Calculate Kp

Calculations involving gas phase equilibrium systems use
 
  • the fact that, for a system at equilibrium at a particular temperature, that the equilibrium constant Kp can be related to the partial pressures of the gases.
     

    Kp =    		p(C)cp(D)d
    p(A)ap(B)b
    The general relationship between these is shown for the reaction
    aA + bB cC + dD.
    where substances A, B, C and D are all gases.
    a,b,c,d are the coefficients required to balance the equation.

  • the stoichiometry of the reaction occurring to reach equilibrium
     
    The ratio of the changes in partial pressures in the reaction occurring to reach equilibrium is the same as the ratio of the coefficients in the balanced equation for the reaction.

Kp can calculated from
  • known initial partial pressures of all gases in the Kp expression and the known equilibrium partial pressure of one of them.
     
    Thus the change in pressure due to reaction can be calculated for one of the gases.
    The changes in the pressures of the other gases involved can be deduced from this and the balanced equation for the reaction.
     
  • OR known initial pressures of all gases in the Kp expression and the total pressure at equilibrium.
     
    This problem uses Dalton's Law (The total pressure is the sum of the partial pressures of the individual gases.) in addition to the above.
Dr Sheila Woodgate
© The University of Auckland 2002