Balancing Chemical Reactions Presentation

Chemical reactions

Balancing Chemical Reactions

A balanced chemical equation is a particle-level accounting statement: every atom present before the reaction must still be present after the reaction.

2H₂ + O₂ → 2H₂O

Learning target: use coefficients to balance atoms while keeping chemical formulas unchanged.

Key idea

Atoms are conserved

Reactions rearrange atoms; they do not create or destroy atoms.

Main skill

Count atoms carefully

Use coefficients and subscripts together to compare both sides.

Rule

Never change subscripts

Changing a subscript changes the substance, not just the amount.

Why it matters

Balanced equations connect particles to real amounts

In a lab, a chemical equation is not only a sentence. It is also a recipe. The coefficients tell how many particles, moles, or relative amounts react.

N₂ + 3H₂ → 2NH₃

This equation says that 1 molecule or 1 mol of N₂ reacts with 3 molecules or 3 mol of H₂ to form 2 molecules or 2 mol of NH₃.

Balanced equations are used to predict

how much reactant is required,
how much product can form,
which reactant limits a reaction,
whether a written equation obeys conservation of mass.

Core model

A reaction rearranges atoms into new combinations

The law of conservation of mass means that the number of atoms of each element must be the same on the reactant side and the product side.

2H₂ + O₂ → 2H₂O

Left side: 4 H atoms and 2 O atoms. Right side: 4 H atoms and 2 O atoms.

Vocabulary

Know what can change and what cannot

Term	Meaning	Example	Can it change during balancing?
Reactants	Starting substances	H₂ and O₂	No, the substances stay the same.
Products	Substances formed	H₂O	No, the product formulas stay the same.
Coefficient	Large number placed before a formula	2 in 2H₂O	Yes, coefficients are changed to balance atoms.
Subscript	Small number inside a formula	2 in H₂O	No, changing it changes the substance.

Important rule: balance with coefficients only. For example, changing H₂O into H₂O₂ changes water into hydrogen peroxide.

Atom-counting rule

Multiply the coefficient by each subscript

For each element, the number of atoms contributed by a formula is:

\[ \text{atoms of an element} = \text{coefficient} \times \text{subscript} \]

When a formula has no written coefficient, the coefficient is 1. When an element symbol has no written subscript, the subscript is 1.

Example 1

3CO₂ contains 3 C atoms and 6 O atoms.

Example 2

2Al₂O₃ contains 4 Al atoms and 6 O atoms.

Example 3

4NH₃ contains 4 N atoms and 12 H atoms.

Interactive simulation

Adjust coefficients until both sides match

Water formation

2H₂ + O₂ → 2H₂O

Coefficient of H₂ 2

Coefficient of O₂ 1

Coefficient of H₂O 2

Atom count

Left H atoms 4

Right H atoms 4

Left O atoms 2

Right O atoms 2

Balanced: both H and O have equal atom counts on both sides.

2 H2 + 1 O2 → 2 H2O

Static fallback: the default coefficients already show the balanced equation 2H₂ + O₂ → 2H₂O.

Dynamic relationship

The graph shows why balance means equality

Each bar compares the number of atoms on the reactant side with the number of atoms on the product side. A balanced equation makes each pair equal.

Interpretation: changing a coefficient changes atom counts in whole-number jumps. A reaction is balanced only when every element has matching counts on both sides.

Worked example

Balance methane combustion step by step

CH₄ + O₂ → CO₂ + H₂O

1. Count carbon first. Left: 1 C in CH₄. Right: 1 C in CO₂. Carbon is already balanced.
2. Count hydrogen. Left: 4 H in CH₄. Right: 2 H in H₂O. Put coefficient 2 before H₂O. CH₄ + O₂ → CO₂ + 2H₂O
3. Count oxygen last. Right side has 2 O atoms in CO₂ and 2 O atoms in 2H₂O, for 4 O atoms total. Put coefficient 2 before O₂.
Final balanced equation: CH₄ + 2O₂ → CO₂ + 2H₂O

Common mistake

Do not balance by changing subscripts

Incorrect

H₂ + O₂ → H₂O₂

This changes the product from water, H₂O, to hydrogen peroxide, H₂O₂. That is a different substance.

Correct

2H₂ + O₂ → 2H₂O

The formula H₂O stays unchanged. Only the coefficient changes to show how many water molecules form.

Reason: coefficients change the amount of a substance. Subscripts change the identity of the substance.

Practice check

Balance aluminum reacting with oxygen

Balance this chemical equation using coefficients only:

Al + O₂ → Al₂O₃

Think through oxygen first because O₂ has 2 oxygen atoms and Al₂O₃ has 3 oxygen atoms. The least common multiple of 2 and 3 is 6.

Show answer and reasoning

Use 3O₂ to get 6 O atoms on the left. Use 2Al₂O₃ to get 6 O atoms on the right.

Now the product side has 4 Al atoms, so place coefficient 4 before Al.

4Al + 3O₂ → 2Al₂O₃

Apply the topic

Use a repeatable balancing strategy

Step 1

Write the correct formulas

Make sure reactant and product formulas are chemically correct before balancing.

Step 2

Count each element

Create a simple atom-count table for both sides of the equation.

Step 3

Change coefficients

Adjust large numbers in front of formulas. Never change subscripts.

Step 4

Check every element

The final equation is balanced only when all atom counts match.

This method works for synthesis, decomposition, combustion, single-replacement, and double-replacement reactions. More complex equations may need more trial-and-check, but the conservation rule is always the same.

Summary

What to remember

Balanced means conserved

Each element must have the same number of atoms on both sides.

Coefficients can change

The coefficient tells how many units of a substance are involved.

Subscripts cannot change

A subscript is part of the chemical formula and identity of the substance.

Always check the final equation

Use atom counting to confirm the equation is truly balanced.

2H₂ + O₂ → 2H₂O