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 quadratic equation