<p>I read in PR somewhere Ice has a lower potential energy state? What does this mean?
Also, in the second PR practice test's last question is weird. </p>
<ol>
<li>Which of the following is necessarily true of a nonionic substance with a high boiling point? (A) It has a large vapor pressure.
(B) It has strong intermolecular forces</li>
</ol>
<p>Answer is B, but why isn't A also right?</p>
<p>thanks</p>
<p>Ice has less potential energy than water, you mean? The freezing of water is exothermic: during the reaction, energy (heat) is released, so the product (ice) ends up at a lower energy state than the reactant (liquid water).</p>
<p>A substance with a high boiling point has low vapor pressure. It takes a lot of energy to overcome strong intermolecular forces, so less of the substance can evaporate.</p>
<p>^ Explained perfectly. I don’t want to be redundant, though I’ll add something to Daisie’s second answer:</p>
<p>Note the question says the substance is “non-ionic” that means the intermolecular forces are not very strong (pertinent to only SAT Chem, not the strongest) but the question states that the substance has a high boiling point, and thus the intermolecular forces are quite strong, though not as strong as ionic bonds.</p>
<p>Vapor Pressure depends on 3 factors: volatility of substance, temperature, molecular mass. In this case:</p>
<p>Statement: Boiling Point is very high
Inference: Intermolecular forces are quite strong
Why? Because a high boiling point means a lot of heat (energy) is required for the constituent particles of the substance to overcome the intermolecular forces and thus undergo phase change.
Thus: The particle will have low vapor pressure. Because, not many particles on the surface of the substance will be able to escape the surface of the substance (due to inference) and are not able to add to the vapor pressure.</p>
<p>I hope this helps, please PM me if you need more help with SAT Chem - I’m prepping for the same!</p>
<p>thank you both</p>
<p>anybody know why this is true though?</p>
<p>“NaCl having more ionic character than AlCl3”</p>
<p>it might be because of the dissociation into ions? I’m not 100% sure, but if you look at Na and look at Al, Na is a more reactive metal. It’s placed in the alkaline earth metals group. So NaCl dissociates into Na and Cl ions more readily. </p>
<p>That maybe why it has more of an ionic character than AlCl3, but I’m not sure…</p>
<p>
</p>
<p>Because the electronegativity difference between Na and Cl is larger than the difference between Al and Cl.</p>