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(over the first forward arrow).
(under the first reverse arrow).
%
%
22.
23.
Model 4 – Concentration as a Function of Time
Concentration (mol∙L–1) or (M)
0.12
0.1
0.08
[A] (M)
0.06
[B] (M)
0.04
0.02
0
0
10
20
30
40
50
60
time (s)
24.
The data in Model 4 can be divided into two regions – the kinetic region and the equilibrium region.
Indicate these regions (time intervals) on the graph in Model 4.
a) Use a grammatically correct English sentence to describe the kinetic region.
b) Use a grammatically correct English sentence to describe the equilibrium region.
25.
From the graph only, estimate the value of
[B]
at:
[A]
a) t = 1 s
b) t = 4 s
c) t = 15 s
d) t = 20 s
26.
In which region (kinetic or equilibrium)
[B]
is the quantity
a constant?
[A]
e) t = 40 s
8
Model 5 – The Law of Mass Action
The Law of Mass Action states for a chemical system described by the balanced chemical equation…
aA + bB
…the ratio
cC + dD
[C]c[D]d
is constant at a given temperature.
[A]a[B]b
The ratio is called the equilibrium constant expression, and
the numerical value is called the equilibrium constant, Kc .
[C]c[D]d
Kc =
[A]a[B]b
By convention, the equilibrium constant values are given without units, even though [ ] is molarity, M.
For example:
the reaction: 2 H2(g) + O2(g)
2 H2O(g)
[H2O]2
Kc =
[H2]2[O2]
the equilibrium expression:
Kc = 1083 (determined by experiment)
the equilibrium constant:
(at 25oC)
27.
When a mixture of H2(g) , O2(g) , and H2O(g) reaches equilibrium, which species is present in the
largest amount? Explain your reasoning.
28.
Write the equilibrium expression for the reaction in Model 4.
29.
Consider the graph of reaction 2 A(g)
(B(g)
below.
M
[B]
[A]
a b
time
c
a) Which is larger at time “a”, the forward rate or the reverse rate? _____________
b) Which is larger at time “b”, the forward rate or the reverse rate? _____________
c) Which is larger at time “c”, the forward rate or the reverse rate? _____________
9
Exercises
1.
The state if equilibrium is often described as “dynamic equilibrium.”
To what does the word “dynamic” refer?
2.
Write the equilibrium expression, Kc , for each of the following reactions:
a) 2 HI(aq)
H2(g) + I2(g)
b) 3 H2(g) + N2(g)
2 NH3(g)
c) 3/2 H2(g) + ½ N2(g)
d) cis-2-butene(g)
e) O3(g)
trans-2-butene(g)
O2(g) + O(g)
f) Xe(g) + F2(g)
3.
NH3(g)
XeF4(g)
Describe the mathematical relationship between the reactions in b) and c) above.
Describe the mathematical relationship between the equilibrium expressions reaction b) and c) above.
4.
When the following reaction reaches equilibrium,
A(aq) + 2 B(aq)
C(aq)
The following concentrations are measured: [A] = 0.60 M ; [B] = 0.20 M ; [C] = 0.55 M .
What is the value of Kc for this reaction?
OVER
10
5.
An equilibrium mixture of PCl5 , PCl3 , and Cl2 , at a certain temperature in a 5.0 L container consists
of 0.80 mol PCl5 , 0.55 mol PCl3 , and 1.2 mol Cl2 . Calculate Kc for the reaction:
PCl3(g) + Cl2(g)
6.
PCl5(g)
Kc = 150.0 at a certain temperature for the reaction:
2 NO(g) + O2(g)
2 NO2(g)
What is the equilibrium concentration of NO2 if the equilibrium concentrations of NO and O2 are
0.00100 M and 0.0500 M , respectively?
7.
The following gases are added to a 1.00 L container: 2.0 moles of A and 4.0 moles of B.
These gases react as follows:
A(g) + 3 B(g)
C(g) + 2 D(g)
At equilibrium, the container has 0.4 moles of D.
a) Calculate the moles of A, B, and C in the container at equilibrium.
b) Calculate the concentrations of A, B, C, and D at equilibrium.
c) Calculate the value of the equilibrium constant, Kc , for this reaction.
11
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