dissolves like".</p>
<p>So the tests in Princeton Review are too easy?? do yougusy recommend reading princeton review and taking the tests from the barrons book?? also is the barrons book harder than the actual exam??</p>
<p>also how do you guys recommend studying for the part 2 (free response) section of the test???</p>