CHM 2046C Module 10 Sample Exam Answers
Part E: Experimental Determination of Equilibrium Constants 10 points
Calculate the equilibrium constant at 25oC for the reaction:
2 NOCl(g) ç è 2 NO (g) +
Cl2 (g)
when 2.00 moles of NOCl were placed in a 1.00-L flask, and the concentration of NO after equilibrium was achieved was 0.66 moles/liter.
|
|
[NOCl(g)] |
[NO (g)] |
[Cl2 (g)] |
|
Initial M |
2.00 M |
0 |
0 |
|
Change |
-0.66 M |
+ 0.66 M |
+½∙(0.66 M) |
|
Equilibrium M |
1.34M |
0.66 M |
0.33M |
|
[NO]2 [Cl2]1 |
|
Kc = ------------------- |
|
[NOCl]2 |
|
[0.66]2 [0.33]1 |
|
Kc = -------------------
= 0.80 |
|
[1.34]2 |
Kotz 6th Reference section 16.3 p 770-772 See Example
16.3 p771 using Kc and Example
16.4 using Kp and
Partial pressures. Try Exercise 16.5 p772; then go to the end of the chapter on
page 790 and try #7-#12. The Solutions manual for these exercises have been
distributed.
Part F. Calculations based on Kc 10 points
At 25oC, Kc=0.090 for the reaction:
H2O (g) + Cl2O (g) ç è 2 HOCl (g)
Calculate the concentration of all species if 2.0 mole of pure H2O and 2.0 mole of pure Cl2O are placed in a 2.0 L flask and the system is allowed to come to equilibrium.
|
|
[H2O (g)] |
[Cl2O (g)] |
[HOCl (g)] |
|
Initial M |
2.0 mol/2.0 L= 1.0M |
1.0M |
0 |
|
Change |
-x |
-x |
+2x |
|
Equilibrium M |
1.0 - x |
1.0 - x |
2x |
|
[HOCl]2 |
|
Kc = --------------------- |
|
[H2O ]1 [Cl2O]1 |
|
[2x]2 |
|
0.090 = --------------------- |
|
[1.0-x ]1
[1.0-x]1 |
|
[2x]2 |
|
0.090 = -------------- |
|
[1.0-x ]2 |
Take the square root of both sides:
|
[2x] |
|
(0.090)½ = -------------- |
|
[1.0-x ] |
0.30 (1.0 – x) = 2x
0.30 – 0.30x = 2x
0.30 = 2.30x
x = 0.13
Substituting for x:
|
|
[H2O (g)] |
[Cl2O (g)] |
[HOCl (g)] |
|
Initial M |
2.0 mol/2.0 L= 1.0M |
1.0M |
0 |
|
Change |
-.13 |
-.13 |
+.26 |
|
Equilibrium M |
0.87 |
0.87 |
0.26 |
Kotz 6th Reference section 16.4 p 772-777 See Example
16.5 p771 using Kc without a quadratic
equation to solve and 16.6 using Kc
and needing the quadratic expression. Try Exercise 16.6 p774 for the first type
and Exercise 16.7 p777 for the second type; then go to the end of the chapter
on page 790-791 and try #13-#18. The Solutions manual for these exercises have
been distributed.
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