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Combination & Decomposition Reactions

Understanding how substances join together and break apart

Overview: The Five Major Types

Millions of chemical reactions exist, making it impossible to memorize them all. Instead, we group them into five major categories based on their reactants to help us predict products.

Our Objectives:

  • Identify reactions by their reactants
  • Predict the products that will form

For now, we focus on the first two: Combination and Decomposition.

1. Combination Reactions

Definition

A combination reaction happens when two or more reactants join together to form one single product.

A + B → AB

Key Examples:

Water Formation

2H2 + O2 → 2H2O

Iron(II) Sulfide

Fe + S → FeS

Calcium Hydroxide

CaO + H2O → Ca(OH)2

Magnesium Nitride

3Mg + N2 → Mg3N2

Aluminium Oxide

4Al + 3O2 → 2Al2O3

Ammonia Formation

N2 + 3H2 → 2NH3

2. Decomposition Reactions

Definition

A decomposition reaction happens when one compound breaks down into two or more simpler substances.

AB → A + B

Key Examples:

Potassium Chlorate

2KClO3 → 2KCl + 3O2

Mercury(II) Oxide

2HgO → 2Hg + O2

Calcium Carbonate

CaCO3 → CaO + CO2

Hydrogen Peroxide

2H2O2 → 2H2O + O2

Water (Electrolysis)

2H2O → 2H2 + O2

Silver Bromide (Light)

2AgBr → 2Ag + Br2

Why Energy is Needed

Decomposition reactions almost always require energy (heat, electricity, or light) to break the strong chemical bonds in the compound.

H2O → H2 + O2

Requires electricity (Electrolysis).

H2O → Steam

A physical phase change using heat, not chemical decomposition.

Complex Decomposition (3+ Elements)

If a compound has more than two elements, predicting products is difficult unless you are given one of the products. You can find the second product by identifying the "missing pieces."

Nickel Carbonate

NiCO3 → CO2 + NiO

Carbonic Acid

H2CO3 → CO2 + H2O
Logic: CaCO3 → CaO + ?
In (CaCO3): 1 Ca, 1 C, 3 O
Out (CaO): 1 Ca, 1 O
Missing: 1 C, 2 O → CO2

Comparison & Identification

The Difference

  • Combination: Substances join. Usually has multiple reactants forming one product.
  • Decomposition: Substance breaks apart. Usually has one reactant forming multiple products.

Identify the Type:

N2 + 3H2 → 2NH3 Combination
2HgO → 2Hg + O2 Decomposition
C + O2 → CO2 Combination

Practice: Balanced Equations

Skeletal Balanced
H2 + Cl2 → HCl H2 + Cl2 → 2HCl
Na + O2 → Na2O 4Na + O2 → 2Na2O
KClO3 → KCl + O2 2KClO3 → 2KCl + 3O2

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