⚗️ Synthesis & Decomposition Reactions

Part 1 of 7 — Building Up and Breaking Down

Chemistry is fundamentally about rearranging atoms. The simplest patterns involve either combining simple substances into a complex product or breaking a complex substance into simpler pieces. These two reaction types — synthesis and decomposition — are mirror images of each other.

Synthesis (Combination) Reactions

A synthesis reaction occurs when two or more reactants combine to form a single product.

General Form

$A + B \rightarrow AB$

Classic Examples

Key Pattern

Look for multiple reactants forming one product. The number of reactants is always greater than the number of products (which is 1).

Real-World Example

Rust formation is a synthesis reaction: $4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$

Decomposition Reactions

A decomposition reaction occurs when a single reactant breaks down into two or more simpler products.

General Form

$AB \rightarrow A + B$

Classic Examples

Key Pattern

Look for one reactant producing multiple products. Decomposition often requires energy input (heat, electricity, or light).

Common Triggers

• Heating (symbol: $\Delta$ above arrow) — thermal decomposition
• Electrolysis — using electrical energy
• Photodecomposition — light-driven breakdown (e.g., AgBr in photography)

Synthesis vs. Decomposition — A Comparison

Memory Trick

• Synthesis = "put together" (syn- means together)
• Decomposition = "break apart" (de- means undo, compose means put together)

These reactions are essentially reverses of each other. If a synthesis reaction forms a compound, the decomposition of that compound yields the original elements or simpler compounds.

Synthesis & Decomposition Concept Quiz 🎯

Identify the Reaction Type 🧮

For each reaction, type synthesis or decomposition.

1. $2\text{Al} + 3\text{Br}_2 \rightarrow 2\text{AlBr}_3$

2. $2\text{HgO} \xrightarrow{\Delta} 2\text{Hg} + \text{O}_2$

3. $\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$

Fill in the Blanks — Synthesis & Decomposition 🔽

Exit Quiz — Synthesis & Decomposition ✅

🔄 Single & Double Replacement Reactions

Part 2 of 7 — Trading Partners

In replacement reactions, atoms or ions switch places. Understanding these reactions requires knowing which elements can displace others — that's where the activity series comes in.

Single Replacement (Single Displacement)

A more reactive element displaces a less reactive element from a compound.

General Form

$A + BC \rightarrow AC + B$

Element A replaces element B in compound BC, but only if A is more reactive than B.

Examples

No Reaction Example

$\text{Cu} + \text{ZnSO}_4 \rightarrow \text{No Reaction}$

Copper is less reactive than zinc, so it cannot displace zinc from solution.

The Activity Series

The activity series ranks metals (and hydrogen) by their ability to lose electrons and form cations. A higher metal can replace a lower one.

Metal Activity Series (Most → Least Reactive)

Key Rules

1. A metal above H₂ can react with acids to produce H₂ gas
2. A metal below H₂ (Cu, Ag, Au) cannot dissolve in common acids
3. A metal can replace any metal below it in solution
4. A halogen can replace any halogen below it: F₂ > Cl₂ > Br₂ > I₂

Memory Aid

Little Kids Bake Cakes Naturally → Makes All Zealous Fellows... → Nice Snacks Pbring Happiness → Curious Humming Agent Ptays Auto

Double Replacement (Double Displacement / Metathesis)

Two ionic compounds in solution exchange ions to form two new compounds.

General Form

$AB + CD \rightarrow AD + CB$

The cations and anions switch partners.

Driving Forces

A double replacement reaction occurs when one of the products is:

1. A precipitate (insoluble solid, denoted ↓)
2. Water (a molecular compound)
3. A gas (escapes the solution, denoted ↑)

Examples

Neutralization

The reaction of an acid with a base to form salt and water is a special case of double replacement:

$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$

Replacement Reactions Quiz 🎯

Predict the Reaction 🧮

For each scenario, indicate if a reaction occurs. Type the chemical formula of the solid metal produced, or type NR if no reaction occurs.

1. $\text{Fe} + \text{CuSO}_4 \rightarrow$ ? (Iron is above copper in the activity series)

2. $\text{Cu} + \text{AgNO}_3 \rightarrow$ ? (Copper is above silver in the activity series)

3. $\text{Ag} + \text{HCl} \rightarrow$ ? (Silver is below hydrogen in the activity series)

Replacement Reactions — Fill in the Blanks 🔽

Exit Quiz — Single & Double Replacement ✅

🔥 Combustion Reactions

Part 3 of 7 — Burning It Down

Combustion is one of the most important reaction types in chemistry and everyday life. From powering car engines to heating homes, combustion reactions release energy by reacting substances with oxygen gas.

