1 CHEMICAL EQUILIBRIUM

• Equilibrium is the term to what we might call a ""balance of forces"
• Chemical Equilibrium occurs when a reaction and its reverse reaction proceed at the same rate
• "synthesis" means creating
• "dissociation" means decomposing (from compounds to elements)
• Molarity is the common concentration
• Only gases and aqueous are included in chemcal equilibrium expression
• 2 ASPECTS OF THE LAW OF MASS ACTION

  1. The equilibrium aspect, concerning thecomposition of a reaction mixture atequilibrium
  2. The kinetic aspect concerning the rateequations for elementary reactions
• K>>1 means that the reaction is product-favored; product predominates at equilibrium
• K<<1 means that the reaction is reactant-favored; reactant predominates at equilibrium
• Equilibrium


  Balance of forces
• Chemical equilibrium

  • Reaction and its reverse reaction proceed at the same rate
  • Not always reactants to products, but it can also be products to reactants
  • Where it will go depends on the different forces, different factors affecting the equilibrium
  • Where your arrow is pointing at, that is your product
  • Wherever your arrow is coming from, that is your reactant

  
• A chemical reaction is in equilibrium when there is no tendency for the quantities of reactants and products to change
• The direction in which we write a chemical reaction is arbitrary
• Dissociation

  Decomposing (from compounds to elements)
• As a system approaches equilibrium, both the forward and reverse reactions are occurring
• At equilibrium, the forward and reverse reactions are proceeding at the same rate
• Once equilibrium is achieved, the amount of each reactant and product remains constant
• Equilibrium equation


  Depicted with a double arrow
• Chemical equilibrium

  • The state in which the chemical activities or concentrations of the reactants and products have no net change over time
  • Usually, this would be the state that results when the forward chemical process proceeds at the same rate as their reverse reaction
  • The reaction rates of the forward and reverse reactions are generally not zero but, being equal, there are no net changes in any of the reactant or product concentrations. This process is called dynamic equilibrium
• Law of Mass Action


  • Concerning the composition of a reaction mixture at equilibrium
  • Concerning the rate equations for elementary reactions
• Equilibrium constant (K)


  The ratio of the rate constants is a constant at that temperature
• Specific names of Keq

  • Gas-phase reactions that use units of partial pressure: Kp
  • Dissociation of water: dissociation constant of water: Kw
  • Dissociation of acids: acid dissociation constant: Ka
  • Reaction fo bases with water: base dissociation constant: Kb
  • Solubility of precipitates: solubility product: Ksp
• Meaning of K value

  • If K >> 1, the reaction is product-favored; product predominates at equilibrium
  • If K << 1, the reaction is reactant-favored; reactant predominates at equilibrium
• Calculating K from known equilibrium amounts

  1. Write the equilibrium expression

  2. Convert equilibrium amounts to molarity

  3. Substitute concentrations into equilibrium expression and calculate K

  
• Calculating K from initial amounts and one known equilibrium amount

  1. Write the equilibrium expression

  2. Use stoichiometry to determine equilibrium concentrations

  3. Substitute concentrations into equilibrium expression and calculate K

  
• Reaction quotient (Q) is a quantitative measure of the extent of reaction, the relative proportion of products and reactants present in the reaction mixture at some instant of time
• Le Chatelier's Principle


  If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance

  
• Effect of concentration

  • When the concentration of a product is increased, the reaction proceeds in reverse to decrease the concentration of the products
  • When the concentration of a reactant is increased, the reaction proceeds forward to decrease the concentration of reactants
• Effect of pressure

  • In reactions where gases are produced and increase in pressure will force the reaction to move to the left (in reverse)
  • If pressure is decrease, the reaction will proceed forward to increase pressure
• Effect of temperature

  • When the temperature increases, Le Chatelier's principle says the reaction will proceed in such a way as to counteract this change, i.e. lower the temperature
  • Therefore, endothermic reactions will move forward, and exothermic reactions will move backwards (thus becoming endothermic)
• A catalyst does not effect either Kc or the position of equilibrium, it only effects the rate of reaction
• Catalyst

  Makes the equilibrium faster, but the equilibrium composition remains unaltered
• LAW OF MASS ACTION

  Stem from research by Cato Guldberg and Peter Waage (1864-1879)