1.
Understanding Chemical Equations
A chemical equation represents a chemical reaction where reactants are converted into
products. It uses chemical symbols and formulas to show the relationship between substances
before and after a reaction.
2. Components of a Chemical Equation
● Reactants: Substances that undergo a reaction (on the left side of the equation).
● Products: Substances formed as a result of the reaction (on the right side of the
equation).
● Arrow (→): Indicates the direction of the reaction.
● Coefficients: Numbers placed before chemical formulas to balance the equation.
● State Symbols: Indicate the physical state of substances:
○ (s) = solid
○ (l) = liquid
○ (g) = gas
○ (aq) = aqueous (dissolved in water)
3. Writing a Word Equation
Before writing a chemical equation, start with a word equation. For example:
Magnesium + Oxygen → Magnesium oxide
4. Writing a Chemical Equation
Convert the word equation into a chemical equation using chemical symbols:
Mg + O₂ → MgO
5. Balancing Chemical Equations
Chemical equations must be balanced to follow the Law of Conservation of Mass, which
states that mass cannot be created or destroyed in a reaction. This means the number of atoms
for each element must be the same on both sides of the equation.
Steps to Balance a Chemical Equation:
1. Write down the unbalanced equation.
○ Example: H₂ + O₂ → H₂O
2. Count the number of atoms of each element.
○ Hydrogen: 2 (left), 2 (right)
○ Oxygen: 2 (left), 1 (right)
3. Balance the atoms by adjusting coefficients.
○ Adjust to: 2H₂ + O₂ → 2H₂O
4. Check the balance.
○ Hydrogen: 4 (left), 4 (right)
○ Oxygen: 2 (left), 2 (right)
6. Types of Chemical Reactions
Understanding reaction types helps in writing equations correctly:
● Combination Reaction: A + B → AB (e.g., 2H₂ + O₂ → 2H₂O)
● Decomposition Reaction: AB → A + B (e.g., 2H₂O → 2H₂ + O₂)
● Single Displacement Reaction: A + BC → AC + B (e.g., Zn + CuSO₄ → ZnSO₄ + Cu)
● Double Displacement Reaction: AB + CD → AD + CB (e.g., AgNO₃ + NaCl → AgCl +
NaNO₃)
● Combustion Reaction: Hydrocarbon + O₂ → CO₂ + H₂O (e.g., CH₄ + 2O₂ → CO₂ +
2H₂O)
7. Using State Symbols
Always include state symbols where necessary:
● 2H₂(g) + O₂(g) → 2H₂O(l)
● NaCl(s) + H₂O(l) → NaCl(aq)
8. Common Pitfalls and Mistakes
● Incorrect balancing: Ensure that the total number of atoms is equal on both sides.
● Forgetting state symbols: Include (s), (l), (g), or (aq) where required.
● Using incorrect formulas: Memorize common compounds and polyatomic ions.
9. Practice Problems
1. Write and balance the equation for the reaction between iron and oxygen to form iron(III)
oxide.
2. Write the balanced equation for the decomposition of hydrogen peroxide (H₂O₂) into
water and oxygen.
3. Write and balance the combustion reaction of propane (C₃H₈).
10. Conclusion
Writing chemical equations correctly is crucial for understanding chemical reactions. By
following the steps outlined above, you can master balancing equations and applying them to
different reaction types. Practice regularly to reinforce these skills.
