Balancing Chemical Equations

Part of: Class 10 Science Notes - Chemical Reactions and Equations

1. Chemical Reactions

A chemical reaction is a process in which one or more substances (reactants) are converted into one or more different substances (products). These new chemicals are formed through a chemical change involving the breaking and forming of chemical bonds.

Note

Example: $2H_2+O_2\to 2H_{2}O$

2. Chemical Equations

A chemical equation is the symbolic representation of a chemical reaction using chemical formulas, symbols, and signs. It consists of two primary parts:

• Reactants: The starting substances (written on the left).
• Products: The substances formed (written on the right).

Case Study: The Combustion of Methane

Consider the "Skeletal" (unbalanced) equation for burning methane:

$C{H}_4+O_2\to C{O}_2+H_{2}O$

Mass Verification Table:

Using atomic masses ($C\approx 12$, $H\approx 1$, $O\approx 16$):

Result: The masses are not equal, which violates the fundamental laws of physics.

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3. The Law of Conservation of Mass

• Author: Antoine Lavoisier
• Scientific Statement: "Mass can neither be created nor be destroyed in a chemical reaction."
• Implication: The total mass of the reactants must exactly equal the total mass of the products.

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4. Why must we balance Chemical Equations?

We balance equations to satisfy the Law of Conservation of Mass. An unbalanced equation is often called a Skeletal Equation.

• The number of atoms of each element must be identical on both the Left Hand Side (LHS) and the Right Hand Side (RHS).
• Balancing ensures that no matter is "lost" or "magically created" during the reaction.

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5. How to Balance Chemical Equations Step-by-Step

Follow this Hit and Trial method (systematic hierarchy) to balance any equation:

1. Write the Skeletal Equation: Use correct chemical formulas (e.g., $Mg+O_2\to MgO$).
2. Draw an Inventory Table: Count atoms of each element on the Reactant (LHS) and Product (RHS) sides.
3. Balance Metals First: Adjust coefficients for elements like $Mg$, $Fe$, or $Na$.
4. Balance Non-Metals: Next, balance $C$, $S$, or $Cl$ (excluding $H$ and $O$).
5. Balance Hydrogen and Oxygen Last: These often auto-balance if the others are correct.
6. Verify the Atom Count: Ensure LHS = RHS to satisfy the Law of Conservation of Mass.

6. Practice Problems

Q) Try balancing the following:

1. $\text{Mg}+{\text{O}}_2\to \text{MgO}$
2. $\text{Na}+{\text{Cl}}_2\to \text{NaCl}$
3. ${\text{H}}_2+{\text{O}}_2\to {\text{H}}_2\text{O}$
4. $\text{Al}+\text{HCl}\to {\text{AlCl}}_3+{\text{H}}_2$
5. $\text{Fe}+{\text{CuSO}}_4\to {\text{FeSO}}_4+\text{Cu}$
6. ${\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2$
7. $\text{NaOH}+{\text{H}}_2{\text{SO}}_4\to {\text{Na}}_2{\text{SO}}_4+{\text{H}}_2\text{O}$
8. $\text{KOH}+{\text{H}}_3{\text{PO}}_4\to {\text{K}}_3{\text{PO}}_4+{\text{H}}_2\text{O}$
9. ${\text{Al}}_2({\text{SO}}_4{)}_3+{\text{Ca(OH)}}_2\to {\text{Al(OH)}}_3+{\text{CaSO}}_4$
10. ${\text{C}}_3{\text{H}}_8+{\text{O}}_2\to {\text{CO}}_2+{\text{H}}_2\text{O}$

Note

Note: Solutions to these problems will be soon available on our site.

Common Mistakes to Avoid

• Changing Subscripts: Never change the small numbers in a formula (e.g., $H_{2}O\to H_2{O}_2$). Only change the coefficients (the numbers in front).
• Forgetting Diatomic Molecules: Remember that elements like Oxygen ($O_2$), Hydrogen ($H_2$), and Chlorine ($C{l}_2$) always exist as molecules in their natural state.

Frequently Asked Questions (FAQs)

Q1: What is a skeletal chemical equation?
A: A skeletal equation is an unbalanced chemical equation where the mass of reactants is not equal to the mass of products.

Q2: Which law is satisfied by balancing a chemical equation?
A: The Law of Conservation of Mass, which states that matter cannot be created or destroyed.

Q3: Can we use fractions to balance equations?
A: While you can use fractions (like $1/2$) during the process, the final equation should always have whole number coefficients.