1.6 equilibria

• Reversible reactions


  Reactions where the forward and backward reactions occur simultaneously, represented by a double-headed arrow
• Equilibrium


  1. Reactants used up quickly initially

  2. Reactant concentration decreases over time

  3. Product concentration increases over time

  4. Equilibrium reached when forward and backward reaction rates are equal

  5. Concentrations of reactants and products remain constant
• Equilibrium only occurs in closed systems
• Equilibrium constant (Kc)


  Ratio of product concentrations to reactant concentrations, raised to the power of their stoichiometric coefficients
• Changing conditions and Le Chatelier's Principle

  1. Increase in reactant/product concentration shifts equilibrium to oppose change

  2. Increase in pressure shifts equilibrium to side with fewer gas molecules

  3. Increase in temperature shifts equilibrium in endothermic direction
• Catalysts have no effect on the position of equilibrium, only the rate of reaching equilibrium
• Ethanol production


  1. Exothermic reaction

  2. High pressure increases yield but is expensive

  3. High temperature increases rate but decreases yield

  4. Compromise between yield and rate
• The equilibrium constant (Kc) is calculated from the equilibrium concentrations of reactants and products, raised to their stoichiometric coefficients
• s of C and D reactants a and B look at the numbers the molar numbers here to a this turns into a power because you can see there look at a squared so the number of moles in front of it is now a power in a KC expression
• Concentration


  Concentration is very important, it must be used, we can't use moles
• Writing the KC expression

  1. Write the products and reactants

  2. Square brackets mean concentration

  3. Put the numbers as powers
• The calculation is easy, just put the numbers in the KC expression
• Units


  Put the units on the top and bottom, cancel out the units to get the final unit
• Calculating equilibrium concentrations before KC

  1. Use an ICE table

  2. Look at the change in moles

  3. Use the equilibrium moles to calculate concentrations
• Rearranging the KC expression to solve for unknown concentrations
• Factors affecting KC


  • Temperature - KC changes with temperature
  • Concentration - KC is unaffected by changes in concentration
  • Catalyst - KC is unaffected by adding a catalyst
• If temperature increase causes equilibrium to shift right, KC increases. If temperature increase causes equilibrium to shift left, KC decreases.
• Catalysts speed up the forward and reverse reactions but do not affect the position of equilibrium or the value of KC.