Question

#79

H2O, to predict the normal boiling point that water would have if not for hydrogen bonding. 79. Hydrogen bonding in water raises the expected boiling point 1½ to 2 times more than it raises the expected boiling point in HF or NH. Give a possible explanation. 80. Figure 12.7 shows how the four base pairs of DNA We've updated our read aloud feature! Give it a try here. Ma ST TTS 504 TTTE Aa

Answer 1

Each HF molecule possesses 3 lone pairs on F and 1 H. HF molecules on average make 2 H-bonds.

Each NH3 molecule possesses 1 lone pair on N and 3 H. NH3 molecules on average also make 2 H-bonds.

The polarity of the O-H bonds in H2O is intermediate between that of H-F and N-H bonds. It is expected that the individual H-bonds between H2O molecules will also be intermediate in strength.

Each H2O molecule possesses 2 lone pairs on O and 2 H. H2O molecules are thus able to form an average of 4 H-bonds.

H2O has a higher boiling point than NH3 because (i) the H-bonds are stronger and (ii) it contains twice as many H-bonds.

H2O has a higher boiling point than HF because it contains twice as many H-bonds, despite these being individually weaker.