Intermolecular force
The above shows two types of intermolecular forces. Dipole-dipole interactions are electrostatic interactions between permanent dipoles in molecules. The van der Waals' force (or van der Waals' interaction), named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds, the hydrogen bonds, or the electrostatic interaction of ions with one another or with neutral molecules or charged molecules. Van der Waals' forces are relatively weak compared to dipole-dipole forces. Which of the following can be explained by this relative weakness of the Van der Waals' forces?
a) The boiling point of H S is higher than that of O .
b) The boiling point of H O is higher than that of HF.
c) The boiling point of NH is higher than that of PH .
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1 solution
H F is F having 7 electrons at outer most shells to make one covalent bond with hydrogen atom; there left with 3 pairs of unpaired (with atom(s)) electrons which give rise to maximized hydrogen bonds.
H 2 O and H 2 S are O and S of both having 6 electrons at outer most shells to make two covalent bonds with hydrogen atoms respectively; there left with 2 pairs of unpaired (with atom(s)) electrons which give rise to strong hydrogen bonds.
N H 3 and P H 3 are N and P of both having 5 electrons at outer most shells to make three covalent bonds with hydrogen atomes respectively; there left with 1 pair of unpaired (with atom(s)) electrons which give rise to hydrogen bonds.
Only O 2 of double bonding which gives two pairs of unpaired (with atom(s)) electrons which could not pair up with opposite charges with absence of hydrogen. Comparatively stronger's hydrogen bond is absent. Only momentary dipole from polarization which is weaker than permanent dipoles is present to increase boiling point. This is the Van der waal's force; without which, oxygen cannot become liquid.
Answer: a ) T h e b o i l i n g p o i n t o f H 2 S i s h i g h e r t h a n t h a t o f O 2 .