Using an ICE table to determine equilibrium concentrations

The reaction quotient expression is used to calculate the acidity constant K_a.  The reaction of the acid with water defines the form of the reaction quotient.

acid + H₂O conjugate base + H₃O⁺
 

Ka =	[H3O+][conjugate base]
	[acid]
reaction quotient

For a particular acid-base conjugate pair, irrespective of the initial concentration of the acid and the conjugate base, at equilibrium [acid], [conjugate base] and [H₃O⁺] combine to give the same number.
 

	reactant	product 1	product 2
initial (before reaction)	c(reac)	c(pro1)	c(pro2)
change (due to reaction)	– change	+ change	+ change
equilibrium (after reaction)	c(reac) – change	c(pro1) + change	c(pro2) + change

 

• known initial concentrations.
   
• the pH (used to calculate [H₃O⁺]).
   
• the reaction that occurs to reach equilibrium.
  Deduce the reaction to reach equilibrium in solutions where a weak acid or base is dissolved from the pH.
  If the pH is less than 7, the predominant reaction occurring to reach equilibrium is:
  weak acid + H₂O conjugate base + H₃O⁺
  If the pH is greater than 7, the predominant reaction occurring to reaction equilibrium is:
  weak base + H₂O  conjugate acid + OH^–
   
  • The change in concentration of one component is calculated from the initial and equilibrium concentrations.
  • The magnitude of the change in other components is the same if all coefficients in the balanced equation for the reaction occurring to reach equilibrium are 1.
  • Changes are positive if the species is a product in the reaction to reach equilibrium.
  • Changes are negative if the species is a reactant in the reaction to reach equilibrium.
     
• Equilibrium concentrations = initial + change (change = equilibrium – initial).