Equilibrium

• Reversible Reactions


  A reversible reaction is one where reactants can change into products and the products can change back into the reactants
• Reversible Reaction and Equilibrium

  • The reactants start with the reactants and as the products form, some react to form reactants again
  • Equilibrium is achieved when the amount of reactants and products remain constant
  • An equilibrium is described as dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction = amount of reactants and products remain constant
  • Where only the reactants and products are present, it is a closed system. Heat energy can be exchanged and external factors such as pressure can change
• Understanding dynamic equilibrium example

  • 100 year 11 pupils (a) and 100 year 12 pupils (b) moving randomly around an assembly hall which is the closed system
  • When they collide with enough energy, they may pair up to form C (Pair). This is a reaction
  • Not all collisions may result in a reaction if there isn’t enough energy
  • If a pair collides with another they may spilt up again.
  • Overtime the number of pairs (c) and singles (a and b) remain constant as the rate of formation is equal to rate of break ups
  • Initially only singles and at equilibrium there is a mixture of pairs and singles
• Dynamic Equilibrium Reactions


  A (gas) + B (gas) <-> C (gas)
• Dynamic Equilibrium Reactions


  1. A reacts with B and form C

  2. C breaks down to form A and B
• A and B move around randomly. When they collide they may react if they collide with sufficient energy to form C. C can break up to release one A and B
• Reaction starts with A and B only present and as the reaction starts, some C appear
• Equilibrium


  The number of A,B and C remain constant
• A,B and C present aren't always the same ones even though the number remains the same. They are constantly forming and breaking up
• Dynamic equilibrium


  A constant amount of the reactants and products and the rates of forward reaction and reverse reaction are the same
• Example Question 


  Hydrogen reacts with iodine to form hydrogen iodide in a reversible reaction. Write an equation for the reversible reaction

  • H2 + I2 <-> 2HI
• Features of a reversible reaction

  • Shown with a reverse arrow
  • direction of reversible reaction can be changed by altering the reaction conditions such as temperature and pressure