Equilibria

Equilibria

1. Chemical Equilibrium

   • Dynamic equilibrium is a reaction where the forward and reverse reactions both occur at the same rate, so that although many molecules are reacting, there is no overall change in concentration
     • Dynamic Equilibrium can only occur at a constant temperature and in a closed system
   • The Equilibrium Constant (Kc)
     • The equilibrium constant is the ratio of the concentrations of the product and reactants in a reaction after equilibrium is reached
     • Kc is different for different reactions and it changes with temperature.
       • If Kc is greater than 1, there is more product than reactant so the position of equilibrium is over to the right
       • If Kc is less than 1, there is more reactant than product so the position of equilibrium is over to the right
     • The units of Kc vary and need to be worked out for each reaction

2. Calculations using the Equilibrium Constant Expressions

   • To Calculate the Equilibrium Constant
     • STEP 1: Find out how many moles of each reactant and product there are at equilibrium
       • STEP 2: Calculate the molar concentration of each reactant and product by dividing each number of moles by the volume of the reaction
         • STEP 3: Substitute the values of the concentrations into the expression for Kc
   • The expression of Kc can be rearranged to find unknown concentrations of either products or reactants

3. The effect of changing conditions on Equilibria

   • A change in temperature changes the value of Kc
     • Depending on whether the reaction is exothermic or endothermic a change in temperature will shift the equilibrium either to the left or to the right changing the concentration of the reactants and products
       • For an exothermic reaction (∆H is negative) increasing the temperature decreases the equilibrium constant
       • For an endothermic reaction (∆H is positive) increasing the temperature increases the equilibrium constant
   • A change in concentration does not change the value of the equilibrium constant
     • If the concentration of any one species involved in an equilibrium is changed, then the concentrations of all the other species will change so that the value of Kc remains constant
   • Addition of a catalyst does not change the value of the equilibrium constant
     • A catalyst is a substance that increases the rate of reaction without itself being used up in the reaction
     • A catalyst does not affect the position of equilibrium as it increases the rate of the forward and reverse reaction equally
       • It can speed up the attainment of equilibrium by providing an alternative path for the reaction with a lower activation energy

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