Reversible reactions and dynamic equilibrium

• Reversible reactions


  Reactions where the products can react to produce the original reactants
• Reversible reactions


  Reaction can be changed by changing conditions (e.g. hot for forwards, cool for reverse)
• Reversible reaction example(the haber process)


  • Hydrogen + nitrogen ⇌ ammonia
• Reversible reaction is endothermic one way


  It is exothermic in the opposite direction
• The same amount of energy is transferred each way in a reversible reaction
• Equilibrium


  When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction
• Effect of changing conditions on equilibrium

  • The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction
  • If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change (Le Chatelier's principle)
• Concentration of reactants is increased


  Position of equilibrium shifts towards products so more product is produced until equilibrium is reached again
• Concentration of products is increased


  Position of equilibrium shifts towards reactants so more reactant is produced until equilibrium is reached again
• Temperature is increased


  Equilibrium moves in the direction of the endothermic reaction
• Temperature is decreased


  Equilibrium moves in the direction of the exothermic reaction
• Effect of temperature on yield for exothermic and endothermic reactions

  • Exothermic: Increase in temperature decreases yield, Decrease in temperature increases yield
  • Endothermic: Increase in temperature increases yield, Decrease in temperature decreases yield
• Pressure is increased in a gaseous reaction


  Equilibrium shifts to the side of the equation with fewer moles of gas
• Pressure is decreased in a gaseous reaction


  Equilibrium shifts to the side of the equation with more moles of gas
• Effect of pressure on yield for reactions with larger or smaller volume of gas

  • Larger volume of gas: Increase in pressure decreases yield, Decrease in pressure increases yield
  • Smaller volume of gas: Increase in pressure increases yield, Decrease in pressure decreases yield