## 7.3 Classifying Chemical Reactions

### Learning Objectives

By the end of this section, you will be able to:

• Define five common types of chemical reactions (single-replacement, double-replacement, composition, decomposition, and combustion).
• Classify chemical reactions as one of these three types given appropriate descriptions or chemical equations.

Up to now, we have presented chemical reactions as a topic, but we have not discussed how the products of a chemical reaction can be predicted. Here we will begin our study of certain types of chemical reactions that allow us to predict what the products of the reaction will be.

## Composition Reactions

A composition reaction (sometimes also called a combination reaction or a synthesis reaction) produces a single substance from multiple reactants. A single substance as a product is the key characteristic of the composition reaction. There may be a coefficient other than one for the substance, but if the reaction has only a single substance as a product, it can be called a composition reaction. In the reaction

$\large{\text{2 H}}_{2}\text{(}g\text{)}+{\text{O}}_2\text{(}g\text{)}\rightarrow{\text{2 H}}_{2}\text{O(}l\text{)}$

water is produced from hydrogen and oxygen. Although there are two molecules of water being produced, there is only one substance—water—as a product. So this is a composition reaction.

## Decomposition Reactions

A decomposition reaction starts from a single substance and produces more than one substance; that is, it decomposes. One substance as a reactant and more than one substance as the products is the key characteristic of a decomposition reaction. For example, in the decomposition of sodium hydrogen carbonate (also known as sodium bicarbonate),

$\large{\text{2 NaHCO}}_{3}\text{(}s\text{)}\rightarrow{\text{ Na}}_{2}\text{CO}_{3}\text{(}s\text{)}+{\text{ CO}}_{2}\text{(}g\text{)}+{\text{H}}_{2}\text{O(}l\text{)}$

sodium carbonate, carbon dioxide, and water are produced from the single substance sodium hydrogen carbonate.

Composition and decomposition reactions are difficult to predict; however, they should be easy to recognize.

### Example 1:  Composition and Decomposition Reactions

Identify each equation as a composition reaction, a decomposition reaction, or neither.

1. $\large{\text{Fe}}_{2}\text{O}_{3}\text{(}s\text{)}+\text{3 SO}_{3}\text{(}g\text{)}\rightarrow {\text{Fe}}_{2}{\text{(}{\text{SO}}_{4}\text{)}}_{3}$
2. $\large{\text{NaCl}}\text{(}aq\text{)}+{\text{ AgNO}}_{3}\text{(}aq\text{)}\rightarrow{\text{NaNO}}_{3}\text{(}aq\text{)}+\text{AgCl(}s\text{)}$
3. $\large{\text{(}{\text{NH}}_{4}\text{)}}_{2}{\text{Cr}}_{2}\text{O}_{7}\text{(}s\text{)}\rightarrow\text{Cr}_{2}\text{O}_{3}\text{(}s\text{)}+{\text{4 H}}_{2}{\text{O}}\text{(}l\text{)}+{\text{N}}_{2}\text{(}g\text{)}$

Identify the equation as a composition reaction, a decomposition reaction, or neither.

$\large{\text{C}}_{3}\text{H}_{8}\text{(}g\text{)}\rightarrow {\text{C}}_{3}\text{H}_{4}\text{(}g\text{)}+\text{H}_{2}\text{(}g\text{)}$

## Single-Replacement Reactions

A single-replacement reaction (sometimes referred to as a single-displacement reaction) is a chemical reaction in which one element is substituted for another element in a compound, generating a new element and a new compound as products. For example,

$\large\text{2 HCl}\text{(}aq\text{)}+{\text{Zn}}\text{(}s\text{)}\rightarrow{\text{ZnCl}}_{2}\text{(}aq\text{)}+{\text{H}}_{2}\text{(}g\text{)}$

is an example of a single-replacement reaction. The hydrogen atoms in HCl are replaced by Zn atoms, and in the process a new element—hydrogen—is formed. Another example of a single-replacement reaction is

$\large\text{2 NaCl}\text{(}aq\text{)}+{\text{F}}_{2}\text{(}g\text{)}\rightarrow{\text{2 NaF}}\text{(}aq\text{)}+{\text{Cl}}_{2}\text{(}g\text{)}$

## Double-Replacement Reactions

A double-replacement reaction (sometimes referred to as a double-displacement reaction) occurs when parts of two ionic compounds are exchanged, making two new compounds. A characteristic of a double-replacement equation is that there are two compounds as reactants and two different compounds as products. An example is

