GCSE Chemistry - How to Solve: Balancing Chemical Equations

How to Solve: Balancing Chemical Equations

Complete Guide For GCSE/A-Level Chemistry (Edexcel, AQA, OCR) – Exam-Ready in 60 Minutes

Introduction

On camera: "Balancing equations isn’t just a box-ticking exercise—it’s the key to predicting reaction yields, calculating masses, and even understanding climate change. Miss this, and you lose 10–15% of your Chemistry exam marks. Today, we’ll turn this from a panic-inducing puzzle into a 30-second habit."

WHAT YOU NEED TO KNOW FIRST

1. Law of Conservation of Mass: Atoms aren’t created or destroyed in a reaction—they just rearrange.
2. Chemical Formulae: You must know (or be given) the correct formulae for reactants and products (e.g., H₂O, CO₂, NaCl).
3. State Symbols: (s), (l), (g), (aq) are not part of balancing—ignore them for now.

KEY TERMS & FORMULAS

Formula to Remember: Total atoms of element X on left = Total atoms of element X on right (This is the only "formula" you need—no maths required!)

STEP-BY-STEP METHOD

On camera: "Follow these 5 steps in order. No shortcuts. Let’s go."

1. Write the unbalanced equation
2. List reactants → products with correct formulae.

3. Example: Methane burns in oxygen → carbon dioxide + water. Unbalanced: CH₄ + O₂ → CO₂ + H₂O

4. Count atoms of each element on both sides

5. Make a table under the equation.

6. Example: | Element | Left Side | Right Side | |---------|-----------|------------| | C | 1 | 1 | | H | 4 | 2 | | O | 2 | 3 |

7. Balance one element at a time

8. Start with the most complex molecule (usually the one with the most elements).

9. Example: Balance H first (4 on left, 2 on right).

   • Add a coefficient of 2 to H₂O → H₂O becomes 2H₂O.
   • Update the table: | Element | Left Side | Right Side | |---------|-----------|------------| | C | 1 | 1 | | H | 4 | 4 | (Balanced!) | O | 2 | 4 |

10. Balance the next element

11. Now balance O (2 on left, 4 on right).

    • Add a coefficient of 2 to O₂ → O₂ becomes 2O₂.
    • Update the table: | Element | Left Side | Right Side | |---------|-----------|------------| | C | 1 | 1 | | H | 4 | 4 | | O | 4 | 4 | (Balanced!)

12. Check all elements

13. Recount every atom. If any don’t match, go back to Step 3.
14. Final balanced equation: CH₄ + 2O₂ → CO₂ + 2H₂O

On camera: "That’s it. No guessing, no skipping steps. Now let’s try three examples—one easy, one medium, and one exam-style."

WORKED EXAMPLES

Example 1 – Basic: Hydrogen + Oxygen → Water

Unbalanced: H₂ + O₂ → H₂O

1. Count atoms: | Element | Left | Right | |---------|------|-------| | H | 2 | 2 | | O | 2 | 1 |

2. Balance O first (2 on left, 1 on right).

3. Add coefficient 2 to H₂O → 2H₂O.

4. Update table: | Element | Left | Right | |---------|------|-------| | H | 2 | 4 | | O | 2 | 2 | (O balanced!)

5. Balance H (2 on left, 4 on right).

6. Add coefficient 2 to H₂ → 2H₂.
7. Final table: | Element | Left | Right | |---------|------|-------| | H | 4 | 4 | | O | 2 | 2 |

Balanced equation: 2H₂ + O₂ → 2H₂O What we did and why: We balanced O first because it appeared in only one molecule on each side. Then we fixed H by doubling the H₂.

Example 2 – Medium: Aluminium + Hydrochloric Acid → Aluminium Chloride + Hydrogen

Unbalanced: Al + HCl → AlCl₃ + H₂

1. Count atoms: | Element | Left | Right | |---------|------|-------| | Al | 1 | 1 | | H | 1 | 2 | | Cl | 1 | 3 |

2. Balance Cl first (1 on left, 3 on right).

3. Add coefficient 3 to HCl → 3HCl.

4. Update table: | Element | Left | Right | |---------|------|-------| | Al | 1 | 1 | | H | 3 | 2 | | Cl | 3 | 3 | (Cl balanced!)

5. Balance H (3 on left, 2 on right).

6. Add coefficient 1.5 to H₂ → 1.5H₂.
7. But coefficients must be whole numbers! Multiply everything by 2:
   • 2Al + 6HCl → 2AlCl₃ + 3H₂
8. Final table: | Element | Left | Right | |---------|------|-------| | Al | 2 | 2 | | H | 6 | 6 | | Cl | 6 | 6 |

Balanced equation: 2Al + 6HCl → 2AlCl₃ + 3H₂ What we did and why: We hit a fraction (1.5), so we doubled all coefficients to keep whole numbers. Always check for fractions!

Example 3 – Exam-Style: Propane (C₃H₈) Combustion

Question: Balance the equation for the complete combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O

1. Count atoms: | Element | Left | Right | |---------|------|-------| | C | 3 | 1 | | H | 8 | 2 | | O | 2 | 3 |

2. Balance C first (3 on left, 1 on right).

3. Add coefficient 3 to CO₂ → 3CO₂.

4. Update table: | Element | Left | Right | |---------|------|-------| | C | 3 | 3 | | H | 8 | 2 | | O | 2 | 7 | (3×2 + 2 = 8 O on right!)

5. Balance H (8 on left, 2 on right).

6. Add coefficient 4 to H₂O → 4H₂O.

7. Update table: | Element | Left | Right | |---------|------|-------| | C | 3 | 3 | | H | 8 | 8 | | O | 2 | 10 | (3×2 + 4×1 = 10 O on right!)

8. Balance O (2 on left, 10 on right).

9. Add coefficient 5 to O₂ → 5O₂.
10. Final table: | Element | Left | Right | |---------|------|-------| | C | 3 | 3 | | H | 8 | 8 | | O | 10 | 10 |

Balanced equation: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O What we did and why: We balanced C and H first (they appeared in only one molecule on each side), then fixed O last. This order avoids fractions!

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP

On camera (fast-paced, direct to student): "Listen up—this is your 60-second cheat sheet for balancing equations. Step 1: Write the unbalanced equation with correct formulae. Step 2: Count atoms of each element on both sides. Step 3: Balance one element at a time, starting with the most complex molecule. Step 4: If you get a fraction, double everything. Step 5: Check all atoms again. Remember: Never change subscripts, always balance C and H before O, and treat polyatomic ions as one chunk. That’s it. Now go balance 5 equations tonight—you’ve got this!"