Types of Chemical Reactions

⚗️ Synthesis & Decomposition Reactions

Part 1 of 7 — Building Up and Breaking Down

Topics in This Part

🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.

What You'll Master in Part 1

• Understanding the core concepts covered in Part 1
• Applying these ideas to solve practice problems
• Building toward AP exam readiness for this topic

🎯 Synthesis (Combination) Reactions

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

General Form

$\boxed{A + B \rightarrow AB}$

Classic Examples

Key Pattern

🔑 Key Concept: 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: $\boxed{4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3}$

⚗️ Decomposition Reactions

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

General Form

$\boxed{AB \rightarrow A + B}$

Classic Examples

Key Pattern

🔑 Key Concept: 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

💡 Tip: Use word roots to remember:

• 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

Topics in This Part

🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.

What You'll Master in Part 2

• Understanding the core concepts covered in Part 2
• Applying these ideas to solve practice problems
• Building toward AP exam readiness for this topic

📌 Single Replacement (Single Displacement)

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

General Form

$\boxed{A + BC \rightarrow AC + B}$

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

Examples

No Reaction Example

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

⚠️ Warning: Copper is less reactive than zinc, so it cannot displace zinc from solution. Always check the activity series before predicting products — if the free element is below the element in the compound, no reaction occurs.

📌 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

🔑 Key Concept: The activity series determines which replacement reactions will occur:

1. A metal above $H_{2}$ can react with acids to produce $H_{2}$ gas
2. A metal below $H_{2}$ (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: $F_{2}$ > $Cl_{2}$ > $Br_{2}$ > $I_{2}$

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

$\boxed{AB + CD \rightarrow AD + CB}$

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:

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

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

Topics in This Part

🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.

What You'll Master in Part 3

• Understanding the core concepts covered in Part 3
• Applying these ideas to solve practice problems
• Building toward AP exam readiness for this topic

📌 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

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

Common Examples

Key Recognition

💡 Tip: If you see a hydrocarbon + $O_{2}$ → it's combustion. The products are always $CO_{2}$ + $H_{2}O$ for complete combustion.

📌 Combustion of Oxygenated Compounds

Compounds containing C, H, and O (like alcohols and sugars) also undergo combustion. The products are still $CO_{2}$ and $H_{2}O$.

Examples

Cellular Respiration

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

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

📌 Complete vs. Incomplete Combustion

Complete Combustion

• Occurs with excess oxygen
• Products: $CO_{2}$ + $H_{2}O$
• Blue flame
• Maximum energy released

Incomplete Combustion

• Occurs with limited oxygen
• Products may include: CO (carbon monoxide), C (soot), or both, along with $H_{2}O$
• Yellow/orange flame
• Less energy released

⚠️ Warning: Carbon monoxide (CO) is toxic and odorless — this is why proper ventilation matters. CO binds to hemoglobin more strongly than $O_{2}$, preventing oxygen transport.

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 $O_{2}$ needed for complete combustion. Enter just the number.

1) $\text{C}_2\text{H}_2 + \underline{\hspace{1em}}\,\text{O}_2 \rightarrow 2\text{CO}_2 + \text{H}_2\text{O}$ → coefficient of $O_{2}$ = ?

2) $2\text{C}_4\text{H}_{10} + \underline{\hspace{1em}}\,\text{O}_2 \rightarrow 8\text{CO}_2 + 10\text{H}_2\text{O}$ → coefficient of $O_{2}$ = ?

3) $\text{C}_2\text{H}_5\text{OH} + \underline{\hspace{1em}}\,\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$ → coefficient of $O_{2}$ = ?

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

Topics in This Part

🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.

What You'll Master in Part 4

• Understanding the core concepts covered in Part 4
• Applying these ideas to solve practice problems
• Building toward AP exam readiness for this topic

⚖️ Rules for Balancing Equations

What You CAN Do

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

What You CANNOT Do

⚠️ Warning: Never change subscripts — that changes the identity of the compound! You also cannot add or remove substances. Only adjust coefficients.

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

💡 Tip: 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:

• $PO_{4}$ appears on both sides — treat it as one unit
• Balance $PO_{4}$: need 2 on left → coefficient of $H_{3}PO_{4}$ = 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 $H_{2}O$ = 6
• Check O from OH and $H_{2}O$: left 3(2) = 6 from OH; right 6 from $H_{2}O$ ✓

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

Common Polyatomic Ions to Watch For

🧪 Worked Examples

Example 1

Problem: Balance $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$

Solution:

• 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 $O_{2}$ 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: $\boxed{4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3}$

Check: Fe: 4 = 4 ✓, O: 6 = 6 ✓

Example 2

Problem: Balance $\text{Al} + \text{HCl} \rightarrow \text{AlCl}_3 + \text{H}_2$

Solution:

• 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: $\boxed{2\text{Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2}$

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) $\underline{\hspace{1em}}\,\text{N}_2 + \underline{\hspace{1em}}\,\text{H}_2 \rightarrow \underline{\hspace{1em}}\,\text{NH}_3$ → coefficient of $H_{2}$ = ?

2) $\underline{\hspace{1em}}\,\text{Al} + \underline{\hspace{1em}}\,\text{HCl} \rightarrow \underline{\hspace{1em}}\,\text{AlCl}_3 + \underline{\hspace{1em}}\,\text{H}_2$ → coefficient of HCl = ?

