notes: ch 8-section 3 “classifying chemical reactions”

Notes: Ch 8-Section 3“Classifying Chemical Reactions”

Classifying reactions

reasons for classification:• helps to predict what products

will form• recognizing patterns can help

when balancing equations5 different types of reactions are

introduced in chapter 8• additional types exist• note---some reactions can be

classified as more than 1 type• note---some reactions do not “fit” any

type

Reaction Types: Combustion Reactions

• (def)- the oxidation reaction of an organic compound in which heat is released

• often used to generate energy; occurs when a substance combines with oxygen releasing a large amount of energy in the form of heat and light

Many Combustible Compounds are Hydrocarbons

• hydrocarbon (def)- a compound composed of only carbon and hydrogen

• the combustion of gasoline produces energy used for transportation

• example: burning of propane

  C3H8 + 5O2 3CO2 + 4H2O

Combustion Reaction---General Pattern:hydrocarbon + oxygen carbon dioxide +

water

Figure 9 p. 276

The complete combustion of any hydrocarbon, such as methane, yields only carbon dioxide and water

Combustion Reactions (cont.)

• some combustible compounds are not hydrocarbons

• alcohols (compounds made of carbon, hydrogen and oxygen) will also combust

• example: combustion of ethanol

  CH3CH2OH + 3O2 2CO2 + 3H2O

Combustion Reactions (cont.)

• if enough oxygen is not available, combustion reactions will be incomplete and carbon monoxide and unburned carbon (soot) will be produced along with the carbon dioxide and water

Reaction Types: Synthesis Reactions• (def)- a reaction in which 2 or more

substances combine to form a new compound

• “synthesis” (from Greek) means “to put together”

• general pattern:

  A + X AXA, X can be elements or compoundsAX is a compound

Synthesis Reactions (cont.)• two elements will form a binary compound• examples:

Na + Cl NaCl (binary ionic compound)

C + O2 CO2 (binary molecular compound)

Figure 10 p. 277

When the elements magnesium and oxygen react, they combine to form the binary compound magnesium oxide

Synthesis reactions (cont.)• compounds can form a ternary

compound ( a compound composed of 2 or more elements)

  CaO(s) + H2O(l ) Ca(OH)2(s)

• some oxides of non-metals can combine with water to produce acids

CO2(g) + H2O(l) H2CO3(aq) (carbonic acid)

Reaction Types: Decomposition Reactions

• (def)- a reaction in which a single compound breaks down to form two or more simpler substances

• general pattern:

AX A + XAX is a compoundA, X can be elements or compounds

• are the opposite of synthesis reactions

• often need heat or electricity to proceed

Decomposition Reactions (cont.)• binary compounds usually decompose

back into the 2 elements that compose them

• example: decomposition of water  2H2O(l) electricity 2H2(g) + O2(g)Figure 11 p. 278

Nitrogen triiodide is a binary compound that decomposes into the elements nitrogen and iodine.

Decomposition Reactions (cont.)• compounds made up of 3 or

more elements usually do not decompose back into those elements

• example: decomposition of limestone (CaCO3)

CaCO3(s) heat CaO(s) + CO2(g)

• many synthesis reactions can be reversed to become decomposition reactions

Reaction Types: Displacement Reactions (Single Replacement Reactions)• (def)- a reaction that occurs when 1 element

replaces a similar element in a compound• general pattern:

A + BX AX + B (or)Y + BX BY + X

A, B, Y and X are elementsAX, BX and BY are compounds

• commonly take place in aqueous solutions• usually require a smaller amount of energy

than synthesis or decomposition reactions

Reactivity is Ranked by Activity Series• activity series

(def)- a series of elements that have similar properties and that are arranged in descending order of chemical reactivity

Activity Series (cont.)• elements are arranged in order, with

the most active ones on top (see Table 4 p. 281 and Appendix A p. 832)

• in general, any element listed can displace those below it, but not above it

• allows predictions about displacement reactions to be made

Figure 12 page 280

Copper is the more active metal and displaces silver from the silver nitrate solution. So copper is higher on the activity series than silver is. The Cu2+ formed gives the solution a blue color

Reaction Types: Double-Displacement Reactions (Double Replacement Reactions)

• (def)- a reaction that occurs when the ions of 2 compounds exchange places in an aqueous solution to form 2 new compounds

• general pattern:

AX + BY AY + BXA, X, B and Y as reactants are ionsAY and BX as products are compounds

Double-Displacement Reactions (cont.)

• takes place in aqueous solution• one of the compounds

produced is usually a precipitate, an insoluble gas or a molecular compound

• the other compound is often soluble and remains in solution

Figure 13 page 283

• example: the formation of solid lead(II) iodide from an aqueous solution of potassium iodide and lead(II) nitrate

• equation: 2KI(aq) + Pb(NO3)2(aq) PbI2(s) + 2KNO3(aq)

Figure 13 page 283