• For the Equilibrium Constant Expression relates the concentrations of reactants and products once the reaction has reached equilibrium.  ○ Equilibrium Constant Expression:

○ In questions, the equilibrium constant expression can be written as Kc, or Ka/Kb (for acids and bases)

• If asked to find Kc expression only include gasses and (aq), NOT solids (s) or liquids (l)

Equilibrium Expressions with Pressure

• For reactions that occur all in the gas phase, the equilibrium expression can be written in terms of partial pressures

○ → Only include gases in the Kp expression

Reaction Quotient: Q

• Calculated the same as the equilibrium constant, but for a system not at equilibrium (usually initial conditions)
• Ex:

○ The subscript zeros indicate initial concentrations

• To justify which direction the system will shift to reach equilibrium, compare Q to K
1. Q = K. The system is at equilibrium; no shift will occur.
2. Q > K → too many products, will shift to consume products and produce more reactants (shift reverse reaction/left)
1. Q < K → not enough products; will shift to consume reactants and produce more products (shift forward reaction/right )
• Do not say shift right/left on AP exam (won’t receive credit)