Problem 28 - Chemistry Class 9

Question

A reaction of decomposition of a compound \(\mathrm{AB}\) is accompanied by absorption of some heat energy. What is the sign of \(\Delta \mathrm{H}\) ?

Step-by-Step Solution

Answer

Answer: The sign of the enthalpy change is positive because the reaction is endothermic and heat energy is absorbed.

Step 1: Recognize the reaction type

The decomposition of a compound \(\mathrm{AB}\) is a reaction where the compound breaks down into its constituent elements, \(\mathrm{A}\) and \(\mathrm{B}\).

Step 2: Determine the nature of the reaction

The reaction is accompanied by the absorption of heat energy, indicating an endothermic process.

Step 3: Endothermic reactions and Enthalpy change

For endothermic reactions, the enthalpy change (\(\Delta H\)) is positive because heat energy is absorbed by the system from the surroundings.

Step 4: Conclusion

Since the decomposition reaction of compound \(\mathrm{AB}\) absorbs heat energy, it is an endothermic reaction, and the sign of the enthalpy change (\(\Delta H\)) is positive.

Key Concepts

Decomposition Reaction

In chemistry, a decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more simpler substances. This process is often represented by the general formula: \[\text{AB} \rightarrow \text{A} + \text{B}\]- **Simple substances:** The products of a decomposition reaction are usually simpler molecules or elements compared to the original compound.- **Types of decomposition reactions:** They can be classified based on the type of energy input that drives the decomposition.

• **Thermal decomposition:** Requires heat.
• **Electrolytic decomposition:** Uses electricity.
• **Photodecomposition:** Driven by light.

This makes decomposition reactions crucial in understanding how compounds interact and break down, whether in academic study or applied fields like material science and environmental chemistry.

Enthalpy Change

Enthalpy change (\(\Delta H\)) refers to the heat content change of a system during a chemical reaction at constant pressure. In essence, it tells us how much heat energy is absorbed or released throughout a reaction. - **Representation:** It is represented by the symbol \(\Delta H\) which measures the difference in enthalpy between products and reactants.- **Measurement:** The unit of measurement is usually in kilojoules per mole (kJ/mol).- **Types of reactions associated with enthalpy changes:**

• If \(\Delta H > 0\), the reaction is endothermic, meaning heat is absorbed.
• If \(\Delta H < 0\), the reaction is exothermic, indicating heat is released.

Understanding enthalpy change is important to determine if energy must be supplied for a reaction to occur, or if it will release energy, thus making it crucial for practical applications like energy management and synthesis of materials.

Heat Absorption

Heat absorption in a chemical reaction occurs when the system takes in heat energy from the surrounding environment. This phenomenon is associated with endothermic reactions and results in a measurable enthalpy change. - **Characteristics of heat absorption:**

• The substance undergoing the reaction feels cold because heat is taken from it.
• The system gains energy, which can lead to the breaking of chemical bonds.

- **Example in decomposition:** During the decomposition of a compound, the heat absorbed can help in breaking apart the molecules into simpler elements or compounds. The process is crucial in reactions such as melting ice or this exercise's breakdown of compound \(\mathrm{AB}\).Being aware of how and why heat is absorbed in reactions helps chemists predict reaction behavior and balance reactions in terms of energy requirement or release.