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13.7: Classifying Reactions

Last updated

Jul 16, 2024
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- 13.6: Entropy and Chemical Reactions
- 13.8: Acid Base Reactions

Jamie MacArthur
Madera Community College

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Learning Objectives

Categorize a reaction as combination, decomposition, single replacement, or double replacement.

There are millions of chemical reactions that occur in the world. Just like we use the periodic table to help us remember properties of the elements, we can group chemical reactions into categories based on similar aspects. We will not look at all the possible reaction types in this section, but we will introduce some important categories for simple reactions. In the following sections we will discuss some more specific reaction types based on how they influence our lives.

Combination Reactions

A combination reaction is a reaction in which two or more substances combine to form a single new substance. Combination reactions can also be called synthesis reactions. The general form of a combination reaction is:

$\ce{A} + \ce{B} \rightarrow \ce{AB}\nonumber$

One combination reaction is two elements combining to form a compound. Solid sodium metal reacts with chlorine gas to produce solid sodium chloride.

$2 \ce{Na} \left( s \right) + \ce{Cl_2} \left( g \right) \rightarrow 2 \ce{NaCl} \left( s \right)\nonumber$

Video $\PageIndex{1}$ : Sodium reacting with chlorine to make table salt. (Source)

In order to write and balance the equation correctly, it is important to remember the seven elements that exist in nature as diatomic molecules: $\ce{H_2}$ , $\ce{N_2}$ , $\ce{O_2}$ , $\ce{F_2}$ , $\ce{Cl_2}$ , $\ce{Br_2}$ , and $\ce{I_2}$ .

One sort of combination reaction that occurs frequently is the reaction of an element with oxygen to form an oxide. Metals and nonmetals both react readily with oxygen under most conditions. Magnesium reacts rapidly and dramatically when ignited, combining with oxygen from the air to produce a fine powder of magnesium oxide.

$2 \ce{Mg} \left( s \right) + \ce{O_2} \left( g \right) \rightarrow 2 \ce{MgO} \left( s \right)\nonumber$

Decomposition Reactions

A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances. The general form of a decomposition reaction is:

$\ce{AB} \rightarrow \ce{A} + \ce{B}\nonumber$

Most decomposition reactions require an input of energy in the form of heat, light, or electricity.

Binary compounds are compounds composed of just two elements. The simplest kind of decomposition reaction is when a binary compound decomposes into its elements. Mercury (II) oxide, a red solid, decomposes when heated to produce mercury and oxygen gas.

$2 \ce{HgO} \left( s \right) \rightarrow 2 \ce{Hg} \left( l \right) + \ce{O_2} \left( g \right)\nonumber$

Video $\PageIndex{2}$ : Mercuric oxide decomposing into mercury and oxygen (Source)

A reaction is also considered to be a decomposition reaction even when one or more of the products are still compounds. A metal carbonate decomposes into a metal oxide and carbon dioxide gas. For example, calcium carbonate decomposes into calcium oxide and carbon dioxide:

$\ce{CaCO_3} \left( s \right) \rightarrow \ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right)\nonumber$

Metal hydroxides decompose on heating to yield metal oxides and water. Sodium hydroxide decomposes to produce sodium oxide and water:

$2 \ce{NaOH} \left( s \right) \rightarrow \ce{Na_2O} \left( s \right) + \ce{H_2O} \left( g \right)\nonumber$

Some unstable acids decompose to produce nonmetal oxides and water. Carbonic acid decomposes easily at room temperature into carbon dioxide and water:

$\ce{H_2CO_3} \left( aq \right) \rightarrow \ce{CO_2} \left( g \right) + \ce{H_2O} \left( l \right)\nonumber$

Single-Replacement Reactions

A single-replacement reaction is a reaction in which one element replaces a similar element in a compound. The general form of a single-replacement (also called single-displacement) reaction is:

$\ce{A} + \ce{BC} \rightarrow \ce{AC} + \ce{B}\nonumber$

In this general reaction, element $\ce{A}$ is a metal and replaces element $\ce{B}$ (also a metal) in the compound. When the element that is doing the replacing is a nonmetal, it must replace another nonmetal in a compound, and the general equation becomes:

$\ce{Y} + \ce{XZ} \rightarrow \ce{XY} + \ce{Z}\nonumber$

(Where $\ce{Y}$ is a nonmetal and replaces the nonmetal $\ce{Z}$ in the compound with $\ce{X}$ .)