$\large{\text{CuCl}}_{2}\text{(}aq\text{)}+{\text{2 AgNO}}_{3}\text{(}aq\text{)}\rightarrow \text{Cu}{\text{(}{\text{NO}}_{3}\text{)}}_{2}\text{(}aq\text{)}+\text{2 AgCl(}s\text{)}$

There are two equivalent ways of considering a double-replacement equation: either the cations are swapped, or the anions are swapped. (You cannot swap both; you would end up with the same substances you started with.) Either perspective should allow you to predict the proper products, as long as you pair a cation with an anion and not a cation with a cation or an anion with an anion.

### Example 2:  Replacement Reactions

Predict the products of this double-replacement equation:

$\large{\text{Na}}_{2}\text{SO}_{4}\text{(}aq\text{)}+\text{BaCl}_{2}\text{(}g\text{)}\rightarrow{\text{?}}$

Predict the products of this double-replacement equation:

$\large{\text{KBr}}\text{(}aq\text{)}+{\text{ AgNO}}_{3}\text{(}aq\text{)}\rightarrow{\text{?}}$

Predicting whether a double-replacement reaction occurs is somewhat more difficult than predicting a single-replacement reaction. However, there is one type of double-replacement reaction that we can predict: the precipitation reaction. A precipitation reaction occurs when two ionic compounds are dissolved in water and form a new ionic compound that does not dissolve; this new compound falls out of solution as a solid precipitate. The formation of a solid precipitate is the driving force that makes the reaction proceed.

To judge whether double-replacement reactions will occur, we need to know what kinds of ionic compounds form precipitates. For this, we use solubility rules, which are general statements that predict which ionic compounds dissolve (are soluble) and which do not (are not soluble or insoluble).  Table 1 “Some Useful Solubility Rules” lists some general solubility rules. We need to consider each ionic compound (both the reactants and the possible products) in light of the solubility rules in Table 1 “Some Useful Solubility Rules”. If a compound is soluble, we use the (aq) label with it, indicating it dissolves. If a compound is not soluble, we use the (s) label with it and assume that it will precipitate out of solution. If everything is soluble, then no reaction will be expected.

Table 1 Some Useful Solubility Rules

 These compounds generally dissolve in water (are soluble): Exceptions: All compounds of Li+, Na+, K+, Rb+, Cs+, and NH4+ None All compounds of NO3− and C2H3O2− None Compounds of Cl−, Br−, I− Ag+, Hg22+, Pb2+ Compounds of SO42 Hg22+, Pb2+, Sr2+, Ba2+ These compounds generally do not dissolve in water (are insoluble): Exceptions: Compounds of CO32− and PO43− Compounds of Li+, Na+, K+, Rb+, Cs+, and NH4+ Compounds of OH− Compounds of Li+, Na+, K+, Rb+, Cs+, NH4+, Sr2+, and Ba2+

A vivid example of precipitation is observed when solutions of potassium iodide and lead nitrate are mixed, resulting in the formation of solid lead iodide:

$\large2\text{KI(}aq\text{)}+\text{Pb}{\text{(}{\text{NO}}_{3}\text{)}}_{2}\text{(}aq\text{)}\rightarrow{\text{PbI}}_{2}\text{(}s\text{)}+2{\text{KNO}}_{3}\text{(}aq\text{)}$

This observation is consistent with the solubility guidelines: The only insoluble compound among all those involved is lead iodide, one of the exceptions to the general solubility of iodide salts.

Lead iodide is a bright yellow solid that was formerly used as an artist’s pigment known as iodine yellow (Figure 1). The properties of pure PbI2 crystals make them useful for fabrication of X-ray and gamma ray detectors.

The solubility guidelines discussed above may be used to predict whether a precipitation reaction will occur when solutions of soluble ionic compounds are mixed together. One merely needs to identify all the ions present in the solution and then consider if possible cation/anion pairing could result in an insoluble compound.

