Chemical Reactions: Synthesis, Decomposition

• Chemical reactions involve the rearrangement of atoms and molecules

Synthesis Reactions

• Elements or simpler compounds combine to form a single, more complex compound
• General form: A + B → AB
• Example: 2Na(s) + Cl₂(g) → 2NaCl(s), where sodium and chlorine combine to form sodium chloride

Decomposition Reactions

• A compound breaks down into simpler substances, such as elements or smaller compounds
• General form: AB → A + B
• Example: 2H₂O₂(aq) → 2H₂O(l) + O₂(g), where hydrogen peroxide decomposes into water and oxygen

Single Replacement Reactions

• An element replaces a similar element within a compound
• General form: A + BC → AC + B
• Example: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g), where zinc replaces hydrogen in hydrochloric acid

Double Replacement Reactions

• Ions in two compounds switch places, leading to the formation of two new compounds
• General form: AB + CD → AD + CB
• Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq), where barium sulfate precipitates out of the solution

Combustion Reactions

• A substance reacts with oxygen, rapidly producing heat and light
• Commonly involves a hydrocarbon reacting with oxygen to produce carbon dioxide and water
• General form: Hydrocarbon + O₂ → CO₂ + H₂O
• Example: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g), the combustion of methane

Balancing Equations & States of Matter

• Balance chemical equations carefully using the order: metals, nonmetals, oxygen, then hydrogen
• Pay attention to the states of matter: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water)

Tips for Identifying Reactions

• Synthesis reactions involve combining substances
• Decomposition reactions involve breaking down a substance
• Replacement reactions involve switching places

Common Mistakes

• When balancing equations, only change coefficients, not subscripts
• Practice identifying reaction types quickly to avoid mixing them up