CR - Reaction Types (Lesson)

Reaction Types

In high school chemistry you probably learned the five basic types of chemical reactions:

• Synthesis
• Decomposition
• Single Displacement
• Double Displacement (Metathesis)
• Combustion

When you can recognize the reaction type, the products of that reaction can be predicted, even if the reaction isn't performed. 

This list will be simplified for AP Chemistry as only three reaction types are necessary to understand. These reaction types will be explained below, but a simple list of reactions to understand looks like this:

• Double Displacement (Metathesis)
• Neutralization
• Oxidation/Reduction (Redox)

Metathesis Reactions

Metathesis reactions (also referred to as double displacement or double replacement reactions) involve the reaction between aqueous solutions of two ionic compounds. In most cases, a reaction is said to occur when a precipitate, or insoluble product, is formed. The reaction shown below, as well as the image depicting, it is an example of this type of reaction.

Pb(NO₃)₂ (aq) + 2 KI (aq)  $\longrightarrow$  2 KNO₃ (aq) + PbI₂ (s)

The products of these reactions can be predicted using simple solubility rules. Please note: The AP curriculum no longer requires the memorization of extensive solubility rules. The rules shown below are all that you are responsible for. As a result, there are many cases where you will need to be told that a precipitate occurs. You may not be told what the identity of that precipitate is, but using the simple rules below you can easily determine it.

Acid/Base Neutralizations

As the name would suggest, neutralization reactions involve the reaction between an acid and a base resulting in a neutralization of each reactant. For the purposes of this chapter, acids will be described as compounds that, when placed in aqueous solution, produce H⁺ ions as shown in the example below. Acids can easily be recognized due to the fact that the first element written in the chemical formula is hydrogen (H).

HCl (g) $\longrightarrow$ H⁺ (aq) + Cl^- (aq)

In reality, there is a much more comprehensive view of acids, but that will not be discussed until later modules.

Likewise, bases are substances that produce hydroxide (OH^-) ions when dissolved in aqueous solution:

NaOH (s) $\longrightarrow$ Na⁺ (aq) + OH^- (aq)

When an acid and a base are allowed to react with one another the products of this reaction are a salt and liquid water as shown below. While the NaCl in this example really is table salt, the term "salt" is a generic term. In this context, a "salt" is simply one of the products of an acid/base neutralization.  It can be said that in the reaction below the sodium salt of hydrochloric acid is formed.

HCl (aq) + NaOH (aq) $\longrightarrow$ NaCl (aq) + H₂O (l)

Oxidation/Reduction Reactions

The final reaction type that must be understood is oxidation/reduction reactions or "redox" reactions for short. These reactions involve the transfer of electrons. For these reactions, there is always one substance that loses electrons (oxidation) and another that gains or receives those electrons (reduction). These reactions are called redox reactions because these two processes must occur in tandem. We cannot have a reaction where a substance only loses electrons and vice versa; therefore, if oxidation occurs then reduction must occur as well.

In order to determine if a reaction is a redox reaction, there must be a change in oxidation number. In order to determine if there is a change in oxidation number, it must first be understood how to assign oxidation numbers to elements in a substance. The rules listed below will help you as you practice determining oxidation numbers.

1. 1. Any atom in its elemental form has an oxidation state of 0
   2. Any monatomic ion has the same oxidation number as charge
   3. Fluorine always has an oxidation number of -1
   4. Alkali metal always has oxidation state of +1 (no exceptions)
   5. Oxygen most often has an oxidation state of -2 (unless bonded to fluorine or in substances that contain the peroxide (O—O-2) polyatomic ion
   6. In most cases, hydrogen has an oxidation state of +1, except when bound to metals or boron
   7. All oxidation numbers must add up to the total charge for the compound or ion

You Try It: Oxidation Numbers Self-Assessment

In the self-assessment activity below, assign oxidation states to the indicated elements for each substance. Click on the plus sign to check yourself!

Once oxidation numbers have been mastered, it can easily be determined whether a reaction is a redox reaction or not.  Looking at the example shown below it can be seen that two different elements have a change in oxidation state.  The magnesium loses electrons to become positively charged while the hydrogen gains those electrons to obtain an oxidation state of zero.

You Try It: Reaction Types Self-Assessment

Before you move on, let's do a quick review of types of reactions.

Reaction Types Review

In case the self-assessment activity above did not go well, use the following activity to help refresh your memory on the types of reactions from previous chemistry courses.

You Try It

Predict the type of reaction and products for the following reactions. Click on the plus sign to check yourself!

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