Reaction Types Section 5.2

Objectives Distinguish among five general types of chemical reactions Predict the products of some reactions based on the reaction type Describe reactions that transfer or share electrons between molecules, ions, or atoms

But why? Scientists are lazy. Classifying reactions allows us to make predictions about what will happen in a reaction without thinking about it too hard. Good news for you: Knowing the reaction type will help you make decisions on a test regarding the outcomes of such equations.

The 5 Reaction Types Synthesis Decomposition Combustion Single-displacement Double-displacement

Synthesis Reactions Definition: Reaction of at least two substances that forms a new, more complex compound General Reaction format: A + B  AB Example: Na + Cl2  NaCl (not balanced) 2Na + Cl2  2NaCl (balanced!)

More about Synthesis Reactions Synthesis  Synthesize  Make something new What kind of reaction is “Photosynthesis?” What does photosynthesis synthesize? 6CO2 + 6 H2O  C6H12O6 + 6O2

Decomposition Reactions Definition: A reaction in which one compound breaks into at least two products General Reaction format: AB  A + B Example: H2O  H2 + O2 (not balanced) 2H2O  2H2 + O2 (balanced!)

More about Decomposition Reactions Decomposition  decompose  break something down How do synthesis and decomposition compare? Electrolysis (using an electric current) is a type of decomposition reaction.

Combustion Reactions Definition: A reaction in which a compound and oxygen burn General Reaction format: A + oxygen  stuff (usually water, CO2) Example: CH4 + O2  CO2 + H2O (not balanced) 2CH4 + 4O2  2CO2 + 4H2O (balanced!)

More about Combustion Reactions Combustion  combust  burn or explode Can synthesis and combustion reactions be alike? Sure. The reaction of hydrogen and oxygen to produce water is both a synthesis and combustion reaction. As oxygen in a combustion reaction is reduced, the products contain more carbon monoxide, then finally solid carbon (soot).

Single-displacement Reactions Definition: A reaction in which atoms of one element take the place of atoms of another element in a compound General Reaction format: XA + B  BA + X Example: CuCl2 + Al  AlCl3 + Cu (not balanced) 3CuCl2 + 2Al  2AlCl3 + 3Cu (balanced!)

More about Single-displacement reactions A locked-up atom is released while a free atom is locked-up. (false arrest?) Potassium and water exhibit a single-displacement reaction K + H2O  KOH + H2 How about the Thermite reaction? Iron oxide + Aluminum  Aluminum oxide + iron

Double-displacement Reactions Definition: A reaction in which a gas, a solid precipitate, or a molecular compound is formed from the apparent exchange of ions between two compounds prisoner exchange? General Reaction format: AX + BY  AY + BX Example: Pb(NO3)2 + K2CrO4  PbCrO4 + 2KNO3

Loose ends Reduction-oxidation (redox) reaction: A reaction that occurs when electrons are transferred from one reactant to another (useful in batteries) Radicals: Fragments of molecules that have at least one electron available for bonding (useful for making polymers like Styrofoam)