🧪 Molecular vs. Ionic Equations

Part 1 of 7 — Writing Full Molecular Equations

When ionic compounds dissolve in water, they dissociate into ions. To properly describe reactions in aqueous solution, chemists use three levels of detail: molecular equations, complete ionic equations, and net ionic equations. In this lesson, we start with the most familiar form — the full molecular equation.

Full Molecular Equations

A molecular equation (also called a formula equation) shows all reactants and products as complete, neutral formulas — just like the balanced equations you've written before.

Example

$\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)$

Key Features

Phase Labels Review

Electrolytes and Dissociation

To move beyond molecular equations, you must know which substances dissociate (break apart) into ions when dissolved.

Strong Electrolytes — Fully Dissociate

$\text{NaCl}(aq) \rightarrow \text{Na}^+(aq) + \text{Cl}^-(aq)$

Weak Electrolytes — Partially Dissociate

These remain mostly as intact molecules in solution and are written in molecular form.

Non-Electrolytes — Do Not Dissociate

Critical Rule

Only strong electrolytes in aqueous solution are written as separated ions. Solids, liquids, gases, weak electrolytes, and non-electrolytes stay as complete formulas.

Writing Balanced Molecular Equations

Steps

1. Identify reactants and products (predict products using reaction type rules)
2. Write correct formulas for each compound (use charges to determine subscripts)
3. Balance the equation
4. Add phase labels — $(aq)$ for dissolved species, $(s)$ for precipitates, etc.

Example: Silver Nitrate + Sodium Chloride

Step 1: Reactants: AgNO₃ and NaCl (both in aqueous solution)

Step 2: Products: The cations and anions swap partners → AgCl and NaNO₃

Step 3: Already balanced: $\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)$

Step 4: AgCl is insoluble (precipitate → solid), everything else is aqueous.

Common Pitfall

Always check solubility to assign correct phase labels. A reaction may not occur if no precipitate, gas, or water forms!

Molecular Equations Concept Quiz 🎯

Electrolyte Classification 🧮

Classify each substance as strong, weak, or non (electrolyte).

1. KBr dissolved in water

2. CH₃COOH (acetic acid) in water

3. C₂H₅OH (ethanol) in water

Fill in the Blanks — Molecular Equations 🔽

Exit Quiz — Molecular Equations ✅

🧪 Complete Ionic Equations

Part 2 of 7 — Breaking Strong Electrolytes into Ions

Now that you can write balanced molecular equations, it's time to show what's really happening in solution. In a complete ionic equation, every strong electrolyte dissolved in water is written as its constituent ions. This reveals which species actually participate in the reaction.

Rules for Writing Complete Ionic Equations

What Gets Split into Ions?

Only aqueous strong electrolytes are written as separated ions:

The Six Strong Acids (memorize these!)

$\text{HCl, HBr, HI, HNO}_3\text{, H}_2\text{SO}_4\text{, HClO}_4$

Strong Bases

$\text{LiOH, NaOH, KOH, Ca(OH)}_2\text{, Sr(OH)}_2\text{, Ba(OH)}_2$

Important

Weak acids (HF, CH₃COOH, H₂CO₃, H₃PO₄) and weak bases (NH₃) are NOT split into ions — they stay as complete formulas.

Solubility Rules

To determine if an ionic compound is $(aq)$ or $(s)$, use the solubility rules:

Generally Soluble (aqueous)

Generally Insoluble (solid precipitate)

Using Solubility Rules

If an ionic compound is soluble → label $(aq)$ → split into ions

If an ionic compound is insoluble → label $(s)$ → keep as formula

Worked Example

Write the complete ionic equation for:

$\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)$

Step 1: Identify what splits

Step 2: Write the complete ionic equation

$\text{Ag}^+(aq) + \text{NO}_3^-(aq) + \text{Na}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s) + \text{Na}^+(aq) + \text{NO}_3^-(aq)$

Step 3: Verify

• Atoms balanced ✓
• Charges balanced: $(+1) + (-1) + (+1) + (-1) = 0$ on left; $0 + (+1) + (-1) = 0$ on right ✓

Complete Ionic Equations Quiz 🎯

Solubility Predictions 🧮

For each ionic compound, type soluble or insoluble.

1. PbCl₂

2. Na₂CO₃

3. Fe(OH)₃

Complete Ionic Equation Practice 🔽

Exit Quiz — Complete Ionic Equations ✅

🧪 Net Ionic Equations

Part 3 of 7 — Removing Spectator Ions

The complete ionic equation shows every ion in solution, but many of those ions don't actually do anything — they're just along for the ride. By removing these spectator ions, we reveal the net ionic equation: the essence of what's really happening in the reaction.