Complete Combustion of Hydrocarbons

When a hydrocarbon (a compound containing only C and H) burns in excess oxygen, the products are always carbon dioxide and water.

General Form

$\text{C}_x\text{H}_y + \left(x + \frac{y}{4}\right)\text{O}_2 \rightarrow x\text{CO}_2 + \frac{y}{2}\text{H}_2\text{O}$

Common Examples

Key Recognition

If you see a hydrocarbon + O₂ → it's combustion. The products are always CO₂ + H₂O for complete combustion.

Combustion of Oxygenated Compounds

Compounds containing C, H, and O (like alcohols and sugars) also undergo combustion. The products are still CO₂ and H₂O.

Examples

Cellular Respiration

Glucose combustion is chemically identical to cellular respiration — your body "burns" glucose to release energy!

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy}$

Complete vs. Incomplete Combustion

Complete Combustion

• Occurs with excess oxygen
• Products: CO₂ + H₂O
• Blue flame
• Maximum energy released

Incomplete Combustion

• Occurs with limited oxygen
• Products may include: CO (carbon monoxide), C (soot), or both, along with H₂O
• Yellow/orange flame
• Less energy released
• Carbon monoxide is toxic — this is why proper ventilation matters

Comparison

Incomplete Combustion Example

$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$

With even less oxygen:

$\text{CH}_4 + \text{O}_2 \rightarrow \text{C} + 2\text{H}_2\text{O}$

Combustion Reactions Quiz 🎯

Balance the Combustion Equations 🧮

Find the coefficient of O₂ needed for complete combustion. Enter just the number.

1. \text{C}_2\text{H}_2 + \text{__O}_2 \rightarrow 2\text{CO}_2 + \text{H}_2\text{O} → coefficient of O₂ = ?

2. 2\text{C}_4\text{H}_{10} + \text{__O}_2 \rightarrow 8\text{CO}_2 + 10\text{H}_2\text{O} → coefficient of O₂ = ?

3. \text{C}_2\text{H}_5\text{OH} + \text{__O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} → coefficient of O₂ = ?

Round all answers to 3 significant figures.

Combustion Concepts — Fill in the Blanks 🔽

Exit Quiz — Combustion Reactions ✅

⚖️ Balancing Chemical Equations

Part 4 of 7 — The Coefficient Method

A balanced equation has equal numbers of each type of atom on both sides. This is required by the Law of Conservation of Mass — atoms are neither created nor destroyed in a chemical reaction.

Rules for Balancing Equations

What You CAN Do

• Change coefficients (the big numbers in front of formulas)

What You CANNOT Do

• Change subscripts (that changes the compound itself!)
• Add or remove substances

Step-by-Step Method

1. Write the unbalanced equation
2. Count atoms of each element on both sides
3. Balance one element at a time using coefficients
4. Start with the most complex substance or elements that appear in only one reactant and one product
5. Balance hydrogen and oxygen last (they often appear in multiple compounds)
6. Check that all atoms are balanced
7. Simplify coefficients to smallest whole numbers

Example: Balancing $\text{Al} + \text{O}_2 \rightarrow \text{Al}_2\text{O}_3$

The Polyatomic Ion Shortcut

When a polyatomic ion appears unchanged on both sides of an equation, treat it as a single unit rather than balancing individual atoms.

Example

$\text{Ca(OH)}_2 + \text{H}_3\text{PO}_4 \rightarrow \text{Ca}_3(\text{PO}_4)_2 + \text{H}_2\text{O}$

Instead of counting individual H, O, and P atoms, notice:

• PO₄ appears on both sides — treat it as one unit
• Balance PO₄: need 2 on left → coefficient of H₃PO₄ = 2 ✓
• Balance Ca: need 3 on right → coefficient of Ca(OH)₂ = 3 ✓
• Count H: left has 3(2) + 2(3) = 12. Right needs 12 → coefficient of H₂O = 6
• Check O from OH and H₂O: left 3(2) = 6 from OH; right 6 from H₂O ✓

