By Riti Gupta | Updated Aug 30, 2022

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A chemical reaction occurs when one or more molecules transform into new species. Chemists represent these transformations with chemical equations, using symbols from the periodic table to denote reactants and products.

What Is a Chemical Equation?

A chemical equation is a concise, symbolic depiction of a reaction. The symbols to the left of the arrow are the reactants; the symbols to the right are the products. The arrow indicates the direction of the reaction.

Example: Combustion of Methane

Unbalanced form:

\(CH_4 + O_2 \rightarrow CO_2 + H_2O\)

Balanced form:

\(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\)

Here, the coefficients (the numbers before each compound) ensure that every element has the same number of atoms on both sides of the arrow.

The Law of Conservation of Mass

According to the Law of Conservation of Mass, atoms are neither created nor destroyed in a chemical reaction. Therefore, the total count of each type of atom must remain identical on both sides of a balanced equation. Coefficients adjust the number of molecules to satisfy this principle.

Step‑by‑Step Guide to Balancing Equations

1. Count the atoms of each element on both sides.
2. Choose an element that appears most frequently and balance it first.
3. Re‑count atoms after adding coefficients.
4. Proceed to the next element, repeating steps 2‑3.
5. When all elements are balanced, verify that any charges (for ionic compounds) also balance.
6. Finally, double‑check the entire equation.

Practical Example: Photosynthesis

Unbalanced reaction:

\(CO_2 + H_2O \rightarrow C_6H_{12}O_6 + O_2\)

Balancing Process

1. Hydrogen: 6 H₂O gives 12 H atoms, matching the product side. Update: CO₂ + 6H₂O → C₆H₁₂O₆ + O₂
2. Carbon: Add a coefficient of 6 to CO₂: 6CO₂ + 6H₂O → C₆H₁₂O₆ + O₂
3. Oxygen: 6CO₂ supplies 12 O atoms and 6H₂O supplies another 6, totaling 18. To balance, add a coefficient of 6 to O₂: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
4. Verify: C 6/6, H 12/12, O 18/18 – the equation is balanced.

This systematic approach guarantees that every atom and charge is accounted for.

Key Takeaways

• Always begin with the element that appears most often.
• Use coefficients, not fractions, for simplicity.
• Check for polyatomic ions and ionic charges as a final step.