Spectator Ions

Spectator ions are ions that appear in the same form on both sides of the complete ionic equation. They don't participate in the actual reaction.

Example

Complete ionic equation:

$\text{Ag}^+(aq) + \underset{\text{spectator}}{\text{NO}_3^-(aq)} + \underset{\text{spectator}}{\text{Na}^+(aq)} + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s) + \underset{\text{spectator}}{\text{Na}^+(aq)} + \underset{\text{spectator}}{\text{NO}_3^-(aq)}$

• Na⁺ appears on both sides → spectator
• NO₃⁻ appears on both sides → spectator
• Ag⁺ reacts with Cl⁻ to form AgCl(s) → NOT spectators

Removing Spectators

Cancel ions that appear identically on both sides:

$\text{Ag}^+(aq) + \cancel{\text{NO}_3^-(aq)} + \cancel{\text{Na}^+(aq)} + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s) + \cancel{\text{Na}^+(aq)} + \cancel{\text{NO}_3^-(aq)}$

Net Ionic Equation

$\text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s)$

This is the simplest representation showing only what changes.

Steps to Write a Net Ionic Equation

The Complete Process

Driving Forces

A reaction occurs (and a net ionic equation exists) when at least one of these forms:

If none of these forms, there is no reaction (NR) and all ions are spectators.

Worked Example: No Reaction

Mix NaCl(aq) and KNO₃(aq)

Step 1 — Predict products: Swapping ions gives NaNO₃ and KCl.

Step 2 — Check solubility: NaNO₃ is soluble. KCl is soluble. No precipitate forms!

Step 3 — Complete ionic equation:

$\text{Na}^+(aq) + \text{Cl}^-(aq) + \text{K}^+(aq) + \text{NO}_3^-(aq) \rightarrow \text{Na}^+(aq) + \text{NO}_3^-(aq) + \text{K}^+(aq) + \text{Cl}^-(aq)$

Step 4: Every ion is a spectator! All ions appear identically on both sides.

Conclusion: No net ionic equation — no reaction (NR).

Key Lesson

Just because you can write a double-replacement equation doesn't mean a reaction occurs. You must have a driving force (precipitate, water, or gas).

Net Ionic Equations Concept Quiz 🎯

Identify Spectator Ions 🧮

For the reaction: $\text{CaCl}_2(aq) + \text{Na}_2\text{CO}_3(aq) \rightarrow \text{CaCO}_3(s) + 2\text{NaCl}(aq)$

List the spectator ions (type the ion formula without charge, in alphabetical order separated by a comma — e.g., "Cl, Na"):

1. The two spectator ions are:

For the net ionic equation $\text{Ca}^{2+}(aq) + \text{CO}_3^{2-}(aq) \rightarrow \text{CaCO}_3(s)$:

1. The sum of charges on the reactant side is:

2. The sum of charges on the product side is:

Net Ionic Equation Concepts 🔽

Exit Quiz — Net Ionic Equations ✅

🧪 Precipitation Reactions

Part 4 of 7 — Using Solubility Rules to Predict Precipitates

Precipitation reactions are among the most common types you'll encounter in AP Chemistry. When two aqueous solutions are mixed, an insoluble solid (precipitate) may form. Your job is to predict if and what precipitates, then write the net ionic equation.

Predicting Precipitation Reactions

Strategy: The Ion-Swap Method

When two ionic compounds in solution are mixed:

1. List all ions present in solution
2. Swap partners — combine each cation with each new anion
3. Check solubility of each possible product
4. If any product is insoluble → precipitation occurs!

Example: Mix AgNO₃(aq) and NaCl(aq)

Ions present: Ag⁺, NO₃⁻, Na⁺, Cl⁻

Possible new combinations:

• Ag⁺ + Cl⁻ → AgCl — Check: Cl⁻ with Ag⁺ is an exception → insoluble ✓
• Na⁺ + NO₃⁻ → NaNO₃ — Check: Na⁺ always soluble → soluble

Result: AgCl precipitates!

$\text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s)$

Common Precipitates on the AP Exam

Net Ionic Patterns

All precipitation reactions follow the same net ionic pattern:

$\text{cation}(aq) + \text{anion}(aq) \rightarrow \text{insoluble salt}(s)$

The spectator ions are always the "other" pair that forms a soluble compound.

