Problem 4 - A Complete Resource Book in Chemistry for JEE Main

Question

If the value of \(\mathrm{AH}\) in a reaction is positive, then the reaction is called (a) exothermic (b) endothermic (c) polymorphic (d) polytropic

Step-by-Step Solution

Answer

The reaction is called endothermic (option b).

Step 1: Understand the Concept

An exothermic reaction releases heat to the surroundings, which means it would have a negative change in enthalpy (ΔH < 0). On the other hand, an endothermic reaction absorbs heat, which results in a positive change in enthalpy (ΔH > 0). Understanding these definitions is crucial for solving the problem.

Step 2: Analyze the Given Information

The exercise states that the value of ΔH is positive. This indicates that the reaction absorbs heat from the surroundings.

Step 3: Identify Reaction Type

Given that a positive ΔH indicates heat absorption, we can conclude that the reaction is endothermic.

Step 4: Select the Correct Option

Based on the definitions, we select the option that matches an endothermic reaction. Hence, the correct option is (b) endothermic.

Key Concepts

Enthalpy Change

Enthalpy change refers to the difference in energy between the reactants and the products during a chemical reaction. It indicates how much energy is either absorbed or released as the reaction progresses. Understanding enthalpy is critical because it helps us determine whether a reaction is endothermic or exothermic.

The symbol for enthalpy change is usually \( \Delta H \) (delta H). A positive \( \Delta H \) means that the system has sucked in energy from its surroundings, while a negative \( \Delta H \) shows that energy is being released. Think of \( \Delta H \) as the reaction's "energy balance" sheet. It tells us if the reaction is energy hungry or generous with its energy.

Heat Absorption

Heat absorption is a key feature of endothermic reactions. In such reactions, heat is absorbed from the surroundings, meaning the overall energy within the system increases. This absorption results in a positive enthalpy change, as the energy gained outweighs what was originally present.

Imagine an ice pack you use to soothe an injury. When you squeeze it to activate, chemicals inside the pack react by absorbing heat, making the surrounding area feel cold. This is a practical, everyday example of a reaction absorbing heat. Other reactions that absorb heat often appear as increases in temperature because the system pulls energy from its environment.

Remember these key points about heat absorption:

• It results in a positive \( \Delta H \)
• It is typical of endothermic reactions
• It leads to energy being stored within the system

Exothermic Reaction

Exothermic reactions release heat and are often observed in everyday life. These reactions let off energy into their surroundings causing a rise in the temperature of the environment.

Combustion is a classic example of an exothermic process, like when you burn wood in a fireplace. This energy release leads to a negative enthalpy change or \( \Delta H < 0 \), as the system releases more energy than it initially contained.

Key characteristics of exothermic reactions include:

• Negative \( \Delta H \), showing energy release
• Typically results in a temperature increase of the surroundings
• Can occur quickly, releasing a large amount of energy in a short time

Understanding these concepts will help you identify reaction types confidently based on their energy profiles.