chemical equillibrium

• Equilibrium


  Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate
• Approach to equilibrium


  1. Forward and reverse reactions are occurring

  2. At equilibrium, forward and reverse reactions are proceeding at the same rate

  3. At equilibrium, the amount of each reactant and product remains constant
• Equilibrium reaction equation


  Represented with a double arrow (⇌)
• Forward reaction rate


  Rate = kf [reactant]
• Reverse reaction rate


  Rate = kr [product]2
• Equilibrium constant (Keq)


  Ratio of forward and reverse reaction rates at equilibrium
• Equilibrium constant (Kc)


  Equilibrium constant expressed in terms of concentrations
• Equilibrium constant (Kp)


  Equilibrium constant expressed in terms of partial pressures
• Equilibrium can be reached from either the forward or reverse direction
• Magnitude of equilibrium constant (K)

  • If K>>1, reaction favours products
  • If K<<1, reaction favours reactants
• Homogeneous equilibrium


  All reactants and products are in the same phase
• Heterogeneous equilibrium


  Something in the equilibrium is in a different phase
• Concentration of pure liquids and solids do not appear in Kc expression
• Numerical value of Kc is meaningful only when associated with a particular chemical equation
• Kc is temperature dependent
• Unit of Kc will vary depending on the reaction
• Reaction quotient (Q)


  Ratio of current concentrations, used to determine if a system is at equilibrium
• Comparing Q and K

  • If Q < K, reaction will proceed to products
  • If Q = K, system is at equilibrium
  • If Q > K, reaction will proceed to reactants
• Le Châtelier's Principle


  If a system at equilibrium is disturbed, it will shift to counteract the disturbance
• Change in reactant/product concentration


  Adding a component will be used up, removing a component will be produced
• Change in volume/pressure


  Higher volume/lower pressure favours side with more moles of gas
• Change in temperature

  • Endothermic: adding heat drives reaction to products
  • Exothermic: adding heat drives reaction to reactants
• Catalysts increase rate of forward and reverse reactions, but do not change equilibrium composition
• Arrhenius acid


  Substance that increases H+ concentration in water
• Arrhenius base


  Substance that increases OH- concentration in water
• Brønsted-Lowry acid


  Proton donor
• Brønsted-Lowry base


  Proton acceptor
• Water is amphiprotic, can act as both acid and base
• Conjugate acid-base pair


  Acid and base formed from each other by the loss/gain of a proton
• Stronger the acid, weaker its conjugate base
• Autoionisation of water


  Some water molecules act as acids and some as bases
• Ion product constant (Kw)


  Equilibrium constant for autoionisation of water, [H3O+][OH-] = 1.0 x 10-14 at 25°C
• Neutral, acidic, basic solutions


  • Neutral: [H3O+] = [OH-]
  • Acidic: [H3O+] > [OH-]
  • Basic: [H3O+] < [OH-]
• Bases below the line with H2O as an acid are strong bases; their conjugate acids do not act as acids in water
• The stronger the acid, the weaker is its conjugate base
• Autoionisation


  In pure water, a few molecules act as bases and a few act as acids
• Ion product (or water ionisation) constant for water, Kw


  • Equilibrium expression: Kw = [H3O+][OH–]
  • At 25 °C, Kw = 1.0 × 10–14
• Neutral solution


  [H3O+] = [OH–]
• Acidic solution


  [H3O+] > [OH–]
• Basic solution


  [H3O+] < [OH–]