7.1 Equilibrium

Equilibrium Reactions in Chemistry

  • Many chemical reactions are reversible and exist in state of equilibrium
  • Dynamic Equilibrium: The forward and reverse reactions occur at equal rates
  • Single arrows are for non-reversible reactions
  • Double headed arrow are for reversible reactions which are in equilibrium

Chemical Systems

  • The terms “Reactants” and “Products” implies a reaction goes to completion when in reality, many reactions are in equilibrium
  • Relative rates of a reaction depend on:
    • Temperature and Pressure
    • Concentration of reactants and products
    • Presence of catalyst
  • At equilibrium:
    • Forward and reverse reactions occur at equal rates
    • No change in concentration of reactants or products

Equilibrium Law

  • Law: At a given temperature, the ratio of the concentration of products to the concentration of reactants (each raised to the power of their molar coefficients) is a constantEquilibrium Law
  • Constant is called equilibrium constant denoted by Kc
  • This constant changes at different temperatures
  • Large Kc value (Kc > 1) means products are favored over reactants
  • Small Kc value (Kc < 1) means reactants are favored over products
  • Pure solids and liquids are not included in when calculating the Kvalue
  • Homogenous equilibrium: reactants and products are present in one phase. (Common one is reactions in gaseous phase)
  • Heterogeneous equilibrium: reactants and products exist in more than one phase
  • Kc of the reverse reaction is the same as 1/Kc of the forward reaction
    • Kc (reverse) = 1/Kc (forward)

Effect of conditions on equilibrium constant

Conditions Table



  • If there are 4 mols of gaseous reactants and 3 mols of gaseous products, an increase in pressure would result in forward reaction being favored to reduce number of mols overall


  • In exothermic reaction, heat is a product. Increase in heat would cause equilibrium position to move left to counter the imbalance caused by heat. A summary of all possibilities is in table below

Temperature on Equilibrium


  • Catalyst reduces activation energy
  • It increases rate of forward and reverse reaction by equal amount thus no change in Kc

Le Châtelier principle

  • Le Châtelier principle: If a change is made to a system that is in equilibrium, the balance between the forward and reverse reactions will shift to offset this change and return system to equilibrium
    • If concentration of reactant is increased, forward reaction will be favored to counteract this. Vice Versa
    • Value of Kc does not change

Reaction Quotient

Reaction Quotient

  • If system has not reached equilibrium yet, then
  • Q is for reaction quotient and helps determine the progress of reaction as it moves toward equilibrium

Reaction Quotient Kc


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