Chemical Equilibrium

• Le Chatelier's Principle states that when an external influence is applied to a system, it will shift towards counteracting the effect of the change.
• If more product is added to a reaction, the equilibrium shifts back to produce more reactant.
• Adding more reactants shifts the equilibrium to the right, while adding more products shifts the equilibrium to the left.
• The equilibrium constant (K) can be calculated using the equation K = [product]/[reactant].
• A catalyst speeds up both the forward and backward reactions equally, so there is no net effect on the position of equilibrium.
• The equilibrium constant (K) can be calculated using the concentrations of reactants and products at equilibrium.
• Changing pH affects the position of equilibrium by changing the concentration of hydrogen ion (H+).
• Chemical equilibrium


  Balance where forward reaction rate = reverse reaction rate, but does not mean it's zero because there is no tendency for the reactants and products to change
• Chemical equilibrium


  • Occurs in gases and aqueous solutions
  • Occurs at the same time
  • Equilibrium achieved = constant
  • No net change
• Synthesis


  R → P
• Dissociation


  P → R
• Law of mass action

  Reverse reaction rate is proportional to the product of the concentrations of each reactant
• Law of mass action proposed by Cato Guldberg and Peter Waage

  March 15, 1864
• Equilibrium constant (Kc)


  Kc = [C]^c[D]^d / [A]^a[B]^b, where a, b, c, d are coefficients and A, B, C, D are concentrations of substances
• K > 1: product-favored, K < 1: reactant-favored
• Calculating Kc given equilibrium concentrations

  Find concentration of each qualified substance

  2. Substitute concentrations into Kc formula
• Calculating Kc using ICE method

  I = Initial Concentration

  C = Change

  E = Equilibrium Concentration

  1. Devise formula for C using coefficients

  2. Find x (unknown) by using substance that completes ICE table

  3. Substitute x into C row

  4. Find E = I + C

  5. Plug E values into Kc formula
• ICE method is used only if initial concentration is given, and no or incomplete equilibrium concentration is given
• Equilibrium constant formulas


  • SnO2(s) + 2CO(g) ⇌ Sn(s) + 2CO2(g), Kc = [CO2]^2 / [CO]^2

  CaCO3 (s) ⇌ CaO(s) + CO2 (g), Kc = [CO2]

  Zn(s) + Cu2+(aq) ⇌ SnO2(s) + Zn2+(aq), Kc = [Zn2+] / [Cu2+]