For example, mixing solutions of silver nitrate and sodium fluoride will yield a solution containing Ag+, NO, Na+, and F ions. Aside from the two ionic compounds originally present in the solutions, AgNO3 and NaF, two additional ionic compounds may be derived from this collection of ions: NaNO3 and AgF. The solubility guidelines indicate all nitrate salts are soluble but that AgF is one of the exceptions to the general solubility of fluoride salts. A precipitation reaction, therefore, is predicted to occur, as described by the following equation:

$\large{\text{NaF}}\text{(}aq\text{)}+{\text{ AgNO}}_{3}\text{(}aq\text{)}\rightarrow{\text{AgF}}\text{(}s\text{)}+{\text{ NaNO}}_{3}\text{(}aq\text{)}$

Figure 1. A precipitate of PbI2 forms when solutions containing Pb2+ and I are mixed. (credit: Der Kreole/Wikimedia Commons)

### Example 3: Predicting Precipitate Reactions

Will a double-replacement reaction occur? If so, identify the products.

1. $\large{\text{KBr}}\text{(}aq\text{)}+\text{Ca}{\text{(}{\text{NO}}_{3}\text{)}}_{2}\text{(}aq\text{)}\rightarrow{\text{?}}$
1. $\large{\text{NaOH}}\text{(}aq\text{)}+\text{Fe}\text{Cl}_{2}\text{(}aq\text{)}\rightarrow{\text{?}}$

Will a double-replacement equation occur? If so, identify the products.

$\large\text{Sr}{\text{(}{\text{NO}}_{3}\text{)}}_{2}\text{(}aq\text{)}+\text{K}\text{Cl}\text{(}aq\text{)}\rightarrow\text{?}$

## Combustion Reactions

combustion reaction occurs when a reactant combines with oxygen, many times from the atmosphere, to produce oxides of all other elements as products; any nitrogen in the reactant is converted to elemental nitrogen, N2. Many reactants, called fuels, contain mostly carbon and hydrogen atoms, reacting with oxygen to produce CO2 and H2O. For example, the balanced chemical equation for the combustion of methane, CH4, is as follows:

$\large{\text{CH}}_{4}\text{(}g\text{)}+{\text{ 2 O}}_2\text{(}g\text{)}\rightarrow{\text{CO}}_{2}\text{(}g\text{)}+{\text{2 H}}_{2}\text{O(}g\text{)}$

Kerosene can be approximated with the formula C12H26, and its combustion equation is

$\large\text{2 C}_{12}\text{H}_{26}\text{(}l\text{)}+{\text{ 37 O}}_2\text{(}g\text{)}\rightarrow{\text{24 CO}}_{2}\text{(}g\text{)}+{\text{26 H}}_{2}\text{O(}g\text{)}$

Sometimes fuels contain oxygen atoms, which must be counted when balancing the chemical equation. One common fuel is ethanol, C2H5OH, whose combustion equation is

$\large\text{C}_{2}\text{H}_{5}\text{OH}\text{(}l\text{)}+{\text{ 3 O}}_2\text{(}g\text{)}\rightarrow{\text{2 CO}}_{2}\text{(}g\text{)}+{\text{3 H}}_{2}\text{O(}g\text{)}$

If nitrogen is present in the original fuel, it is converted to N2, not to a nitrogen-oxygen compound. Thus, for the combustion of the fuel dinitroethylene, whose formula is C2H2N2O4, we have

$\large\text{2 C}_{2}\text{H}_{2}\text{N}_{2}\text{O}_{4}\text{(}l\text{)}+{\text{ O}}_2\text{(}g\text{)}\rightarrow{\text{4 CO}}_{2}\text{(}g\text{)}+{\text{ 2 H}}_{2}\text{O(}g\text{)}+\text{ N}_{2}\text{(}g\text{)}$

### Example 4: Combustion Reactions

Complete and balance each combustion equation.

1. The combustion of propane, C3H8
2. The combustion of NH3

Complete and balance the combustion equation for cyclopropanol, C3H6O.

### Key Takeaways

• A composition reaction produces a single substance from multiple reactants.
• A decomposition reaction produces multiple products from a single reactant.
• Combustion reactions are the combination of some compound with oxygen to make oxides of the other elements as products (although nitrogen atoms react to make N2).
• A single-replacement reaction replaces one element for another in a compound.
• A double-replacement reaction exchanges the cations (or the anions) of two ionic compounds.
• A precipitation reaction is a double-replacement reaction in which one product is a solid precipitate.
• Solubility rules are used to predict whether some double-replacement reactions will occur.

