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13.3: Types of Reactions

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Lesson Objectives

The student will:

  • describe what is occurring in synthesis, decomposition, single replacement, double replacement, and combustion reactions.
  • classify a chemical reaction as a synthesis, decomposition, single replacement, double replacement, or a combustion reaction.
  • predict the products of simple reactions.

Vocabulary

  • combustion reaction
  • decomposition reaction
  • double replacement reaction
  • hydrocarbon
  • single replacement reaction
  • synthesis reaction

Introduction

Chemical reactions are classified into types to help us analyze them and to help us predict what the products of the reaction will be. The five major types of chemical reactions are synthesis, decomposition, single replacement, double replacement, and combustion.

Synthesis Reactions

A synthesis reaction is one in which two or more reactants combine to make one product. The general equation for a synthesis reaction is:

A + B \rightarrow AB

Synthesis reactions occur as a result of two or more simpler elements or molecules combining to form a more complex molecule. We can always identify a synthesis reaction because there is only one product. If you are given elemental reactants and told that the reaction is a synthesis reaction, you should be able to predict the products. For example, consider the equation below. Two elements (hydrogen and oxygen) combine to form one product (water).

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

You should also be able to write the chemical equation for a synthesis reaction if you are given a product by picking out its elements and writing the equation. As a result, we can write the synthesis reaction for sodium chloride just by knowing the elements that are present in the product.

2 \ \text{Na}_{(s)} + \text{Cl}_{2(g)} \rightarrow 2 \ \text{NaCl}_{(s)}

Example:

  1. Write the chemical equation for the synthesis reaction of silver bromide, \text{AgBr}.
  2. Predict the products for the following reaction: \text{CO}_{2(g)} + \text{H}_2\text{O}_{(l)}.
  3. Predict the products for the following reaction: \text{Li}_2\text{O}_{(s)} + \text{CO}_{2(g)}.

Solution:

  1. 2 \ \text{Ag}_{(s)} + \text{Br}_{2(l)} \rightarrow 2 \ \text{AgBr}_{(s)}
  2. \text{CO}_{2(g)} + \text{H}_2\text{O}_{(l)} \rightarrow \text{H}_2\text{CO}_{3(aq)}
  3. \text{Li}_2\text{O}_{(s)} + \text{CO}_{2(g)} \rightarrow \text{Li}_2\text{CO}_{3(s)}

Decomposition Reactions

When one type of reactant breaks down to form two or more products, we have a decomposition reaction. The best way to remember a decomposition reaction is that for all reactions of this type, there is only one reactant. The general equation for a decomposition reaction is:

AB \rightarrow A + B

Look at the equation below for an example of a decomposition reaction. In this reaction, ammonium nitrate breaks down to form dinitrogen oxide and water.

\text{NH}_4\text{NO}_{3(s)} \rightarrow \text{N}_2\text{O}_{(g)} + 2 \text{H}_2\text{O}_{(g)}

Notice that there is only one reactant, \text{NH}_4\text{NO}_3, on the left of the arrow and that there is more than one on the right side of the arrow. This is the exact opposite of a synthesis reaction.

When studying decomposition reactions, we can predict the reactants in a similar manner as we did for synthesis reactions. Look at the formula for magnesium nitride, \text{Mg}_3\text{N}_2. What elements do you see in this formula? You see magnesium and nitrogen. Now we can write a decomposition reaction for magnesium nitride.

\text{Mg}_3\text{N}_{2(s)} \rightarrow 3 \ \text{Mg}_{(s)} + \text{N}_{2(g)}

Notice there is only one reactant.

Example:

Write the chemical equation for the decomposition of the following compounds into their individual elements:

  1. \text{Al}_2\text{O}_3
  2. \text{Ag}_2\text{S}
  3. \text{MgO}

Solution:

  1. 2 \ \text{Al}_2\text{O}_3 \rightarrow 4 \ \text{Al} + 3 \ \text{O}_2
  2. \text{Ag}_2\text{S} \rightarrow 2 \ \text{Ag} + \text{S}
  3. 2 \ \text{MgO} \rightarrow 2 \ \text{Mg} + \text{O}_2

Single Replacement Reactions

A third type of reaction is the single replacement reaction. In single replacement reactions, one element reacts with one compound to form products. The single element is said to replace an element in the compound when the products form, hence the name single replacement.

