Equations Describing Solution Chemistry

Going Into Solution

If an ionic compound dissolves, simply write the ions that comprise the compound.  (This sometimes is called a dissociation equation.)

Example: aluminum nitrate

Al(NO₃)_3(s) --H₂O-->  Al⁺³_(aq) + 3NO₃^-_(aq)

To represent how a covalent compound could dissolve as a whole molecule, we could show the following:

Example:  sucrose dissolving

                    C₁₂H₂₂O_11(s) --H₂O--> C₁₂H₂₂O_11(aq)  

To represent how a covalent compound could break into ions, we will represent it with an ionization equation. Notice we need to use the solvent explicitly and typically we will end up forming a hydronium ion (an ion we usually describe as a hydrogen ion) .

Example:  hydrogen chloride ionizing

HCl + H₂O -->  H₃O⁺_(aq)    +  Cl^-_(aq)

Explaining Phenomenon

Precipitation
To explain what happens when solids precipitate out of solution, we will write ionic equations.  These equations portray how the solute looks in solution, before the reaction and after.

Example:  

Word equation:
silver nitrate (aq) + sodium chloride(aq) -->  silver chloride(s) + sodium nitrate(aq)

Formula equation:

 AgNO_3(aq) + NaCl_(aq) -->  AgCl_(s) + NaNO_3(aq)  

Ionic equation:
Ag⁺_(aq) + NO₃^-_(aq) + Na⁺_(aq) + Cl^-_(aq) --> AgCl_(s) + Na⁺_(aq) + NO₃^-_(aq)

The net ionic equation simply takes the ionic equation and lines out those ions that are unchanged on both sides of the equation, the so-called spectator ions.  Rewriting the ionic equation without the spectator ions leaves the so called net ionic equation.  This equation often describes the kinds and numbers of ions that come together to form a precipitate.  

Ag⁺_(aq) + NO₃^-_(aq) + Na⁺_(aq) + Cl^-_(aq) --> AgCl_(s) + Na⁺_(aq) + NO₃^-_(aq)

 

    The net ionic equation:  

Ag⁺_(aq) +  Cl^-_(aq) --> AgCl_(s) 

Conductivity
Since we will be associating conductivity with the presence of ions, if we see predict that a solution will have many ions by writing an equation, we will be able to predict it is a good conducting solution.  If we see that ions are disappearing, we will be able to predict that the solution is a poor conducting solution.

Example:  

Ba(0H)_2(aq) + H₂SO_4(aq) -->  BaSO_4(s) + 2H₂O_(l)

Ba⁺²_(aq) + 2OH^-_(aq) + 2H⁺+ SO₄^-2_(aq) --> BaSO_4(s) + 2H₂O_(l)
We can see that there are fewer ions in solution after the reaction takes place.  As a result, we would predict, the resulting solution would have a poorer conductivity.  

Colligative Properties
An equation will also help us to investigate properties that are dependent on the number of particles in solution.  We can simply count the number of particles that are present.  

For instance.... 

Glucose when it dissolves gives us 1 particle in solution for one particle of solid dissolving...
C₆H₁₂O_6(s) --H₂O--> C₆H₁₂O_6(aq)

Calcium chloride gives us 3 particles in solution for every one particle of solid dissolving...
CaCl_2(s) --H₂O-->  Ca⁺²_(aq) + 2Cl^-_(aq)

As a result, a calcium chloride solution of the same concentration as a glucose solution, could have 3 times the effect on the melting/boiling points, on the vapor pressures or other colligative properties.