Chemistry · StudentHub Lesson
Balancing Chemical Reactions
How to balance chemical equations to satisfy conservation of mass.
What you will learn
- Explain the law of conservation of mass
- Identify reactants and products in an equation
- Balance simple chemical equations
- Use coefficients correctly
- Check balanced equations for accuracy
Watch the lesson
Balancing chemical equations | Chemistry | Khan Academy · Khan Academy
Watch on YouTubeTopic notes
Main Idea
Balancing chemical equations ensures the number of atoms of each element is equal on both sides, following conservation of mass.
Key Concepts
- Reactants are on the left, products on the right
- Coefficients (not subscripts) are changed to balance equations
- Atoms cannot be created or destroyed in a reaction
Definitions
- Coefficient: number placed before a formula to balance an equation
- Subscript: number showing atoms within a molecule (never changed when balancing)
- Law of conservation of mass: matter is neither created nor destroyed
Processes
- Write the unbalanced equation
- Count atoms of each element on both sides
- Add coefficients to balance one element at a time
- Recheck all elements until balanced
Examples
Balancing aluminum + oxygen -> aluminum oxide: 4Al + 3O2 -> 2Al2O3.
Common Mistakes
- Changing subscripts instead of coefficients
- Forgetting to balance polyatomic ions as units
- Not rechecking all elements after adjustments
Key concepts
Important terms
- Coefficient
- A number placed in front of a chemical formula to balance an equation.
- Reactant
- A starting substance in a chemical reaction.
- Product
- A substance formed as a result of a chemical reaction.
Worked examples
Problem
Balance the equation: H2 + O2 -> H2O
- 1. Count atoms: left has 2 H, 2 O; right has 2 H, 1 O
- 2. Add coefficient 2 in front of H2O: H2 + O2 -> 2H2O
- 3. Recount: right now has 4 H, 2 O; left needs 4 H
- 4. Add coefficient 2 in front of H2: 2H2 + O2 -> 2H2O
- 5. Recheck: 4 H and 2 O on both sides
Answer: 2H2 + O2 -> 2H2O
Quick revision
- Balance equations using coefficients only
- Never change subscripts
- Conservation of mass must hold
- Count atoms on both sides
- Balance one element at a time
- Polyatomic ions can be balanced as units
- Always recheck final equation
Check your understanding
Question 1 · Multiple choice
What can be changed to balance a chemical equation?
Question 2 · Multiple choice
Which law requires that chemical equations be balanced?
Question 3 · True or false
Changing a subscript is an acceptable way to balance an equation.
Question 4 · Short answer
Balance: N2 + H2 -> NH3
Question 5 · Short answer
What term describes substances formed in a reaction?
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