$3\text{Ca(OH)}_2 + 2\text{H}_3\text{PO}_4 \rightarrow \text{Ca}_3(\text{PO}_4)_2 + 6\text{H}_2\text{O}$

Common Polyatomic Ions to Watch For

Worked Examples

Example 1: $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$

• Balance Fe: $2\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$ → Fe: 2 = 2 ✓
• Balance O: need 3 O on left, but O₂ gives even numbers → use LCM of 2 and 3 = 6
• $2\text{Fe} + 1.5\text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$ → multiply all by 2: $4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$

Check: Fe: 4 = 4 ✓, O: 6 = 6 ✓

Example 2: $\text{Al} + \text{HCl} \rightarrow \text{AlCl}_3 + \text{H}_2$

• Balance Cl: $\text{Al} + 3\text{HCl} \rightarrow \text{AlCl}_3 + \text{H}_2$
• Balance H: 3H on left, 2H on right → LCM = 6
• $\text{Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2$ → wait, now Cl: 6 vs 6 ✓
• Balance Al: need 2 on left → $2\text{Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2$

Check: Al: 2 = 2 ✓, H: 6 = 6 ✓, Cl: 6 = 6 ✓

Balancing Equations Quiz 🎯

Find the Missing Coefficients 🧮

Balance each equation. Enter the coefficient of the indicated substance.

1. \text{__N}_2 + \text{__H}_2 \rightarrow \text{__NH}_3 → coefficient of H₂ = ?

2. \text{__Al} + \text{__HCl} \rightarrow \text{__AlCl}_3 + \text{__H}_2 → coefficient of HCl = ?

3. \text{__Fe}_2\text{O}_3 + \text{__CO} \rightarrow \text{__Fe} + \text{__CO}_2 → coefficient of CO = ?

Balancing Strategy — Fill in the Blanks 🔽

Exit Quiz — Balancing Equations ✅

🔮 Predicting Products

Part 5 of 7 — Using Reaction Types to Predict What Forms

One of the most important skills in chemistry is being able to look at reactants and predict what the products will be. The key is to first identify the reaction type, then apply the pattern for that type.

Product Prediction Strategy

Step 1: Classify the Reactants

Step 2: Apply the Pattern

Step 3: Balance the Equation

After predicting the products, balance the equation using the coefficient method.

Predicting Synthesis Products

Metal + Nonmetal → Ionic Compound

Use charges to determine the formula:

Nonmetal Oxide + Water → Acid

Metal Oxide + Water → Base (Metal Hydroxide)

Predicting Decomposition Products

Common Decomposition Patterns

Key Memory Tool

Think of decomposition as "un-synthesizing" — the reverse of combining:

• Carbonates → oxide + CO₂
• Hydroxides → oxide + H₂O
• Chlorates → chloride + O₂

Predicting Replacement Products

Single Replacement

1. Check the activity series — will the reaction occur?
2. The free element replaces the corresponding element in the compound

$\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}$

Zn replaces Cu because Zn is higher in the activity series.

Double Replacement

1. Identify the cations and anions
2. Swap partners: cation₁ + anion₂ and cation₂ + anion₁
3. Write correct formulas using ion charges
4. Check for a driving force (precipitate, gas, or water)

$\text{Pb(NO}_3)_2 + 2\text{KI} \rightarrow \text{PbI}_2\downarrow + 2\text{KNO}_3$

• Pb²⁺ pairs with I⁻ → PbI₂ (insoluble precipitate — driving force!)
• K⁺ pairs with NO₃⁻ → KNO₃ (soluble)

Product Prediction Quiz 🎯

Predict the Products 🧮

Write the chemical formula of the main product. Use standard notation (no spaces, no charges).

1. Synthesis: $\text{K} + \text{Cl}_2 \rightarrow$ ? (K has charge +1, Cl has charge −1)

2. Decomposition: $\text{MgCO}_3 \xrightarrow{\Delta}$ ? + CO₂ — what is the other product?

3. Combustion: $\text{CH}_4 + 2\text{O}_2 \rightarrow$ ? + 2H₂O — what is the other product?

Prediction Strategy — Fill in the Blanks 🔽

Exit Quiz — Predicting Products ✅

🛠️ Problem-Solving Workshop

Part 6 of 7 — Identifying, Balancing, and Predicting

It's time to put together everything you've learned about reaction types. In this workshop, you'll practice the complete workflow: identify the reaction type, predict the products, and balance the equation.

