ES 3

• Graph representation of reversible reactions

  Reactants being used up initially, then flattening out over time due to decreased collision chances
• Reversible reactions

  Reactions go forward and backward, represented by a double harpoon symbol
• The slides used in the video are available for purchase
• Backward reactions

  Initially slow due to few products, then speeds up as product concentration increases
• Kc expression

  Products on top, reactants on the bottom, with matching powers for molar values
• Dynamic equilibrium is reached in closed systems where reactants or products cannot escape
• Reversible reactions go forward and backward, represented by a double harpoon symbol
• Equilibria is the topic of this video for revision
• Generic example of reversible reaction


  • A and B reacting to form C and D, and C and E forming A and B
• Dynamic equilibrium

  Rate of forward reaction equals rate of backward reaction, concentration of each substance remains constant
• Equilibrium can be quantified using Kc, based on molar concentrations in a reaction
• Graph representation of reversible reactions

  Products being made quickly initially, then flattening out as reactants are used up
• If KC is much greater than 1, equilibrium lies well over to the right (more products than reactants)
• Temperature affects the value of KC
• Calculating KC value

  For the given reaction, KC expression is [SO3]^2 / [O2][SO2], plug in the concentrations and calculate the value
• KC expression

  Products on the top, reactants on the bottom, concentration in moles per decimetre cubed
• If temperature increases, equilibrium shifts to the endothermic direction and KC decreases
• Example of calculating KC value

  • Concentration of SO2 is 0.4 mol/dm^3, concentration of O2 is 0.2 mol/dm^3, concentration of SO3 is 0.8 mol/dm^3
• For an exothermic forward reaction, the reverse reaction is endothermic
• If temperature decreases, equilibrium shifts to the exothermic direction and KC increases
• KC value is 20 in the provided example
• If KC is just a bit bigger than 1

  Equilibrium lies slightly to the right
• If KC is less than 1


  Equilibrium lies to the left
• If KC is bang-on 1

  Equilibrium is in the middle, neither to the left nor to the right
• If temperature is decreased

  Equilibrium shifts in the exothermic direction
• KC


  Value that indicates the position of equilibrium
• If pressure is increased

  Equilibrium shifts to the side with the fewest number of gas particles
• If temperature is increased

  Equilibrium shifts in the endothermic direction
• If KC is much greater than 1

  Equilibrium lies well over to the right, indicating more products than reactants
• If KC is well less than 1


  Equilibrium lies well over to the left
• The value of KC is unaffected by changes in concentration
• If pressure is decreased


  Equilibrium shifts to the side with the most number of gas particles