There are actually three different types of single replacement reactions: 1) the single element is a metal and replaces the metal in the second reactant, 2) the single element is a metal and replaces the hydrogen in the second reactant, which is always an acid, and 3) the single element is a nonmetal and replaces the nonmetal in the second reactant.

Replacement of a Metal with a Metal

In this section, we will focus on single replacement reactions where an elemental metal reactant replaces the metal (or the cation) of a second compound. The general equation for this reaction is:

A + BC \rightarrow B + AC

Consider the following example. Notice there is only one reactant that is an element and one reactant that is a compound.

\text{Zn}_{(s)} + \text{Cu(NO}_3)_{2(aq)} \rightarrow \text{Zn(NO}_3)_{2(aq)} + \text{Cu}_{(s)}

When studying single replacement reactions, we can predict reactants in a similar manner as we did for synthesis and decomposition reactions. Suppose that we know a single replacement reaction will occur between solid aluminum and solid iron(III) oxide.

\text{Al}_{(s)} + \text{Fe}_2\text{O}_{3(s)}

In order to predict the products, we need to know that aluminum will replace iron and form aluminum oxide. Aluminum has a charge of +3 (it is in Group 3A), and oxygen has a charge of -2 (it is in Group 6A). The compound formed between aluminum and oxygen, therefore, will be \text{Al}_2\text{O}_{3(s)}. Since iron is replaced in the compound by aluminum, the iron product will now be a single element. The unbalanced equation will be:

\text{Al}_{(s)} + \text{Fe}_2\text{O}_{3(s)} \rightarrow \text{Al}_2\text{O}_{3(s)} + \text{Fe}_{(s)}.

The balanced equation will be:

2 \ \text{Al}_{(s)} + \text{Fe}_2\text{O}_{3(s)} \rightarrow \text{Al}_2\text{O}_{3(s)} + 2 \ \text{Fe}_{(s)}

Example:

  1. Write the chemical equation for the single replacement reaction between zinc solid and lead(II) nitrate solution to produce zinc nitrate solution and solid lead.
  2. Predict the products for the following reaction: \text{Fe} + \text{CuSO}_4.
  3. Predict the products for the following reaction: \text{Al} + \text{CuCl}_2.

Solution:

  1. \text{Zn}_{(s)} + \text{Pb(NO}_3)_{2(aq)} \rightarrow \text{Pb}_{(s)} + \text{Zn(NO}_3)_{2(aq)}
  2. \text{Fe}_{(s)} + \text{CuSO}_{4(aq)} \rightarrow \text{Cu}_{(s)} + \text{FeSO}_{4(aq)}
  3. 2 \ \text{Al} + 3 \ \text{CuCl}_2 \rightarrow 3 \ \text{Cu} + 2 \ \text{AlCl}_3

Replacement of Hydrogen with a Metal

These reactions are the same as those studied in the last section, except the compound in the reactant side of the equation is always an acid. Since you may not have studied acids yet, you should consider an acid to be a compound in which hydrogen is combined with an anion. Therefore, in this section, we will consider single replacement reactions where the element reactant replaces the hydrogen in the acid compound. The general reaction is:

\mathrm{A} + 2 \ \mathrm{HX} \rightarrow \mathrm{AX}_2 + \mathrm{H}_2

The chemical equation below is an example of this type of reaction:

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

When studying these single replacement reactions, we can predict reactants in a similar manner as we did for the other types of single replacement reactions. Look at the reaction below. Since \text{HCl} is a compound that has hydrogen combined with an anion, it is an acid.

