CHEMICAL EQUILIBRIUM
1. Colorless N₂O₄ gas breaks down into brown NO₂ gas. Write the equilibrium equation
for this reaction.
N₂O₄(g) ↔ 2NO₂
2. How is existence of equilibrium indicated in a chemical equation?
The arrows which represent forward and reverse indicated the existance of equilibrium
in a chemical equation.
3. Because the two gases in this equilibrium are different colors, their concentrations can
be determined using a spectrophotometer. The following chart indicates what happens
over time following the addition of 1 mol N₂O₄ to an empty 1 L vessel. Plot the data
(include title and tabel axes).
Time
(min)
0
1
2
3
4
5
6
7
8
9
10
11
12
[N₂O₄] 1.00 0.70 0.45 0.35 0.30 0.27 0.25 0.23 0.22 0.21 0.20 0.20 0.20
[NO₂]
0
0.60 1.10 1.30 1.40 1.46 1.50 1.54 1.56 1.58 1.60 1.60 1.60
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
8
9
10
11
12
4. Was all of the N₂O₄ used up in the reaction? How does the graph show this?
Not all of the N₂O₄ used up in the reaction after 10 min, the amount of the N₂O₄ is stable.
5. According to the graph, how many moles of NO₂ were produced?
1.60 g/mol
6. How many moles of NO₂ would have been produced if all of the N₂O₄ was used up?
If all of the N₂O₄ was used up, we would have 2 mole of NO₂
7. Assume the spectrophotometer cannot directly measure [N₂O₄], but can measure
[NO₂] because of its brown color. Explain how the [N₂O₄] values would be calculated.
According to equilibrium conditions if I know the initial concentration of the N₂O₄ and
NO₂ gases and final concentration of NO₂, I would calculate the [N₂O₄] which have been
consumed and remaining.
8. Define reaction reversibility.
Forward and reverse reaction takes of the same time.
9. Can the same equilibrium be reached starting with pure NO₂? If so, how much NO₂
would be required?
2 mol
10. HI can form via the equation H₂(g) + I₂(g) ↔ 2H(g). 1 mol H₂ and 1 mol I₂ were placed
in a 1 L vessel (no HI was represent initially). At equilibrum [HI] was measured to be
1.56 mol/L. Sketch a graph for the equilibrium reaction, clearly indicating the satarting
and equilibrium concentrations of all chemicals. (Be carefull to draw the general shape
of the lines correctly: straight vs. curving up vs. curving down).
INTRODUCTION TO EQUILIBRIUM
PART 1
Table 1: Volume after x transfers (2 large straws)
X
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
ml
50
43
43,1
37,2
38,2
33
34,9
30
32
34
29,9
31,8
27,9
30
26,4
29
25,4
26,2
25,1
26
23,9
25,1
23
25
24,9
0
6,9
6,7
12,2
11,2
16,9
15
19,9
17,4
15,9
20
18
21,9
19,2
23
20,2
24
22,9
19
22
25,1
23,8
26
24
24,9
ml
Part 2:
Table 2: Volume after X transfers (1 large straw, 1 small straw)
X
0
ml
50
1
2
42,9 42,9
3
37
4
0
7
7
13
6
7
37,1 32,1 32,9 28,8
18,2 14,3 15,4 12,4 13,8
ml
5
16
8
9
29
25
10
11
12
13
25,9 22,1 23,1 19,5 20,7
15
17
17
12,9 17,3 16,9 21,1 20,9 24,8 23,9 27,8 26,7 30,2 26,1 32,8
31,7 35,5 34,5 37,5 36,1
34
33
1) Draw a line across a piece of graph paper. Plot two graphs – one for each table.
Note that “number of transfers” is the independent variable (i.e. it goes on the
x – axis). Connect values so that there are two continuous lines on each of the
two graphs (Note: sequential data points should be connected with straight
lines.)
14
Graph of table 1:
60.00
50.00
40.00
Series1
30.00
Series2
20.00
10.00
0.00
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
Graph of table 2:
60
50
40
Series1
30
Series2
20
10
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
2) Assuming that Part 1 simulates the reaction A (reactant) – B (product) what do
these represent:
i.
Volume of water in cylinder #1 (50 ml to start), concentration of forward
reaction
ii.
Volume of water from #1 to #2, concentration of backward reaction
iii.
Transferring water from #1 to #2, forward reaction
iv.
Transferring water from #2 to #1, backward reaction
v.
Number of transfers, duration of reactions
vi.
Diameter of straws, rate of reaction
3) What are the two important defining characteristics of a dynamic equilibrium?
How can you tell by looking at your graphs when equilibrium has been
stablished?
There are two important characteristics of a dynamic equilibrium. These are
reversibility and not going completion.
4) How can you tell by looking at a graph which reaction is (forward or reverse) is
favored?