Equations & Net Ionic Equations

Learning Objective 4.2.A Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.

Quick Notes

All changes in matter — physical or chemical — can be represented using balanced equations.
Chemical equations must obey:
- Conservation of mass — same number of atoms of each element on both sides
- Conservation of charge — same total, overall charge on both sides
Types of equations:
- Molecular equation – shows all compounds as neutral formulas
- Complete ionic equation – shows all soluble ionic compounds as dissociated ions
- Net ionic equation – shows only the species involved in the actual chemical change
Spectator ions do not participate in the reaction and are omitted from the net ionic equation.

Full Notes

Balanced equations are the symbolic language of chemistry. They are used to represent both physical and chemical changes, and they help us understand how matter behaves and transforms.

Balanced Chemical Equations

In a chemical reaction, atoms are rearranged into new substances. A balanced equation reflects this by showing:

The correct chemical formulas of all reactants and products
Equal numbers of each atom on both sides of the equation
Equal net charges on both sides if ions are involved

Example: H₂ + Cl₂ → 2HCl
Each side has 2 hydrogen atoms and 2 chlorine atoms. The mass and charge are balanced.

Physical Change Equations

Even physical changes, such as phase changes, can be written symbolically:

Example: H₂O(s) → H₂O(l)
No new substance is formed — just a change in physical state.

Types of Equations in Aqueous Reactions

In aqueous ionic reactions, we often write three different forms of an equation:

Molecular equation: Shows all substances as if they were molecules.

Example:Na₂SO₄(aq) + BaCl₂(aq) → 2NaCl(aq) + BaSO₄(s)
Complete ionic equation: Shows all ions dissociated from soluble ionic compounds.

Example:2Na⁺(aq) + SO₄²⁻(aq) + Ba²⁺(aq) + 2Cl⁻(aq) → 2Na⁺(aq) + 2Cl⁻(aq) + BaSO₄(s)
Net ionic equation: Removes spectator ions (ions that appear unchanged on both sides).

Example:SO₄²⁻(aq) + Ba²⁺(aq) → BaSO₄(s)

Spectator Ions

Spectator ions do not participate in the chemical reaction. They appear unchanged on both sides of the complete ionic equation and are omitted from the net ionic equation.

Example:Reaction between NaOH and HCl
Full equation: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Net ionic equation: OH⁻(aq) + H⁺(aq) → H₂O(l)

Conservation Laws in Equations

Conservation of mass: total number of atoms of each element is constant
Conservation of charge: total charge of all species is the same before and after the reaction

These principles apply to all types of chemical equations.

Matt’s exam tip

If you’re asked to write a net ionic equation, start by writing the full molecular equation, breaking all strong electrolytes into ions, identifying and cancelling spectator ions, and checking that both atoms and charges are balanced.

Summary

Balanced equations symbolically represent how matter changes — whether through a physical change or a chemical reaction.
Molecular, ionic, and net ionic equations offer different levels of detail depending on context.
All forms must obey the laws of conservation of mass and charge.
Mastering these representations is essential for understanding and communicating chemical changes clearly.