Network Structures and the Boiling Points of Water, NH3, and HF
1.
The boiling point for H2O (see Figure 1.24 in the text) is out of line with those of other
group VI compounds. Sketch a model to show how hydrogen bonding can account
for its relatively high boiling point.
2.
Work with your group to construct about a dozen ball-and-stick
models of water molecules. Use the shortest (pink) sticks in your
set for the hydrogen–oxygen bonds. Connect one of your water
molecules to four others by hydrogen bonds. Use the longer (white)
sticks in your set for the hydrogen bonds, as in Figures 1.25(c) and
1.27 in the text. The picture to the right shows how to begin such
connections. Continue to add water molecules to see how a
network of hydrogen-bonded water molecules might form.
3.
Can you form a ring of hydrogen-bonded water molecules? If so, how many molecules
are required to form a ring without bending any of the sticks?
4.
Is it possible to link such a ring to a second ring of water molecules to extend the
network of water molecules? Compare your extended structure with the
representations shown in Figure 1.2 in the text. What conclusion(s) can you draw?
5.
What happens when you break a few of the hydrogen bonds in your extended
structure of water molecules? What does this suggest about the volumes of liquid
and solid water? Compare your structure with the representations of Figure 1.2 in
the text. What conclusion(s) can you draw?
6.
The boiling points for NH3 and HF (see Figure 1.24 in the text) are, like water, out of
line with other group V and VII compounds, respectively. Sketch models to show
how hydrogen bonding can account for their relatively high boiling points.
7.
Work with your group to construct about a dozen ball-and-stick models of ammonia
molecules. Use the shortest (pink) sticks in your set for the hydrogen–nitrogen
bonds. Connect one of your ammonia molecules to four others by hydrogen bonds.
Use the longer (white) sticks in your set for the hydrogen bonds, as in step 2.
Continue to add ammonia molecules to see how a network of hydrogen-bonded
ammonia molecules might form. What are your conclusions?
8.
Can you form a ring of hydrogen-bonded ammonia molecules? If so, how many
molecules are required to form a ring without bending any of the sticks?
9.
Is it possible to link such a ring to a second ring of ammonia molecules to extend the
network of ammonia molecules? What conclusion(s) can you draw?
10. Repeat steps 7-9, but substitute HF molecules for ammonia. What do you conclude?
11. Do your results explain why water has a higher boiling point than either NH 3 or HF?
What is the evidence for your answer?
12. If we have a large number, N, of water molecules, there are 2N hydrogen atoms and
2N nonbonding pairs that can interact to form hydrogen bonds . Since it takes one
hydrogen atom and one nonbonding pair to form each hydrogen bond, the maximum
number of hydrogen bonds that can be formed among these molecules is 2N. If we
have N ammonia molecules, how many hydrogen atoms are available to form
hydrogen bonds? How many nonbonding pairs? What is the maximum number of
hydrogen bonds that can be formed among these N ammonia molecules? Do the
same analysis for N hydrogen fluoride molecules. Are your results consistent with
your answer to step 11?
Individual Results and Conclusions
Group Results and Conclusions
Names of group members who participated in the activity. (Circle the recorder’s name).
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Instructor Notes
This Guided Inquiry provides an alternative pathway through material covered in 1.26.
Consider This and Section 1.8 Further Structural Effects of Hydrogen Bonding in
Water. You can follow this activity with 1.30. Investigate This What are the
temperatures in an ice-water mixture?