The Nature of Chemical Reactions

Chemical Reactions Occur Everywhere  Words like:  Grow  Ripen  Burn  Cook  Digest  Rust  These are all a result of chemical reactions

Chemical Reactions Change the Composition of a Substance  Evidence a chemical reaction has occurred include:  Formation of a precipitate  Gas production  Color change  Heat released  Heat absorbed  Light produced  Electricity produced

Energy is Usually Needed to Start a reaction  Energy is needed to break bonds and rearrange the molecule or compound.  Energy is released when new bonds are reformed.  Energy needed the start a reaction is called activation energy.  Reactions that do not require outside energy to start are called spontaneous reactions.

 Conservation of Energy.  Energy is never created or destroyed. It changes form.  Chemical energy stored in bonds called chemical potential energy.  Chemical potential energy is converted to thermal energy and light when wood is burned.  Chemical reactions that release thermal energy are called exothermic reactions.  Chemical reactions that absorb energy from their surroundings are called endothermic reactions.

Reactants and Products  C 6 H 12 O 6 + 6O 2 6H 2 O + 6CO 2  Reactantproducts  If the reaction is endothermic it will store more chemical potential energy in the products than the reactants.  If the reaction is exothermic it will store more chemical potential energy in the reactants than the products.

Classifying Reactions  Five Main Types:  Synthesis  Decomposition  Combustion  Single Displacement  Double Displacement

Synthesis  When two substances are joined to form a single more complex compound. A + B A B 2Na + Cl 2 Na Cl 2 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 Products store more chemical potential energy

 Na 2 Cl 2 2NaCl 2  Na 2 Cl 2 NaCl 2 NaCl 2

Decomposition  When one compound or molecule is broken down into parts. AB A + B 2H 2 O 2H 2 + O 2 2H 2 O 2H 2 + O 2 Disacharride monosacharride + monosacharride

Combustion  One of the reactants is always oxygen and the other is usually carbon based.  The products are usually CO 2 and H 2 O. 2CH 4 + 4O 2 2CO 2 + 4H 2 O

Single Displacement  When one element of a compound is replaced by a more reactive element. XA + B BA + X 3CuCl 2 + 2Al 2AlCl 3 + 3Cu All alkali and some other metals undergo single displacement reactions with H 2 O and can be explosive because hydrogen gas is produced. All alkali and some other metals undergo single displacement reactions with H 2 O and can be explosive because hydrogen gas is produced.

Double Displacement  When two compounds exchange components with each other. AX + BY AY + BX Pb(NO 3 ) + K 2 CrO 4 PbCrO 4 + 2KNO 3

Another Way to Classify Reactions  Look at the electrons that have been transferred from one compound to another.  Oxidation-Reduction (REDOX) reactions.  Substances that receive electrons are said to have been reduced.  Substances that lose electrons are said to have been oxidized.  These reactions are linked.  If something is oxidized than something else in the reaction has to have been reduced

REDOX Reactions  When iron reacts with oxygen to form rust Fe 2 O 3 :  Each iron atom loses 3 electrons to form the ion Fe +3.  Each Oxygen atom gains two electrons to form the ion O -2.  Some redox reactions do not involve ions.  In this case oxidation is the gain of oxygen or loss of hydrogen.  Reduction is the loss of oxygen and gain of hydrogen.  Respiration and Combustion are redox reactions because O 2 reacts with carbon compounds to produce CO 2.  Carbon atoms in CO 2 are oxidized while O 2 atoms are reduced.