How is a chemical equation written, read & balanced by a starter?

by  Maryam Hussain

What is a chemical equation?

A chemical equation is the symbolic representation of a chemical reaction in terms of chemical formulas. Before writing any chemical equation, a step is there that symbols and formulas are made available for the substances involved in chemical processes, in addition to the names. These symbols and formulas are analogous to symbol and variables used in equation of physics, like that used in Newton’s second law of motion F = ma, and provide a type of chemical shorthand that chemists have found valuable because of their convenience. 

When the multipliers of reactants and products, called coefficients, in a chemical equation, are correctly given, the numbers of atoms of each element are equal on both sides of the arrow. The equation is then said to be a balanced chemical equation.

We will illustrate the concept through a step-by-step procedure starting from writing & completing it to balancing of it.

Step#1

In English narration, for example, the burning of sodium in chlorine to produce sodium chloride is written as

Sodium     +    Chlorine    =    Sodium chloride

Step#2

In the language of chemistry, the same reaction may be written as

Na    +   Cl     ⇀    NaCl

Step#3

Correct the equation for the elemental or combined state of reactants & products, because chlorine is found as a diatomic molecule in nature

Na    +    Cl₂    ⇀    NaCl

Step#4

Perform the balancing of the equation, by multiplying each reactant with a numeral, where required, to make it in agreement with the law of conservation of mass

2Na    +    Cl₂    ⇀    2NaCl

Step#5

Calculate the mass of reactants and products to satisfy the law of conservation of mass

Reactants	Products
Na = 2 x Atomic weight of Na  = 2 x 23 g = 46 g Cl =  2 x Atomic weight of Cl   =   2 x 35.5 g = 71 g   Total weight of reactants  =  117 g	NaCl = 2 x formula weight of NaCl = 2 x (Atomic weight of Na + Atomic weight of Cl) = 2 x (23 g + 35.5 g) = 2 x 58.5 g                                                              Total weight of products  =  117 g

 

Because the total mass of all reactants (117 grams) is equal to the total mass of all products (117 grams) so the law of conservation of mass has been followed and the given chemical equation has been balanced.

Step#6

Label the state of each reactant and product in an equation. You do this by placing appropriate labels indicating the state within parentheses following the formulas of the substances. You use the following phase labels

solid (s)

liquid (l)

gas (g)

2Na (s)    +    Cl₂ (g)     ⇀     2NaCl (s)

Now read the equation as “two gram-atoms of sodium metal react with one-gram mole of chlorine gas to produce 2 grams of formula units of sodium chloride solid”.

The art of balancing a chemical equation is a trial-and-error method by which we try various coefficients (multiplier present on the left side of any reactant or any product) on both sides, going back and forth, and so on until the equation is balanced. The general process of balancing a chemical equation has been described in the above illustration. Now balancing of some single-lined complex chemical reactions will be elaborated.

Illustration-02

Balance the following chemical equation:

Pb(NO₃)₂ + AlCl₃ → Al(NO₃)₃ + PbCl₂ 

Solution

Put arbitrary symbols for each of the reactants and products.

A        +   B     →       C        +    D

Where

A  : Pb(NO₃)₂

B  : AlCl₃

C  : Al(NO₃)₃

D  : PbCl₂

Now either construct the following table or directly go to the trick of balancing complex equations.

Specie	Initial counts		Suggested manipulation	Resultant counts after implementing the suggestion		Remarks (balanced, not balanced)
	Reactant side	Product side	3 x A  +  2 x B  → 2 x C  + 3 x D	Reactant side	Product side
Pb	01	01		03	03	Balanced
N	02	03		06	06	Balanced
O	06	09		18	18	Balanced
Al	01	01		02	02	Balanced
Cl	03	02		06	06	Balanced

Trick:

Start with the radical, but not with the element, that is present on both sides. Try to balance this radical as a group rather than doing the component elements.

Re-considering the given equation:

The nitrate radical (NO₃−) is seen on both sides of this equation. Keeping thoughts of common & identical items on both sides of the arrow, let us balance the equation.

Starting with the first element on the left and proceeding to the right, as before. There is one lead (Pb) on both sides, so they are already balanced, with one on each side. Next is the nitrate radical. There are two on the left but three on the right.

a). If we multiply coefficient 3 with Pb(NO₃)₂  then we get 3Pb(NO₃)₂ .

b). If we multiply coefficient 2 with Al(NO₃)₃  then we get 2Al(NO₃)₃ .

From a) & b) we see that the nitrate radical (NO₃) is balanced with six on each side.

3Pb(NO₃)₂ + AlCl₃ → 2Al(NO₃)₃ + PbCl₂

Proceeding to the aluminum, we see that there are two moles on the right-hand side, so we need to multiply a coefficient 2 with AlCl₃ to balance the two aluminum on the left-hand side.

3Pb(NO₃)₂ + 2AlCl₃ → 2Al(NO₃)₃ + PbCl₂

This gives us six chlorines on the left-hand side; so we need to multiply a coefficient 3 with PbCl₂ to balance the six chlorines on the right-hand side.

3Pb(NO₃)₂ + 2AlCl₃ → 2Al(NO₃)₃ + 3PbCl₂

By rechecking the equation, we see that all elements are now balanced.