Dr Romano Challenge Problem Solution.....

[DAT Destroyer]

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Ok nobody bagged it..... Why does H2 have a higher boiling point than He ? Ok.... normally the first thing we will look for on the DAT is a molecule with H bonding,,,this will cause a higher boiling point, higher viscosity, higher surface tension, higher heat of fusion and vaporization BUT a LOWER Vapor Pressure...... If you compared I2 Br2 Cl2 one would argue that size is the main criteria.....this is true,,,since as the halogen or Noble gas size increases,so does the Dispersion forces consequently the boiling points! Ok,,,,but how does one explain this anomaly? H2 is lighter than He yet boils higher,,,,,, you didnt think about it hard enough....The Van Der Waals attraction depends not only on the number of electrons in a molecule, but on the AREA of Contact. H2 boils 16 degrees higher than He because hydrogen has an elongated symmetry while Helium is spherical. The elongated form means that the electron cloud is more diffuse resulting in greater contact area. Helium on the other hand is spherical and has less contact area. Imagine gluing two dimes together..... the more surface area that is in contact the stronger the bond !!! Helium is spherical Hydrogen is diffuse . For the DAT I think if you remember that braching lowers the boiling point { since a branched molecule has less surface area } you are in good shape....also remember that a cyclic molecule has a higher BP than non cyclic,,,for example, cyclopentane has a greater boiling point than pentane,,,,,more areas of contact,,,,,, analogous to the H2 He problem..... Melting Points are not always so clear cut,,,,,for example,,,,,branching will lower the BP but .....you arent going to like this,,,,,it may actually raise the melting point because as we increase braching we may also increase symmetry,,,thus increase the abilty to pack into a crystal...... Ok,,,,I hope you liked the answer,,,,any onther questions email me at orgoman.com For next time,,,, Why is it so important to phosphorylate glucose at the start of glycolysis? Good Luck guys Dr Romano

 

[DrGeek]

Ok nobody bagged it.....

If you don't understand my answer that doesn't mean I didn't get it right:
http://forums.studentdoctor.net/showthread.php?t=284524

"...van der Waals force will be larger for hydrogen molecules than for rare gas atoms. This includes forces that arise from fixed or angle-averaged dipoles and free or rotation dipoles as well as shifts in electron cloud distribution.

Charge fluctuation in a hydrogen molecule can cause the probability amplitude function of an adjacent molecule to shift with a wave function of only slightly different energy.

Because a helium atom has both its 2s states filled, such distortion requires superposition of 2s and 2p states in response to random charge fluctuation. Perturbation theory indicates that such higher order contribution to wavefunction distortion is not as great."

 

[dat_student]

...If you compared I2 Br2 Cl2 one would argue that size is the main criteria.....this is true,,,since as the halogen or Noble gas size increases,so does the Dispersion forces consequently the boiling points! Ok,,,,but how does one explain this anomaly? H2 is lighter than He ...

No anomaly, H2 is larger than He. The boiling point of H2 is higher than the boiling point of He.

size:
He < H2
boiling point:
He < H2

size:
F2 < Cl2 < Br2 < I2
boiling points:
F2 (84.95 K) < Cl2 (239.1 K) < Br2 (332.25 K) < I2 (458.4K)

I don't see the anomaly 😕

 

[fancymylotus]

phosphorylation of glucose traps it inside the cell(since phosphorylated stuff cant cross the cell membrane)i think and doesnt it make it more reactive,etc?