Equilibrium
Law
Introduction to the Equilibrium law
• Read 14.3 to PE1
Equilibrium
constant
Kc =
[H2O]2
[H2]2[O2]
Mass action
expression
2H2(g) + O2(g)  2H2O (g)
Step 1: Set up the “equilibrium law” equation
Step 2: Product concentrations go in numerator
Step 3: Concentration in mass action expression
is raised to the coefficient of the product
Step 4: Reactant concentrations go in denominator
Step 5: Concentrations in mass action expression
are raised to the coefficients of reactants
Equilibrium law: important points
• State (g, l, s, aq) may or may not be added at
this point since we will only be dealing with
gasses for this section. Later it will matter.
• The equilibrium law includes concentrations of
products and reactants in mol/L (M)
• The value of Kc will depend on temperature,
thus this is listed along with the Kc value
• Tabulated values of Kc are unitless
• By substituting equilibrium concentrations into
equilibrium law, we can calculate Kc …
• Do RE 14.31, 35, 36, 37 (pg. 589)
Equilibrium law: RE 14.31, 14.35
CO(g) + 2H2(g)  CH3OH(g)
[CH3OH]
[0.00261]
Kc =
,Kc=
[CO] [H2]2
[0.105] [0.250]2
=0.398
C2H4(g) + H2O(g)  C2H5OH(g)
[C2H5OH]
[0.150]
Kc =
,Kc=
= 300
[C2H4] [H2O]
[0.0222] [0.0225]
Equilibrium law: RE 14.36, 14.37
CO(g) + 2H2(g)  CH3OH(g)
[CH3OH]
[x]
= 0.500
Kc =
,Kc=
[CO] [H2]2
[0.210] [0.100]2
x = [0.210] [0.100]2 (0.500) = 0.00105 M
N2(g) + 3H2(g)  2NH3(g)
[NH3]2
2
[0.280]
Kc =
,Kc=
=
64
3
[N2] [H2]
[0.00840] [x]3
x3 = [0.280]2 / [0.00840] (64) = 0.146
x3 = 0.146
x = 0.53 M
When Kc  mass action expression
• We can use the equilibrium law to determine if
an equation is at equilibrium or not
• If mass action expression equals equilibrium
constant then equilibrium exists
Q - consider: C2H4(g) + H2O(g)  C2H5OH(g)
If Kc = 300, []s = 0.0197 M, 0.0200 M, 0.175 M
which direction will the reaction need to shift?
[C2H5OH]
[0.175]
Kc =
,Kc=
= 300
[C2H4] [H2O]
[0.0197] [0.0200]
444
 300
444 must be reduced to 300. Thus, the top
must decrease and the bottom must increase. A
shift to left is required to establish equilibrium.
More equilibrium law problems
• Do RE 14.32, 33 (pg. 589). For each, state in
which direction the reaction needs to shift
PCl3 (g) + Cl2(g)  PCl5(g)
[PCl5]
[0.00600]
Kc =
,Kc=
= 0.18
[PCl3] [Cl2]
[0.0520] [0.0140]
8.24
 0.18
Top must , bottom must  - shift to left is needed
SO2(g) + NO2(g)  NO(g) +SO3(g)
[NO] [SO3]
[0.0100][0.0400]
Kc =
,Kc=
= 85
[SO2] [NO2]
[0.00150] [0.00300]
88.9
 85
Top must , bottom must  - shift to left is needed
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