Balancing chemical equations

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Lesson notes

Why balance equations?

Atoms are **rearranged** in a chemical reaction, not created or destroyed.
The **number of atoms** of each element must be the same on both sides of the equation.
An unbalanced equation does not obey the **Law of Conservation of Mass**.

Writing chemical equations

Use **chemical symbols** and **formulae** for reactants and products.
Diatomic elements (e.g., O₂, H₂, Cl₂) are written with a subscript 2.
Example: copper + oxygen → copper oxide is written as Cu + O₂ → CuO.

Identifying imbalance

Count atoms of each element on both sides of the arrow.
In Cu + O₂ → CuO: left has 1 Cu, 2 O; right has 1 Cu, 1 O → **oxygen is unbalanced**.
Only **coefficients** (numbers in front of formulae) can be changed to balance.

Balancing by adjusting coefficients

Place a coefficient in front of a compound to multiply all atoms in that compound.
Example: 2CuO means 2 Cu atoms and 2 O atoms.
Never change subscripts inside a formula (e.g., CuO to CuO₂) – that changes the substance.

Step-by-step balancing example

Start with the unbalanced equation: Cu + O₂ → CuO.
Balance oxygen: put 2 before CuO → Cu + O₂ → 2CuO.
Now copper is unbalanced (1 left, 2 right). Put 2 before Cu → **2Cu + O₂ → 2CuO**.
Check: left 2 Cu, 2 O; right 2 Cu, 2 O → balanced.

Working with polyatomic ions

Treat polyatomic ions (e.g., OH⁻, CO₃²⁻) as a **single unit** when balancing.
Example: Ca(OH)₂ contains 1 Ca, 2 O, 2 H.
Coefficient multiplies the entire ion group: 2Ca(OH)₂ gives 2 Ca, 4 O, 4 H.

Practice tips

Balance **metals** first, then **nonmetals**, then **hydrogen and oxygen** last.
If an element appears in more than one compound on a side, leave it for later.
Check your work by recounting atoms after each coefficient change.

Bohr model of a copper atom (Cu). Used to understand the number of atoms in compounds like CuO.

Bohr model of an oxygen atom (O). Oxygen forms diatomic molecules (O₂) with two oxygen atoms.

Slides

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Practice questions

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1.How many hydrogen atoms are there in one molecule of H2O?
Easy
- A1
- B2
- C3
- D4
2.How many hydrogen atoms are there in 2H2O?
Easy
- A2
- B4
- C6
- D8
3.How many hydrogen atoms are there in 4C2H4?
Medium
- A4
- B8
- C12
- D16
4.How many hydrogen atoms are there in 2C7H16?
Medium
- A14
- B16
- C28
- D32
5.What is the chemical formula for oxygen?
Easy
- AO
- BO2
- CO3
- D2O
6.In the balanced equation 2Mg + O2 → 2MgO, how many magnesium atoms are on each side?
Medium
- A1
- B2
- C3
- D4
7.Balance the equation: CH4 + ? O2 → CO2 + 2H2O. What number should replace the question mark?
Hard
- A1
- B2
- C3
- D4
8.Balance the equation: C2H4 + 2O2 → ? CO + 2H2O. What number should replace the question mark?
Hard
- A1
- B2
- C3
- D4

Revision notes

## Balancing Chemical Equations - **Law of Conservation of Mass**: Atoms are not created or destroyed in a chemical reaction. The number of each type of atom must be the same on both sides of the equation. - **Word equations** describe reactions using words, e.g., copper + oxygen → copper oxide. - **Symbol equations** use chemical symbols and formulae. Example: Cu + O₂ → CuO (unbalanced). - **Diatomic molecules**: Oxygen exists as O₂, not O. - **Balancing steps**: 1. Count atoms of each element on both sides. 2. Change coefficients (numbers in front of compounds) to balance atoms. Never change subscripts (e.g., CuO to CuO₂) as that changes the substance. 3. Multiply all atoms in a compound by the coefficient. Example: 2H₂O means 4 H atoms and 2 O atoms. - **Balanced example**: 2Cu + O₂ → 2CuO (2 Cu atoms, 2 O atoms each side). ### Practice Balancing: - Oxidation of zinc: 2Zn + O₂ → 2ZnO - Oxidation of magnesium: 2Mg + O₂ → 2MgO - Complete combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O - Incomplete combustion of ethene: C₂H₄ + 2O₂ → 2CO + 2H₂O - Oxidation of silver: 4Ag + O₂ → 2Ag₂O - Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ - Reaction of sodium and chlorine: 2Na + Cl₂ → 2NaCl

Chemical equation: A representation of a chemical reaction using symbols and formulae.
Balancing: Adjusting coefficients to ensure the same number of each atom on both sides of the equation.
Coefficient: The number placed in front of a chemical formula to indicate how many molecules or atoms are involved.
Diatomic molecule: A molecule composed of two atoms, e.g., O₂.
Subscript: A number written below and to the right of an element symbol indicating the number of atoms in a molecule.

Flashcards

What is a diatomic molecule?

A molecule composed of two atoms of the same element, e.g., O2.

Why must chemical equations be balanced?

Because atoms are neither created nor destroyed in a chemical reaction; the number of each type of atom must be the same on both sides.

What is the only way to balance a chemical equation?

By changing the number in front of a compound (coefficient), not by changing subscripts within a formula.

In the equation Cu + O2 → CuO, how many oxygen atoms are on each side?

Left: 2 (from O2), Right: 1 (from CuO).

How do you balance Cu + O2 → CuO?

Place a 2 in front of Cu and a 2 in front of CuO: 2Cu + O2 → 2CuO.

How many hydrogen atoms are in H2O?

How many hydrogen atoms are in 2H2O?

4 (2 × 2).

How many hydrogen atoms are in 10H2O?

20 (10 × 2).

How many hydrogen atoms are in CH4?

How many hydrogen atoms are in C2H4?

How many hydrogen atoms are in 4C2H4?

16 (4 × 4).

How many hydrogen atoms are in 2C7H16?

32 (2 × 16).

How many hydrogen atoms are in Ca(OH)2?

Write a word equation for the oxidation of zinc.

zinc + oxygen → zinc oxide

Balance: ___ Zn + O2 → 2ZnO

2 Zn + O2 → 2ZnO

Balance: 2Mg + O2 → ___ MgO

2Mg + O2 → 2MgO

Balance: CH4 + ___ O2 → CO2 + 2H2O

CH4 + 2 O2 → CO2 + 2H2O

Balance: C2H4 + 2O2 → ___ CO + 2H2O

C2H4 + 2O2 → 2CO + 2H2O

Balance: ___ Ag + O2 → 2Ag2O

4 Ag + O2 → 2Ag2O

Balance: 6CO2 + 6H2O → C6H12O6 + ___ O2

6CO2 + 6H2O → C6H12O6 + 6 O2

Balance: ___ Na + ___ Cl2 → ___ NaCl

2 Na + 1 Cl2 → 2 NaCl

What does the coefficient in front of a compound represent?

The number of molecules or formula units of that compound.

If you have 2H2O, how many oxygen atoms do you have?

2 (2 × 1).

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