Converting Between Kc
and Kp
To convert between Kc to Kp use the
following equation which is based on the relationship between molarities
and gas pressures.
Kp = Kc(RT)Dn
Dn is the difference in the number of moles
of gases on each side of the balanced equation for the reaction.
Dn = (number of moles of gaseous products
- number of moles of gaseous reactants)
Converting
Kc to Kp
-
Calculate the difference in the number of moles of
gases.
-
Subsititute Dn, R, and T into the equation and
solve.
Example: Calculate the value of Kp for
the following reaction, at 333 K.
-
Calculate the difference in the number of moles of gases, Dn.
Dn = (2 moles of gaseous
products - 0 moles of gaseous reactants) = 2
-
Substitute the values into the equation and calculate Kp.
Kp = (6.96 x 10-5)[(0.0821)(333)]2 =
0.052
Note: because we do not choose to use units for Kc and Kp
, we cannot cancel units for R and T. However, be careful to use the
value of R consistent with the units of pressure used in the problem, and
T in Kelvin.
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Converting
Kp to Kc
-
Calculate the change in the number of moles of gases.
-
Subsititute Dn, R, and T into the equation and
solve.
Example: Calculate the value of Kc at 373
K for the following reaction:
-
Calculate the change in the number of moles of gases, Dn.
Dn = (2 moles
of gaseous products - 3 moles of gaseous reactants) =
- 1
-
Substitute the values into the equation and calculate Kc.
2.40 = Kc[(0.0821)(373)]-1
Kc = 73.5
Note: because we do not choose to use units for Kc and Kp
, we cannot cancel units for R and T. However, be careful to use the
value of R consistent with the units of pressure used in the problem, and
T in Kelvin.
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