Molecular sizes

1. Why is the molecular size of ethanol and ethane similar? How do we define two molecules of two different compounds to be similar in size?

The molecular size can be regarded as the molar mass of the molecule,
Molar mass of ethane ( CH3CH3 )
=12x2+1x6
=30

Molar mass of ethanol ( CH3CH2OH )
=12x2+1x6+16x1
=46

As you see, the their molar mass is similar with compared to the large hydrocanbon molecule.

分子或其他化合物有多大，可以從它們的摩爾質量得知，當然，前題是它們有相同的結構，因為除了分子，還有巨型離子結構，巨型共僄結構......

2. Also, why has HBr got a higher melting and boiling point than HCl? As I recall,
the electronegativity difference between the bonding atoms in HCl is larger than
that in HBr, the dipole-dipole attractions in HCl should be larger than that in HBr.

It is correct that the melting point can be depend on the strength of the bonding between the molecule. However, we consider the molecular size if the strength of force between the molecue is similar.

In the case of HBr and HCl, both H--Cl and H--Br is polar, but HBr has larger molecular size (79.9+1)than that of HCl(35.5+), the larger the molecular size, the stronger the force is. Thus, the van der Waals's force between HBr is stronger even though H--Br bond is less polar than H--Cl.

因為HBr 比 HCl 大，所以有較強的范德華力，所以有較高的融點及沸點

3. Also, why has H2Te got a higher melting and boiling point than H2S, as I recall,
the polarity of S-H bond is larger than that polarity of Te-H bond?

Same to the answer of Q2, the molecular size of H2Te(127.6+1x2) is much larger than H2S(32.1+1x2), so H2Te have stronger van der Waals' force. More energy is needed to beark stronger bond during melting or boiling.

因為H2Te 比 H2Sl 大，所以有較強的范德華力，所以有較高的融點及沸點

Q2:
The b.p depends on the eletrostatic force between molecules ,the force is van der Waals force.
van der Waals force depends on mass of molecules, the higher mass means the higher b.p.
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Q3 is similar to Q2