Lecture 3: Combination and Decomposition Reactions

Ready for the third lecture! We will now explore the fundamental Types of Chemical Reactions, starting with how substances join together and how they break apart.

1. Combination Reactions

A combination reaction is defined as a reaction where two or more substances (elements or compounds) combine to form a single, new product.

General Form: A + B \rightarrow C

Examples:

1. Formation of Slaked Lime: Calcium oxide (quick lime) reacts vigorously with water to form calcium hydroxide (slaked lime). \text{CaO}(s) + \text{H}_2\text{O}(l) \rightarrow \text{Ca}(\text{OH})_2(aq) + \text{Heat}
2. Burning of Coal: Carbon reacts with oxygen to form carbon dioxide. \text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)
3. Formation of Water: Hydrogen gas combines with oxygen gas. 2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)

Exothermic Reactions

In the reaction where quick lime combines with water, a large amount of heat is evolved, making the mixture warm.

• Definition: Reactions that release heat, along with the formation of products, are called exothermic chemical reactions.
• Examples: The burning of natural gas (\text{CH}_4) and the decomposition of vegetable matter into compost are exothermic.
• Respiration: The process of respiration, where glucose (\text{C}<em>6\text{H}</em>{12}\text{O}_6) combines with oxygen to provide energy, is considered an exothermic process.

2. Decomposition Reactions

Decomposition reactions are considered the opposite of combination reactions.

• Definition: In a decomposition reaction, a single reactant breaks down to yield two or more simpler products.

General Form: C \rightarrow A + B

Decomposition reactions require energy—in the form of heat, light, or electricity—to break the bonds between the atoms of the reactant. Reactions that absorb energy are known as endothermic reactions.

A. Thermal Decomposition (Using Heat)

When decomposition is carried out by heating, it is called thermal decomposition.

1. Limestone (\text{CaCO}_3): Heating limestone yields calcium oxide (quick lime, used in cement manufacture) and carbon dioxide gas. \text{CaCO}_3(s) \xrightarrow{\text{Heat}} \text{CaO}(s) + \text{CO}_2(g)
2. Lead Nitrate: Heating lead nitrate powder results in the emission of brown fumes of nitrogen dioxide (\text{NO}_2) along with lead oxide and oxygen gas. 2\text{Pb}(\text{NO}_3)_2(s) \xrightarrow{\text{Heat}} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)

B. Decomposition by Light (Photolysis)

Light energy can cause decomposition.

1. Silver Chloride (\text{AgCl}): White silver chloride turns grey when placed in sunlight, decomposing into silver metal and chlorine gas. 2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)
2. Silver Bromide (\text{AgBr}): This decomposition reaction is also caused by light and is used in black and white photography.