Let's analyze the given reactions to identify which one is a decomposition reaction. A decomposition reaction is a type of chemical reaction where a single compound breaks down into two or more simpler substances.

Analyzing Chemical Reactions to Find Decomposition

We will examine each option provided and determine the type of chemical reaction it represents.

1. $\text{CH}_4 \text{ (g) + 2O}_2\text{(g)} \rightarrow \text{CO}_2\text{ (g) + 2H}_2\text{O(g)}$

   This reaction involves a substance (methane, $\text{CH}_4$) reacting rapidly with oxygen ($\text{O}_2$), producing heat and light. This is a typical example of a combustion reaction.

2. $\text{2AgCl(s)} \rm \xrightarrow{Sunlight} \text{2Ag(s) + Cl}_2\text{(g)}$

   In this reaction, a single compound (silver chloride, $\text{AgCl}$) breaks down into two simpler substances (silver, $\text{Ag}$, and chlorine gas, $\text{Cl}_2$) upon exposure to sunlight. This matches the definition of a decomposition reaction. Specifically, this is an example of photolytic decomposition, where decomposition is caused by light energy.

3. $\text{CuO + H}_2 \rm \xrightarrow{Heat} \text{Cu + H}_2\text{O}$

   Here, hydrogen ($\text{H}_2$) reacts with copper(II) oxide ($\text{CuO}$). Hydrogen removes oxygen from copper(II) oxide, reducing $\text{CuO}$ to $\text{Cu}$. Hydrogen itself gains oxygen, getting oxidized to $\text{H}_2\text{O}$. This reaction is an example of a redox reaction (reduction-oxidation reaction). It can also be classified as a single displacement reaction where $\text{H}_2$ displaces $\text{Cu}$.

4. $\text{Fe(s) + CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq) + Cu(s)}$

   In this reaction, iron ($\text{Fe}$) displaces copper ($\text{Cu}$) from its sulfate solution ($\text{CuSO}_4$). A more reactive metal displaces a less reactive metal from its salt solution. This is a clear example of a single displacement reaction.

Summary of Reaction Types

Based on the analysis, the reaction where a single compound, silver chloride ($\text{AgCl}$), breaks down into simpler substances, silver ($\text{Ag}$) and chlorine ($\text{Cl}_2$), is the decomposition reaction.

Revision Table: Key Reaction Types

Additional Information on Decomposition Reactions

Decomposition reactions often require energy input in the form of heat, light, or electricity.

• Thermal decomposition: Decomposition caused by heat. Example: Heating calcium carbonate ($\text{CaCO}_3$) to form calcium oxide ($\text{CaO}$) and carbon dioxide ($\text{CO}_2$).
• Electrolytic decomposition: Decomposition caused by electricity. Example: Electrolysis of water ($\text{H}_2\text{O}$) to form hydrogen gas ($\text{H}_2$) and oxygen gas ($\text{O}_2$).
• Photolytic decomposition: Decomposition caused by light. Example: Decomposition of silver chloride ($\text{AgCl}$) in sunlight.

Decomposition reactions are important in various chemical processes, including extraction of metals, manufacturing of cement, and photographic development.