Chemical Reactions And Equations Worksheet With Answer Key - The University Of Sydney Page 32
ADVERTISEMENT
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40CHEM1101
2007-J-8
June 2007
Marks
A major disadvantage of hydrogen as a fuel is that it is a gas, and therefore hard to
5
store. There is an enormous world-wide effort, including research performed in the
University of Sydney, to develop novel chemical structures in which H
can be stored
2
much more efficiently. One of the structures being tested in the School of Chemistry
is shown below.
H
2
2.39 Å
O
2+
Cu
CH
~
2
C
O
2+
Cu
H
2
What type of intermolecular force (or forces) are responsible for the binding between
2+
the Cu
and the H
?
2
2+
There are ion-induced dipole forces between Cu
and the H
molecules as well
2
as weak dispersion (London or induced dipole-induced dipole) forces.
In order that such a material be useful for fuel storage, the binding of the H
must be
2
reversible:
cage(s) + H
(g)
cage·H
(s)
2
2
One simple way to reverse the binding is to increase the temperature, so that at low
temperature the equilibrium lies to the right and at high temperature to the left. Use
this information, plus any chemical knowledge or intuition to infer the sign of ∆G,
∆H and ∆S at “low” and “high” temperatures. (You may assume that ∆H and ∆S do
not change greatly with temperature.)
At low temperature, the equilibrium lies to the right favouring products:
∆G < 0. At high temperature, it lies to the left favouring reactants: ∆G > 0.
The reaction involves formation of a solid from a solid and a gas. There is
therefore a decrease in the entropy: ∆S < 0. This is true at all temperatures.
The forward reaction becomes less favourable as the temperature is increased.
Le Chatelier’s principle therefore suggests that the reaction is exothermic:
∆H < 0. This is true at all temperatures. (If the temperature is increased, the
equilibrium shifts to remove heat by increasing the backward reaction.)
∆G
∆S
∆H
Temperature
low
< 0
< 0
< 0
high
> 0
< 0
< 0
ADVERTISEMENT
0 votes
Related Articles
Related forms
Related Categories
Parent category: Education