CHEMICAL EQUILIBRIA

• Reversible reactions


  Also known as equilibrium reactions, where both forward and reverse reactions occur simultaneously
• Irreversible reactions


  Reactions that proceed to completion, represented by a unidirectional arrow
• Chemical equilibrium


  State where the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant
• Le Chatelier's principle


  • If an external stress is applied to a system at equilibrium, the system adjusts to partially offset the stress as it reaches a new equilibrium position
• Factors affecting equilibrium


  • Changes in concentration
  • Changes in temperature
  • Changes in volume
  • Changes in pressure
  • Presence of a catalyst
• Effect of increasing concentration of reactant

  1. System favors forward reaction to reduce reactant concentration

  2. Rate of forward reaction is greater than reverse reaction until new equilibrium is reached
• Effect of removing product

  1. System favors forward reaction to replenish removed product

  2. Reaction shifts to the right until new equilibrium is reached
• Solids and liquids do not affect equilibrium position
• Increase in pressure


  Decrease in volume (for gaseous systems)
• Effect of increasing pressure on N2O4 ⇌ 2NO2 reaction

  1. Equilibrium shifts towards formation of fewer moles of gas (N2O4)

  2. Color changes from brown (NO2) to nearly colorless (N2O4)
• Temperature only affects exothermic and endothermic reactions
• N2O4


  • Greater number of moles than N2O4
  • Equilibrium shifts to the right during expansion
  • Initially nearly colorless gas that turns darker brown due to the equilibrium shift favoring the formation of NO2
• Compression


  • Equilibrium reaction shifts toward the formation of gas with fewer moles
  • Equilibrium will shift towards the formation of colorless N2O4, and you will see it visually in the syringe as a change in color from brown to nearly colorless
• Exothermic reaction


  Heat released is treated as if it is a product of the chemical reaction
• Endothermic reaction


  Heat absorbed is treated as if it is a reactant
• Reaction of N2O4 and NO2 with heat

  heat + N2O4(g) ⇌ 2NO2(g)
• Increasing temperature


  Reaction shifts to the right, favoring the formation of brown NO2 gas
• Decreasing temperature


  Reaction shifts to the left, favoring the formation of colorless N2O4 gas
• Effect of temperature on equilibrium

  • -78.4°C: Equilibrium shifts towards colorless N2O4 gas
  • 20°C: Equilibrium shifts towards brown NO2 gas
  • -9.3°C: Appreciable concentrations of both N2O4 and NO2 exist
• Catalysts increase the rate of both the forward and reverse reactions in an equilibrium reaction, but do not alter the equilibrium constant or shift the equilibrium
• Equilibrium constant expression (Keq)

  Product of the equilibrium concentration (or partial pressure for gaseous species) of the products raised to their coefficients, over the product of the equilibrium concentration (or partial pressure for gaseous species) of the reactants raised to their coefficients
• Equilibrium constant expressions


  • S(s) + O2(g) ⇌ SO3(g): Keq = [SO3]

  NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq): Keq = [NH4+][OH-] / [NH3]

  H2O(l) ⇌ H3O+(aq) + OH-(aq): Keq = [H3O+][OH-]