The Reaction Quotient (College Board AP® Chemistry)

2ZnS (s) + 3O₂ (g) ⇌ 2ZnO (s) + 2SO₂ (g)

Write the equilibrium expression for the equilibrium constant, K_c, for the reaction above.

Answer:

• For the generic equation

aA + bB ⇌ cC + dD

The equilibrium expression for K_c 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 K_c 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

Remember, an equilibrium constant only expresses the state of a system at equilibrium.

When writing a K_c or Q_c expression, always be sure to include the brackets, as brackets around a species represent the concentration of that species. However, when writing a K_P or Q_P 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 K_c and K_P. We need to remember that the equations for Q_c and Q_P follow the same format.