How to balance chemical equation?

How to balance chemical equation?

Since, no matter is destroyed or created during chemical reaction, i.e. energy is never destroyed nor created, there must be equal number of element present in product side as it was in reactant side. That is the reason why we need to balance chemical reaction. Balancing a chemical equation is done by putting suitable integer before the elements or compounds in both reactant and product side.

There are two methods of balancing chemical equation:

1. Hit and Trail Method
2. Partial equation method.

Hit and Trail method: In this method the equation is balanced by trial and error methods. There is no definite rules to balance the equation but simple guessing. Generally hit and trial method is applicable for simple chemical reaction.

H₂ + Cl_{2    →}   2HCL

Following step should be noted while balancing a chemical equation:

1. The atom which occurs at minimum number of places on both sides should be selected first and the one occurring maximum number of times should be taken last of all. Let’s illustrate this by following equation:

Na+ H₂O  →  NaOH +H₂

Let’s look at each element, Na and oxygen is in same number in both product and reactant side, but there is two hydrogen atom in reactant side while in product side there is three.  So let’s try to balance the number of hydrogen atom.

If we multiply H₂O by 2 in L.H.S, then number of hydrogen becomes 4, and that of oxygen becomes 2. IN the product side, if we also multiply NaOH by 2, then total hydrogen becomes 4 and oxygen becomes 2. So hydrogen and oxygen in both side becomes equal, i.e balanced. However, while multiplying NaOH in product side by 2, the number of Na becomes 2 in product side, but it is only one in reactant side. So, to balance this, we again multiply Na in reactant side by two, so that overall number of atoms of element becomes equal.

So, final balanced chemical equation becomes,

2Na + 2H₂O →  2NaOH +H₂

Partial equation method:

Partial equation method is used for balancing complex chemical equations. There are chemical equation which involves many steps and reacting elements occur in more than one products in the product side. In such cases, balancing by Hit and Trial method is difficult. That is why partial equation method is used to balance such chemical equations.

In Partial equation method, equation is first divided into partial equation, which are simply probable steps that might occur in the chemical reaction. These probable steps are then balanced by hit and trail method and finally added. Following steps are applied in balancing chemical equation by partial equation method.

1. At first, different probable steps are written for the given chemical equation. These probable steps are simply the partial chemical equation which we can write for easily balancing the reactants and product. Note that the probable steps may or may not occur in real.
2. After breaking down the probable steps for the chemical equation, the partial equations are individually balanced by using Hit and Trial Method.
3. Such balanced partial equations are multiplied by suitable integer. This is done if required, so that the elements which are not formed in the product side of the overall chemical equation is canceled out.
4. Finally, the balanced partial equations are added to get the final overall balanced equation.

Lets’ illustrate some examples:

PbS + O₃    →PbSO₄ + O₂

In the given reaction, the atom of Pb and S is balanced in both reactant and product side. But there is 3 atom of oxygen in reactant side and 6 oxygen atom in product side. So let’s balance this chemical equation by partial equation method.

• First let’s think the possible step for occurring of the given chemical reaction. There is lead sulphide reacting with ozone, it means ozone must be acting as oxidizing agent which liberates nascent oxygen. So the first step can be generation of nascent oxygen by decomposition of ozone molecule.

Step-1

O₃  →O₂ + [O]

This liberated nascent oxygen can react with PbS to give lead sulphate and oxygen as;

Step-2

PbS + [O] → PbSO₄

This partial step is not balanced reaction. There is equal number of Pb and S atom, but no. of oxygen is one in reactant side but four in product side. So we can multiply nascent oxygen in reactant side by four.

PbS + 4[O] → PbSO₄

If we make four nascent oxygen in second probale step, then we must also need to make it equal number of nascent oxygen liberated in step one. So we need to multiply step one with integer four

{ O₃  →O₂ + [O]    }    × 4

Now lets add both partial equation:

{ O₃   →O₂ + [O]    }  × 4

PbS + 4[O] → PbSO₄

PbS  + 4O₃   PbSO₄ + 4O₂

This is the balanced chemical equation.

Other equations:

KMnO₄ + H₂SO₄  + H  →K₂SO4 + MnSO₄ + H₂0

In The given equation, except Mn all other atoms are not balanced. So let’s first break down the equation into partial equations and then balance them by Hit and Trial method.

Potassium permanganate is strong oxidizing agent, so it always discharge nascent oxygen.

1.  KMnO₄  → K₂O + MnO + O

Balancing this equation by Hit and Trial method.

2KMnO₄   → K₂O + 2MnO + 5O

 

2. K₂O + H₂SO₄ →K₂SO₄ +H₂O

This is already balanced so we don’t need to balance this equation.

3. MnO + H₂SO₄  →MnSO₄ +H₂O

This is also balanced chemical equation.

4. H+ O →H₂O

We can simply balance this reaction by simply putting 2 in the hydrogen in reactant side.

2H+ O  →H₂O

Now,

Lets’ add all the partial equation and cancel out any common atoms which don’t appear in product. In the product side there is no MnO and O, so we need to cancel out these. To cancel out these we can make them equal in number by multiplying the whole reaction with suitable integer. In this case, we have multiplied last and second last partial reaction by 5 and 2 to make O and MnO equivalent in both product and reactant side. Once all the reaction is balanced, we can get the balanced chemical equation by adding partial equation.

2KMnO₄   →K₂O + 2MnO + 5O

K₂O + H₂SO₄  →K₂SO₄ +H₂O

[MnO + H₂SO₄  → MnSO₄ +H₂O ] ×2

[2H+ O  →H₂O]  ×5

2KMnO₄ + 3H₂SO₄ + 10 H   →K₂SO₄ + 2MnSO₄ + 8H₂O

This is the balanced chemical equation.