Multi-Step Example

Mix: $\text{FeCl}_3(aq) + 3\text{NaOH}(aq) \rightarrow$ ?

Step 1: Identify ions

Fe³⁺, Cl⁻, Na⁺, OH⁻

Step 2: Swap partners

• Fe³⁺ + OH⁻ → Fe(OH)₃ — Solubility? OH⁻ insoluble with most metals → insoluble
• Na⁺ + Cl⁻ → NaCl — Solubility? Na⁺ always soluble → soluble

Step 3: Molecular equation

$\text{FeCl}_3(aq) + 3\text{NaOH}(aq) \rightarrow \text{Fe(OH)}_3(s) + 3\text{NaCl}(aq)$

Step 4: Complete ionic equation

$\text{Fe}^{3+}(aq) + 3\text{Cl}^-(aq) + 3\text{Na}^+(aq) + 3\text{OH}^-(aq) \rightarrow \text{Fe(OH)}_3(s) + 3\text{Na}^+(aq) + 3\text{Cl}^-(aq)$

Step 5: Net ionic equation (cancel Na⁺ and Cl⁻)

$\text{Fe}^{3+}(aq) + 3\text{OH}^-(aq) \rightarrow \text{Fe(OH)}_3(s)$

Charge check: (+3) + 3(−1) = 0 on left; 0 on right ✓

Precipitation Reactions Quiz 🎯

Predict the Precipitate 🧮

For each pair of solutions mixed, write the formula of the precipitate that forms. If no reaction occurs, type NR.

1. $\text{CaCl}_2(aq) + \text{Na}_2\text{CO}_3(aq) \rightarrow$

2. $\text{KNO}_3(aq) + \text{NaBr}(aq) \rightarrow$

3. $\text{Cu(NO}_3)_2(aq) + 2\text{NaOH}(aq) \rightarrow$

Precipitation Concepts 🔽

Exit Quiz — Precipitation Reactions ✅

🧪 Acid-Base Net Ionic Equations

Part 5 of 7 — Strong and Weak Acid-Base Reactions

Acid-base reactions are one of the most important categories for net ionic equations on the AP exam. The driving force is the formation of water ($\text{H}_2\text{O}$). How you write the net ionic equation depends on whether the acid and base are strong or weak.

Strong Acid + Strong Base

The Universal Net Ionic Equation

When a strong acid reacts with a strong base:

Molecular: $\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$

Complete ionic: $\text{H}^+(aq) + \text{Cl}^-(aq) + \text{Na}^+(aq) + \text{OH}^-(aq) \rightarrow \text{Na}^+(aq) + \text{Cl}^-(aq) + \text{H}_2\text{O}(l)$

Net ionic (cancel Na⁺ and Cl⁻): $\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$

Key Point

Every strong acid–strong base reaction has the same net ionic equation! The identity of the acid and base doesn't matter because the salt ions are always spectators.

Weak Acid + Strong Base

When a weak acid reacts with a strong base, the weak acid is NOT split into ions — it stays molecular.

Example: Acetic acid + Sodium hydroxide

Molecular: $\text{CH}_3\text{COOH}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCH}_3\text{COO}(aq) + \text{H}_2\text{O}(l)$

Complete ionic: $\text{CH}_3\text{COOH}(aq) + \text{Na}^+(aq) + \text{OH}^-(aq) \rightarrow \text{Na}^+(aq) + \text{CH}_3\text{COO}^-(aq) + \text{H}_2\text{O}(l)$

Net ionic (cancel Na⁺): $\text{CH}_3\text{COOH}(aq) + \text{OH}^-(aq) \rightarrow \text{CH}_3\text{COO}^-(aq) + \text{H}_2\text{O}(l)$

Why Is This Different?

The weak acid doesn't fully dissociate, so it cannot be written as H⁺ + anion. It must appear as the complete molecule. The net ionic equation is unique to each weak acid.

Strong Acid + Weak Base

When a strong acid reacts with a weak base, the weak base stays molecular.

Example: HCl + Ammonia

Molecular: $\text{HCl}(aq) + \text{NH}_3(aq) \rightarrow \text{NH}_4\text{Cl}(aq)$

Complete ionic: $\text{H}^+(aq) + \text{Cl}^-(aq) + \text{NH}_3(aq) \rightarrow \text{NH}_4^+(aq) + \text{Cl}^-(aq)$

Net ionic (cancel Cl⁻): $\text{H}^+(aq) + \text{NH}_3(aq) \rightarrow \text{NH}_4^+(aq)$

Summary Table

Acid-Base Net Ionic Quiz 🎯

Acid-Base Classification 🧮

For each net ionic equation scenario, type strong-strong, weak-strong, or strong-weak.

1. HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

2. HF(aq) + KOH(aq) → KF(aq) + H₂O(l)

3. HClO₄(aq) + NH₃(aq) → NH₄ClO₄(aq)

Acid-Base Net Ionic Concepts 🔽

Exit Quiz — Acid-Base Net Ionic ✅

🧪 Problem-Solving Workshop

Part 6 of 7 — Writing Net Ionic Equations for Various Reaction Types

Time to practice the complete process — from identifying the reaction type, through molecular and complete ionic equations, to the final net ionic equation. This workshop covers precipitation, acid-base, and gas-forming reactions.

Gas-Forming Reactions

Some double-replacement reactions produce an unstable compound that decomposes into a gas and water. These are important driving forces.

Common Gas-Forming Patterns

Example: Acid + Carbonate

$2\text{HCl}(aq) + \text{Na}_2\text{CO}_3(aq) \rightarrow 2\text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$

Net ionic: $2\text{H}^+(aq) + \text{CO}_3^{2-}(aq) \rightarrow \text{H}_2\text{O}(l) + \text{CO}_2(g)$

The fizzing you see when acid hits baking soda is CO₂ gas escaping!

Systematic Problem-Solving Approach

The 5-Step Method

Decision Tree for Splitting

Ask: Is this species...

• A solid, liquid, or gas? → Keep molecular
• A weak electrolyte? → Keep molecular
• An aqueous strong electrolyte? → Split into ions

Common Mistakes to Avoid

1. Splitting a precipitate into ions (NO — it's a solid!)
2. Splitting a weak acid like HF or CH₃COOH (NO — weak electrolyte!)
3. Forgetting to balance the molecular equation first
4. Not checking charge balance in the net ionic equation

Mixed Practice Quiz 🎯

Write the Products 🧮

Identify the precipitate or gas formed. Type the chemical formula (no phase labels).

1. What precipitate forms when Pb(NO₃)₂(aq) is mixed with KI(aq)?

2. What gas is produced when HCl(aq) reacts with NaHCO₃(aq)?

3. What precipitate forms when CuSO₄(aq) is mixed with NaOH(aq)?

Reaction Classification 🔽

Challenge Problems 🏆

Exit Quiz — Problem-Solving Workshop ✅

🧪 Synthesis & AP Review

Part 7 of 7 — AP-Style Net Ionic Equation Problems

This final lesson brings together everything: molecular equations, complete ionic equations, net ionic equations, solubility rules, acid-base chemistry, and gas-forming reactions. The problems mirror the style and difficulty of AP Chemistry free-response questions.

AP Exam Tips for Net Ionic Equations

What the AP Exam Expects

On the AP Chemistry exam, you may be asked to:

• Write the balanced net ionic equation for a described reaction
• Identify spectator ions
• Predict whether a reaction occurs
• Identify driving forces

Common AP Formats

Scoring Tips

1. Correct formulas — wrong formula = wrong answer
2. Correct charges — always include charges on ions
3. Correct phases — $(s)$, $(l)$, $(g)$, $(aq)$ matter
4. Balanced — both atoms and charges must balance
5. Lowest whole-number coefficients — simplify the equation

Complete Review: Types of Net Ionic Equations

Summary of All Patterns

Quick Solubility Reminders

• Always soluble: Na⁺, K⁺, NH₄⁺, NO₃⁻
• Usually soluble: Cl⁻, Br⁻, I⁻ (except Ag⁺, Pb²⁺, Hg₂²⁺)
• Usually soluble: SO₄²⁻ (except Ba²⁺, Pb²⁺, Ca²⁺, Sr²⁺)
• Usually insoluble: OH⁻, S²⁻, CO₃²⁻, PO₄³⁻ (except Group 1, NH₄⁺)

AP-Style Questions — Set 1 🎯

AP-Style Free Response Practice 🧮

For each scenario, identify the precipitate formed (write formula) or type NR if no reaction occurs.

1. Solutions of iron(III) nitrate and sodium hydroxide are mixed.

2. Solutions of ammonium chloride and potassium nitrate are mixed.

3. Solutions of lead(II) nitrate and sodium sulfate are mixed.

AP Concept Review 🔽

AP-Style Questions — Set 2 🏆

Final Exit Quiz — Net Ionic Equations Mastery ✅