Reaction Quotient and Equilibrium Constant
- For
the Equilibrium Constant Expression relates the concentrations of reactants and products once the reaction has reached equilibrium.- Equilibrium Constant Expression:
(on RFS)- In some tests K is written as Kc
- Equilibrium Constant Expression:
- 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 determine in which direction a system will shift to reach equilibrium, we compare the values of Q and K. There are three possible cases
- Q = K. The system is at equilibrium; no shift will occur.
- Q > K → too many products, will shift to consume products and produce more reactants (shift reverse reaction/left)
- 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)