急determining the appropriate strength of hydragon bonding

1.
Chloroform and ethyl ethanoate can dissolve polystyrene.


2.
Why hydrogen bonds are formed between XH molecules ?
X is much more electronegative than H. In an XH molecule, due to the great difference in electronegativities, most of the electron clouds of the bond pair of electrons are drawn towards X. Therefore, a strong partial negative charge is built up on X, and a strong partial positive charge is built up on H, i.e. δ-X-Hδ+.
Since the H atom has no inner shell of electrons, its nucleus is unusually “bare” and is attracted to a lone pair of electrons on an atom X of a neighbouring molecule.
...... δ-X-Hδ+......δ-X-Hδ+......
This type of intermolecular attraction is called hydrogen bond.


Why hydrogen bonds are formed in the mixture of chloroform and ethyl ethanoate ?
Consider a chloroform molecule (HCCl3). The 3 chlorine atoms are highly electronegative, which withdraw electrons and thus build up a strong partial positive charge on H atom (-Hδ+).
Consider an ethyl ethanoate molecule (CH3COOCH2CH­3). The O atom in C=O group can withdraw the σ electrons by inductive electrons and withdrawing the π electrons by the resonance effect. Therefore, a strong partial negative charge on the O atom is built up on the O atom of the C=O group (C=Oδ-) .
Similar, hydrogen bond is formed between the H atom of chloroform and the O atom of the C=O group of ethyl ethanoate molecule.

圖片參考：http://ndc.hkcampus.net/~ndc-007/Hbond.jpg

1. chloroform and ethyl ethanoate will dissolve polystyrene cup.
so we should use other container such as glass beaker to
hold them.
2. chloroform contains three chlorine atoms which are also
electronegative. three electronegative atoms is as strong as
one F , O or N atom. so it forms H bonding .