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