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

(b) endothermic

Step 1: Understanding the Concept

In thermodynamics, the enthalpy change, \(\Delta H\), indicates whether a reaction absorbs or releases heat. If \(\Delta H\) is positive, it means that the reaction absorbs heat from the surroundings.

Step 2: Identifying the Type of Reaction

A reaction that absorbs heat is described as endothermic. The term 'endothermic' refers to the process of taking in energy in the form of heat, whereas 'exothermic' refers to the release of heat.

Step 3: Conclusion

Given that the enthalpy change \(\Delta H\) is positive, the reaction is classified as endothermic. Therefore, the correct answer is (b) endothermic.

Key Concepts

Thermodynamics

Thermodynamics is a branch of physics that deals with the relationships between heat and other forms of energy. At its core, it studies how energy is transferred within a system and how it affects matter. It is all about the movement of heat and how that affects physical systems.

In thermodynamics, there are laws that describe how energy interacts in a closed system. The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or transformed. This means that the total energy in a closed system remains constant.

• Energy is exchanged between systems and surroundings.
• Heat can be transferred into or out of a system.
• Thermodynamics helps us understand why certain reactions occur.

The concept of enthalpy ( 2 H), a central topic in thermodynamics, is essential for understanding the heat exchange in chemical reactions. Enthalpy change tells us if heat is released or absorbed during a reaction, offering insight into the energy dynamics of the process.

Endothermic Reactions

Endothermic reactions are processes that absorb energy from the surroundings. In the context of chemical reactions, this energy is typically in the form of heat. When we say a reaction is endothermic, it means that energy is required for the reaction to proceed.

These reactions can be seen in everyday life, such as in the process of photosynthesis where plants absorb sunlight to produce glucose and oxygen.

• Endothermic reactions have a positive 2 H (enthalpy change).
• They feel cold to the touch as they absorb heat from the surroundings.
• In an endothermic reaction, the energy of the products is higher than that of the reactants.

The enthalpy change, 2 H, is crucial for confirming whether a reaction is endothermic or not. If 2 H is positive, it indicates that the reaction requires heat absorption to move forward.

Heat Absorption

Heat absorption is a key aspect of endothermic reactions. It refers to the process by which a substance takes in heat energy from its surroundings.

In the context of chemistry, when a reaction absorbs heat, it is necessary to facilitate the endothermic process.

• Heat absorption leads to an increase in the enthalpy of a system.
• This energy intake can break bonds in the reactants, forming new bonds in the products.
• Reactions like melting ice or evaporating water exemplify heat absorption.

Understanding how heat absorption works is vital for predicting how reactions behave under different conditions. By monitoring how much heat is absorbed, scientists can deduce whether a reaction will proceed spontaneously or require external energy input.