### Exercises

1. Which is a composition reaction and which is not?

a. NaCl + AgNO3 → AgCl + NaNO3

b.  CaO + CO2 → CaCO3

2.  Which is a composition reaction and which is not?

a. H2 + Cl2 → 2 HCl

b.  2 HBr + Cl2 → 2 HCl + Br2

3.  Which is a composition reaction and which is not?

a.  2 SO2 + O2 → 2 SO3

b.  6 C + 3 H2 → C6H6

4.  Which is a composition reaction and which is not?

a.  4 Na + 2 C + 3 O2 → 2 Na2CO3

b.  Na2CO3 → Na2O + CO2

5.  Which is a decomposition reaction and which is not?

a.  HCl + NaOH → NaCl + H2O

b.  CaCO3 → CaO + CO2

6.  Which is a decomposition reaction and which is not?

a.  3 O2 → 2 O3

b.  2 KClO3 → 2 KCl + 3 O2

7.  Which is a decomposition reaction and which is not?

a.  Na2O + CO2 → Na2CO3

b.  H2SO3 → H2O + SO2

8.  Which is a decomposition reaction and which is not?

a.  2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C

b.  C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

9.  Which is a combustion reaction and which is not?

a.  C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

b.  2 Fe2S3 + 9 O2 → 2 Fe2O3 + 6 SO2

10.  Which is a combustion reaction and which is not?

a.  CH4 + 2 F2 → CF4 + 2 H2

b.  2 H2 + O2 → 2 H2O

11.  Which is a combustion reaction and which is not?

a.  P4 + 5 O2 → 2 P2O5

b.  2 Al2S3 + 9 O2 → 2 Al2O3 + 6 SO2

12.  Which is a combustion reaction and which is not?

a.  C2H4 + O2 → C2H4O2

b.  C2H4 + Cl2 → C2H4Cl2

13.  Is it possible for a composition reaction to also be a combustion reaction? Give an example to support your case.

14.  Is it possible for a decomposition reaction to also be a combustion reaction? Give an example to support your case.

15.  Complete and balance each combustion equation.

a.  C4H9OH + O2 → ?

b.  CH3NO2 + O2 → ?

16.  Complete and balance each combustion equation.

a.  B2H6 + O2 → ? (The oxide of boron formed is B2O3.)

b.  Al2S3 + O2 → ? (The oxide of sulfur formed is SO2.)

c.  Al2S3 + O2 → ? (The oxide of sulfur formed is SO3.)

17. Assuming that each single-replacement reaction occurs, predict the products and write each balanced chemical equation.

a.  Zn + Fe(NO3)2 → ?

b.  F2 + FeI3 → ?

18.  Assuming that each single-replacement reaction occurs, predict the products and write each balanced chemical equation.

a. Li + MgSO4 → ?

b.  NaBr + Cl2 → ?

19.  Assuming that each single-replacement reaction occurs, predict the products and write each balanced chemical equation.

a.  Sn + H2SO4 → ?

b.  Al + NiBr2 → ?

20.  Assuming that each single-replacement reaction occurs, predict the products and write each balanced chemical equation.

a.  Mg + HCl → ?

b.  HI + Br2 → ?

21.  Assuming that each double-replacement reaction occurs, predict the products and write each balanced chemical equation.

a.  Zn(NO3)2 + NaOH → ?

b.  HCl + Na2S →

22.  Assuming that each double-replacement reaction occurs, predict the products and write each balanced chemical equation.

a.  Ca(C2H3O2)2 + HNO3 → ?

b.  Na2CO3 + Sr(NO2)2 → ?

23.  Assuming that each double-replacement reaction occurs, predict the products and write each balanced chemical equation.

a.  Pb(NO3)2 + KBr → ?

b.  K2O + MgCO3 → ?

24.  Assuming that each double-replacement reaction occurs, predict the products and write each balanced chemical equation.

a.  Sn(OH)2 + FeBr3 → ?

b.  CsNO3 + KCl → ?

25.  Use the solubility rules to predict if each double-replacement reaction will occur and, if so, write a balanced chemical equation.

a.  Na2CO3 + Sr(NO2)2 → ?

b.  (NH4)2SO4 + Ba(NO3)2 → ?

26.  Use the solubility rules to predict if each double-replacement reaction will occur and, if so, write a balanced chemical equation.

a.  KC2H3O2 + Li2CO3 → ?

b.  KOH + AgNO3 → ?

27.  Use the solubility rules to predict if each double-replacement reaction will occur and, if so, write a balanced chemical equation.

a.  K3PO4 + SrCl2 → ?

b.  NaOH + MgCl2 → ?