3) $\underline{\hspace{1em}}\,\text{Fe}_2\text{O}_3 + \underline{\hspace{1em}}\,\text{CO} \rightarrow \underline{\hspace{1em}}\,\text{Fe} + \underline{\hspace{1em}}\,\text{CO}_2$ → 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

Topics in This Part

🔑 Key Concept: Mastering this material will strengthen your foundation for both the AP Chemistry exam and more advanced chemistry topics.

What You'll Master in Part 5

• Understanding the core concepts covered in Part 5
• Applying these ideas to solve practice problems
• Building toward AP exam readiness for this topic

🎯 Product Prediction Strategy

🔑 Key Concept: The key to predicting products is to first identify the reaction type, then apply the pattern for that type.

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

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

• Carbonates → oxide + $CO_{2}$
• Hydroxides → oxide + $H_{2}O$
• Chlorates → chloride + $O_{2}$

🔮 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.

⚠️ Warning: A common error is forgetting to check the activity series. If the free element is below the element in the compound, write NR (no reaction).

Double Replacement

1. Identify the cations and anions
2. Swap partners: $cation_{1}$ + $anion_{2}$ and $cation_{2}$ + $anion_{1}$
3. Write correct formulas using ion charges
4. Check for a driving force (precipitate, gas, or water)

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

• $Pb^{2+}$ pairs with $I^{-}$ → $PbI_{2}$ (insoluble precipitate — driving force!)
• $K^{+}$ pairs with $NO_{3}^{-}$ → $KNO_{3}$ (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: $\text{MgCO}_3 \xrightarrow{\Delta}$ ? + $CO_{2}$ — what is the other product?

3) Combustion: $\text{CH}_4 + 2\text{O}_2 \rightarrow$ ? + $2H_{2}O$ — 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

Practice Makes Perfect

This workshop features multi-step problems that mirror the AP Chemistry exam format. Each problem requires you to combine concepts from previous parts and show your work clearly.

🔑 Why this matters: The AP Chemistry exam rewards students who can apply concepts to unfamiliar problems — structured practice is the best preparation.

What You'll Master in Part 6

• Working through complete multi-step problems from start to finish
• Building problem-solving strategies you can apply on the AP exam
• Identifying which concepts to apply and in what order

📌 The Complete Workflow

Step 1: Identify the Reaction Type

💡 Tip: On the AP exam, quickly classify reactions by counting reactants/products and looking for $O_{2}$, free elements, or ionic compounds exchanging partners.

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 $CO_{2}$ + $H_{2}O$

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

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

Solution:

1. Type: Two elements combining → Synthesis
2. Product: $Mg^{2+}$ + $N^{3-}$ → $Mg_{3}N_{2}$ (cross charges: 3 Mg, 2 N)
3. Balance:

$\boxed{3\text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2}$

Example 2

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

Solution:

1. Type: Element + compound → Single replacement
2. Activity series: Fe is above Ag → reaction proceeds
3. Product: Fe replaces Ag. Fe has charge +2 → $Fe(NO_{3})_{2}$ + Ag
4. Balance:

$\boxed{\text{Fe} + 2\text{AgNO}_3 \rightarrow \text{Fe(NO}_3)_2 + 2\text{Ag}}$

Example 3

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

Solution:

1. Type: Two ionic compounds → Double replacement
2. Swap: $Na^{+}$ with $Cl^{-}$ → NaCl; $Ca^{2+}$ with $CO_{3}^{2-}$ → $CaCO_{3}$
3. Driving force: $CaCO_{3}$ is insoluble → precipitate forms ✓
4. Balance:

$\boxed{\text{Na}_2\text{CO}_3 + \text{CaCl}_2 \rightarrow \text{CaCO}_3\downarrow + 2\text{NaCl}}$

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

Bringing It All Together

This comprehensive review connects every concept from Parts 1–6 with AP-style problems. The questions are designed to mirror what you'll see on the actual exam — multi-step, multi-concept, and requiring clear written explanations.

🔑 Why this matters: AP Chemistry exam questions rarely test one concept in isolation — success requires connecting ideas across topics.

What You'll Master in Part 7

• Solving AP-style questions that integrate multiple concepts from this unit
• Writing clear, concise explanations using proper chemistry terminology
• Identifying and avoiding common AP exam traps and mistakes

📋 Complete Reaction Type Summary

🔑 Key Concept: To classify any reaction, follow this decision tree: (1) Only one reactant? → Decomposition. (2) Only one product? → Synthesis. (3) $O_{2}$ + fuel → $CO_{2}$ + $H_{2}O$? → Combustion. (4) Free element + compound? → Single replacement. (5) Two ionic compounds? → Double replacement.

📌 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

💡 Tip: On the AP exam, quickly scan for visual cues: a lone reactant (decomposition), $O_{2}$ on the left with $CO_{2}$ + $H_{2}O$ on the right (combustion), a free element mixed with a compound (single replacement), or two aqueous ionic compounds (double replacement).

⚠️ Warning: Common errors when predicting products: (1) writing incorrect ionic formulas by not cross-checking charges, (2) forgetting to consult the activity series for single replacement, (3) predicting a double replacement when no driving force exists (no precipitate, gas, or water), and (4) submitting unbalanced equations.

🔄 Net Ionic Equations — Quick Review

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

📌 Step-by-Step Method

🧪 What to Split vs. Keep Whole

💡 Spectator ions appear unchanged on both sides — 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 ✅