quick chem question

[elz787]

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NH3 + H2O -> NH4+ + OH

the ammonia is donating its electron pair right?

 

[dat_student]

NH3 + H2O -> NH4+ + OH

the ammonia is donating its electron pair right?

Ammonia donates an electron

 

[elz787]

another quick question, i just read that the boiling and melting points of a cis molecule are higher than a trans.

But I thought the boiling point of trans is higher while the melting point of cis is higher.....

which is it?

 

[dat_student]

another quick question, i just read that the boiling and melting points of a cis molecule are higher than a trans.

But I thought the boiling point of trans is higher while the melting point of cis is higher.....

which is it?

both statements are wrong.

 

[elz787]

so what would you suggest is a correct statement. I think its funny how both these statements were in my DAT study guides....sigh.

 

[dat_student]

so what would you suggest is a correct statement. I think its funny how both these statements were in my DAT study guides....sigh.

not funny, 90% of DAT materials are garbage.

in general, mp is about compaction & bp: interaction

 

[mochafreak]

Like dat_student said: mp is about compaction, so (for the dat) trans has the higher mp, because they can pack together better. BP is about dipole moments. Since the cis has a stronger dipole it has the higher BP (for the dat).

 

[dat_student]

Like dat_student said: mp is about compaction, so (for the dat) trans has the higher mp, because they can pack together better. BP is about dipole moments. Since the cis has a stronger dipole it has the higher BP (for the dat).

Mocha, not quite right. it really depends on many factors*:

Few points about bp:

Boiling point is the point at which there is no further rise in temperature, and the additional heat energy supplied is absorbed as latent heat of vaporization to transform the liquid into gas. This transformation occurs not only at the surface of the liquid (as in the case of evaporation) but also throughout the volume of the liquid, where bubbles of gas are formed.

In terms of intermolecular interactions, the boiling point represents the point at which the liquid molecules possess enough heat energy to overcome the various intermolecular attractions binding the molecules into the liquid (eg. dipole-dipole attraction, instantaneous-dipole induced-dipole attractions, and hydrogen bonds). Therefore, the boiling point is also an indicator of the strength of these attractive forces.

Now, does a better compaction always result in higher bp? no. Does a better interaction always result in higher bp? No. A good compaction helps increase the number of instantaneous-dipole induced-dipole attractions and hydrogen bonds and various other intermolecular forces. This may result in a highr boiling point. A stronger dipole-dipole attraction also increases the strength of intermolecular forces. Thus, this also can help increase the bp. There is no one rule that applies to all scenarios. Each scenario must be analyzed thoroughly, independently, and scientifically. *Mocha, you are absolutely right.

 

[mochafreak]

Yes, I should have pointed out that BP depends on dipole-dipole, and van der waals as well. But, as far as the depth which you go into (in the first paragraph)...it isn't necessary for the dat. What Kaplan says on the chem and ochem sections of their study guide is what the dat expects for the answer...this isn't chemical engineering. This is why examkrackers chemistry is overkill for the dat...because it goes into the depth you speak of...wish I hadn't wasted my time with it. 🙂 For the dat you don't have to research each question, it is actually simple and straightforward (if you let it be). I hope you are going for the PhD, because I think you're going to be one bored general dentist. 😀