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Chemical Equation

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The chemical reaction which is expressed by using molecular formulae of reactants and products is called chemical equation. The chemical equation is also called formula equation. This note provides information about chemical equation and its types and irreversible reaction and reversible chemical reactions.
Chemical Equation

Chemical equations

The chemical reaction which is expressed by using molecular formulae of reactants and products is called chemical equation. The chemical equation is also called formula equation. The representation of chemical reaction becomes easy.

2 H2 + O2→2 H2O

Skeletal chemical equation

The chemical equation in which the total number of atoms of each element in reactant and product is not equal is called an unbalanced chemical equation or skeletal chemical equation. Some examples:

  • Sodium + Chlorine→Sodium chloride
  • Nitrogen + Hydrogen→Ammonia
  • Zinc + Hydrochloric acid →Zinc chloride + Hydrogen

Drawbacks of skeletal chemical equation

  • Skeletal chemical equation does not follow the law of conservation of mass.
  • It does not tell about the ratio of reactant and product molecules.
  • It does not give information about the total number of atoms of each element in reactants and products.
  • It is essential to balance the chemical equation because a balanced chemical equation satisfies the law of conservation of mass in a chemical reaction.

Balanced Chemical Equation

There are many unbalanced chemical equations which need to be balanced. Mostly, we balance the chemical equation by comparing the numbers of atoms in reactant and product molecules and then equalizing the both sides. Balanced chemical equation is a type of equation in which a total number of atoms in reactants and products are equal. It is based on the law of conservation of mass.
For example;
Hydrogen and oxygen react with eachother to produce water.
Hydrogen + Oxygen→ Water
H2 + O2→ H2O (Skeletal equation)..........(i)

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2H2 +O2→ 2H2O (Balanced chemical equation).............(Ii)

In the above equation (ii), the total number of atoms in reactant and product are equal so it is a balanced chemical equation.

Balancing a Chemical Equation

There are various ways to balancea chemical equation. But simplest one is Hit and trial method which is explained below:

  1. First of all, we write the correct word equation to express the chemical reaction.
  2. In the second step, we translate the word equation into the skeletal chemical equation using symbols and molecular formula.
  3. In the third step, the numbers of atoms of different elements other than oxygen and hydrogen are counted on each side of the chemical equation.
  4. For balancing the number should be added as a coefficient.
  5. In the last step, the atoms of the biggest molecule of the chemical equations are first balanced whereas the hydrogen and the oxygen atoms of the equation are balanced at last.

Example:

When ethane is burnt in oxygen, carbon dioxide gas and water are produced.

Ethane + Oxygen → Carbon dioxide + water(Word Equation)

C2H6 + O2 → CO2 + H2 (Skeletal Equation)

2C2H6 + 7O2 → 4CO2 + 6 H2O (Balanced Equation)

Irreversible reaction and reversible chemical reactions

The chemical reaction that occurs only in one direction is called irreversible reaction. For example, when calcium carbonate is being strongly heated, it decomposes to calcium oxide and carbon dioxide.

Calcium carbonate → Calcium oxide + Carbon dioxide

CaCO3 → CaO + CO2↑

The chemical reaction that occurs both in forward and backward directions is called reversible reaction.

Nitrogen + Hydrogen \(\rightleftharpoons\) Ammonia

N2 + H2 \(\rightleftharpoons\) NH3

Things to remember
  • The chemical reaction which is expressed by using molecular formulae of reactants and products is called chemical equation. The chemical equation is also called formula equation.
  • The chemical equation in which the total number of atoms of each element in reactant and product is not equal is called an unbalanced chemical equation or skeletal chemical equation.
  • Skeletal chemical equation does not follow the law of conservation of mass.
  • Balanced chemical equation is a type of equation in which a total number of atoms in reactants and products are equal. It is based on the law of conservation of mass.
  • The chemical reaction that occurs only in one direction is called irreversible reaction. 
  • The chemical reaction that occurs both in forward and backward directions is called reversible reaction.
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Videos for Chemical Equation
A Beginner's Guide to Balancing Equations
Balancing Chemical Equations
Irreversible Reaction
What are Reversible Reactions?
Questions and Answers
The chemical equation written by balancing the number of atoms of elements on both sides of the reactants and products is called balanced chemical equation.
For example:
  • Na2O + H2O → 2NaOH
  • CaO + H2O → Ca (OH)2

Following information can be obtained from a balanced chemical equation:

  1. Name if the reactants and the products.
  2. The type of chemical equation.
  3. Symbols and molecular formulae of the reactants and the products.
  4. Number of atoms and molecules of the reactants and the products.
  5. The ratio of weight of atoms or molecules of reactants or products.
  • 2Na + 2H2O → 2NaOH + H2↑
  • 2KCl3→Heat 2KCl + 3O2
  • Zn + 2HCl (dil) → ZnCl2 + H2↑
The reaction which occurs in forward and backward directions is called reversible reaction. For example:
N2 + 3H22NH3
The chemical reaction which occurs only in forward direction is called irreversible reaction. For example:
CaCO3 → CaO + CO2
The reaction which absorbs heat during the chemical change is called endothermic reaction. For example:
2KClO3 + Heat → 2KCl + 3O2
CaCO3 + Heat → CaO + CO2
NaNO2 + NH4Cl + Heat → NaCl + 2H2O + N2
The reaction which evolves heat during the chemical change is called exothermic reaction. For example:
C + 2H2 → CH4 + Heat
C + O2 → CO2 + Heat

The information that cannot be obtained from a balanced chemical equation are:

  1. The physical state of reactants and products.
  2. Rate of chemical reaction.
  3. Concentration of reactants and products.
  4. Duration of chemical reaction.
  5. The conditionsnecessaryfor chemical reaction.

The method for writing balanced chemical equation are:

  1. First of all, chemical reaction should be written correctly in the form of word equation.
  2. Then, the word equation should be written correctly in the form of a formula equation.
  3. The number of atoms of each element should be made equal disturbing the molecular formula.
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