Metal Replacement

Magnesium is a more reactive metal than copper. When a strip of magnesium metal is placed in an aqueous solution of copper (II) nitrate, it replaces the copper. The products of the reaction are aqueous magnesium nitrate and solid copper metal.

$\ce{Mg} \left( s \right) + \ce{Cu(NO_3)_2} \left( aq \right) \rightarrow \ce{Mg(NO_3)_2} \left( aq \right) + \ce{Cu} \left( s \right)\nonumber$

This subcategory of single-replacement reactions is called a metal replacement reaction because it is a metal that is being replaced (copper).

Hydrogen Replacement

Many metals react easily with acids and when they do so, one of the products of the reaction is hydrogen gas. Zinc reacts with hydrochloric acid to produce aqueous zinc chloride and hydrogen (figure below).

$\ce{Zn} \left( s \right) + 2 \ce{HCl} \left( aq \right) \rightarrow \ce{ZnCl_2} \left( aq \right) + \ce{H_2} \left( g \right)\nonumber$

In a hydrogen replacement reaction, the hydrogen in the acid is replaced by an active metal.

Video $\PageIndex{3}$ : Zinc reacting with hydrochloric acid produces bubbles of hydrogen gas. (Source)

Some metals are so reactive that they are capable of replacing the hydrogen in water. The products of such a reaction are the metal hydroxide and hydrogen gas. All group 1 metals undergo this type of reaction. Sodium reacts vigorously with water to produce aqueous sodium hydroxide and hydrogen (see figure below).

$2 \ce{Na} \left( s \right) + 2 \ce{H_2O} \left( l \right) \rightarrow 2 \ce{NaOH} \left( aq \right) + \ce{H_2} \left( g \right)\nonumber$

Explosion from a reaction of sodium metal with water that creates hydrogen gas. — Figure $\PageIndex{1}$ : Sodium metal reacts vigorously with water, giving off hydrogen gas. A large piece of sodium will often generate so much heat that the hydrogen will ignite. (Credit: User:Ajhalls/Wikimedia Commons; Source; License: Public Domain)

Double-Replacement Reactions

A double-replacement reaction is a reaction in which the positive and negative ions of two ionic compounds exchange places to form two new compounds. The general form of a double-replacement (also called double-displacement) reaction is:

$\ce{AB} + \ce{CD} \rightarrow \ce{AD} + \ce{CB}\nonumber$

In this reaction, $\ce{A}$ and $\ce{C}$ are positively-charged cations, while $\ce{B}$ and $\ce{D}$ are negatively-charged anions. Double-replacement reactions generally occur between substances in aqueous solution. In order for a reaction to occur, one of the products is usually a solid precipitate, a gas, or a molecular compound such as water.

Formation of a Precipitate

A precipitate forms in a double-replacement reaction when the cations from one of the reactants combine with the anions from the other reactant to form an insoluble ionic compound. When aqueous solutions of potassium iodide and lead (II) nitrate are mixed, the following reaction occurs:

$2 \ce{KI} \left( aq \right) + \ce{Pb(NO_3)_2} \left( aq \right) \rightarrow 2 \ce{KNO_3} \left( aq \right) + \ce{PbI_2} \left( s \right)\nonumber$

There are very strong attractive forces that occur between $\ce{Pb^{2+}}$ and $\ce{I^-}$ ions and the result is a brilliant yellow precipitate (see figure below). The other product of the reaction, potassium nitrate, remains soluble.

Video $\PageIndex{4}$ : A yellow precipitate of solid lead (II) iodide forms immediately when solutions of lead (II) nitrate and potassium iodide are mixed. (Source)

Section Summary

Combination reactions occur when two or more substances combine to form a new substance.
A definition of decomposition reaction and example reactions are given.
The activity series describes the relative reactivities of metals and halogens.
The double-replacement reaction generally takes the form of AB + CD → AD + CB where A and C are positively-charged cations, while B and D are negatively-charged anions.
In a double replacement reactions, typically one of the products is a precipitate, a gas, or a molecular compound.

Glossary

combination reaction: A reaction in which two or more substances combine to form a single new substance.

decomposition reaction: A reaction in which a compound breaks down into two or more simpler substances.
single-replacement reaction: A reaction in which one element replaces a similar element in a compound.
double-replacement reaction: A reaction in which the positive and negative ions of two ionic compounds exchange places to form two new compounds.