Chemical Reactions - Part II

2. How to balance a reaction
A chemical reaction is always written as an equation. The left side of the equation represents the reacting compounds or reactants and the right side of the equation represents the end result of the reaction or the products. A chemical equation should give a reader all the information about the reaction.

While writing a chemical equation the following considerations have to be strictly observed :

• The number of atoms on the reactant side should be the same as the number of atoms on the product side. The reaction should be balanced.

          Example :  Ag₂O         Ag + O₂  (unbalanced)

                        2Ag₂O        4Ag   + O₂  (balanced)

• A horizontal arrow should indicate in which direction the reaction is proceeding. Normally the arrow is from left to right  . Only for a reversible reaction, the arrow can be shown from the right to left.

• In case of precipitation of a compound after reaction, a vertically downward arrow is indicated 
  (). In case  a product is gaseous a vertically upward arrow is indicated (  ).  

• Physical state of the reactants and the products can be shown. For example (s) or (l) means the compound is in solid or liquid form, (aq) means the compound is dissolved in water.

          Example :  Zn (s) + H₂SO₄  (aq)         ZnSO₄ (aq)  + H₂

                     Ca(OH)₂ (aq)   +  CO₂ (g)         CaCO₃  + H₂O (l)  

• Conditions under which the reaction is taking place can also be indicated. For example if heat is required to initiate the reaction (endothermic reaction) it is written over the horizontal arrow or on the left side of the equation. If heat is given out (exothermic reaction) then it is written on the right side of the equation. If any catalyst is used for increasing the reaction rate, then it is written below the horizontal arrow.

                                                     (heat)
           Example : 2KClO₃  (s)                  2KCl (s) + 3O₂   (g)
                                                      MnO₂
                                                    (catalyst)

                                                                         sunlight
                         6CO₂   (g)      +     6H₂O  (l)              C₆H₁₂O₆  (s)    +   6O₂
                                                                        chlorophyll

•  While writing down a chemical reaction,  the chemical formula written down should indicate stable chemicals.

    Example :   Mg   +   O         MgO

          is incorrect even if the equation is balanced. Oxygen is a stable molecule as O₂ and not as O.
          The correct way to write the above equation is

          2Mg    +    O₂        2MgO

A chemical reaction can be written in terms of ions also. Such chemical equations are called ionic equations. For balancing an ionic equation, same procedures have to be followed as above. But in this case an additional criteria has to be kept in mind : the ionic charges have to be balanced also.

Example :     Al      +     H⁺                Al³⁺      +  H₂

First balance the atoms on both the sides of the reaction.  We see that we need two atoms of H on the left hand side.

                     Al      +            2H⁺              Al³⁺      +  H₂

The ionic charges on both the sides are not balanced. The left side has 2+ and the right side has 3+. If we multiply the charges by 3 and 2 on both the sides respectively, we see that  we get

                     Al      +            6H⁺               2Al³⁺    +  H₂

Now the charges are balanced. But there is an imbalance in hydrogen. The correct ionic equation will be

                   Al      +            6H⁺               2Al³⁺    +  3H₂

The equation is balanced completely on both the sides as far as the number of atoms and ionic charges are concerned.

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