\text{Mg}_{(s)} + 2 \text{HCl}_{(aq)}

In order to predict the products, we need to know that magnesium will replace hydrogen and form magnesium chloride. Magnesium has a charge of +2 (it is in Group 2A), and chlorine has a charge of -1 (it is in group 7A). Therefore, the compound formed will be \text{MgCl}_2. The replaced hydrogen forms the product elemental hydrogen, \text{H}_2. After the products are determined, all that remains is to balance the equation.

\text{Mg}_{(s)} + 2 \ \text{HCl}_{(aq)} \rightarrow \text{MgCl}_{2(s)} + \text{H}_{2(g)}

Notice that one reactant is the element \text{Mg} and one reactant is an acid compound. The \text{Mg} has replaced the hydrogen in \text{HCl} in the same manner as \text{Zn} replaced the hydrogen in the \text{HBr} in the example above.

Example:

  1. Write the chemical equation for the single replacement reaction between iron solid and hydrochloric acid solution to produce iron(II) chloride solution and hydrogen gas.
  2. Predict the products for the following reaction: \text{Zn}_{(s)} + \text{H}_2\text{SO}_{4(aq)}.
  3. Predict the products for the following reaction: \text{Al}_{(s)} + \text{HNO}_{3(aq)}.

Solution:

  1. \text{Fe}_{(s)} + 2 \ \text{HCl}_{(aq)} \rightarrow \text{FeCl}_{2(aq)} + \text{H}_{2(g)}
  2. \text{Zn}_{(s)} + \text{H}_2\text{SO}_{4(aq)} \rightarrow \text{ZnSO}_{4(aq)} + \text{H}_{2(g)}
  3. 2 \ \text{Al}_{(s)} + 6 \ \text{HNO}_{3(aq)} \rightarrow 2 \ \text{Al(NO}_3)_{3(aq)} + 3 \ \text{H}_{2(g)}

Replacement of a Nonmetal with a Nonmetal

In this section, we will focus on the final type of single replacement reactions where the element reactant replaces the nonmetal (or the anion) in a compound. The general equation for this type of reaction is:

A + BC \rightarrow C + BA

Notice in the equation below that the chlorine replaced the iodine to produce solid iodine as a product. We can predict the products for these single replacement reactions in a similar manner as for all other single replacement reactions. The only difference here is that we have to remember that we are replacing the anion of the compound rather than the cation.

\text{Cl}_{2(g)} + 2 \ \text{KI}_{(aq)} \rightarrow 2 \ \text{KCl}_{(aq)} + \text{I}_{2(s)}

Look at the reaction between chlorine gas and sodium bromide shown below. This is an actual method for extracting bromine from ocean water found to contain sodium bromide. Can you complete the reaction?

\text{Cl}_{2(g)} + \text{NaBr}_{(aq)} \rightarrow ?

In order to predict the products of this reaction, we need to know that chlorine will replace bromine and form sodium chloride. Sodium has a charge of +1 (it is in Group 1A) and chlorine has a charge of -1 (it is in group 7A). The compound formed will be \text{NaCl}.

\text{Cl}_{2(g)} + 2 \ \text{NaBr}_{(aq)} \rightarrow 2 \ \text{NaCl}_{(aq)} + \text{Br}_{2(l)}

Notice, as with all of the other single replacement reactions, that the reactants include one element and one compound, and the products contain one element and one compound. This is the determining factor for identifying whether you have a single replacement reaction.

Example:

  1. Write the chemical equation for the single replacement reaction between sodium iodide solution and liquid bromine to produce sodium bromide solution and solid iodine.
  2. Predict the products for the following reaction: \text{Br}_{2(aq)} + \text{KI}_{(aq)}.
  3. Predict the products for the following reaction: \text{MgI}_{2(aq)} + \text{Cl}_{2(aq)}.

