Equilibrium

• Dynamic Equilibrium 


  The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction
• Le Chatelier's Principle

  When a system in dynamic equilibrium is subject to change, the equilibrium position will shift to minimise the change
• The Equilibrium Law


  When a system is at equilibrium, the product of the concentrations of the products raised to the power of their stochiometric coefficients divided by the product of the concentrations of the reactants raised to the power of their stoichiometric coefficients is a constant at constant temperature
• K꜀

  The Equilibrium constant in terms of concentration at a constant pressure
• Kₚ

  The equilibrium constant in terms of partial pressure at a constant temperature
• Partial pressure


  The pressure that would be exerted by a gas in a mixture of gases if it occupied the same volume on its own at the same temperature
• Mole fraction


  The amount in moles of a component in a mixture divided by the total amount of all components in moles in the mixture
• Homologous equilibrium 


  Reactants and products are present in the same phase. Examples include reactions where everything is a gas, or everything is present in the same solution
• Heterogenous equilibrium


  Reactants and products are present in more than one phase. Usual examples include reactions involving solids and gases, or solids and liquids
• ⇌

  Represents a reversible reaction
• Factors that may alter the position of equilibrium


  • Concentration (of a solution in equilibrium)
  • Pressure (of gasses in equilibrium)
  • Temeprature
• Explain why a system at equilibrium has a constant colour


  • At equilibrium, rate of forward reaction = rate of reverse reaction
  • ∴ conc of reactants and products remains constant
• A + B ⇌ C + D

  Describe and explain the effect of increasing the conc of [A] on the position of equilibrium

  • Equilibrium position will shift to the right
  • To decrease the conc of [A]
  • Amount of C & D will increase (inc yield)
• A + B ⇌ C + D

  Describe and explain the effect of decreasing [A] on the position of equilibrium

  • Equilibrium will shift to the left
  • To inc the conc of A
  • Amount of A & B willl increase (lower yield)
• Consider 2A + 3B ⇌ C + 2D

  If the pressure was increased

  • Equilibrium position will shift to the right
  • To decrease the pressure
  • Because there are fewer moles of gas on the right (product) side
  • Amount of C + D will increase (inc yield)
• Consider 2A + 3B ⇌ C + 2D

  If the pressure was decreased

  • Equilibrium position will shift to the left
  • To increase the pressure
  • Because there are fewer moles of gas on the left (reactant) side
  • Amount of A + B will increase (dec yield)
• The effect of increasing the temperature on equilibrium

  By heating up the system, the heat energy in the system is increased. The reaction will shift to favour the endothermic reaction (ΔH is +ve), allowing the system to take heat energy in and minimise the increase in temperature
• Consider N₂ + O₂ ⇌ 2NO ΔH = +180

  The above mixture is cooled down. State and explain the effect on the equilibrium position

  • Equilibrium will shift to the left
  • To release heat energy
  • Because the reverse reaction is exothermic
• State and explain the effect on equilibrium position of adding a catalyst to a system in equilibrium

  • The equilibrium position does not change
  • Rate of forward and reverse reactions increase by equal amounts
• The time taken to reach equilibrium was measured. The experiment was repeated with a catalyst. State and explain the effect on time taken to establish equilibrium

  • The time taken to reach equilibrium is reduced
  • Catalyst provides alternate energy pathway with lower activation energy
  • Greater proportion of particles have energy above the new lower activation energy
  • Greater number of successful collisions per unit time
  • Increases the rate of reaction
• Other than increased ROR, state an advantage to using a catalyst in a chemical reaction

  • Reaction take place at lower temperatures with lower energy demand so less carbon dioxide is emitted into the atmosphere (fossil fuels last longer)
  • Different reactions can be used with better atom economy and less waste
  • Catalysts can be re-used
• If the reactants and products are in the same physical state, the equilibrium mixture is homogenous, and all reactants and products are put into the same expression for the equilibrium constant
• If the reactants and products in a mixture are in different physical states, the reaction is said to be heterogenous. In a heterogenous mixture only gases or aqueous substances go into the expression for the equilibrium constant (Any solids or liquids are left out, their concentrations are considered to be constant)
• $K꜀ =$$\frac{[products]ᴾ}{[reactants]ᴿ}$