Equilibrium Constant
Example
H2 + F2  2HF
The equilibrium constant for the reaction
above is found to be 2.1 x 103 at a specific
temperature. At equilibrium, the
concentrations of both hydrogen and
fluorine are found to be 0.0045 M. What is
the equilibrium concentration of hydrogen
fluoride?
Example
• A mixture of 5.00 x 10-3 mole of H2 and
1.00 x 10-2 mole of I2 is placed in a 5.00L
container and allowed to come to
equilibrium. Analysis of the equilibrium
mixture shows the concentration of HI is
1.87 x 10-3 M. Calculate the Kc at 448°C
for this reaction.
H2 + I2  2HI
Example
• Sulfur trioxide decomposes at high
temperatures in a sealed container as
shown:
2SO3  2SO2 + O2
Initially the vessel is charged at 1000K
with 6.09 x 10-3 M SO3. At equilibrium
the SO3 concentration is 2.44 x 10-3 M.
Calculate the Kc at 1000K.
Kc and Kp
• When concentration is expressed in M use
Kc as the constant.
• When concentration is expressed in atm
use Kp
• aA  cC + dD
• Kp = Pcc PDd / PAa
Example
• The reaction for the formation of nitrosyl
chloride
2NO(g) + Cl2(g)  2NOCl(g)
was studied at 25°C. The pressures at
equilibrium were found to be PNOCl=
1.2atm, PNO = 5.0x10-2atm, and PCl =
.3atm. Calculate the Kp for this reaction.
Kp from Kc
• Kp = Kc (RT)n
• n – total coefficients of gaseous products
minus total coefficients of gaseous
reactants
• R = .0821 L atm/mol K
Example
• The Kc for the reaction
N2 + 3H2  2NH3
is .105. Calculate the Kp at 472°C.
Solids and Liquids
• The concentrations of pure solids and
liquids in a reaction does not really change
• They are not included in the equilibrium
expression
Example
• Write the equilibrium expression for:
CO2(g) + H2(g)  CO(g) + H2O(l)
SnO2(s) + 2CO(g)  Sn(s) + 2CO2(g)