chapter 21

Chapter 21

Types of Reactions

21.1 Classifying Reactions

Most of the products you use everyday are the result of chemical reactions.

This chapter will help you recognize the types of reactions and predict what types of substances will be produced.

Symbols You Should Learn..

(s) = The substance is a solid.(l) = The substance is a liquid.(g) = The substance is a gas.(aq) = The substance is dissolved in

solution (aqueous).

Addition Reactions

In an Addition Reaction, two or more substances combine to form a new compound.

For example:– Fe(s) + O2(g) Fe2O3(s)

So… A + B AB

Addition Reactions cont…

Polymerization, or the formation of polymers, is a series of addition reaction taking place to produce a very large molecule.

Polymers are made by joining smaller molecules called monomers.

monomerspolymer + monomer large polymer

Decomposition Reactions

A chemical reaction in which a single compound is broken down to produce two or more smaller compounds is called a Decomposition Reaction.

For Example:– 2H2O(l) 2H2(g) O2(g)

Decomposition Reactions cont…Larger compounds can also

decompose to produce other compounds, as in the decomposition of baking soda with heat:

For example:– 2NaHCO3(s) CO2(g) + Na2CO3 + H2O

So… AB A + B

Single-Displacement Reactions

In Single-Displacement Reactions, one element replaces a similar element in a compound.

For example, if you place an iron nail into a beaker of copper (II)chloride, you will begin to see a reddish copper forming on the iron nail. In this reaction, iron replaces copper in the solution and the copper falls out of the solution as a metal.

Single-Displacement Reactions cont…Equation:

– Fe(s) + CuCl2(aq) FeCl2(aq) + Cu(s)

So…AX + B BX + A

AX is the compoundB is the elementBX is the compoundA is the element

Double-Displacement Reactions

In Double-Replacement Reactions, ions from two compounds in solution exchange places to produce two new compounds.

The formation of a precipitate occurs when one of the compounds formed in a double-displacement reaction is insoluble, or does not dissolve in water.

Double-Displacement cont…

For example: – Pb(NO3)2(aq) + 2KI(aq) PbI2(s) +

2KNO3(aq)So… AB + CD AD + CB

Combustion Reactions

In a Combustion Reaction, a substance combines with oxygen to release energy.

For example, in the combustion of natural gas to heat a house, methane (natural gas) combines with oxygen to produce carbon dioxide and water:

CH4 + 2O2 CO2 + 2H2O So… Carbon Compound + O2(g) CO2 +

H2O(g)

21.2 Energy In Reactions

In many reactions, less energy is required to break the bonds in the reactants than is released when bonds are formed to make new products.

In these types of reactions, called Exothermic reactions, some type of energy is released.

Exothermic cont…

For example: The combustion of gasoline to run automobiles is an exothermic reaction.

Exothermic reactions are used every day to heat our homes.

Exothermic reactions can be detected by measuring a rise in temperature.

Endothermic Reactions

Sometimes more energy is required to break the bonds in the reactants than is released from the formation of new bonds in the products.

In these reactions, called Endothermic reactions, more energy must be provided for the reaction to take place than is released.

Endothermic reactions are measured by a decrease in temperature.

Endothermic Reactions cont…

An instant “cold pack” is an example of an endothermic reaction. To activate the cold pack, you squeeze the plastic bag to release the water.

When the water contacts the ammonium nitrate crystals, a reaction occurs and the pack becomes icy cold.

Endothermic Reaction cont…

Besides being endothermic, this reaction is also called a dissolution reaction.

This reaction occurs when an ionic compound dissolves in water to make an ionic solution.

THE END!!!!!