Solution:

  1. 2 \ \text{NaI}_{(aq)} + \text{Br}_{2(l)} \rightarrow 2 \ \text{NaBr}_{(aq)} + \text{I}_{2(s)}
  2. \text{Br}_{2(aq)} + 2 \ \text{KI}_{(aq)} \rightarrow 2 \ \text{KBr}_{(aq)} + \text{I}_{2(s)}
  3. \text{MgI}_{2(aq)} + \text{Cl}_{2(aq)} \rightarrow \text{MgCl}_{2(aq)} + \text{I}_{2(s)}

Double Replacement

For double replacement reactions, two reactants will react by having the cations exchange places. The key to identifying this type of reaction is to recognize that it has two compounds as reactants. This type of reaction is more common than any of the others, and there are many different types of double replacement reactions. Some double replacement reactions are more common than others. For example, precipitation and neutralization reactions are two of the most common double replacement reactions. Precipitation reactions are ones where two aqueous reactants combine to form products where one of the products is an insoluble solid. A neutralization reaction is one where the two reactant compounds are an acid and a base and the two products are a salt and water.

Example:

The following is a precipitation reaction because \text{AgCl}_{(s)} is formed.

\text{AgNO}_{3(aq)} + \text{NaCl}_{(aq)} \rightarrow \text{AgCl}_{(s)} + \text{NaNO}_{3(aq)}

The following is a neutralization reaction because the acid, \text{H}_2\text{SO}_4, is neutralized by the base, \text{NaOH}.

2 \ \text{NaOH}_{(aq)} + \text{H}_2\text{SO}_{4(aq)} \rightarrow \text{Na}_2\text{SO}_{4(aq)} + 2 \ \text{H}_2\text{O}_{(l)}

In order to write the products for a double displacement reaction, you must be able to determine the correct formulas for the new compounds. Consider this common laboratory experiment that involves the reaction between lead(II) nitrate and sodium iodide, both clear solutions. Here is the start of the reaction:

\text{Pb(NO}_3)_{2(aq)} + \text{NaI}_{(aq)}

Now, predict the products based on what you know about charges. We know that the cations exchange anions. We now have to look at the charges of each of the cations and anions to see what the products will be.

We should presume the charge of the lead will remain +2, and since iodine forms ions with a charge of -1, one product will be \text{PbI}_2. The other product will form between the sodium ion, whose charge is +1, and the nitrate ion, whose charge is -1. Therefore, the second product will be \text{NaNO}_3. Once the products are written in, the equation can be balanced.

\text{Pb(NO}_3)_{2(aq)} + 2 \ \text{NaI}_{(aq)} \rightarrow \text{PbI}_{2(s)} + 2 \ \text{NaNO}_{3(aq)}

The experiment produces a brilliant yellow precipitate. If you have use of a solubility table, it is easy to determine that the precipitate will be the lead(II) iodide. Even without a solubility table, knowing that lead compounds tend to precipitate and sodium compounds are always soluble, we would still be able to determine that the \text{PbI}_2 is the brilliant yellow precipitate.

Look at the reaction between acetic acid and barium hydroxide below.

\text{HC}_2\text{H}_3\text{O}_{2(aq)} + \text{Ba(OH)}_{2(aq)} \rightarrow \text{?}

Try to predict the products by having the cations exchange places and writing the correct formulas for the products formed.

\text{HC}_2\text{H}_3\text{O}_{2(aq)} + \text{Ba(OH)}_{2(aq)} \rightarrow \text{Ba(C}_2\text{H}_3\text{O}_2)_{2(aq)} + \text{H}_2\text{O}_{(l)} \ \text{(not balanced)}

Therefore, the final balanced equation will be:

2 \ \text{HC}_2\text{H}_3\text{O}_{2(aq)} + \text{Ba(OH)}_{2(aq)} \rightarrow \text{Ba(C}_2\text{H}_3\text{O}_2)_{2(aq)} + 2 \ \text{H}_2\text{O}_{(l)} \ \text{(balanced)}

This is an acid-base reaction yielding salt, barium acetate, and water. Notice that \text{HOH} and \text{H}_2\text{O} are the same.

