TOPIC 7 SL: EQUILIBRIUM

TOPIC 7 SL: EQUILIBRIUM

1. Physical equilibrium

   • Equilibrium is established when the rate of one state equals the rate of another different state.

2. Chemical equilibrium

   • Equilibrium is reached when the concentrations of reactants and products become constant
     • For example: 2HI(g) < === > H2(g) + I2(g)
       • Kc
         • The equilibrium constant Kc has a fi xed value for a particular reaction at a specifi ed temperature
         • Magnitude
           • Kc very small: equilibrium lies to the left in favor of reactants
           • Kc very large: equilibrium lies to the right in favor of products
         • Reaction quotient Q
           • The reaction quotient, Q, is a measure of the relative amounts of reactants and products present in a reaction at a particular time.
           • Q= Kc
             • reaction is at equilibrium, no net reaction occurs
           • Q < Kc
             • reaction proceeds to the right in favour of products;
           • Q > Kc
             • reaction proceeds to the left in favour of reactants
   • Characteristics
     • Equilibrium is dynamic
       • The reaction has not stopped but both forward and backward reactions are still occurring at the same rate.
     • Equilibrium is achieved in a closed system
       • A closed system has no exchange of matter with the surroundings, so equilibrium is achieved where both reactants and products can react and recombine with each other.
     • The concentrations of reactants and products remain constant at equilibrium
       • They are being produced and destroyed at an equal rate.
     • At equilibrium there is no change in macroscopic properties
       • Macroscopic properties are observable properties such as colour and density. These do not change as they depend on the concentrations of the components of the mixture.
     • Equilibrium can be reached from either direction
       • The same equilibrium mixture will result under the same conditions, no matter whether the reaction is started with all reactants, all products, or a mixture of both.
   • Le Chatelier’s principle
     • When a system at equilibrium is subjected to a change, it will respond in such a way as to minimize the effect of the change.
     • Changes in concentration
       • If the concentration of reactants increases, equilibrium shifts to the left; and vice-versa.
         • No change in value of Kc
     • Changes in pressure
       • If pressure increases, equilibrium shifts to the side with fewer gaseous molecules
         • No change in value of Kc
     • Changes in temperature
       • If temperature increases, equilibrium shifts to the endothermic side; and vice-versa
         • Kc increases if equilibrium shifts to the right, and vice-versa.
     • Addition of a catalyst
       • No change in equilibrium position
         • No change in value of Kc

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