[tuyetnhungdhd]

I have an exercise which I cannot answer. Please kindly help me to resolve it. I highly appreciate! Thanks!


Exercise: The Van der Waals constants are :
a=19.01 L2.atm/mol; b=0.1460 L/mol for pentane
and
a=18.05 L2.atm/mol; b=0.1417 L/mol for isopentane

a) Basing your reasoning on intermolecular forces, why would you expect a for pentane to be greater?
b) Basing your reasoning on molecular size, why would

you expect b for pentane to be greater?

Note: Val der Waals equation for real gas:
(P + n2a/V2)(V-nb) = nRT

Which a and b in this equation are Val der Waals constants.

I am looking forward the answer as soon as possible!

[Borek]

Look for physical meaning of a and b coefficients.

[tuyetnhungdhd]

I've just found the answer:

a for pentane is higher as intermolecular forces in pentane is stronger than those in isopentane.

Pentane and isopentane have the same molecular formula (C5H12), but different in shapes of molecules. Pentane has the shape of "sausage" while isopentane has the shape of a nearly sphere. So, the molecular surface of pentane is larger than those of isopentane. Therefore, the strength of London force in pentane is stronger than in isopentane.

As a result of the difference in shape between the two molecules, pentane has larger molecular size compared to isopentane, therefore b for pentane if higher than b for isopentane.

Thank Borek for your instructions!

[tuyetnhungdhd]

I have another question in intermolecular forces:

Consider the strengths and formation enthalpies of two hydrogen bonds in two substances:

0_H...0 (in H20-water): 2.8Ao; 25kJ/mol
0_H...0 (in RC00H-acid carboxylic): 2.5Ao; 30kJ/mol

Explain the differences in strengths and formation enthalpies in two cases while they are all hydrogen bonds between H and 0.

Answer:

0 in H20 is hybridized sp3 while in acid carboxylic C is hybridized sp2, so the characters of the bonds are different.

I dont' understand while they mention hybridization in C atom, but not in O atom (in the acid molecule) as H is not bound to C directly.

Please kindly explain more clearly so that I can understand better.

[Donaldson Tan]

Note: Val der Waals equation for real gas:
(P + n2a/V2)(V-nb) = nRT

btw your VDW equation is wrong. See attached

[tuyetnhungdhd]

I mean the same to you, but I write a bit different. As yours is more correct, OK

[mike]

btw your VDW equation is wrong. See attached

They both look the same to me mathematically.

[Donaldson Tan]

what does a and b represent?

think along the line regarding the difference between a real gas and an ideal gas.

hint 1: the volume of the molecules are significant when compared to the volume of the system.

hint 2: there are intermoleculer forces between the molecules.