Example:

  1. Write a chemical equation for the double replacement reaction between calcium chloride solution and potassium hydroxide solution that produces potassium chloride solution and a precipitate of calcium hydroxide.
  2. Predict the products for the following reaction: \text{AgNO}_{3(aq)} + \text{NaCl}_{(aq)}.
  3. Predict the products for the following reaction: \text{FeCl}_{3(aq)} + \text{KOH}_{(aq)}.

Solution:

  1. \text{CaCl}_{2(aq)} + 2 \ \text{KOH}_{(aq)} \rightarrow \text{Ca(OH)}_{2(s)} + 2 \ \text{KCl}_{(aq)}
  2. \text{AgNO}_{3(aq)} + \text{NaCl}_{(aq)} \rightarrow \text{AgCl}_{(s)} + \text{NaNO}_{3(aq)}
  3. \text{FeCl}_{3(aq)} + 3 \ \text{KOH}_{(aq)} \rightarrow \text{Fe(OH)}_{3(s)} + 3 \ \text{KCl}_{(aq)}

Combustion

A special type of single replacement reaction deserves some attention. These reactions are combustion reactions. In a combustion reaction, oxygen reacts with another substance to produce carbon dioxide and water.

In a particular branch of chemistry, known as organic chemistry, we study compounds known as hydrocarbons. A hydrocarbon is an organic substance consisting of only hydrogen and carbon. Combustion reactions usually have a hydrocarbon reacting with oxygen to produce \text{CO}_2 and \text{H}_2\text{O}. In other words, the only part that changes from one combustion reaction to the next is the actual hydrocarbon involved in the reaction. The general equation for this reaction is given below. Notice oxygen, carbon dioxide, and water are listed in the general equation to show that these reactants and products remain the same from combustion reaction to combustion reaction.

\text{hydrocarbon} + \text{O}_{2(g)} \rightarrow \text{CO}_{2(g)} + \text{H}_2\text{O}_{(l)}

Look at the the combustion reaction of octane, \text{C}_8\text{H}_{18}, shown below. Octane has 8 carbon atoms, hence the prefix “oct-.”

2 \ \text{C}_8\text{H}_{18(l)} + 25 \ \text{O}_{2(g)} \rightarrow 16 \ \text{CO}_{2(g)} + 18 \ \text{H}_2\text{O}_{(l)}

This reaction is referred to as complete combustion. Complete combustion reactions occur when there is enough oxygen to burn the entire hydrocarbon. This is why the only products are carbon dioxide and water.

Have you ever been in a lab and seen black soot appear on the bottom of a heated beaker? Or, have you ever seen the black puffs of smoke come out from the exhaust pipe of a car? If there is not enough oxygen, the result is an incomplete combustion reaction with \text{CO}_{(g)} and \text{C}_{(s)} (in the form of soot) also formed as products. Incomplete combustion reactions are actually quite dangerous because one of the products in the reaction is carbon monoxide, not carbon dioxide. Carbon monoxide is a gas that prevents oxygen from binding to the oxygen transport proteins in our blood cells. When the concentration of carbon monoxide in the blood becomes too high, not enough oxygen can be transported and the person can die.

Example:

Identify whether each of the following reactions are complete or incomplete combustions, and then balance the equation.

  1. \text{C}_7\text{H}_{16(l)} + \text{O}_{2(g)} \rightarrow \text{CO}_{2(g)} + \text{H}_2\text{O}_{(l)}
  2. \text{C}_3\text{H}_{8(g)} + \text{O}_{2(g)} \rightarrow \text{CO}_{2(g)} + \text{H}_2\text{O}_{(l)}
  3. \text{CH}_{4(g)} + \text{O}_{2(g)} \rightarrow \text{CO}_{(g)} + \text{H}_2\text{O}_{(l)}
  4. \text{C}_5\text{H}_{12(l)} + \text{O}_{2(g)} \rightarrow \text{CO}_{2(g)} + \text{H}_2\text{O}_{(l)}
  5. \text{C}_2\text{H}_{6(g)} + \text{O}_{2(g)} \rightarrow \text{CO}_{(g)} + \text{H}_2\text{O}_{(l)}

