Chapter 15

Chapter 15: Chemical Equilibrium

Dynamic Equilibrium

• All reactions are reversible
• Reaches equilibrium when the concentration of R and P are no longer changing; Rate F = Rate R
  • Reactions are still occurring

Equilibrium constant ($k$)

• Never has units
• k = products over reactants
• Pure liquids and pure solids are NOT included in equilibrium expression
• $k_c$ when using concentration
• Significance of k
  • When k > 1, more products than reactants
  • When k < 1, more reactants than products
  • When k = 1, approximately equal R + P
• Reaction shifts with k
  • k is for reaction forward and 1/k is for the reaction reversed
  • If reaction coefficients are multiplied or divided by a common factor, k gets raised to that power

K in terms of pressure ($k_p$)

• $P_A V = n_A RT$

Heterogeneous Equilibrium

If only one species in equilibrium is aqueous or a gas, the k value is based on just that species

Calculating k from equilibrium concentrations

• ICE tables
• Initial, Change, Equilibria

Predicting the direction of a reaction

• Q = products over reactants
  • It is the concentrations at their initial value
  • If Q is less than k, form products, reaction shifts to the right
  • If Q is greater than k, form reactants, reaction shifts to the left

Finding equilibrium concentration

• Solve for 1 unknown
• From only initial concentrations
• Steps
  1. Write the equation and k
  2. Q vs k
  3. ICE table
     • Use x for the change
     • Direction
     • Stoichiometry
  4. Use the equilibrium values from the table to solve for x
     • May have to use the quadratic equation
       • Discard the negative answer
       • If you get two positive numbers, check your answer using both to find the correct one
  5. Calculate the equilibrium concentration and check k
• Assume "small x"
  • The "change" is terribly small
  • If k is already small, assume that x is small
  • the ratio of x to the initial value must be less than 5%

Le Chatelier's Principle

• When a stress is put on the system at equilibrium, the system will respond to lessen that stress
• The effect of concentration changes
  • Add reactant
    • Make products
  • Add products
    • Make reactant
  • Remove reactant
    • Make reactant
  • Remove products
    • Make products
• The effect of pressure and volume changes
  • Increase pressure due to a decrease in volume
    • Shift to the side with the fewer moles of gas
  • Decrease in pressure due to an increase in volume
    • Shift to the side with more moles of gas
  • Increase pressure (w/ an inert gas) at a constant volume
    • No effect
  • Decrease pressure (w/ an inert gas) at a constant volume
    • No effect
• The effect of temperature changes: change k 
  • Exothermic reaction
    • If the temperature increases, shift left and make reactants
      • Decreases k
    • If the temperature decreases, shift right and make products
      • Increases k
  • Endothermic reaction
    • If the temperature increases, shift right and make products
      • Increases k
    • If the temperature decreases, shift left and make reactants
      • Decreases k