Kinetics and Equilibrium

• Collision Theory


  • Particles must collide with each other
  • Particles must collide with sufficient energy
  • Particles must collide with the correct orientation of molecular geometry
• Reaction rate


  Affected by a change in the number of effective collisions
• Factors affecting reaction rate

  • Concentration of reactants
  • Temperature
  • Surface area
  • Pressure
  • Volume
• Higher concentrations of reactants

  • More particles are present
  • More collisions
  • Reaction is faster
• Increasing temperature


  • More energy
  • More collisions
  • Reaction is faster
• Increasing surface area


  • More area for contact
  • More collisions
  • Reaction is faster
• Catalyst


  • Helps to increase the number of effective collisions
  • Lowers the activation energy

  
• Reversible Reaction


  • Can go forward and backward
  • Shown with a reversible yield sign
• Types of Reversible Reactions

  • Reversible Reaction - Products can react together to form reactants
  • Complete Reaction - Solid is formed and precipitates out of solution
  • Complete Reaction - Gas is formed and escapes from solution
  • Dynamic Equilibrium - Particles in constant motion, reactants collide to form products, products collide to form reactants, reactants and products are produced at equal rates
• Equilibrium


  A reaction is at equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction
• Equilibrium Constant (Keq)


  1. "constant", eq: "equilibrium", Used for a reaction that is at equilibrium, Shows which are favored; reactants or products

  
• Equilibrium Constant (Keq) values

  • If Keq > 1, products are favored
  • If Keq < 1, reactants are favored
  • If Keq = 1, neither is favored
• How to write a Keq expression

  1. Write a balanced equation

  2. Cross out the pure solids and liquids, their value in the constant = 1

  3. Write the formulas of products (in [ ]'s) over formulas of reactants (in [ ]'s)

  4. Write the coefficients as exponents
• Formula in [ ]'s means molarity of the substance
• Keq is Temperature Specific, if the temperature changes, then the Keq changes
• K' or Keq'


  When the original equilibrium equation is changed by reversing the equation or by changing the coefficients, the K value changes to K'
• Reaction Quotient (Q)


  Calculated the same as Keq, for concentration values which are not necessarily at equilibrium

  
• Relationship between Q and K

  • When Q = K, the system is at equilibrium
  • When Q < K, the system shifts to make more products
  • When Q > K, the system shifts to make more reactants
• Le Chatelier's Principle


  Whenever a stress is placed on a reaction at equilibrium, the reaction will shift to restore equilibrium by relieving that stress
• Factors that stress a system

  • Change in concentration
  • Change in temperature
  • Change in pressure from a change in volume affects gases
• Stress: Concentration


  System shifts toward the lower concentration
• Stress: Temperature


  • Endothermic - Increasing temperature shifts toward the product
  • Exothermic - Increasing temperature shifts toward the reactants
• Stress: Pressure change from a volume change


  • Increase in pressure shifts toward the side with less moles of gas
  • Decrease in pressure shifts toward the side with more moles of gas
• Addition of a catalyst causes no shift because it affects both sides equally
• Increased pressure due to addition of inert gas causes no shift
• To solve Le Chatelier's problems

  1. Identify the stress

  2. Determine the shift (toward decreases, away from increases)

  3. Use the shift to determine the effects