CR_Net Ionic Reactions Lesson

Net Ionic Reactions

Netionic.jpegIn order for us to explore chemical reactions more closely, we need to look at them in a different way. The balanced equations we have written in the past are not the complete picture of what is going on in the reaction. These balanced equations are called molecular reactions. In these types of equations all the substances are written as molecules or compounds.  

For example, LaTeX: NaCl_(aq)+AgNO_{3(aq)} \rightarrow AgCl_(s)+NaNO_{3(aq)}NaCl(aq)+AgNO3(aq)AgCl(s)+NaNO3(aq) is called a "molecular equation" even though there are no molecules. The reason is because the formulas are shown with no charges even though the compounds contain ions  with charges.

The reality is that often reactions take place in solution (symbolized by aq for aqueous), allowing the chemicals to dissolve (if they are able).  Remember that in order for a reaction to occur, the particles must collide.  When chemicals are in solution, the particles are freer to move around than they would be in another state.

So, if a compound is dissolved in water to form a solution, the ions separate. If we were to look at an ionic compound that is dissolved in water, we would not see individual compound units. Instead, we would see individual ions surrounded by water molecules. Note that most of the time these water molecules act as the vessel and do not actually participate in the reaction. When an aqueous chemical is involved in a chemical reaction, it is more accurate to write ions instead of the compound.

To begin writing ionic reactions, first we will write what are called dissociation reactions. Dissociation is the breaking apart of an ionic compound when dissolved in water. Remember that the water acts as the vessel, but is usually not written in the equation.

Ex: The dissociation of calcium chloride in water is
CaCl2 (s) Ca+2 (aq) + 2Cl(aq)
Notice the charges on the ions. You will have to "un criss-cross" the formula & check the periodic table to determine these charges.
The (aq) means hydrated, surrounded by water.
Also notice that the reaction is balanced!

When regular chemical reactions are written using ions, we call them ionic equations. Watch this video to learn how to write ionic equations as well as net ionic equations.

Here is another example using the same process (as explained in the video above) to write a net ionic equation.

Ex: Solutions of lead II nitrate and potassium iodide react to form a precipitate of lead iodide and a potassium nitrate solution.
Start by writing and balancing the molecular reaction.
Pb(NO3)2 (aq) + 2 KI (aq) → Pbl2 (s) + 2 K(NO3) (aq)
Notice that the phases were included in the equation above. Including them will help you to do the next step. Notice that lead II iodide is a solid. We know this from the word precipitate.
Now, write the ionic reaction. To do this, replace each aqueous chemical with its ions. (These would be the products if you were simply writing a dissociation reaction.) Remember to keep the reaction balanced.
Pb(NO3)2 (aq) + 2 KI (aq) → Pbl2 (s) + 2 K(NO3) (aq)
Pb2+ (aq) + 2(NO3) (aq) + 2K+ (aq) + 21 (aq) → Pbl2 (s) + 2K+ (aq) + 2(NO3) (aq)
Lastly, we can write what is called a net ionic reaction.
To do this, simply cancel out anything that is identical on both the reactant and product side. Then write the equation leaving out what you have crossed out.
Pb2+ (aq) + 2(NO3) (aq) + 2K+(aq) + 21 (aq) → Pbl2 (s) + 2K*(aq) + 2(NO3)¯(aq)
Pb2+ (aq) + 21 (aq) → Pbl2 (s)
The chemicals that were cancelled out are called spectator ions. These ions do not actively participate in the reaction. So, the net ionic reaction shows us exactly what is going on in the reaction.

Now, let's apply the solubililty rules you learned earlier to write a net ionic equation.

Ex. Solutions of silver nitrate and sodium sulfide are mixed. Write the molecular equation, ionic equation, & net ionic equation.
First, write the formula for each chemical. Determine the type of reaction to predict the products. Balance the equation.
Molecular Equation: 2AgNO3(aq) + Na2S(aq) →→ Ag2S and 2NaNO3
Use the solubility rules to determine the solubility of the products.
According to rule #6, silver sulfide is insoluble. This means that it will be a precipitate (s).
According to rule #1, all sodium and all nitrate compounds are soluble (aq).
2AgNO3(aq) + Na2S(aq) → Ag2S(s) and 2NaNO3(aq)
Now, write the aqueous compounds as ions.
lonic Equation:
2Ag+(aq) + 2NO3(aq) + 2Na+(aq) + S2-(aq) → Ag2S(s) + 2Na+(aq) + 2NO3(aq)
Cross out the spectator ions to write the net ionic equation.
Net lonic Equation: 2Ag+(aq) + S2-(aq) → Ag₂S(s)

What Should be Written as Ions?

So far you have learned that salts that are dissolved are written as ions. What else should be written this way? Strong acids and bases should also be written as ions. The term strong refers to the extent that the chemical breaks into the ions. Strong acids and bases break completely, or almost completely, into their ions. We have been assuming that in every reaction we have talked about that all of the reactants turn into products. But, this is not always true. In weak acids and bases, the majority of the reactants remain as reactants. So, they will not be written as ions.

The easiest way to remember which acids and bases should be written as ions is to memorize the list of strong acids and bases.

MEMORIZE
Strong acids
HCI - hydrochloric acid
HBr - hydrobromic acid
HI- hydroiodic acid
HNO3 nitric acid
H2SO4 - sulfuric acid
HCIO4 - perchloric acid
HCIO3 - chloric acid
Strong bases
Hydroxides of group IA and IIA

Here is a list of the substances that are NOT written as ions:

  • Elements in their natural state (ex. H2)
  • Molecules (ex. NH3 or C6H12O6, sugar)
  • Non-soluble compounds (use solubility rules to determine this)
  • Weak acids and bases (such as acetic acid and ammonia)
  • Any other non-electrolyte
  • Solids
  • Pure liquids
  • Gases (memorize the list given earlier)

Watch this video to see lots of examples of writing net ionic equations.

You should not only watch - you should try these reactions along with the teacher. Writing net ionic equations takes practice. The more you do, the easier it will get. You will not only use the concepts learned on this page, but you will also apply concepts you learned about specific reactions on previous pages. It would be an excellent idea to go back through your notes (or the content pages) and make a complete list of reactions.

Remember to work on the module practice problems as you complete each section of content.  

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