T YPES OF C HEMICAL R EACTIONS AND E NERGY IN R EACTIONS

T YPES OF C HEMICAL R EACTIONS There are many types of chemical reactions They have been classified into five types Synthesis, Combustion Decomposition, Single Replacement, and Double Replacement Each type of reaction follows a certain set of rules

S YNTHESIS R EACTIONS A synthesis reaction is when two or more substances combine to form another substance A general formula for a synthesis reaction is A + B -> AB 2Na + Cl 2 ->2NaCl

D ECOMPOSITION R EACTION

S INGLE R EPLACEMENT R EACTION Also called a single displacement reaction A single replacement reaction is when one element replaces another in a reaction The general formula of a single replacement reaction is A + BC -> AC + B 2Na + 2H 2 O--> 2NaOH + H 2

D OUBLE R EPLACEMENT R EACTION Also called a double displacement reaction A double replacement is when the positive ion of one compound replaces that of another to create two new compounds A double replacement reaction takes place if a precipitate, water, or a gas forms when two ionic compounds in a solution are combined A precipitate is an insouluble compound that comes out of a solution The general fromula for a double replacement reaction is AB + CD -> AD + CB AgNO 3 + NaCl → AgCl + NaNO 3

C OMBUSTION R EACTIONS A combustion reaction occurs when a substance reacts with oxygen to produce energy in the form of heat and light Combustion reactions also produce one or more products that contain the elements of the reactants Many combustion reactions can also be classified as other types 2 H 2 + O 2 → 2 H 2 O(g) + heat

O XIDATION -R EDUCTION R EACTIONS Many reactions cause substances to gain or lose electrons Oxidation describes the loss of electrons Reduction describes the gain of electrons Many reactions involving electron transfer involve oxygen pulling electrons from metallic elements The substance that gains electrons becomes more negative and and we say it is reduced The substance that loses electrons becomes more positive and we say it is oxidized Reduction always happens with oxidation, and is commonly called redox.

E NERGY E XCHANGE IN C HEMICAL R EACTIONS Chemical bonds are the source of energy in a chemical reaction When bonds are broken this requires energy When new bonds are formed, energy is released Combustion reactions require much less energy to break the bonds than the energy released when the new bonds are formed

M ORE E NERGY O UT Chemical reactions that release energy are called exergonic reactions In these reactions, less energy is required to break the bonds than is released As a result of this, some form of energy is released, such as light or heat When the energy given off by a reaction is primaraly heat, the reaction is called exothermic Burning wood, exploding dynamite, and rusting metal are all exothermic reactions Exothermic reactions provide most of the power used in homes and industry

M ORE E NERGY I N A recation that requires more energy to break bonds than is released is called endergonic The energy absorbed can be in the form of light, heat, or electricity When the energy needed for a reaction is in the form of heat, the reaction is called endothermic Endothermic reactions can absorb enough heat to freeze water Any form of cooking is usually an endothermic reaction, photosynthesis

C ATALYSTS AND I NHIBITORS A catalyst is a substance that speeds up a chemical reaction without being permanently changed itself Catalysts can be reused in another chemical reaction Inhibitors are substances that are used to slow a chemical reaction