Essential Question: What is taking place during chemical reactions?
Types of Chemical Reactions
One skill that chemists learn over time is that of writing and balancing equations. The first task is deciding what type of reaction is taking place. In this chapter we study three types:
 Precipitation Reactions: In these reactions two soluble salts usually react to form to an insoluble salt (the precipitate!) and a soluble salt. The cations of the reacting salts exchange anions. It is a double replacement reaction that forms a solid.
 Acid-Base Reactions: Most commonly an acid of the type HX or H2X reacts with a basic hydroxide to form a salt plus water. Alternatively, the acid may react with ammonia (NH3) to form an ammonium salt (but no water). These are proton transfer reactions in which H+ (the proton) is transferred from the acid to the base.
 Oxidation-Reduction Reactions: These are reactions in which one type of atom increases in oxidation number (is oxidized) and another type of atom decreases in oxidation number (is reduced). A large number of oxidation-reduction (redox) reactions contain one or more reactants or products, which are pure elements.
Note that hydroxides can react with acids in acid-base reactions, and also with other salts in precipitation reactions.
Writing Balanced Ionic Equations
The first step in writing a balanced equation is predicting the products of the reaction as discussed above.
Then the steps below are completed in sequence:
 Balance the Molecular Equation: In the “molecular” equation, nothing is broken up into ions.
Salt formulas are written so that the cation charges exactly balance out the anion charges so that the salt is neutral. Then the equation is balanced for atoms. This is no different that what we have been doing!
 Balance the Total Ionic Equation: The first step in writing an ionic equation is to decide what species should be broken up into ions. The rules below should help!
Break up into Ions

Strong Acids. HCl, HBr, HI, HNO3, HClO4, and H2SO4 are the most common examples; assume other acids are weak.

Strong Bases. All Group 1 and 2 hydroxides
(except for Be and Mg); assume other bases are weak.

Soluble Salts (aq). Salts of the alkali metals, salts containing the NH
4
+ ion, the NO
3
ion, and other salts as specified on your solubility rules (on the back of the periodic table)
Do NOT break up! Leave as is!

Weak Acids. Nearly all acids are weak.

Weak Bases. Nearly all bases are weak.

Insoluble Salts. Most salts are insoluble (solid).

Non-electrolytes or Weak Electrolytes.
Examples include H
2
O, gases, pure elements, hydrocarbons, and alcohols.

 Balance the Net Ionic Equation: Identify all spectator ions: these are ions that are identical on both sides of the balanced total ionic equation. Remove the spectator ions from the equation.
What remains is the net ionic equation. Finally, simplify the stoichiometric coefficients if all of them are divisible by a common factor.
If all the ions are spectator ions so that nothing is left for your net ionic equation, no reaction has taken place!
Exercises: For each of the following reactions, complete the chart. Be sure to balance all of your equations.
1. Mg(OH)
2
(s) + HCl( aq )
(a) Reaction type: Formulas of Products Formed:
(b) Molecular Equation
(c) Total Ionic Equation
(d) Net Ionic Equation
2. AgNO
3
( aq ) + K
2
Cr
2
O
7
( aq )
(a) Reaction type: Formulas of Products Formed:
(b) Molecular Equation
(c) Total Ionic Equation
(d) Net Ionic Equation

3. HCl( aq ) + NaOH( aq )
(a) Reaction type: Formulas of Products Formed:
(b) Molecular Equation
(c) Total Ionic Equation
(d) Net Ionic Equation
4. LiOH( aq ) + H2SO4( aq )
(a) Reaction type: Formulas of Products Formed:
(b) Molecular Equation
(c) Total Ionic Equation
(d) Net Ionic Equation
5. Na2S( aq ) + Ba(OH)2( aq )
(a) Reaction type: Formulas of Products Formed:
(b) Molecular Equation
(c) Total Ionic Equation
(d) Net Ionic Equation