Types of Reactions

Chemical reactions can be classified into a handful of categories based on the pattern of how reactants rearrange into products. Recognizing these patterns lets you predict products without memorizing every individual reaction.

Combination (Synthesis) Reactions

Two or more simple substances combine to form one more complex substance.

Pattern: A + B → AB

Examples:

• 2 Na + Cl₂ → 2 NaCl (metal + nonmetal → ionic compound)
• 2 Mg + O₂ → 2 MgO
• SO₃ + H₂O → H₂SO₄ (nonmetal oxide + water → acid)
• CaO + H₂O → Ca(OH)₂ (metal oxide + water → base)

Decomposition Reactions

One complex substance breaks down into two or more simpler substances. This is the reverse of combination.

Pattern: AB → A + B

Examples:

• 2 H₂O₂ → 2 H₂O + O₂ (hydrogen peroxide decomposes)
• CaCO₃ → CaO + CO₂ (limestone heated)
• 2 KClO₃ → 2 KCl + 3 O₂ (potassium chlorate decomposes)

Single Displacement (Replacement) Reactions

One element replaces another element in a compound. The displaced element is “kicked out” because the incoming element is more reactive.

Pattern: A + BC → AC + B

Examples:

• Zn + CuSO₄ → ZnSO₄ + Cu (zinc displaces copper)
• Fe + CuSO₄ → FeSO₄ + Cu (iron displaces copper)
• Cl₂ + 2 NaBr → 2 NaCl + Br₂ (chlorine displaces bromine)

The activity series determines whether a displacement reaction will occur. A more active metal displaces a less active metal from solution. If the incoming element is less active than the element in the compound, no reaction occurs.

Double Displacement (Metathesis) Reactions

Two compounds exchange partners. The cations trade anions.

Pattern: AB + CD → AD + CB

Examples:

• AgNO₃ + NaCl → AgCl(s) + NaNO₃ (precipitation)
• HCl + NaOH → NaCl + H₂O (neutralization)
• Na₂CO₃ + 2 HCl → 2 NaCl + H₂O + CO₂ (gas evolution)

Double displacement reactions are driven by the formation of:

1. A precipitate (insoluble solid)
2. Water (neutralization)
3. A gas (that escapes the solution)

If none of these forms, no net reaction occurs.

Combustion Reactions

A substance reacts with oxygen, typically producing heat and light. For organic compounds (containing C and H), the products are always CO₂ and H₂O.

Pattern: Hydrocarbon + O₂ → CO₂ + H₂O

Examples:

• CH₄ + 2 O₂ → CO₂ + 2 H₂O (methane combustion)
• 2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O (ethane combustion)
• C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O (propane combustion)

If the compound also contains oxygen (like ethanol, C₂H₅OH), the products are still CO₂ and H₂O - the oxygen in the compound just reduces the amount of O₂ needed.

Disproportionation Reactions

In a disproportionation reaction, a single element is simultaneously oxidized and reduced. The same species acts as both the oxidizing agent and the reducing agent.

Example:

• 2 H₂O₂ → 2 H₂O + O₂

In hydrogen peroxide (H₂O₂), oxygen has an oxidation state of -1. In the products, oxygen in H₂O is -2 (reduced) and oxygen in O₂ is 0 (oxidized). The same element (-1 oxygen) both gained and lost electrons.

Another example:

• Cl₂ + 2 NaOH → NaCl + NaClO + H₂O

Chlorine starts at 0 and ends up as -1 (in NaCl, reduced) and +1 (in NaClO, oxidized).

Summary Table