Chapter 18.1 – Heterogeneous Equilibria – Lecture Notes 

                              (equilibria is the plural of equilibrium)                                               Std 9


You will need to spend some time with these explanations to really understand this business.  Give yourself time to understand!


This chapter is trying to help you understand that a pure substance cannot be included in an equilibrium expression.  Equilibrium expressions require that we be able to give a concentration for a reactant or product.  So, you have to know:  (1)  what does concentration really mean, and (b)  what a pure substance is and how the chemist writes pure substances.  So let’s get organized and take this stuff one state at a time:


         GAS STATE ­- 

            Gases are always going to have some concentration, because their volume with change with pressure. We never consider them as a pure substance for purposes of writing Keq .  When we write a gas we always write it like these examples:   

H2O (g)  ,  N2 (g) ,  O2  (g) , NH3  (g)  ,  HCl  (g)



            Liquids are written in two different ways. 

(1)  Pure liquids have only one substance in them.  We can write them as shown in these examples with a lower case L in parentheses below and to the right:   H2O (l) ,  N2 (l) ,  O2 (l) ,  NH3 (l) , Br2 (l) ,  H2SO4 (l) .

(2)  Substances that are dissolved in water will be written differently.  They will always be able to be written with a concentration (or M).  Here are a few examples:  NH4OH (aq) ,  H2SO4 (aq) ,  Na2CO3 (aq) ,  HCl (aq) ,  NaCl  (g) .  Notice that  HCl  (g)  is very different from  HCl  (aq) .  Why?  The aqueous HCl is dissolved in water and has a concentration.  It also forms the H+ ion and the Cl- ion.  Ion formation can only happen in water and is therefore considered aqueous.  The ions will have a concentration because we can measure how many moles are in each liter of water.  Notice that H2SO4 (l)  and H2SO4 (aq) are very different.  H2SO4 (l) is pure H2SO4 and is not in water so it cannot have a concentration for the equilibrium constant.  However the H2SO4 (aq) can have a concentration in water and so is part of the equilibrium constant.



            Solids can only be written in one way.  They are shown as follows:   NaCl (s) ,  S8 (s) , KBr (s) , Mg (s) , Al2O3 (s) .  Notice that NaCl (s) means you have the white crystalline stuff from your salt shaker.  However, NaCl (aq) means you have salt water like from the ocean!  There is a concentration of the NaCl (aq)  in the water in which it is dissolved.  Before we put it in the water it was a solid, but once it is in the water, it is aqueous (dissolved in water). 


So, here is a summary of what kinds of things can be put into an equilibrium constant:




NOT included



(lower case L)



Pure liquids have not been dissolved in anything




Solids have not been dissolved in anything




It is a mixture in the air and will have so many moles per liter of gas




It is dissolved in water and will have so many moles per liter of solution



Let us define our two types of equilibria:


1.      homogeneous equilibria – all products and reactants are in the same state. 




                        N2 (g)    +  3 H2 (g)    ----->  2 NH3 (g)  


         Since all reactants and products are gas, all are included in the Keq  and the expression for the equilibrium constant becomes:



                        Keq  =  --------------

                                      [N2] [H2]3



2       heterogeneous equilibria – the reaction has products and/or reactants are in more than one state.. Since pure liquids (l) and pure solids (s) cannot be included in the Keq , the Keq will not include all products and reactants. 



                        2 KClO3 (s)    ------>   2 KCl (s)    +  3 O2 (g)               


         The reactant and product with the (s) subscripts (pure solids) cannot be included, so we substitute a 1 for each position where they would normally show up so the Keq  will look like this:


                                        [1]2 [O2]3

                        Keq  =  --------------  =   [O2]3



      You can see the since the concentrations of  pure liquids  (l)  and pure solids  (s)  are always 1, they have no affect on the equilibrium.  This is why the equilibrium expression is written:


                        Keq  =  [O2]3