1. Types of Equations

2. Symbols seen in equations
In any chemical equation, there are symbols that we need to become acquainted with.
Here is a sample equation:
Ca(NO3)2 (aq) + 2 NaOH (aq)  2 NaNO3 (aq) + Ca(OH)2 (s)
We should already be familiar with the idea that the reactants are on the left and the products are on the right. We should also already know about coefficients and subscripts.

3. Symbols seen in equations
Ca(NO3)2 (aq) + 2 NaOH (aq)  2 NaNO3 (aq) + Ca(OH)2 (s) For the equations we are going to study this chapter there is one more important part of the equation and that is the symbol in ( ) which tells us which phase the material is in. (s) – solid (l) – liquid (g) – gas (aq) – aqueous solution

4. Molecular Equations Molecular Equations
The first type of equation we want to look at are called
Molecular Equations
We are familiar with this type of equation as this is the type that we have seen most of the time in our study of chemistry up to this point.
Every species is written as a molecule no matter what phase it is in.

5. Molecular Equations - examples
Here are some examples of Molecular Equations.
1. CS2 + 3 O2  CO2 + 2 SO2
NaCl (aq) + AgNO3 (aq)  NaNO3 (aq) + AgCl (s)
BaCl2 + Na2CrO4  BaCrO4 + 2 NaCl
Notice that there are no ions present and that whether the phase is written in or not, they are still Molecular Equations.

6. Complete Ionic Equations
The second type of equation is called a Complete Ionic Equation The identifying characteristic about this type of reaction is that any soluble species is written as an ion, anywhere it is found in the equation. Note: 2 KCl written as ions is 2 K+1 not K+2

7. Complete ionic equations - examples
CS2 (l) + 3 O2 (g)  CO2 (g) + 2 SO2 (g)
In this equation, there are no soluble species, so the equation stays the same – there are no aqueous ions to write out.
Na+1 (aq) + Cl-1 (aq) + Ag+1 (aq) NO3 -1 (aq)  Na+1(aq) + NO3-1 (aq) + AgCl (s)
In this equation, both the NaCl and the AgNO3 are soluble on the reactant side. The NaNO3 is soluble on the product side so all 3 get dissociated and written as ions. The AgCl is not soluble so it remains written as a solid.

8. Complete Ionic Equations – Examples2
BaCl2 (aq) + Na2CrO4 (aq)  BaCrO4 (s) + 2 NaCl (aq) What is above is a molecular equation. On a piece of paper, write out what you think the equation would look like if it were written as a complete ionic equation.

9. Is this what you got ?
Ba Cl-1 (aq) + 2 Na+1 + CrO4-2 (aq)  BaCrO4 (s) + 2 Na Cl-1 (aq) Check and compare this carefully with what you have written out. There are some subtle differences. Check out the Cl-1 ions for starters. Did you have 2 of them on each side ?

10. Net Ionic Equations
Net Ionic Equations are used quite often in AP Chemistry. If you were comfortable with Complete Ionic Equations, then you should find writing Net Ionic Equations fairly easy. Remember that all ions are always in aqueous solution, they don’t need the (aq) designation though they often do.
We need to become familiar with spectator ions. These are ions that are found in a soluble species as a reactant and as a product, so these ions are always in solution. Here is a simple example:
Na+1 + Z-1 + A+1 + B-1  Na+1 + B-1 + AZ
Here we see the Na+1 to be a spectator ion because it is an ion on both the reactant and the product side. B-1 is also a spectator ion.

11. Net Ionic Equations - Examples
CS2 (l) + 3 O2 (g)  CO2 (g) + 2 SO2 (g)
There is no change since there are no soluble species.
NaCl (aq) + AgNO3 (aq)  NaNO3 (aq) + AgCl (s)
Now here is the complete ionic equation.
Na+1 (aq) + Cl-1 (aq) + Ag+1 (aq) + NO3-1 (aq)  Na+1(aq) + NO3-1(aq) + AgCl (s)
We eliminate ions found on both sides to get the Net Ionic Eq. which would be the Na+1 and the NO3-1 ions.
Ag+1 (aq) + Cl-1 (aq)  AgCl (s)

12. Net ionic equations – examples2
BaCl2 + Na2CrO4  BaCrO4 + 2 NaCl
Ba+2 (aq) + 2 Cl-1 (aq) + 2 Na+1 (aq) + CrO4-2 (aq)  BaCrO4 (s) + 2 Na Cl-1 (aq)
Here is the net ionic equation:
Ba+2 (aq) + CrO4-2 (aq)  BaCrO4 (s)
A net ionic equation is always going to look more compact than the other type of equations.