Balancing Chemical Equations (Simple and Redox Reactions)

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Types of Chemical Reactions

(1) Synthesis: combination of two or more elements to form a compound General form: A + B -> AB

            element   +    element     ->     Compound

Example: formation of water is a synthesis reaction: 2H2 + O2 --> 2H2O

(2) Decomposition: opposite of synthesis

                  One substance breaks down to form two or more simpler substance

General form: AB --> A + B

          compound   -->    element    +      element

(3) Double Replacement -two ionic compounds form two other ionic compounds

-occur when the elements in compounds replace each other or exchange places

-general formula: AS + XY --> AY + XB (when A + X are positive ions, B+ Y are negative ions)

-usually forms a precipitate--> insoluble solid that forms from solution, sinks to bottom.

Example: 2KI + Pb(NO3)2 --> PbI2 + 2KNO3

         ZnBr2 + 2AgNO3   -->  2AgBr   +  Zn(NO3)2
         Pb(NO3)2  +  2KI  -->  PbI2   +  2KNO3

(4) Single Replacement -Occur when one reactive element (metal or non-metal) is replaced by another element in a compound -general formula A + BC --> AC + B -One element and one compound as reactant, one element and one compounds as product -Example: Cl2 + 2KBr --> 2KCl + Br2

(5) Combustion -Reaction between an acid and base to form a salt and water

-General form: CSCY + O2 -eg. C2H6O3 + O2 --> CO2 + H2O

(6) Neutralization -Reaction of a compound or element with oxygen to form an oxide and produce heat.

-general form : CXCY + O2 --> CO2 + H2O

Simple Reactions

In many instances some simple rules can be employed to balance chemical reactions. These are listed below:

  1. Look for elements that only occur in one compound on each side of the equation and balance those first.
  2. Look for groups of atoms (polyatomic ions) that remain intact from one side of the equation to the other, balance these groups as a unit.
  3. If a reactant or product is present as a free element or monoatomic substance (e.g. Ca(s) or N2(g)) balance that last.

Redox Reactions

(i.e. reactions that involve electron transfers)