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:
- Look for elements that only occur in one compound on each side of the equation and balance those first.
- 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.
- 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)