The Reaction Quotient
- The reaction quotient, Q, is the ratio of products and reactants in a system at any point in the reaction
- The equilibrium constant (K) the ratio of products and reactants in a system at equilibrium
- Q and K are defined by an expression that relates the products’ and reactants’ concentrations (c) or partial pressures (P)
- Consider the generic equation:
aA + bB ⇌ cC + dD
Where a, b, c, and d represent stoichiometric coefficients and A, B, C, and D represent chemical species
- The expressions for Qc and Kc are:
[A] indicates the concentration of species A in mol/L
- The expressions for QP and KP are:
PA indicates the partial pressure of species A in atm
- Solids and pure liquids are omitted from all of these expressions as their concentrations and partial pressures do not depend on the amount of substance present
- As a reversible reaction proceeds towards equilibrium, the value of Q approaches K such that at equilibrium Q = K
Worked example
2ZnS (s) + 3O2 (g) ⇌ 2ZnO (s) + 2SO2 (g)
Write the equilibrium expression for the equilibrium constant, Kc, for the reaction above?
Answer:
- For the generic equation
aA + bB ⇌ cC + dD
The equilibrium expression for Kc is
- Solids are not included in an equilibrium expression as their concentrations are constant
- So, ZnS(s), ZnO(s), and their corresponding stoichiometric coefficients will not be written in the equilibrium expression
- Therefore the equilibrium expression for Kc becomes
- When solving for Q we substitute in values for concentration or partial pressure any point during the reaction, but when solving for K we substitute values for concentration or partial pressure for the system at equilibrium
- Both Q and K are unitless values
- The value of K is the same for a given reversible reaction regardless of the initial amounts of each species but it is temperature dependant and will vary with changes in temperature
Exam Tip
Remember, an equilibrium constant only expresses the state of a system at equilibrium.
When writing a Kc or Qc expression, always be sure to include the brackets, as brackets around a species represent the concentration of that species. However, when writing a KP or QP expression never use brackets as this expression involves partial pressures not concentrations.
Note that the AP Chemistry exam equation and constant sheet only includes the equations for Kc and KP. We need to remember that the equations for Qc and QP follow the same format.