An Introduction to Chemical Reactions

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances into another. At the heart of chemistry is understanding how and why these reactions occur. We can classify most chemical reactions into one of five main types.

1. Synthesis (or Combination) Reaction

In a synthesis reaction, two or more simple substances combine to form a more complex product. Think of it as building something bigger from smaller pieces.

General Formula: A + B ⟶ AB

Example: The reaction between sodium and chlorine gas to produce sodium chloride (table salt).

2Na + Cl₂ ⟶ 2NaCl

2. Decomposition Reaction

A decomposition reaction is the opposite of a synthesis reaction. A complex substance breaks down into two or more simpler substances.

General Formula: AB ⟶ A + B

Example: The decomposition of hydrogen peroxide into water and oxygen gas.

2H₂O₂ ⟶ 2H₂O + O₂

3. Single-Displacement (or Single-Replacement) Reaction

In this type of reaction, a single uncombined element replaces another element in a compound. Essentially, one element "kicks out" another.

General Formula: A + BC ⟶ AC + B

Example: Zinc metal reacts with copper(II) chloride. The zinc replaces the copper.

Zn + CuCl₂ ⟶ ZnCl₂ + Cu

4. Double-Displacement (or Double-Replacement) Reaction

In a double-displacement reaction, the positive and negative ions of two ionic compounds exchange places to form two new compounds. It's like the compounds "swap partners."

General Formula: AB + CD ⟶ AD + CB

Example: When silver nitrate reacts with sodium chloride, they form silver chloride and sodium nitrate.

AgNO₃ + NaCl ⟶ AgCl + NaNO₃

5. Combustion Reaction

A combustion reaction is a rapid reaction between a substance with an oxidant, usually oxygen, to produce heat and light. When the substance is a hydrocarbon (made of carbon and hydrogen), the products are always carbon dioxide and water.

General Formula (for hydrocarbons): Hydrocarbon + O₂ ⟶ CO₂ + H₂O

Example: The combustion of methane (natural gas).

CH₄ + 2O₂ ⟶ CO₂ + 2H₂O

Practice Problems

Instructions: For each of the following reactions, identify which of the five types of chemical reactions it is.

1. H₂ + O₂ ⟶ H₂O

• Type of Reaction: _________________________

2. Mg + 2HCl ⟶ MgCl₂ + H₂

• Type of Reaction: _________________________

3. CaCO₃ ⟶ CaO + CO₂

• Type of Reaction: _________________________

4. C₃H₈ + 5O₂ ⟶ 3CO₂ + 4H₂O

• Type of Reaction: _________________________

5. 2KOH + H₂SO₄ ⟶ K₂SO₄ + 2H₂O

• Type of Reaction: _________________________

Chemical Reactions - ANSWER KEY

1. 2H₂ + O₂ ⟶ 2H₂O

Type of Reaction: Synthesis

Explanation: Two simple substances (hydrogen and oxygen) combine to form a more complex one (water).

2. Mg + 2HCl ⟶ MgCl₂ + H₂

Type of Reaction: Single-Displacement

Explanation: Magnesium (a single element) replaces the hydrogen in hydrochloric acid (HCl).

3. CaCO₃ ⟶ CaO + CO₂

Type of Reaction: Decomposition

Explanation: A complex substance (calcium carbonate) breaks down into two simpler substances (calcium oxide and carbon dioxide).

4. C₃H₈ + 5O₂ ⟶ 3CO₂ + 4H₂O

Type of Reaction: Combustion

Explanation: A hydrocarbon (propane, C₃H₈) reacts with oxygen to produce carbon dioxide and water.

5. 2KOH + H₂SO₄ ⟶ K₂SO₄ + 2H₂O

Type of Reaction: Double-Displacement

Explanation: The potassium (K⁺) from KOH swaps with the hydrogen (H⁺) from H₂SO₄. The positive ions have "swapped partners."