CHEM - Topic 7/17

• Equilibrium 


  when concentrations of reactants and products in a run are not changing and the rate of forward reaction equals the rate of the backward/reverse reaction
• Dynamic state


  when rate of forward and reverse chemical rxns is equal and there are no observable changes to the system despite it continuing to react
• Static state 


  when there appears to be no change therefore is at equilibrium
• physical equilibrium 


  when states of a physical process reach equilibrium
• chemical equilibrium 


  when reaction steadies where there are still reactants and products
• homogenous rxn


  when all the reactants and products are in the same phase
• stress 


  a disturbance of equilibrium which "shifts" equilibrium left or right
• Characteristics of a system at equilibrium

  dynamic state of balance -- no change in concentrations or properties to the naked eye once equilibrium is reached -- closed system, therefore pressure is a constant and so is temperature. ALSO, equilibrium can be reached from either direction
• Equilibrium constant 


  the extent to which reactions are converted into products/ the ratio of products to reactants
• what is the equilibrium constant only dependent on

  temperature
• K will increase and decrease on


  if heat is added or removed and if the reaction is exo or endo
• If a reaction is exo and heat is added, what happens to K?

  It will decrease since there will be more reactants.
• Equilibrium constant Units


  dependent on exponents on concentration.
• If K is big, reaction favours....

  products
• If K is small, reaction favours...

  Reactants
• K of the reverse reaction is

  the inverse of K for forward reaction.
• K gives no indication of 


  the rate of rxn
• change in equilibrium concentration can be neglected if it is less than

  10^−3
• The RXN quotient is

  the extent to which reactants have become products at a given point of time.
• When K=Q 


  rxn is at equilibrium
• When K<Q (K less than Q)

  shifts to reactants
• When K> Q (k is larger than Q) 


  Shifts to products
• Le Chatelier's Principal states: 


  when a system at equilibrium and subject to change, the system will respond to minimize the effect of change.
• If Kc > 1 then:

  the concentration of products is greater than the concentration of reactants and we say that the equilibrium lies to the right hand side
• If Kc < 1 then,

  the concentration of reactants is greater than the concentration of products and we say that the equilibrium lies to the left hand side
• what happens to the Kc  value if we reverse the rxn equation

  Kc  becomes the reciprocal of the original Kc value:
• What is the Reaction quotient?

  Q, is the ratio of products and reactants for a reaction that has NOT yet reached equilibrium
• If Q = Kc then

  the reaction is at equilibrium, no net reaction occurs
• If Q < Kc then

  reaction proceeds to the right in favour of the products
• If Q > Kc then

  reaction proceeds to the left in favour of the reactants
• Position of the equilibrium

  The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture.
• When position of equilibrium shifts to the left

  it means the concentration of reactants increases
• When position of equilibrium shifts to the right

  it means the concentration of products increases
• What does Le Chatelier's principle say

  if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change
• The principle is used to predict changes to the position of equilibrium when there are changes in 


  temperature, pressure or concentration
• Effects of Concentration 


  Increase - equilibrium shits to the RIGHT to reduce hte effect of increase in the concentration of A Reactant. Decrease - equilibrium shift to the left to reduce the effect of a decrease in reactant (or an increase in the concentration of products)
• Effects of pressure, only when.. 


  Changes in pressure only affect reactions where the reactants or products are gases
• Increase in pressure 


  equilibrium shifts in the direction that produces the SMALLER number of molecules of gas to decrease the pressure again
• Decrease in pressure 


  Equilibrium shifts in the direction that produces the LARGER number of molecules of gas to increase the pressure again
• Effects of temperature

  Increase - equilibrium moves in the Endothermic direction to reverse the change. Decrease - equilibrium moves in the exothermic direction to reverse the change.