Topic 4.2 Notes – Net Ionic Equations

1. What a Net Ionic Equation Is

A chemical equation represents a rearrangement of atoms. Nothing disappears. Nothing appears from nowhere. Atoms are just reorganized.

Two rules always apply:

• Conservation of mass → same number of each atom on both sides
• Conservation of charge → total charge must be equal on both sides

A net ionic equation shows only the species that actually undergo chemical change. It removes ions that don’t participate.

The same reaction can be written three ways:

• Molecular equation → compounds written intact
• Complete ionic equation → all strong electrolytes split into ions
• Net ionic equation → spectator ions removed

The form depends on what you’re trying to show. Net ionic equations answer the question: What actually reacted?

2. The Three Forms of a Reaction

Let’s see how the forms connect.

Molecular Equation

This shows substances as neutral formulas with phases.

Example:

$$\ce{BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq)}$$

This tells you what was mixed and what formed, including the solid precipitate $\ce{BaSO4}$.

Complete Ionic Equation

Now separate all strong electrolytes (soluble ionic compounds, strong acids/bases).

$$\ce{Ba^{2}+(aq) + 2Cl^{-}(aq) + 2Na^{+}(aq) + SO4^{2}-(aq) -> BaSO4(s) + 2Na^{+}(aq) + 2Cl^{-}(aq)}$$

Important:

Do NOT dissociate:

• Solids (s)
• Liquids (l)
• Gases (g)
• Weak acids/bases (they stay mostly intact)

This form shows everything in solution.

Net Ionic Equation

Cancel ions that appear unchanged on both sides.

Here, $\ce{Na^{+}}$ and $\ce{Cl^{-}}$ are spectators.

$$\ce{Ba^{2}+(aq) + SO4^{2}-(aq) -> BaSO4(s)}$$

Now you clearly see what changed: the barium and sulfate formed a solid.

Always check:

• Atoms balanced?
• Charge balanced?

Left side charge: $+2 + (-2) = 0$
Right side charge: 0
Balanced ✔️

3. Reactions in Aqueous Solution

Most reactions in this unit happen in water.

Why Ionic Compounds Dissociate

Water is polar. Oxygen has partial negative charge. Hydrogens have partial positive charge.

That polarity stabilizes separated ions through ion-dipole attractions, so soluble ionic compounds break apart.

Here’s what that looks like at the particle level:

NaCl dissolving in water at the macroscopic and microscopic levels

Notice how the oxygen ends of water point toward $\ce{Na^{+}}$, and the hydrogen ends point toward $\ce{Cl^{-}}$. That orientation is what stabilizes the ions in solution.

So when something is labeled (aq), think free-moving ions, not intact formula units.

Solubility Rules You Must Know

The College Board guarantees:

• All Na⁺, K⁺, NH₄⁺, and NO₃⁻ salts are soluble.

If a product is insoluble, it forms a precipitate (s) and stays intact in ionic equations.

Precipitation Reactions

Two aqueous ionic solutions mix. Ions swap partners. If one combination is insoluble, a solid forms.

General pattern:

$$\ce{AB(aq) + CD(aq) -> AD(aq) + CB(s)}$$

The net ionic form usually looks like:

cation (aq) + anion (aq) → solid (s)

On tests, you’re often given reactants and must predict the solid using solubility rules before writing anything else.

4. How to Write a Net Ionic Equation

Here’s the clean process.

1. Write and balance the molecular equation.
   Never skip balancing first.
2. Determine solubility.
   Identify aqueous vs solid.
3. Write the complete ionic equation.
   Dissociate only soluble aqueous compounds.
4. Cancel spectator ions.
5. Write the final net ionic equation.
   Reduce coefficients if possible.
   Re-check atom and charge balance.

If your final equation isn’t balanced for charge, something went wrong.

5. Physical vs Chemical Changes

Every process can be written symbolically.

Physical Changes

No rearrangement of atoms.

Examples:

$$\ce{H2O(l) -> H2O(g)}$$

$$\ce{KBr(s) -> K^{+}(aq) + Br^{-}(aq)}$$

The second looks dramatic, but it’s dissolution. No new substance formed.

Chemical Changes

Atoms rearrange into new substances.

Precipitation example:

$$\ce{Ag^{+}(aq) + Cl^{-}(aq) -> AgCl(s)}$$

Bonds are forming and breaking. That’s chemical change.

On FRQs, you may be asked to write either a full molecular equation or just the net ionic form depending on context. Read carefully.

6. Common AP Mistakes

• Dissociating a solid precipitate
• Forgetting phases (aq vs s matters)
• Not balancing before splitting into ions
• Leaving spectators in the final equation
• Ignoring charge balance

If you’re unsure, imagine the beaker. Which particles actually combine into something new?