topic 3 - equilibrium constant Kp

• when can a Kp value be deduced?


  any homogenous reversible reaction occurring in the gas phase.
• what is the general expression for Kp?

  Kp = (pProducts)^x / (pReactants)^y

  p = the partial pressure

  x + y = number of moles
• what sort of brackets are used for pressure?

  normal brackets to show that we are using pressures not concentrations.
• what are the steps for calculating Kp?

  1. write the expression for Kp from the reactant equation.
  2. calculate the mole fraction for each gaseous reactant and product.
  3. calculate the partial pressure of each gaseous reactant and product.
  4. substitute the values for partial pressures into the expression using number of moles as the power.
  5. calculate the overall value and units for Kp.
• what are the units for pressure?

  kPa
• does pressure have an effect on Kp?

  no.
• why does a change in pressure not effect Kp?

  if the pressure of 1 reactant/product changes then the equilibrium shifts to oppose the change which changes all the pressure of all the other species. The changes to pressure cancel each other out and so the value of Kp doesn't alter.
• does using a a catalyst affect Kp?

  no.
• why does a catalyst not affect Kp?

  it doesn't affect the position of equilibrium so has no effect on Kp value.
• does a change in temperature effect Kp?

  yes
• what does an increase in temperature of the surroundings do?

  equilibrium will shift in the endothermic direction to decrease the temperature again.
• what does a decrease in temperature of the surroundings do?

  equilibrium will shift in the exothermic direction to increase the temperature again.