The Complete Workflow

Step 1: Identify the Reaction Type

Step 2: Predict Products

Apply the rules for that reaction type:

• Synthesis: combine to form one compound
• Decomposition: break into simpler substances
• Single replacement: check activity series, swap element
• Double replacement: swap cations, check for driving force
• Combustion: products are CO₂ + H₂O

Step 3: Write Correct Formulas

Use ion charges for ionic compounds. Don't forget subscripts!

Step 4: Balance the Equation

Use coefficients. Check every atom. Simplify to smallest whole numbers.

Worked Examples

Example 1

Given: $\text{Mg} + \text{N}_2 \rightarrow$ ?

1. Type: Two elements combining → Synthesis
2. Product: Mg²⁺ + N³⁻ → Mg₃N₂ (cross charges: 3 Mg, 2 N)
3. Balance: $3\text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2$ ✓

Example 2

Given: $\text{Fe} + \text{AgNO}_3 \rightarrow$ ?

1. Type: Element + compound → Single replacement
2. Activity series: Fe is above Ag → reaction proceeds
3. Product: Fe replaces Ag. Fe has charge +2 → Fe(NO₃)₂ + Ag
4. Balance: $\text{Fe} + 2\text{AgNO}_3 \rightarrow \text{Fe(NO}_3)_2 + 2\text{Ag}$ ✓

Example 3

Given: $\text{Na}_2\text{CO}_3 + \text{CaCl}_2 \rightarrow$ ?

1. Type: Two ionic compounds → Double replacement
2. Swap: Na⁺ with Cl⁻ → NaCl; Ca²⁺ with CO₃²⁻ → CaCO₃
3. Driving force: CaCO₃ is insoluble → precipitate forms ✓
4. Balance: $\text{Na}_2\text{CO}_3 + \text{CaCl}_2 \rightarrow \text{CaCO}_3\downarrow + 2\text{NaCl}$ ✓

Classify and Analyze 🎯

Full Workflow Practice 🧮

Identify each reaction type. Type: synthesis, decomposition, single, double, or combustion.

1. $4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$

2. $\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3$

3. $\text{C}_2\text{H}_6 + \frac{7}{2}\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$

Predict Products and Balance 🔬

Workshop Review — Fill in the Blanks 🔽

Exit Quiz — Problem-Solving Workshop ✅

🎓 Synthesis & AP Review

Part 7 of 7 — AP-Style Questions on Reaction Classification

This final part brings together everything from the entire topic. You'll face AP-level questions that require you to classify reactions, predict products, balance equations, and apply your knowledge of the activity series, driving forces, and combustion patterns.

Complete Reaction Type Summary

AP Exam Tips

1. Read carefully — the AP exam often describes reactions in words rather than symbols
2. Know the activity series — required for predicting single replacement
3. Know solubility rules — required for identifying precipitates in double replacement
4. Balancing must be correct — even if the products are right, unbalanced equations lose points
5. State symbols matter — (s), (l), (g), (aq) are often tested
6. Net ionic equations — AP frequently asks for these in double replacement reactions

Net Ionic Equations — Quick Review

For double replacement reactions in solution, the AP exam often asks for net ionic equations.

Steps

1. Write the balanced molecular equation
2. Write the complete ionic equation (split soluble ionic compounds into ions)
3. Cancel spectator ions (ions that appear unchanged on both sides)
4. What remains is the net ionic equation

Example

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

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

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

The spectator ions (Na⁺ and NO₃⁻) are removed because they don't participate in the actual reaction.

AP-Style Questions — Set 1 🎯

AP-Style Identification 🧮

Classify each reaction. Type: synthesis, decomposition, single, double, or combustion.

1. $2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}$

2. $\text{Pb(NO}_3)_2(\text{aq}) + 2\text{KI}(\text{aq}) \rightarrow \text{PbI}_2(\text{s}) + 2\text{KNO}_3(\text{aq})$

3. $2\text{C}_8\text{H}_{18} + 25\text{O}_2 \rightarrow 16\text{CO}_2 + 18\text{H}_2\text{O}$

AP-Style Questions — Set 2 🔬

Comprehensive Review — Fill in the Blanks 🔽

Final Exit Quiz — Reaction Types Mastery ✅