1.10. partial pressure and Kp

partial pressure equation

if a mixture of gases contain 3 different gases, toal pressure is sum of all the partial pressure:
P =p1 + p2 + p3

partal pressure of gas 1= mole fraction x total pressure of gas 1

mole fraction

mole fraction = number of moles of a gas/ total number of moles of all gases

expression for Kp

N2 (g) + 3H2 (g) -> 2NH3 (g)

Kp= p(NH3 )2 / p(N2 ) p(H2 )3

p= partial pressure of that gas
Kp= equilibrium constant
Only include gases in the Kp expression. Ignore solids, liquids, and aqueous substances.

unit of Kp

Kp= p(NH3 )2 / p(N2 ) x p(H2 )3

unit= KPa2/ (Kpa x Kpa3)
unit = 1/Kpa2
unit= KPa-2

Effect of changing conditions on value of Kc or Kp

Kc and Kp only change with temperature. They do not change if pressure or concentration are altered. A catalyst also has no effect on Kc or Kp

The larger the Kp the greater the amount of products. If Kp is small we say the equilibrium favours the reactants

effect of temperature on position of equilibrium and Kp

N2 (g) + 3H2 (g) -> 2 NH3 (g) [exothetmic in forward direction]

Both the position of equilibrium and the value of Kc or Kp will change if temperature is altered.

If temperature is increased the reaction will shift to oppose the change and move in the backwards endothermic direction. The position of equilibrium shifts left. The value of Kp gets smaller as there are fewer products.

effect of pressure on position of equilibrium and Kp

N2 (g) + 3H2 (g ) -> 2 NH3 (g)

The position of equilibrium will change if pressure is altered but the value of Kp stays constant as Kp only varies with temperature.

If pressure is increased the reaction will shift to oppose the change and move in the forward direction to the side with fewer moles of gas. The position of equilibrium shifts right. The value of Kp stays the same though as only temperature changes the value of Kp .