CHEM 30A equilibrium worksheet solutions final UCLA

Name: ____________________________ 1
Equilibrium Worksheet SOLUTIONS
Complete the following questions on a separate piece of paper.
1. Write the equilibrium expression, Keq, for each of the following reactions:
a) 2 NO(g) + O2(g) ⇌2 NO2(g)
[ NO 2]2
Keq 
[ NO]2 [O 2]
b) 4 HCl(g) + O2(g) ⇌2 H2O(g) + 2 Cl2(g)
[ H O]2 [Cl ]2
K eq  2 4 2
[ HCl ] [O2 ]
c) NOCl(g)⇌NO(g) + ½ Cl2(g)
1
[ NO ] [Cl 2 ] 2
K eq 
[ NOCl ]
d) Fe3+(aq) + SCN-(aq)⇌FeSCN2+(aq)
[ FeSCN 2]
K eq  3
[ Fe ][ SCN  ]
e) CaCl2(s) ⇌Ca2+(aq) + 2Cl-(aq)
K eq [Ca 2][Cl  ]2
f) HC2H3O2(aq) + H2O(l) ⇌ H3O+(aq) + C2H3O2-(aq)

[ H 3O ][C 2 H 3O2 ]
K eq 
[ HC 2 H 3O2 ]
g) MgO(s) + CO2(g) ⇌ MgCO3(s)
1
K eq 
[CO2 ]
h) C(s) + CO2(g) + 2Cl2(g) ⇌ COCl2(g)
[COCl 2 ]
K eq 
[CO2 ][Cl 2 ]
i) Ca3(PO4)2(s) ⇌ 3 Ca2+(aq) + 2 PO43-(aq)
3
K eq [Ca 2]3 [ PO4 ]2
j) ZnO(s) + CO(g) ⇌ Zn(s) +CO2(g)
[CO2 ]
K eq 
[CO]
2. The equilibrium constant for the equilibrium CO(g) + H2O(g) ⇌CO2(g) + H2(g) is 302 at 600K. What is
the value of the equilibrium constant for the reverse reaction at the same temperature?
1 1
K eq reverse   3.31x10  3
K eq 302

3. Classify the following equilibria as heterogeneous or homogeneous, and write an equilibrium
expression for each.
a) NH4NO2(s) ⇌N2(g) + 2 H2O(g) heterogeneous
Keq [ N 2 ][ H 2 O]2
b) H2O(l) ⇌ H2O(g) heterogeneous
K eq [ H 2 O( g ) ]
c) SO2(g) + ½ O2(g) ⇌ SO3(g) homogeneous

, Name: ____________________________ 2
[ SO3 ]
Keq 
[ SO2 ][O2 ]
d) S8(s) + 8 O2(g) ⇌ 8 SO2(g) heterogeneous
8
[ SO2 ]
Keq 
[O2 ]8

4. At the equilibrium point in the decomposition of phosphorus pentachloride to chlorine and
phosphorus trichloride, the following concentrations are obtained: 0.010 mol/L PCl5, 0.15 mol/L PCl3
and 0.37 mol/L Cl2. Determine the Keq for the reaction.

PCl5(g) ⇌ PCl3(g) + Cl2(g)
C = 0.001 M 0.15 M 0.37 M
[ PCl3 ][Cl 2 ]
K eq 
[ PCl5 ]
(0.15)(0.37)
K eq 
(0.010)
5.55
Therefore the Keq value for the reaction is 5.55.
5. The colourless gas dinitrogen tetroxide decomposes to the brown coloured air pollutant nitrogen
dioxide and exists in equilibrium. A 0.125 mol sample of dinitrogen tetroxide is introduced into a 1.00
L container and allowed to decompose at a given temperature. When equilibrium is reached, the
concentration of the dinitrogen tetroxide is 0.0750 mol/L. What is the value of Keq for this reaction?

N2O4(g) ⇌ 2 NO2(g)
MR 1 2 [ NO2 ]2
K eq 
I 0.125 0 [ N 2O4 ]
C -x +2x (0.10) 2
E 0.125- 2x K eq 
x (0.075)
=0.075 0.13
0
x=0.050
Therefore the Keq value for the reaction is 0.13
6. Phosphorus pentachloride decomposes to phosphorus trichloride and chlorine gas. A 1.10 mol/L
sample of PCl5 was placed into a vessel. At equilibrium it was determined that the concentration was
0.33 mol/L. Calculate the Keq for this reaction.
[ PCl3 ][Cl 2 ]
PCl5(g) ⇌ PCl3(g) + Cl2(g) K eq 
[ PCl5 ]
MR 1 1 1
I 1.10 0 0 (0.77)(0.77)
K eq 
C -x +x +x (0.33)
E 1.10-x x x
1.80
=0.33
x= 0.77

Therefore the Keq value for the reaction is 1.80
7. Gaseous dinitrogen tetroxide is placed in a flask and allowed to decompose to nitrogen dioxide and
reach equilibrium at 100°C. At 100°C, the value of Keq is 0.212. If the concentration of dinitrogen
tetroxide at equilibrium is 0.155 mol/L, what is the concentration of nitrogen dioxide at equilibrium?