“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 equations 5 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  AX A, X can be elements or compounds AX 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 + X AX is a compound A, 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 elements AX, 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 + BX A, X, B and Y as reactants are ions AY 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