CHM 2046C Module 10 Sample Exam
Part G: LaChatelier’s Principle 10 points
Consider the isomerism of butane with an equilibrium constant of Kc = 2.5.
CH3
|
CH3-CH2-CH2-CH3 (n-butane) ç è CH3-CH-CH3 (isobutane)
The system is originally at equilibrium with [n-butane]=1.0 M and [isobutane]=2.5M.
(a) If 0.50 moles/liter of isobutane is suddenly added and the system shifts to a new equilibrium position, what is the equilibrium concentration of each gas?
|
|
[n-butane] |
[isobutene] |
|
Initial M |
1.0 M |
2.5 M |
|
Initial Change |
|
0.5 M |
|
Change from Shift |
+ x |
-x |
|
Equilibrium M |
1.0 + x |
3.0 - x |
|
[isobutane]1 |
|
Kc = --------------------- |
|
[n-butane]1 |
|
[3.0 - x]1 |
|
2.5 = ------------------ |
|
[1.0 + x]1 |
2.5 (1.0 + x) = 3.0 – x
2.5 + 2.5x = 3.0 – x
3.5x = 0.5
x = 0.14 M
|
|
[n-butane] |
[isobutene] |
|
Initial M |
1.0 M |
2.5 M |
|
Initial Change |
|
0.5 M |
|
Change from Shift |
+ .14 |
-.14 |
|
Equilibrium M |
1.14 |
2.86 |
(b) If 0.50 moles/liter of n-butane is suddenly added
and the system shifts to a new equilibrium position, what is the equilibrium
concentration of each gas?
|
|
[n-butane] |
[isobutene] |
|
Initial M |
1.0 M |
2.5 M |
|
Initial Change |
0.5 M |
|
|
Change from Shift |
-x |
+x |
|
Equilibrium M |
1.5 - x |
2.5 + x |
|
[isobutane]1 |
|
Kc = --------------------- |
|
[n-butane]1 |
|
[2.5 + x]1 |
|
2.5 = ------------------ |
|
[1.5 - x]1 |
2.5 (1.5 - x) = 2.5 + x
3.75 - 2.5x = 2.5 + x
1.25 = 3.5x
x = 0.38 M
|
|
[n-butane] |
[isobutene] |
|
Initial M |
1.0 M |
2.5 M |
|
Initial Change |
+0.5M |
|
|
Change from Shift |
- 0.38 |
+0.38 |
|
Equilibrium M |
1.12 |
2.88 |
Notice that both equilibrium concentrations after causing the shift yields approximately the same equilibrium concentrations after equilibrium is re-established.
Kotz 6th Reference section 16.6 p 781-786 See Example 16.8 p784 for the effect of the addition or removal of a reactant or product. Try Exercise 16.11 p785 for the effect of adding or removing of a reactant or product and Exercise 16.12 p786 for the effect of concentration and Volume changes on the Equilibria. Then go to the end of the chapter on page 791 and try #25-#28. The Solutions manual for these exercises have been distributed