CChieh@UWaterloo.ca

Equilibrium Constants and Chemical Equations

Skills to develop

  • Write the expression for the equilibrium constant K for a given chemical reaction equiation.
  • Express the equilibrium constant for a chemical equation in terms of equilibrium constant(s) for one or more related equations.
  • Calculate the equilibrium constant for a chemical equation from equilibrium constant(s) for one or more related equations.
  • Equilibrium Constants and Chemical Equations

    The equilibrium constnat depends on the way we write the chemical reaction equation. When we change the equation, we need to change the value of the equalibrium for the new equation.

    Law of Mass Action, a review

    The law of mass action gives an equilibrium constant expression for any chemical reaction equation. The law suggests that for a general chemical reaction equation between a moles of A and b moles of B producing c moles of C and d moles of D,

    a A + b B --> c C + d D

    where

    A, B - represent reactants
    C, D - represent products
    a, b, c, d - are the coefficients for a balanced chemical equation.

    If the system is at equilibrium at a given temperature, then the following ratio is a constant.

           [C]c    [D]d
        ----------------  =  Keq
           [A]a    [B]b
    
    When a mixture of A, B, C, D is enclosed in a closed system, the law of mass action suggests that the system try to maitain a state such that K is a constant.

    You can write the equilibrium constant (K) expression for any reaction equation according to the las of mass action. This has been done in the module on the Law of Mass Action.

    The following rules are obvious for manipulating the K's when you change the equations.

    Rule 1

    Rule 2.

    Rule 3

    The reasons for the above rules is the relationship between the K and the enthalpy change, dH, of a reaction equation. This relationship is expressed by the Van't Hoff Equation,
                   dH
        ln K  = - ---- + B
                   R T
    
    where B is a constant. For example if you double the number of moles of the reactants and products, the value for dH doubles, resulting in the new equilibrium constant having its value squared. The reasons behind the other rules follow similar arguments.

    Summary of Skills

    Confidence Building Questions

    cchieh@uwaterloo.ca