How to write balanced chemical equations in Chemistry?

In Chemistry, writing a balanced chemical equation is a skill that needs practice. This article will discuss all the tips and tricks to hone that skill.

Introduction to chemical reactions and equations 

A chemical reaction is the process of changing reactants into products. A chemical equation, however, is a symbolic representation of that reaction. Chemical equations denote the molar quantities and the identities of different reactants and products involved in the reaction. 

The reactants are written on the left-hand side, while the products are represented on the right-hand side of an equation. The arrow shows the direction in which the chemical change occurs. More than one reactants or products on either side of the equation are separated by using a plus (+) sign between them:

The most common reaction arrow points from left to right (→), symbolising the reactants changing into products or the reaction occurring in the forward direction. 

What is a balanced chemical equation? 

A balanced chemical equation has an equal number of atoms of each element on both sides of the reaction. This concept is based on the law of conservation of mass, i.e., mass cannot be created or destroyed; it can only be converted from one form into another where the total mass of the system stays constant. 

An equal number of atoms from the same elements are present on both sides of a balanced chemical equation, only their combination changes in transforming from reactants to products. Remember, no elements appear on one side of the equation and not on the other side. 

5 Steps to write a balanced chemical equation

Here are the 5 key steps in writing a balanced chemical equation:

1. Identify the different reactants and products. 
2. Write the equation in words. 
3. Convert the words into chemical formulae. 
4. Add Stoichiometric coefficients. 
5. Add state symbols (this step is optional). 

We will take one example reaction:

Magnesium metal reacts with hydrochloric acid to form magnesium chloride salt and liberate hydrogen gas. 

1). Identify the different reactants and products 

In this reaction, we have identified:
Reactants = magnesium and hydrochloric acid.
Products = magnesium chloride and hydrogen.

2). Write the equation in words 

magnesium + hydrochloric acid → magnesium chloride + hydrogen 

3). Convert words into chemical formulae

Mg + HCl → MgCl₂ + H₂

4). Add Stoichiometric coefficients 

Stoichiometric coefficients are added with the reactants and products to equalise the number of atoms of each element present on both sides.

In this example, there is 1 Mg-atom on both sides of the equation; hence, it is balanced. However, there is 1 Cl-atom and 1 H-atom on the left side, while 2 Cl-atoms and 2 H-atoms on the right side. To balance, we need to add the coefficient 2 with HCl. 

Mg + 2HCl → MgCl₂ + H₂

Now, there are 2 Cl-atoms and 2 H-atoms on both sides. In this way, it is a fully balanced chemical equation. 

Note:
Stoichiometric coefficients are the numbers added at the front of the chemical formulae that denote the number of atoms of each element in the molecule. For example, 2CO₂ represents 2 carbon dioxide molecules, meaning 2 carbon atoms and 4 atoms of oxygen. 

Remember, while balancing chemical equations, we can change the stoichiometric coefficients but not the subscripts of the chemical formula because that would mean changing the chemical composition itself. For example, O₂  is oxygen, but O₃  denotes the ozone gas. 

5). Add state symbols

 Mg(s) + 2HCl(aq) → MgCl₂(aq)+ H₂(g)

This step is optional, depending on whether you are asked to write the state symbols in the question statement or not. If not, it is better to skip the state symbols. 

Balanced chemical equations worked examples 

Worked example 1:

Balance the chemical equation for the reaction of iron (Fe) with oxygen (O₂) to form iron (III) oxide.

Step 1:
Identify the different reactants and products.
Reactants: Iron (Fe) and oxygen (O₂)
Product: Iron (III) oxide (Fe₂O₃)

Step 2:
Write the equation in words. 
Iron + oxygen → Iron (III) oxide 

Step 3:
Convert words into chemical formulae. 
Fe + O₂ → Fe₂O₃

Step 4:
Add Stoichiometric coefficients. 
Only 1 Fe-atom on the reactant side but 2 Fe-atoms on the product side, so add the coefficient 2 in front of Fe. 
2Fe + O₂ → Fe₂O₃

Now, there are 2 Fe-atoms on both sides.
2 O-atoms on the reactant side but 3 O-atoms on the product side, so add the coefficient 3 in front of O₂ and 2 in front of Fe₂O₃.
2Fe + 3O₂ → 2Fe₂O₃

Now, there are 6 O-atoms on both sides. However, it disturbed the balance of Fe-atoms as there are now 4 Fe-atoms on the product side but only 2 Fe-atoms on the reactant side. So, change the coefficient 2 with 4 in front of Fe. 
4Fe + 3O₂ → 2Fe₂O₃

In this way, it is a fully balanced chemical equation with 4 Fe-atoms and 6 O-atoms on both sides: 
4Fe + 3O₂ → 2Fe₂O₃

This example shows that balancing a chemical equation is just a matter of finding the right coefficients. The choice of the correct coefficients comes with practice, similar to a trial and error method. Moreover, the coefficients can be changed from either or both the reactant and product sides in order to equalise the number of different atoms present. 

Worked example 2:

Balance the chemical equation for the complete combustion of pentane that produces carbon dioxide and water.

Step 1:
Identify the different reactants and products.
Reactants: pentane (C₅H₁₂) and oxygen (O₂)
Products: carbon dioxide (CO₂) and water (H₂O)

Step 2:
Write the equation in words. 
pentane + oxygen → carbon dioxide + water 

Step 3:
Convert words into chemical formulae
C₅H₁₂ + O₂ → CO₂ + H₂O 

Step 4:
Add Stoichiometric coefficients 
5 C-atoms are on the reactant side while 1 C-atom is on the product side, so add the coefficient 5 in front of CO₂. 
C₅H₁₂ + O₂ → 5CO₂ + H₂O 

Now, there are 5 C-atoms on both sides. 
12 H-atoms on the reactant side while 2 H-atoms on the product side, so add the coefficient 6 in front of H₂O. 
C₅H₁₂ + O₂ → 5CO₂ + 6H₂O 

This makes a total of 12 H-atoms on both sides. 
2 O-atoms on the reactant side while a total of 5(2)+6 = 16 O-atoms on the product side, so add the coefficient 8 in front of O₂. 
C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

Now, there are 16 O-atoms on both sides. 
So, the fully balanced chemical equation is: 
C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

Summary and conclusion 

Balanced chemical equations ensure that the fundamental law of conservation of mass is followed. They provide the correct ratio of reactants and products, which is essential for determining the amounts of substances involved in a chemical reaction. So many parts of Chemistry depend on this one skill; therefore, make sure you master the skill of writing balanced chemical equations to ace your exams. 

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