7.2 Types of Reactions

The walls of this cave are limestone. When hydrochloric acid is dropped on limestone, a chemical reaction occurs that produces a gas. Geologists use this reaction to determine whether a rock sample contains the mineral calcium carbonate, CaCO 3 .

7.2 Types of Reactions Classifying Reactions What are the general types of chemical reactions?

Some general types of chemical reactions are synthesis reactions, decomposition reactions, single-replacement reactions, double-replacement reactions, and combustion reactions.

7.2 Types of Reactions Classifying Reactions

Just as you can classify matter into different types, you can classify chemical reactions into different types. Reactions are often classified by the type of reactant or the number of reactants and products.

7.2 Types of Reactions Classifying Reactions Synthesis

A

synthesis reaction

is a reaction in which two or more substances react to form a single substance . • The reactants may be either elements or compounds. • The product synthesized is always a compound.

• The general equation for a synthesis reaction is A + B  AB

7.2 Types of Reactions Classifying Reactions Decomposition

A

decomposition reaction

is a reaction in which a compound breaks down into two or more simpler substances.

• The reactant in a decomposition reaction must be a compound.

• The products may be elements or compounds. • The general equation for a decomposition reaction is AB  A + B

7.2 Types of Reactions Classifying Reactions Single Replacement

A

single-replacement reaction

is a reaction in which one element takes the place of another element in a compound. Single replacement reactions have the general form A + BC  B + AC

7.2 Types of Reactions Classifying Reactions

As copper replaces silver in the silver nitrate solution in a single-replacement reaction, the solution turns blue, and silver crystals form on the wire.

Cu + 2AgNO 3  2Ag + Cu(NO 3 ) 2

7.2 Types of Reactions Classifying Reactions Double Replacement

A

double-replacement reaction

is one in which two different compounds exchange positive ions and form two new compounds. • The general form of a double replacement reaction is AB + CD  AD + CB • Two replacements take place in this reaction. A replaces C, and C replaces A.

7.2 Types of Reactions Classifying Reactions

When potassium iodide solution is poured into a solution of lead(II) nitrate, a double-replacement reaction takes place. Lead(II) iodide forms as a yellow precipitate. Potassium nitrate remains in solution.

Pb(NO 3 ) 2 + 2KI  PbI 2 + 2KNO 3

7.2 Types of Reactions Classifying Reactions

When geologists test the calcium carbonate content in a rock, they use a double replacement reaction.

CaCO 3 + 2HCl  CaCl 2 + H 2 CO 3 The product carbonic acid,H 2 CO 3 , then decomposes into carbon dioxide gas and water.

H 2 CO 3  CO 2 + H 2 O

7.2 Types of Reactions Classifying Reactions Combustion

A

combustion reaction

is one in which a substance reacts rapidly with oxygen, often producing heat and light. The main component of natural gas is methane. When methane burns in oxygen, a combustion reaction occurs. CH 4 + 2O 2  CO 2 + 2H 2 O

7.2 Types of Reactions Classifying Reactions

A Bunsen burner generates heat and light by the combustion of natural gas.

7.2 Types of Reactions Reactions as Electron Transfers How did the discovery of subatomic particles affect the classification of reactions?

The discovery of subatomic particles enabled scientists to classify certain chemical reactions as transfers of electrons between atoms.

7.2 Types of Reactions Reactions as Electron Transfers

A reaction in which electrons are transferred from one reactant to another is called an

oxidation-reduction reaction,

or redox reaction.

7.2 Types of Reactions Reactions as Electron Transfers Oxidation

For a long time, people have known that metals react with oxygen. Calcium reacts with oxygen and forms calcium oxide (CaO). Iron reacts with oxygen and forms rust, or iron(III) oxide (Fe 2 O 3 ).

These types of synthesis reactions, in which a metal combines with oxygen, have been classified as oxidations.

7.2 Types of Reactions Reactions as Electron Transfers Reduction

The process in which an element gains electrons during a chemical reaction is called reduction.

As calcium atoms lose electrons during the synthesis of calcium oxide, the oxygen atoms gain electrons. As each neutral oxygen atom gains two electrons, it becomes an ion with a charge of 2 – .

O + 2e –  O 2 –

7.2 Types of Reactions Reactions as Electron Transfers

Oxidation and reduction always occur together. • When one element loses electrons, another element must gain electrons.

• A reactant is said to be reduced if it gains electrons. A reactant is oxidized if it loses electrons.

• Oxidation-reduction reactions do not always involve complete transfers of electrons. For example, in the synthesis of water, hydrogen is oxidized, and oxygen is reduced, but the transfer of electrons is only partial.

7.2 Types of Reactions Assessment Questions

1. Which of these statements best describes a double replacement reaction? a.

b.

c.

d.

Two different compounds exchange positive ions and form two new compounds.

An element takes the place of another element in a compound.

One compound breaks down into two or more simpler substances.

Two or more substances react to form a single substance.

7.2 Types of Reactions Assessment Questions

1. Which of these statements best describes a double replacement reaction? a.

b.

c.

d.

Two different compounds exchange positive ions and form two new compounds.

An element takes the place of another element in a compound.

One compound breaks down into two or more simpler substances.

Two or more substances react to form a single substance.

ANS: A

7.2 Types of Reactions Assessment Questions

2. Which of the following statements about oxidation reduction reactions is true?

a. Oxidation-reduction reactions always involve a transfer of protons between atoms.

b. Oxidation is the process in which electrons are gained.

c. Oxidation and reduction always occur together.

d. Oxidation-reduction reactions always involve oxygen as one of the reactants.

7.2 Types of Reactions Assessment Questions

2. Which of the following statements about oxidation reduction reactions is true?

a. Oxidation-reduction reactions always involve a transfer of protons between atoms.

b. Oxidation is the process in which electrons are gained.

c. Oxidation and reduction always occur together.

d. Oxidation-reduction reactions always involve oxygen as one of the reactants.

ANS: C