Solution:

  1. Complete; \text{C}_7\text{H}_{16(l)} + 11 \ \text{O}_{2(g)} \rightarrow 7 \ \text{CO}_{2(g)} + 8 \ \text{H}_2\text{O}_{(l)}
  2. Complete; \text{C}_3\text{H}_{8(g)} + 5 \ \text{O}_{2(g)} \rightarrow 3 \ \text{CO}_{2(g)} + 4 \ \text{H}_2\text{O}_{(l)}
  3. Incomplete; 2 \ \text{CH}_{4(g)} + 3 \ \text{O}_{2(g)} \rightarrow 2 \ \text{CO}_{(g)} + 4 \ \text{H}_2\text{O}_{(l)}
  4. Complete; \text{C}_5\text{H}_{12(l)} + 8 \ \text{O}_{2(g)} \rightarrow 5 \ \text{CO}_{2(g)} + 6 \ \text{H}_2\text{O}_{(l)}
  5. Incomplete; 2 \ \text{C}_2\text{H}_{6(g)} + 5 \ \text{O}_{2(g)} \rightarrow 4 \ \text{CO}_{(g)} + 6 \ \text{H}_2\text{O}_{(l)}

This video contains classroom demonstrations of several reaction types and then shows the balancing process for the reaction equations (3a) http://www.youtube.com/watch?v=4B8PFqbMNIw (8:40).

Lesson Summary

The Five Types of Chemical Reactions
Reaction Name Reaction Description
synthesis two or more reactants form one product.
decomposition one type of reactant forms two or more products.
single replacement one element reacts with one compound to form products.
double replacement two compounds act as reactants.
combustion a hydrocarbon reacts with oxygen gas.

Further Reading / Supplemental Links

The website below also reviews the different types of chemical reactions.

Review Questions

  1. When balancing combustion reactions, did you notice a consistency relating to whether the number of carbons in the hydrocarbon was odd or even?
  2. Distinguish between synthesis and decomposition reactions.
  3. When dodecane, \text{C}_{10}\text{H}_{22}, burns in excess oxygen, the products would be:
    1. \text{CO}_2 + 2 \ \text{H}_2
    2. \text{CO} + \text{H}_2\text{O}
    3. \text{CO}_2 + \text{H}_2\text{O}
    4. \text{CH}_4\text{O}_2
  4. In the decomposition of antimony trichloride, which of the following products and quantities will be found?
    1. \text{An} + \text{Cl}_2
    2. 2 \ \text{An} + 3 \ \text{Cl}_2
    3. \text{Sb} + \text{Cl}_2
    4. 2 \ \text{Sb} + 3 \ \text{Cl}_2
  5. Acetylsalicylic acid (aspirin), \text{C}_9\text{H}_8\text{O}_{4(s)}, is produced by reacting acetic anhydride, \text{C}_4\text{H}_6\text{O}_{3(l)}, with salicylic acid, \text{C}_7\text{H}_6\text{O}_{3(s)}. The other product in the reaction is acetic acid, \text{C}_2\text{H}_4\text{O}_{2(l)}. Write the balanced chemical equation.
  6. When iron rods are placed in liquid water, a reaction occurs. Hydrogen gas evolves from the container, and iron(III) oxide forms onto the iron rod.
    1. Write a balanced chemical equation for the reaction.
    2. What type of reaction is this?
  7. A specific fertilizer is being made at an industrial plant nearby. The fertilizer is called a triple superphosphate and has a formula \text{Ca}(\text{H}_2\text{PO}_4)_2. It is made by treating sand and clay that contains phosphate with a calcium phosphate solution and phosphoric acid. The simplified reaction is calcium phosphate reacting with phosphoric acid to yield the superphosphate. Write the balanced chemical equation and name the type of reaction.

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