Chemical equilibrium

• Reversible reactions


  Reactions that go forwards and backwards
• Reversible reaction progress


  1. Reactants used up quickly

  2. Concentration of reactants drops dramatically

  3. Products produced quickly

  4. Rate of forward reaction slows as reactants decrease

  5. Equilibrium reached when forward and backward rates are equal
• Dynamic equilibrium


  When the concentrations of reactants and products remain constant, even though the amounts are different
• Equilibrium only occurs in closed systems
• Le Chatelier's Principle


  If a reaction at equilibrium is subject to a change, the position of equilibrium will move to counteract the change
• Increasing concentration of a reactant or product


  Equilibrium shifts to reduce that concentration
• Increasing pressure


  Equilibrium shifts to the side with the fewest number of gas particles
• Increasing temperature


  Equilibrium shifts in the endothermic direction
• Catalysts have no effect on the position of equilibrium, they only speed up the rate at which equilibrium is established
• Making ethanol


  • Reaction is exothermic going forward
  • Conditions: 60 atm pressure, 300°C, phosphoric acid catalyst
  • Lower temperature increases yield but decreases rate, so a compromise is reached
• Equilibrium constant (Kc)


  A measure of the relative concentrations of reactants and products at equilibrium
• Pressure


  How fast we produce it
• High pressure


  Equilibrium shifts to the right
• Increasing pressure


  Increases the rate of successful collisions
• High pressurized equipment is expensive, requiring thicker and more robust vessels and pipes
• The compromise is 60 atmospheres of pressure, balancing yield, speed and cost
• Equilibrium constant (KC)


  Calculated from the molar concentrations of products and reactants in a reaction
• Calculating KC


  1. Products over reactants

  2. Concentrations in square brackets

  3. Exponents match mole values
• KC is only valid for one temperature, as changing temperature shifts the equilibrium
• Exothermic forward reaction


  Increasing temperature shifts equilibrium backwards, decreasing KC
• Endothermic forward reaction


  Increasing temperature shifts equilibrium forwards, increasing KC