Effect of changing conditions on value of Kc or Kp Effect of Temperature on position of equilibrium and Kc Both the position of equilibrium and the value of Kc or Kp will change it temperature is altered N2 (g) + 3H2 (g ) 2 NH3 (g) In this equilibrium which is exothermic in the forward direction 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 Kc gets smaller as there are fewer products. N Goalby chemrevise.org Increasing pressure does not change Kp. The increased pressure increases the pressure terms on bottom of Kp expression more than the top. The system is now no longer in equilibrium so the equilibrium shifts to the right increasing mole fractions of products and decreases the mole fractions of reactants. The top of Kp expression therefore increases and the bottom decreases until the original value of Kp is restored Effect of Pressure on position of equilibrium and Kp The position of equilibrium will change it pressure is altered but the value of Kp stays constant as Kp only varies with temperature N2 (g) + 3H2 (g ) 2 NH3 (g) In this equilibrium which has fewer moles of gas on the product side 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. p 2 NH3 p N2 p 3 H2 Kp= x 2 NH3 . P2 x N2 .P x 3 H2 .P3 Kp= x 2 NH3 . P2 x N2 x 3 H2 . P4 Kp= Where P is total pressure and x mole fraction. The larger the Kc the greater the amount of products. If Kc is small we say the equilibrium favours the reactants Kc and Kp only change with temperature. It does not change if pressure or concentration is altered. A catalyst also has no effect on Kc or Kp
3.1.10 Equilibrium constant K p for homogeneous systems (A-level only)
Students should be able to:
• predict the qualitative effects of changes in temperature and pressure on the position of equilibrium
• predict the qualitative effects of changes in temperature on the value of K p
• understand that, whilst a catalyst can affect the rate of attainment of an equilibrium, it does not affect the value of the equilibrium constant.