Ion–electron method for Basic or Neutral solutions:
For Example: The reaction between permanganate ion and iodide ions
occurs in neutral solutions. The unbalanced skeleton reaction is
The obvious pairs for the half reactions are:
In the second half–reaction the iodine atoms needed to be balanced:
The first half–reaction needs two water molecules to balance the oxygen atoms:
Four hydrogen ions on left side of first–half reaction balances the hydrogen atoms
The next step is the balancing of charges. We find that the first half reaction
has a total charge of +3 on the left and 0 on the right, while the second half–reaction
has a total charge of −2 on the left and 0 on the right. We add three electron to
the first half reaction on the left and two electrons to the second half reaction
on the right:
To equalize the electrons, we, multiply the first half reaction by 2 and the second
half reaction by 3
We add the equations
And cancel the 6 electrons
This reaction is balanced in acidic solution. To convert to a basic solution, we use
step 7 i.e., add 8OH− ions to each side.
The 8H+ and 8OH− on the left are combined to make 8H2O.
We then cancel 4H2O from each side to simplify the final equation:
Therefore, the balanced equation for neutral solution of permanganate ion and iodide ions is thus obtained by the Ion–electron method.
In order to balance the complex oxidation–reduction reactions within a length of time, chemists have developed a simple and most versatile method known as Ion–electron method. The reactions which occur in acid solutions (containing H+ ions), can be balanced by using Ion–electron method, which in turn involves six steps.
Step 1:
In the first step the 2 reactants are separated to give the same product known as skeleton reaction. The first reactant giving the product is called the first half–reaction. The second part of the reactant giving the same product is called the second half reaction.
Step 2:
In the second step, the atoms on the reactant side are balanced by using the coefficient x, which is equal to the number of atoms in the product side (right side) in both (first half and second half) half–reactions.
Step 3:
In the third step, water molecules (H2O) are added to any one or both half–reactions on the reactant side (left side) of the reaction, which contain oxygen, in order to balance the oxygen atoms on the product side (right side) of the reaction.
Step 4:
In the fourth step, hydrogen atoms (H+ ions) on the product side (right side), in one or both half–reactions are balanced by adding the number of H+ ions on the reactant side(left side). Till now all the atoms are balanced in both half–reactions.
Step 5:
In the fifth step, all the ions on both half−reactions are balanced, by adding the number of electrons (e−) either on the product side or reactant side, such that the total charge is zero on both sides of two half–reactions.
Step 6:
In sixth step, the electrons are equalized in the two half–reactions (first–half and second–half), by multiplying either the first–half reaction or second–half reaction with a certain number such that both half–reactions have same number of electrons.
Now, the reactants of first–half reaction and the second half–reaction are written on the reactant side (left side) and the products of first–half reaction and second–half reaction are written on the product side (right side) resulting in a equation.
Cancel the number of electrons from each side, add the number of same atoms on each side and divide by a coefficient 2 to obtain the final balanced equation. Thus, a balanced equation in acid solution (contains H+ ion) is obtained by using Ion–electron method.
The reactions which occur in basic or neutral solutions (containing OH− ions), can be balanced by using Ion–electron method, which in turn involves seven steps. All the six steps are common for balancing all reactions which occur in acidic, basic or neutral solutions but in addition, one more step is added to the six steps for basic or neutral solutions, in order to balance the OH− ions.
Step 7:
Add one OH− ion for each H+ ion to both sides of the equation in step 6. Combine the H+ and OH− ions on one side of the reaction into water molecules. Cancel H2O molecules that appear on both sides of equation, and thus, a balanced equation in basic or neutral solution (containing OH− ion) is obtained by using Ion–electron method.
All the atoms must be balanced in the reaction. In addition, in ionic reaction the charges must also balance. In this equation we count total charge of 0 on both sides of arrow.